Re: 파킨슨 병 .. Visual Dysfunction in Parkinson’s Disease 2023년 논문...

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Brain Sci. 2023 Aug; 13(8): 1173.

Published online 2023 Aug 7. doi: 10.3390/brainsci13081173

PMCID: PMC10452537

PMID: 37626529

Visual Dysfunction in Parkinson’s Disease

Francisco Nieto-Escamez,1,2,* Esteban Obrero-Gaitán,3,* and Irene Cortés-Pérez3

George B. Stefano, Academic Editor and Abdelaziz M. Hussein, Academic Editor

Author information Article notes Copyright and License information PMC Disclaimer

Associated DataData Availability Statement

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Abstract

Non-motor symptoms in Parkinson’s disease (PD) include ocular, visuoperceptive, and visuospatial impairments, which can occur as a result of the underlying neurodegenerative process. Ocular impairments can affect various aspects of vision and eye movement. Thus, patients can show dry eyes, blepharospasm, reduced blink rate, saccadic eye movement abnormalities, smooth pursuit deficits, and impaired voluntary and reflexive eye movements.

Furthermore, visuoperceptive impairments affect the ability to perceive and recognize visual stimuli accurately, including impaired contrast sensitivity and reduced visual acuity, color discrimination, and object recognition. Visuospatial impairments are also remarkable, including difficulties perceiving and interpreting spatial relationships between objects and difficulties judging distances or navigating through the environment.

Moreover, PD patients can present visuospatial attention problems, with difficulties attending to visual stimuli in a spatially organized manner. Moreover, PD patients also show perceptual disturbances affecting their ability to interpret and determine meaning from visual stimuli. And, for instance, visual hallucinations are common in PD patients. Nevertheless, the neurobiological bases of visual-related disorders in PD are complex and not fully understood. This review intends to provide a comprehensive description of visual disturbances in PD, from sensory to perceptual alterations, addressing their neuroanatomical, functional, and neurochemical correlates. Structural changes, particularly in posterior cortical regions, are described, as well as functional alterations, both in cortical and subcortical regions, which are shown in relation to specific neuropsychological results. Similarly, although the involvement of different neurotransmitter systems is controversial, data about neurochemical alterations related to visual impairments are presented, especially dopaminergic, cholinergic, and serotoninergic systems.

파킨슨병(PD)의 비운동 증상으로는 

안구, 시지각 및 시공간 장애가 있으며, 

이는 근본적인 신경 퇴행성 과정의 결과로 발생할 수 있습니다. 

ocular,

visuoperceptive, and

visuospatial impairments

안구 장애는 

시력과 안구 운동의 다양한 측면에 영향을 미칠 수 있습니다. 

따라서 환자는 

안구 건조증, 

안검 경련, 

눈 깜박임 속도 감소, 

안구 운동 이상 saccadic eye movement abnormalities , 

매끄러운 추구 결손, 

자발적 및 반사적 안구 운동 장애를 보일 수 있습니다. 

Thus, patients can show dry eyes, blepharospasm, reduced blink rate, saccadic eye movement abnormalities, smooth pursuit deficits, and impaired voluntary and reflexive eye movements

https://www.youtube.com/watch?app=desktop&v=l_W-ZDvdL_E

또한 

시지각 장애는 

대비 감도 손상, 

시력 저하, 

색상 구분, 

물체 인식 등 

시각 자극을 정확하게 인지하고 인식하는 능력에 영향을 미칩니다. 

visuoperceptive impairments affect the ability to

perceive and recognize visual stimuli accurately, including

impaired contrast sensitivity and

reduced visual acuity,

color discrimination, and

object recognition.

물체 간의 공간적 관계를 인지하고 해석하는 데 어려움을 겪고 

거리를 판단하거나 

환경을 탐색하는 데 어려움을 겪는 등 

시공간 장애도 두드러지게 나타납니다. 

Visuospatial impairments are also remarkable, including

difficulties perceiving and interpreting spatial relationships between objects and

difficulties judging distances or

navigating through the environment

또한 

PD 환자는 

시각적 자극을 공간적으로 체계적으로 처리하는 데 어려움을 겪는 

시공간 주의력 문제를 보일 수 있습니다. 

또한 

PD 환자는 

시각적 자극을 해석하고 의미를 판단하는 능력에 영향을 미치는 

지각 장애를 보이기도 합니다. 

예를 들어, 

시각적 환각은 

PD 환자에게 흔한 증상입니다. 

그럼에도 불구하고 

PD에서 시각 관련 장애의 신경생물학적 기반은 

복잡하고 완전히 이해되지 않았습니다. 

이 리뷰에서는 

감각 장애부터 

지각 장애에 이르기까지 

PD의 시각 장애에 대한 포괄적인 설명을 제공하고 

신경해부학적, 기능적, 신경화학적 상관관계를 다루고자 합니다. 

특히 

후피질 영역의 구조적 변화와 

피질 및 피질하 영역의 기능적 변화가 

특정 신경심리학적 결과와 관련하여 설명되어 있습니다. 

마찬가지로 

다양한 신경전달물질 시스템의 관여에 대해서는 논란이 있지만 

시각 장애와 관련된 신경화학적인 변화, 특히 

도파민계, 콜린계 및 세로토닌계에 대한 

데이터가 제시됩니다.

Keywords: Parkinson’s disease, visual impairment, visuoperceptive deficit, visuospatial deficit, visual hallucinations, dopamine

1. Introduction

Parkinson’s disease (PD) is the second most common neurodegenerative disorder after Alzheimer’s disease, with an estimated preval‎ence for industrialized countries of 1% of the population over 60 years and 3% in people older than 80 years [1]. PD is characterized by the loss of dopaminergic neurons in substantia nigra pars compacta, which modulate the fronto–thalamo–striatal circuit, leading to a wide range of motor and non-motor symptoms (NMSs) [2,3]. Thus, NMS is a broad spectrum of symptoms, including mood disorders [4], sensory and perceptual dysfunction [5], and cognitive disturbances [6], with visuospatial processing impairments among the NMSs having accumulated the most interest in the last years [7,8].

Basic visual processes are affected in PD, including reduced spatial contrast sensitivity and impaired color discrimination [9], oculomotor control defects and diplopia [10,11], dry eyes disease [12], glaucoma [13], and visual hallucinations even in the absence of dementia [14].

Furthermore, visual pathologies are accompanied by a poor performance in tasks requiring high-order processing capabilities, such as object mental rotation [15], perception of space [16], spatial maps representation, visuospatial working memory, effective navigation, and target localization [17,18,19], which can be considered as preclinical markers and predictors of disease development [20].

Impaired visual and visuospatial functions can affect a broad range of essential daily living skills, such as driving, reading, writing, or walking [21,22]. These problems have been reported to be increased throughout the progress of the illness, resulting in a reduction of self-efficacy and quality of life.

This review aims to provide a comprehensive review of ocular, visuoperceptive, and visuospatial disturbances in PD, as well as the consequences of integration deficits between high-level perceptual and cognitive processes. For that purpose, published research employing specific neuropsychological tasks have been included, and the disease’s effects on their performance have been examined, even in different phases of the disease. The neurobiological substrates, including structural and functional alterations, and neurochemical mechanisms will be revised for each condition. Until now, some papers reviewing visual disturbances in PD have been published. Some time ago, an outstanding paper by Weil et al. [20] revised changes in visual function at different stages of visual processing, paying special attention to genetic factors, and the relation between clinical features of PD and visuoperceptual alterations as a biomarker of the disease. And, more recently, another remarkable review by Ghazi-Saidi [23] has analyzed the relation between visuospatial and executive deficits in PD. Our work is an attempt to provide an actual and comprehensive picture of visual dysfunctions in PD.

1. 소개

파킨슨병(PD)은 알츠하이머병 다음으로 가장 흔한 신경 퇴행성 질환으로,

선진국에서는 60세 이상 인구의 1%,

80세 이상에서는 3%의 유병률을 보이는 것으로 추정됩니다[1].

PD는 흑질 흑질의 도파민성 뉴런이 소실되어

전탈라모-선조체 회로를 조절하여

광범위한 운동 및 비운동 증상(NMS)을 유발하는 것이 특징입니다[2,3].

따라서

NMS는

기분 장애[4], 감각 및 지각 기능 장애[5], 인지 장애[6] 등

광범위한 증상을 포함하며,

지난 몇 년 동안 가장 많은 관심이 집중된 것은 NMS 중

시각 공간 처리 장애입니다[7,8].

PD에서는

공간 대비 감도 감소 및 색상 변별력 장애[9],

안구 운동 조절 결함 및 복시[10,11],

안구 건조증[12],

녹내장[13],

치매가 없는 경우에도 시각 환각[14] 등

기본적인 시각 프로세스가 영향을 받습니다.

또한

시각 병리는

물체 정신 회전[15],

공간 지각[16],

공간 지도 표현,

시공간 작업 기억,

효과적인 탐색,

목표물 위치 파악[17,18,19] 등

고차 처리 능력이 필요한 작업의 수행 저하를 동반하며,

이는 질병 발병의 전임상 마커이자 예측 인자로 간주될 수 있습니다[20].

시각 및 시공간 기능 장애는

운전, 읽기, 쓰기, 걷기와 같은

광범위한 필수 일상 생활 기술에 영향을 미칠 수 있습니다[21,22].

이러한 문제는

질병이 진행됨에 따라 증가하여

자기 효능감과 삶의 질을 떨어뜨리는 것으로 보고되었습니다.

이 리뷰는

PD의 안구,

시지각 및 시공간 장애와

고차원 지각 및 인지 과정 간의 통합 결함의 결과에 대한

포괄적인 검토를 제공하는 것을 목표로 합니다.

이를 위해 특정 신경심리학적 과제를 사용한 발표된 연구를 포함했으며, 질병의 여러 단계에서도 질병이 과제 수행에 미치는 영향을 조사했습니다. 구조적 및 기능적 변화를 포함한 신경생물학적 기질과 신경화학적 메커니즘을 각 질환에 맞게 수정할 예정입니다. 지금까지 파킨슨병의 시각 장애를 검토한 논문이 몇 편 발표되었습니다. 얼마 전 Weil 등[20]의 뛰어난 논문은 시각 처리의 여러 단계에서 시각 기능의 변화를 수정하여 유전 적 요인과 질병의 바이오 마커로서 PD의 임상 적 특징과 시지각 변화 사이의 관계에 특별한주의를 기울였습니다. 그리고 최근에는 Ghazi-Saidi의 또 다른 주목할 만한 연구[23]에서 PD의 시공간적 결함과 실행 결함 사이의 관계를 분석했습니다. 이러한 연구는 PD의 시각 기능 장애에 대한 실제적이고 포괄적인 그림을 제공하려는 시도입니다.

2. Ocular and Visual Impairments in Parkinson’s Disease

PD patients show a number of ocular and visual impairments resulting from pathological processes or consequences of medication, and usually get worse during disease progression [24], with up to 70% of patients reporting recurrent visual complaints [25]. Thus, a panoply of oculomotor issues have been described in PD patients [10,26], including poor binocular convergence, double vision (diplopia), bradykinesia and hypokinesia of ocular pursuit, impaired vertical upward and downward glaze, defective saccadic movements with longer reaction times, hypometria, and square wave jerks [24,27,28,29]. These problems have direct consequences on patients’ abilities to perform daily tasks, such as reading. writing, driving, or navigating [5]. Furthermore, visual deficits can even be found in the prodromal stages of PD, as early as decades before the onset of motor symptoms [30].

2. 파킨슨병의 안구 및 시각 장애

PD 환자는

병리학적 과정이나 약물 치료의 결과로 인해

여러 가지 안구 및 시각 장애를 보이며,

일반적으로 질병이 진행되는 동안 악화되며[24],

최대 70%의 환자가 반복적인 시각 불만을 보고합니다[25].

따라서

PD 환자에서는

양안 수렴 불량,

복시(복시),

안구 추구의 서동 운동 및 운동 저하,

수직 상하 운동 장애,

반응 시간이 긴 결함성 안진 운동,

hypometria,

square wave jerks 등[10,26]

다양한 안구 운동 문제가 나타나고 있습니다[24,27,28,29].

Thus, a panoply of oculomotor issues have been described in PD patients [10,26], including

poor binocular convergence,

double vision (diplopia),

bradykinesia and hypokinesia of ocular pursuit,

impaired vertical upward and downward glaze,

defective saccadic movements with longer reaction times,

hypometria, and

square wave jerks

https://www.youtube.com/watch?v=wKQZlPBJQM4

이러한 문제는

환자의 읽기, 쓰기, 운전, 내비게이션과 같은

일상 업무 수행 능력에 직접적인 영향을 미칩니다[5].

또한,

시각적 결손은

운동 증상이 나타나기 수십 년 전인

PD의 전구 단계에서도 발견될 수 있습니다[30].

2.1. Retinal Changes

In the PD population, the main structures that make up the eyeball and the optic nerve are quantitatively and qualitatively affected. High-resolution structural imaging approaches by optical coherence tomography (OCT) of the retina show a decrease of the macular retinal thickness, macular volume, average retinal nerve fiber layer (RNFL), retinal ganglion cell layer (RGC), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), retinal pigment epithelium, and photoreceptor layer in PD patients [31,32,33,34] (See Figure 1). However, macular thickness or volume are not always reduced [33,35].

2.1. 망막 변화

PD 환자군에서는

안구와 시신경을 구성하는 주요 구조가

양적, 질적으로 영향을 받습니다.

망막의 빛간섭단층촬영(OCT)을 통한 고해상도 구조 영상 접근법은

황반 망막 두께,

황반 용적, 평균

망막 신경 섬유층(RNFL),

망막 신경절 세포층(RGC)의 감소를 보여줍니다,

내망막신경섬유층(IPL), 내핵층(INL), 외망막신경섬유층(OPL), 외핵층(ONL), 망막색소상피 및 광수용체층이 감소합니다[31,32,33,34]( 그림 1 참조). 그러나 황반 두께나 부피가 항상 감소하는 것은 아닙니다 [33,35].

Figure 1

Portion of a human retina. A: Amacrine cell; Bi: Bipolar cell; BM: Bruch’s membrane; C: Cone; GCL: Ganglion cell layer; H: Horizontal cell; INL: Inner nuclear layer; IPL: Inner plexiform layer; IS: Inner segment; M: Muller cell; ONL: Outer nuclear layer; OPL: Outer plexiform layer; OS: Outer Segment; P: Pigment epithelial cell; R: Rod; RPE: Retinal pigment epithelium; G: Ganglion cell; AX: Axons. Adapted from Hartmann and Schmid [43]. Image licensed under GFDL by the author. https://de.wikipedia.org/wiki/Datei:Retina.jpg (accessed on 19 June 2023).

Some authors have attempted to correlate retinal layer thinning with clinical scales scores. It has been reported that RNFL thickness reduction correlated to duration and severity of disease [35,36]. However, other authors could not confirm‎ this alteration [37,38,39] nor its relation with disease duration or severity [40]. The RGCIPL layer thickness in the parafovea has been the parameter most frequently correlated with visual outcomes in PD patients [40,41,42].

Along with structural alterations, functional changes assessed through the visual evoked response (VER) and the electroretinogram (ERG) have also been shown to correlate with PD duration and severity [35]. It has also been proposed that electrophysiological alterations begin years before structural changes are observable [44].

Retinal degeneration may be caused by progressive dopamine depletion and α-synuclein-mediated axonal degeneration [13]. Dopamine is a key neurotransmitter in the retina, and its depletion results in a reduced electrical response under different light conditions [45]. IPL and GCL thinning observed in PD has been linked to dopaminergic loss in the left substantia and disease severity [46]. Furthermore, dopamine also influences other retinal neurotransmitters involved in retinal processing, such as glutamate, GABA, and glycine, disrupting these neurotransmission pathways [44].

Moreover, misfolded α-synuclein [47] and phosphorilated α-synuclein [48] have been reported in the inner retinal layer, and retinal α-synucleinopathy density scores positively correlate with brain α-synucleinopathy density scores, pathology stage, and the UPDRS-III motor sub-score [49].

Retinal and macula nerve fiber layer thinning is also linked with minor hallucinations [50] and disease duration and severity [51]. Moreover, retinal layer thinning correlates with frontal and occipital cortex thickness and is linked to lower scores in the Poppelreuter-Ghent overlapping figures test and the RPMT in a visually impaired PD sample [52]. Recently, Hannaway et al. [53] have reported that visual dysfunction is a better predictor than retinal thickness for dementia in PD.

일부 저자는 망막층 얇아짐과 임상 척도 점수의 상관관계를 시도한 바 있습니다. RNFL 두께 감소는 질병의 지속 기간 및 중증도와 상관관계가 있는 것으로 보고되었습니다[35,36]. 그러나 다른 저자들은 이러한 변화[37,38,39]나 질병 지속 기간 또는 중증도와의 관계를 확인하지 못했습니다[40]. PD 환자의 시각적 결과와 가장 자주 상관관계가 있는 파라미터는 주변시야의 RGCIPL 층 두께였습니다 [40,41,42].

구조적 변화와 함께 시각유발반응(VER)과 망막전도(ERG)를 통해 평가된 기능적 변화도 PD 지속 기간 및 중증도와 상관관계가 있는 것으로 나타났습니다 [35]. 또한 전기생리학적 변화는 구조적 변화가 관찰되기 수년 전에 시작된다는 주장도 제기되었습니다 [44].

망막 변성은

점진적인 도파민 고갈과

α-시누클레인 매개 축삭 변성에 의해 발생할 수 있습니다 [13].

도파민은

망막의 주요 신경전달물질로,

도파민이 고갈되면 다양한 빛 조건에서

전기적 반응이 감소합니다 [45].

도파민은 망막의 핵심 신경전달물질로 시각 시스템의 빛 적응 과정에서 중심적인 역할을 합니다. 망막 도파민의 유일한 공급원은 도파민성 아마크린 세포(DAC)입니다. 저희와 다른 연구자들은 이전에 DAC가 막대, 원뿔 및 본질적으로 광과민성 망막 신경절 세포(ipRGC)에 의해 활성화된다는 사실을 입증한 바 있습니다. 그러나 각 종류의 감광성 세포가 DAC에서 어떻게 빛에 대한 반응을 일으키는지는 아직 명확하지 않습니다. 저희는 생쥐에서 원추체 기능을 유전적으로 분리하여 DAC의 원추체 매개 반응과 그 신경 경로를 구체적으로 조사했습니다. 우리는 빛 증가(ON) 양극성 세포로부터 DAC에 대한 흥분성 입력이 보고된 것 외에도, 원뿔이 ipRGC의 역행성 신호 경로를 통해 DAC에 신호를 보내는 것을 발견했습니다. 원뿔은 또한 다른 아마크린 세포에 의해 매개되는 DAC에서 ON 및 광감소(OFF) 억제 반응을 생성하며, 이는 각각 1형 및 2/3a형 OFF 양극성 세포에 의해 주도될 가능성이 높습니다. 염료 주입 결과, DAC는 ON/OFF 억제 유무에 관계없이 유사한 형태학적 프로파일을 보였습니다. 우리의 데이터는 원추세포가 특정 병렬 흥분 및 억제 회로를 활용하여 DAC 활동을 조절하고 도파민 방출과 망막의 빛 적응 상태를 효율적으로 조절한다는 것을 보여줍니다.

https://www.mdpi.com/1422-0067/25/4/2226

PD에서 관찰되는 IPL 및 GCL 얇아짐은

좌측 실질의 도파민 손실 및 질병 중증도와 관련이 있습니다 [46].

또한

도파민은

글루타메이트, GABA, 글리신과 같이 망막 처리에 관여하는

다른 망막 신경전달물질에도 영향을 미쳐

이러한 신경전달 경로를 방해합니다 [44].

또한

망막 내층에서 잘못 접힌 α-시누클레인 [47] 및 인산화 α-시누클레인 [48]이 보고되었으며,

망막 α-시누클레인 병증 밀도 점수는

뇌 α-시누클레인 병증 밀도 점수, 병리 단계 및 UPDRS-III 운동 하위 점수와

양의 상관관계가 있는 것으로 나타났습니다 [49].

망막 및 황반 신경 섬유층 얇아짐은

경미한 환각[50],

질병 지속 기간 및 중증도[51]와도 관련이 있습니다.

또한

망막층 얇아짐은

전두엽 및 후두피질 두께와 상관관계가 있으며,

시각장애 PD 샘플에서 포펠로이터-겐트 겹치기 그림 검사 및 RPMT의 낮은 점수와 관련이 있습니다 [52].

최근 Hannaway 등[53]은

시각 기능 장애가

망막 두께보다

치매에 대한 더 나은 예측 인자라고 보고했습니다.

2.2. Glaucomatous Disturbances

There is also a higher incidence of glaucoma and glaucomatous-like visual field defects, with the majority of cases related to open-angle glaucoma [54,55]. However, epidemiological data are scarce, and the evidence regarding these deficits is low [56,57].

2.2. 녹내장성 장애

녹내장 및 녹내장과 유사한 시야 결손의 발생률도 높으며,

대부분의 경우 개방각 녹내장과 관련이 있습니다 [54,55].

그러나

역학 데이터는 부족하고

이러한 결함에 대한 증거는 부족합니다 [56,57].

2.3. Pupil Reactivity

Alterations in pupil reactivity have been observed in PD patients. PD patients show a significantly lager pupil diameter, with unequal pupil size after light adaptation and longer light reflex latencies and constriction times, along with reduced contraction amplitude [26,58]. Some studies suggest that pupil changes could be independent from dopaminergic deficiency [59], and dopaminergic treatment has no effect on the pupil light reflex [60]. Alhassan et al. [59] suggest that both parasympathetic (cholinergic) and sympathetic (adrenergic) autonomic systems are altered in PD, but the parasympathetic pathway is more affected. The parasympathetic imbalance is considered an early manifestation of PD [61]. You et al. [62] point out that pupillary parasympathetic dysfunction advances with the progression of PD, whereas pupillary sympathetic dysfunction changes slowly.

Pupil reactivity alterations may also reflect a sensory deficit due to impaired retinal or optic nerve function, but it has also been suggested they could be due to the degeneration of subcortical regions, such as the locus coeruleus in the brain stem [59].

Finally, cognitively impaired PD patients show more pupil constriction deficits than those with normal cognitive functions, similarly to pupil dysfunction reported in Alzheimer’s disease [63]. Kahya et al. [64] have reported an increased pupillary response with increased postural demand in PD.

2.3. 동공 반응성

PD 환자에서

동공 반응성의 변화가 관찰되었습니다.

PD 환자는

동공 직경이 상당히 커지고,

빛 적응 후 동공 크기가 불균일하며,

빛 반사 지연 시간과 수축 시간이 길어지고,

수축 진폭이 감소합니다 [26,58].

일부 연구에서는

동공 변화가 도파민 결핍과 무관할 수 있으며[59],

도파민 치료는 동공 빛 반사에 영향을 미치지 않는다고 제안합니다[60].

알하산 등[59]은

부교감(콜린성) 및 교감(아드레날린성) 자율신경계가

PD에서 모두 변화하지만

부교감 경로가 더 많은 영향을 받는다고 제안합니다.

부교감신경 불균형은

PD의 초기 증상으로 간주됩니다[61].

You 등[62]은

동공 부교감 기능 장애는

PD의 진행과 함께 진행되는 반면

동공 교감 기능 장애는 천천히 변화한다고 지적합니다.

동공 반응성 변화는

망막 또는 시신경 기능 장애로 인한 감각 결손을 반영할 수도 있지만,

뇌간 흑질과 같은 피질하 영역의 퇴행으로 인한 것일 수도 있다고 제안되었습니다 [59].

마지막으로,

인지 장애가 있는 PD 환자는

알츠하이머병에서 보고된 동공 기능 장애와 유사하게

정상 인지 기능을 가진 환자보다

더 많은 동공 수축 결손을 보입니다 [63].

Kahya 등[64]은 PD에서 자세 요구가 증가함에 따라 동공 반응이 증가한다고 보고했습니다.

2.4. Eyelid and Blink Reflex

Eyelid impairments, including a reduction of blink rate, apraxia in the opening of the eyelid, blepharospasm, and ptosis of the upper and Meibomian gland disease have been reported [26,65,66,67]. Three types of eyelid movement abnormalities are notable in the experimental models of PD: blink hyperreflexia, impaired blink reflex plasticity, and reduced rate and impaired rhythm of the spontaneous blinks [68].

Blepharospasm (BSP) is a form of focal dystonia that manifests with spasms of the eyelids, involuntary closure of the eye, and enhanced spontaneous blinking, or any combination of the previous ones [69].

Armstrong [70] reported that blink duration and excitability appear to be increased, which may be related to a loss of dopaminergic neurons. Furthermore, many PD patients show a reduced habituation response of the blink reflex, which may improve after treatment with levodopa or amantidine [70], although other authors reported no positive relationship between blink rate and dopamine synthesis capacity [71].

A reduced corneal sensitivity has been reported in PD by Reddy et al. [72]. And, some authors have shown that the lens can show defects in PD [12], with an increased preval‎ence of marked nuclear cataracts and a higher intensity of subscapular cataracts.

One of the most usual oculo-visual conditions is dry eye disease, affecting up to 60% of PD patients [12,67]. Dry eye disease in PD can be the result of corneal hypoesthesia, decreasing blinking rate and reflex lacrimation, autonomic neuropathy leading to decreased tear secretion, increased tear osmolarity, decreased tear mucin, and lipid layer disruption secondary to meibomian gland dysfunction [67].

2.4. 눈꺼풀 및 눈 깜박임 반사

눈 깜박임 속도 감소,

눈꺼풀 열림의 운동실조증,

안검 경련,

상안검 및 메이보미안샘 질환을 포함한

눈꺼풀 장애가 보고되었습니다[26,65,66,67].

PD의 실험 모델에서는

눈 깜박임 과반사,

눈 깜박임 반사 가소성 장애,

자발적 눈 깜박임의 속도 감소 및 리듬 장애의

세 가지 유형의 눈꺼풀 운동 이상이 주목할 만합니다 [68].

안검 경련(BSP)은

눈꺼풀 경련, 눈의 불수의적 감김, 자발적 깜박임 강화 또는

이전 증상들의 조합으로 나타나는

국소 근긴장이상증의 한 형태입니다 [69].

암스트롱[70]은

눈 깜박임 지속 시간과 흥분성이 증가하는 것으로 보이며,

이는 도파민성 뉴런의 손실과 관련이 있을 수 있다고 보고했습니다.

또한, 많은 PD 환자는

눈 깜박임 반사의 습관화 반응이 감소하여

레보도파 또는 아만티딘 치료 후 개선될 수 있지만[70],

다른 저자들은 눈 깜박임 속도와 도파민 합성 능력 사이에 긍정적인 관계가 없다고 보고했습니다[71].

Reddy 등은 PD에서 각막 민감도 감소가 보고되었습니다[72]. 또한 일부 저자는 수정체가 PD에서 결함을 보일 수 있으며[12], 현저한 핵 백내장의 유병률이 증가하고 수정체하 백내장의 강도가 더 높다는 것을 보여주었습니다.

가장 흔한 안구 시각 질환 중 하나는

안구건조증으로,

PD 환자의 최대 60%에 영향을 미칩니다[12,67].

PD의 안구건조증은

각막 감각 저하, 깜박임 속도 및 반사 눈물 감소, 눈물 분비 감소로 이어지는

자율 신경 병증, 눈물 삼투압 증가, 눈물 점액 감소, 마이봄샘 기능 장애로 인한

지질층 파괴의 결과로 발생할 수 있습니다 [67].

2.5. Visual Acuity, Contrast Sensitivity, and Color Vision in PD

PD patients often complain of poor vision. This deficit can result from, besides other reasons, impaired visual acuity, with low-contrast acuity particularly affected [73]. A reduction of contrast sensitivity has been reported, particularly for intermediate and high frequencies [27] in central (foveal) and peripheral locations [74]. Loss of color vision has also been described [75]. Cross-sectional studies show that PD patients self-report poor eyesight [76] and have poorer objectively measured visual function parameters [77].

Polo et al. [77] found that parameters corresponding to visual acuity at different contrast levels and all contrast sensitivity test results were altered in patients with PD in comparison with healthy participants, with contrast sensitivity the most affected variable. These authors also reported that patients had a tendency to protanomaly.

Polo et al. [77] also proposed that RGC loss could be the cause of contrast and color deficiencies in PD, with dopaminergic D1 and D2 receptors playing a role in color vision and contrast sensitivity alterations [32]. And, reduced macular thickness and volume have been associated with poor visual acuity, contrast sensitivity, and color vision [77].

Although a retinal dopamine deficit might be related to contrast sensitivity deficits, the orientation-specific impairment points to cerebral cortex involvement [20]. Contrast sensitivity deficits can be partially reversed with levodopa, and apomorphine has been shown to improve contrast perception at all spatial frequencies [78].

Color discrimination deficits may be an early dopaminergic symptom in PD and a disease-specific feature [75]. Red–green color blindness (protan–deutan axis) produces blurred vision, with reduced perception of monochromatic contours, especially for dark green, light blue, and dark red stimuli [20]. Color vision dysfunction is observed even at early stages of the disease and progresses with the disease [79], affecting the motor speed and core region of the body performance [73].

Recently, dysfunction across all aspects of vision, including visual acuity, contrast sensitivity, color vision, and higher-order visual processes, have been linked with a higher risk of dementia in PD [80].

2.5. PD의 시력, 대비 감도 및 색각

PD 환자는

종종 시력 저하를 호소합니다.

이러한 시력 저하는 다른 이유 외에도 시력 장애로 인해 발생할 수 있으며,

특히 저대비 시력이 영향을 받습니다 [73].

특히 중간 및 고주파수[27]의 경우,

중심(중심와) 및 주변부 위치에서 대비 감도가 감소하는 것으로 보고되었습니다[74].

색각 상실도 설명되었습니다 [75].

단면 연구에 따르면 PD 환자는 시력 저하를 스스로 보고하고[76],

객관적으로 측정된 시각 기능 매개변수가 더 나쁘다고 합니다[77].

Polo 등[77]은 건강한 참가자와 비교하여 PD 환자에서 다양한 대비 수준에서의 시력과 모든 대비 민감도 테스트 결과에 해당하는 매개변수가 변경되었으며, 대비 민감도가 가장 큰 영향을 받는 변수임을 발견했습니다. 이 저자들은 또한 환자들이 변성 경향이 있다고 보고했습니다.

Polo 등[77]은 RGC 손실이 PD의 대비 및 색 결핍의 원인이 될 수 있으며, 도파민성 D1 및 D2 수용체가 색각 및 대비 감도 변화에 중요한 역할을 한다고 제안했습니다[32]. 또한 황반 두께와 부피 감소는 시력, 대비 감도 및 색각 저하와 관련이 있습니다 [77].
망막 도파민 결핍은 대비 감도 결핍과 관련이 있을 수 있지만, 방향에 따른 장애는 대뇌 피질 침범을 가리킵니다 [20]. 대비감도 결핍은 레보도파로 부분적으로 회복될 수 있으며, 아포모르핀은 모든 공간 주파수에서 대비 지각을 개선하는 것으로 나타났습니다 [78].

색채 변별력 결핍은 PD의 초기 도파민성 증상이자 질병 특유의 특징일 수 있습니다 [75]. 적록색맹(프로탄-듀탄 축)은 특히 진한 녹색, 하늘색, 진한 빨간색 자극에 대해 단색 윤곽에 대한 인식이 감소하면서 시야가 흐려집니다 [20]. 색각 기능 장애는 질병의 초기 단계에서도 관찰되며 질병과 함께 진행되어 [79] 운동 속도와 신체 성능의 핵심 영역에 영향을 미칩니다 [73].
최근에는 시력, 대비 감도, 색각 및 고차 시각 과정을 포함한 시각의 모든 측면에 걸친 기능 장애가 PD의 치매 위험 증가와 관련이 있는 것으로 밝혀졌습니다 [80].

2.6. Visual Hallucinations

Visual hallucinations (VHs) are the most common manifestation of psychosis in PD, and have been be associated with rapid cognitive decline in PD patients. Their occurrence takes place in up to 75% of PD patients [81]. Moreover, the pathogenesis of VHs in patients with PD is not well understood. Over the course of the disease, minor hallucinations may first evolve into VHs with retained insight and, subsequently, into multimodality hallucinations with loss of insight and delusions. These usually consist of vividly perceived scenes, including people and animals. Passage hallucinations with objects passing across the peripheral visual field, extracampine hallucinations, and a sense of presence have also been described [81].

VHs in PD have been explained as a result of dysfunction of attentional networks in combination with ambiguous visual input, which may lead to VHs when remembered images intrude into consciousness [82]. Abnormal levels of the default mode network (DMN), a large-scale network that activates during rest, and in daydreaming and musing are observed in PD patients with VHs [83]. According to Weil and Reeves [81], VHs are due to over interpretation of visual input. These authors describe a reduction of white matter in posterior thalamic projections, which may play an important role for network shifting and releasing DMN inhibition [84].

Regarding the contribution of neurotransmitter systems in VHs, it has proved difficult to disentangle, due to the overlapping functional networks involved. It is known that levodopa treatment and dopamine agonists produce VHs [85,86]. It has also been proposed that hypersensitization of nigrostriatal dopaminergic neurons by anti-Parkinson’s drugs contribute to VHs [87]. The early disturbance of serotonergic and cholinergic pathways occurring in PD and glutamatergic and GABAergic changes affecting the overall balance between excitatory and inhibitory signaling may also play a role in the decoupling of the DMN [88]. It results in the perception of priors stored in the unconscious memory and the uncontrolled emergence of intrinsic narratives produced by the DMN [88].

VHs predict a range of poor outcomes, including more rapid cognitive decline and the development of dementia [89,90] and an increased likelihood of a move from independent living to a care home [91].

2.6. 시각적 환각

시각 환각(VH)은

PD에서 정신병의 가장 흔한 증상이며,

PD 환자의 급격한 인지 기능 저하와 관련이 있습니다.

이러한

환각은 PD 환자의 최대 75%에서 발생합니다[81].

또한 PD 환자에서 VH의 발병 기전은 잘 알려져 있지 않습니다. 질병이 진행되는 동안 경미한 환각은 처음에는 통찰력이 유지되는 VH로 발전하고, 이후 통찰력 상실과 망상을 동반하는 다발성 환각으로 발전할 수 있습니다. 환각은 보통 사람과 동물 등 생생하게 지각되는 장면으로 구성됩니다. 주변 시야를 가로질러 물체가 지나가는 통로 환각, 피질 외 환각 및 존재감도 설명되었습니다 [81].

PD의 VH는 모호한 시각적 입력과 함께 주의 네트워크의 기능 장애의 결과로 설명되어 왔으며, 기억된 이미지가 의식에 침입할 때 VH로 이어질 수 있습니다 [82]. 휴식 중과 공상 및 명상 중에 활성화되는 대규모 네트워크인 기본 모드 네트워크(DMN)의 비정상적인 수준이 VH를 가진 PD 환자에서 관찰됩니다 [83]. 웨일과 리브스[81]에 따르면 VH는 시각적 입력에 대한 과도한 해석으로 인해 발생한다고 합니다. 이 저자들은 후방 시상 돌기의 백질 감소를 설명하며, 이는 네트워크 이동 및 DMN 억제 해제에 중요한 역할을 할 수 있습니다 [84].

VH에서 신경전달물질 시스템의 기여와 관련하여, 관련된 기능적 네트워크가 중복되어 있기 때문에 풀기가 어려운 것으로 입증되었습니다. 레보도파 치료와 도파민 작용제가 VH를 생성하는 것으로 알려져 있습니다 [85,86]. 또한 항파킨슨병 약물에 의한 흑질 도파민성 뉴런의 과민화가 VH에 기여하는 것으로 제안되었습니다 [87]. PD에서 발생하는 세로토닌 및 콜린성 경로의 초기 교란과 흥분성 및 억제성 신호 사이의 전반적인 균형에 영향을 미치는 글루탐산 및 GABA성 변화도 DMN의 디커플링에 역할을 할 수 있습니다 [88]. 그 결과 무의식적 기억에 저장된 선행을 지각하고 DMN에 의해 생성된 내재적 내러티브가 통제되지 않은 상태로 출현하게 됩니다 [88].

VH는 더 빠른 인지 기능 저하와 치매 발병 [89,90], 독립 생활에서 요양원으로의 이동 가능성 증가 [91] 등 여러 가지 좋지 않은 결과를 예측합니다.

3. Visual Cognition Impairments in Parkinson’s Disease

Visual cognition deficits have been commonly reported in PD, although there is no consensus regarding frequency, characteristics, and relationships with other variables. Nevertheless, although many authors agree that visual cognition is not the most affected domain in PD [92,93], the majority of studies report a significant decline in visuospatial, visuoperceptive, visuoconstructive, and visual memory functions [94,95,96].

Some authors state that visual cognition deficits in PD are the consequence of central processing dysfunction rather than specific visuospatial impairments, particularly low-level perceptual deficits and executive function impairment [97,98]. Low-level visual dysfunction has important implications for understanding cognitive deterioration, as visual input is required for most of the standard neuropsychological tests. But, visual scenery generation and perception are simultaneously coupled with cognitive processes [99]. Thus, it has been reported that PD patients’ performance in a wide range of neuropsychological tests involving visual cognition can be attributed to abnormalities in low-level visual functions, especially low- and high-contrast visual acuity [96]. And, it has been suggested that lower-level vision acts as a confounder in object identification or in the time needed to interpret visual sceneries [96].

3. 파킨슨병의 시각 인지 장애

시각 인지 결손은 파킨슨병에서 흔히 보고되고 있지만,

그 빈도, 특성 및 다른 변수와의 관계에 대한 합의가 이루어지지 않고 있습니다.

그럼에도 불구하고 많은 저자들은 시각 인지가 PD에서 가장 영향을 많이 받는 영역이 아니라는 데 동의하지만[92,93], 대부분의 연구에서 시각 공간, 시지각, 시구성 및 시각 기억 기능의 현저한 저하를 보고하고 있습니다[94,95,96].

일부 저자는 PD의 시각 인지 결손이 특정 시각 공간 장애, 특히 낮은 수준의 지각 결손과 실행 기능 장애보다는 중추 처리 기능 장애의 결과라고 말합니다 [97,98]. 대부분의 표준 신경심리 검사에는 시각적 입력이 필요하기 때문에 낮은 수준의 시각 기능 장애는 인지 기능 저하를 이해하는 데 중요한 의미를 갖습니다. 그러나 시각적 풍경 생성 및 지각은 인지 과정과 동시에 결합되어 있습니다 [99]. 따라서 시각적 인지와 관련된 광범위한 신경심리 검사에서 PD 환자의 낮은 수행 능력은 낮은 수준의 시각 기능, 특히 저대비 및 고대비 시력의 이상에 기인할 수 있다고 보고되었습니다 [96]. 또한, 낮은 수준의 시력은 물체를 식별하거나 시각적 풍경을 해석하는 데 필요한 시간을 방해하는 요인으로 작용한다고 제안되었습니다 [96].

3.1. Visuospatial Impairment

Visuospatial deficits in PD patients have commonly been assessed with the Judgement of Line Orientation test (JLO), a tool that eval‎uates the ability to estimate angular relationships between line segments. Several studies have reported significant decreases in JLO scores [96,100,101,102], particularly in cognitively impaired PD patients [103]. PD patients are prone to confound oblique lines by two or more spacings [104], and show more severe intraquadrant and horizontal lines errors [105]. Some authors have reported that the interference of the visuospatial sketchpad (a component of working memory involved in the storage and manipulation of visual and spatial information) is relevant only in moderate to severe phases of the disease [106]. Recently, Kawashima et al. [107] showed that visuospatial recognition was impaired in the visuospatial o-back test, which does not involve a memory component. And, Kawabata et al. [8] have showed deficits in position discrimination in the Visual Object and Space Perception Battery (VOSP).

Mental rotation and three-dimensional and visual transformation processes have also been reported to be impaired in PD [15,108]. However, there is no consensus about mental rotation abilities in PD. Thus, some authors have documented impaired mental rotation and suggest a problem of the perception of extra-personal space [15], whereas other studies have reported spared mental rotation abilities in PD [109]. It can be argued that each mode of mental transformation is associated with a distinct network of brain regions, and these networks are likely affected differentially by the neuropathology of PD. Amick et al. [110] reported that PD patients showed an impaired ability to mentally rotate hands, but not objects. According to these authors, frontostriatal motor systems and the parietal lobes would play a necessary role for integrating visuospatial cognition with motor imagery during the mental rotation of hands. And, recently, Bek et al. [111] have proposed that PD patients would present difficulties integrating visual and kinesthetic elements of motor imagery.

3.1. 시각 공간 장애

파킨슨병 환자의 시각 공간 결함은 일반적으로 선분 사이의 각도 관계를 추정하는 능력을 평가하는 도구인 선 방향 판단 테스트(JLO)로 평가해 왔습니다. 여러 연구에서 특히 인지 장애가 있는 PD 환자[103]에서 JLO 점수가 현저히 감소한 것으로 보고되었습니다[96,100,101,102]. PD 환자는 사선을 두 개 이상의 간격으로 혼동하기 쉽고[104], 사분면 내 및 수평선 오류가 더 심하게 나타납니다[105]. 일부 저자는 시각 공간 스케치패드(시각 및 공간 정보의 저장과 조작에 관여하는 작업 기억의 구성 요소)의 간섭이 질병의 중등도에서 중증 단계에서만 관련이 있다고 보고했습니다 [106]. 최근 가와시마 등[107]은 기억 구성 요소를 포함하지 않는 시각 공간 오백 테스트에서 시각 공간 인식이 손상되었음을 보여주었습니다. 그리고 가와바타 등[8]은 시각 물체 및 공간 지각 배터리(VOSP)에서 위치 변별에 결함이 있음을 보여주었습니다.

정신적 회전과 3차원 및 시각적 변환 과정도 PD에서 손상된 것으로 보고되었습니다 [15,108]. 그러나 PD의 정신적 회전 능력에 대한 합의된 결론은 없습니다. 따라서 일부 저자는 정신 회전 장애를 문서화하고 개인 외 공간에 대한 지각 문제를 제안한 반면 [15], 다른 연구에서는 PD에서 정신 회전 능력이 보존되었다고 보고했습니다 [109]. 각 정신적 변형 방식은 서로 다른 뇌 영역 네트워크와 연관되어 있으며, 이러한 네트워크는 PD의 신경 병리에 따라 다르게 영향을 받을 가능성이 있다고 주장할 수 있습니다. Amick 등[110]은 PD 환자가 정신적으로 손을 회전하는 능력은 손상되었지만 물체를 회전하는 능력은 손상되지 않았다고 보고했습니다. 이 저자들에 따르면, 전두정엽 운동 시스템과 두정엽은 손을 정신적으로 회전하는 동안 시각 공간 인지를 운동 이미지와 통합하는 데 필요한 역할을 합니다. 그리고 최근 Bek 등[111]은 PD 환자가 운동 이미지의 시각적 요소와 운동 감각적 요소를 통합하는 데 어려움을 겪을 수 있다고 제안했습니다.

3.2. Visuoperceptive Impairment

The Facial Recognition Test (FRT) has been used to assess the ability to recognize faces in PD patients without involving a memory component. Some PD patients, even cognitively unimpaired ones, present more difficulties on this test than on the JLO [103,112,113]. Another test, the Visual Form Discrimination Test (VFDT), has been used to eval‎uate visual recognition impairment in PD. Raskin et al. [114] showed a gradual impairment of visuospatial functions, and other authors have demonstrated that non-demented PD patients fail in this test [101,112]. Some authors have shown that PD patients have difficulties identifying objects embedded in complex figures and are less accurate and make more mistakes in perceptual judgements on a bistable percept paradigm (BPP) [115]. PD patients also show problems in semantical categorization of visual stimuli [116].

Kawabata et al. [8] investigated the features of visuoperceptual disturbances in PD using the battery VOSP. The authors found that one-third of patients exhibited impaired identification of incomplete letters and showed a reduction of functional connectivity in the primary visual network.

Difficulties in the perception of space and depth have also been observed in PD patients. Stereopsis impairment has been observed in some studies [117,118,119]. It has been explained as a result of basic visual perception alterations, such as color vision and contrast sensitivity deficits [118], and oculomotor behavior [120], which appear linked to the degree of disease deterioration and motor impairment [119].

Difficulties in the detection of motion are also observable in PD [121,122]. This deficit is independent of gait dysfunction and low-level vision changes, and may arise from difficulty perceptually integrating form and motion cues in posterior superior temporal sulcus [121]. These authors reported that PD patients perform significantly worse for human motion than the object motion task.

3.2. 시지각 장애
안면 인식 테스트(FRT)는 기억 요소를 포함하지 않고 PD 환자의 얼굴을 인식하는 능력을 평가하는 데 사용되었습니다. 일부 PD 환자, 심지어 인지 장애가 없는 환자도 이 검사에서 JLO보다 더 많은 어려움을 보입니다 [103,112,113]. 또 다른 테스트인 시각적 형태 변별 테스트(VFDT)는 PD의 시각적 인식 장애를 평가하는 데 사용되었습니다. Raskin 등[114]은 시각 공간 기능의 점진적인 손상을 보여 주었고, 다른 저자들은 치매가 아닌 PD 환자가 이 테스트에서 실패한다는 것을 입증했습니다[101,112]. 일부 저자는 PD 환자가 복잡한 그림에 포함된 물체를 식별하는 데 어려움이 있으며, 이안정 지각 패러다임(BPP)에서 지각 판단의 정확성이 떨어지고 실수가 많다는 것을 보여주었습니다[115]. PD 환자는 또한 시각 자극의 의미론적 범주화에도 문제를 보입니다 [116].

가와바타 등[8]은 배터리 VOSP를 사용하여 PD에서 시지각 장애의 특징을 조사했습니다. 저자들은 환자의 1/3이 불완전한 글자를 식별하는 데 장애를 보였으며 일차 시각 네트워크의 기능적 연결성이 감소한 것으로 나타났습니다.
공간과 깊이에 대한 인식의 어려움도 PD 환자들에게서 관찰되었습니다. 일부 연구에서 입체시 장애가 관찰되었습니다 [117,118,119]. 이는 색각 및 대비감도 결핍[118], 안구 운동 행동[120]과 같은 기본적인 시각 지각 변화의 결과로 설명되었으며, 이는 질병 악화 및 운동 장애 정도와 관련이 있는 것으로 보입니다[119].
움직임 감지의 어려움은 PD에서도 관찰할 수 있습니다[121,122]. 이러한 결손은 보행 기능 장애 및 저수준 시력 변화와는 무관하며, 후상 측두 고랑에서 형태와 동작 단서를 지각적으로 통합하는 데 어려움을 겪기 때문에 발생할 수 있습니다 [121]. 이 저자들은 PD 환자가 물체 동작 과제보다 사람 동작 과제에서 훨씬 더 나쁜 수행을 보인다고 보고했습니다.

3.3. Visuoconstructive Impairment

Visuoconstructive impairment in the block design subtest from the Wechsler Adult Intelligence Scale (WAIS) has been related to worsening of other cognitive domains, and motor and severity in PD [123]. In the Clock Drawing Test (CDT), drawing and copy scores are significantly lower in PD, with the last correlated with high-contrast visual acuity measures [96]. Visuoconstructive abilities have also been assessed using more complex copy tests, such as the Rey–Osterrieth Copy Figure (ROCF) [123,124]. Patients show impaired visual cognition, particularly judgement of line orientation and rotation [124].

PD patients, with or without dementia, show a tendency to copy figures very close to the model, a phenomenon called “closing-in” [125]. Initially, it has been explained as a form of constructional apraxia, and some authors have proposed patients have difficulty in the visuospatial analysis of the model and/or in holding this representation in visual working memory [126]. Others suggest that the closing-in phenomenon would be an extreme manifestation of a default tendency of the motor system, so that the actions would be performed toward the focus of attention [127]. De Lucia et al. [128] have proposed that the closing-in phenomenon is related to frontal-executive impairments in PD dementia.

3.3. 시각 구성 장애
웩슬러 성인 지능 척도(WAIS)의 블록 디자인 하위 테스트에서 시각 구성 장애는 다른 인지 영역의 악화, 운동 및 PD의 중증도와 관련이 있습니다[123]. 시계 그리기 테스트(CDT)에서 그림 그리기 및 복사 점수는 PD에서 현저히 낮으며, 마지막은 고대비 시력 측정과 상관관계가 있습니다 [96]. 시각적 구성 능력은 레이-오스테리에트 복사 그림(ROCF)[123,124]과 같은 보다 복잡한 복사 테스트를 사용하여 평가되기도 했습니다. 환자는 시각적 인지, 특히 선의 방향과 회전에 대한 판단에 장애를 보입니다[124].
치매 유무에 관계없이 치매 환자는 모델에 매우 가깝게 그림을 모사하는 경향을 보이는데, 이를 '클로즈인'이라고 합니다[125]. 처음에는 이 현상을 구성 실어증의 한 형태로 설명했으며, 일부 저자는 환자가 모델의 시각 공간 분석 및/또는 시각 작업 기억에서 이러한 표현을 유지하는 데 어려움을 겪는다고 제안했습니다 [126]. 다른 사람들은 클로징 인 현상이 운동 시스템의 기본 경향의 극단적 인 표현이므로 주의의 초점을 향해 행동이 수행 될 것이라고 제안합니다 [127]. De Lucia 등[128]은 클로징 인 현상이 PD 치매의 전두엽 손상과 관련이 있다고 제안했습니다.

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4. Side-of-Onset and Type of Parkinson’s Disease in Relation to Visual Symptoms

The side of motor symptom onset is an important consideration in the study of PD, as most patients initially present with symptoms on one side of the body, reflecting the loss of dopamine primarily in the contra-lateral hemisphere. The right hemisphere is more responsible than the left for many spatial abilities, and failure to distinguish patients with LPD from RPD may mean that visuospatial deficits that contribute to functional decline are missed in patients with LPD [129].

A factor that has been shown to influence visual processing in people with PD is the body hemifield where the first motor symptoms appeared [130] and their characteristics [131]. Thus, Verreyt et al. [130] reported that LPD patients more often perform worse on tasks of spatial attention and visuospatial orienting. Davidsdottir et al. [132] examined spatial navigation and visuospatial functioning. LPD patients were generally more visually dependent than RPD patients, who in turn were more visually dependent than the control group. Moreover, egocentric midpoint estimation was dependent on visual input biases, with the deviation increasing for LPD and decreasing for RPD. Schendan et al. [133] used a hierarchical perception task in PD, distinguishing between patients whose motor symptoms started on the left side of the body (LPDs) or the right side (RPDs). These authors observed that LPDs showed an abnormal perception of global elements, whereas RPDs perceived worse the local elements that make up an object. According to Schendan et al. [133], the link between the link side of motor symptoms and visuospatial abilities would rely on the contralateral temporoparietal junction.

On the other side, visual deficits have also been analyzed according to the type of motor symptoms that characterize the onset of the disease, defining two phenotypes: tremor dominant-phenotype (T-D) vs. bradykinesia and rigidity dominant-phenotype (B/R-D). The Visual Activities Questionnaire showed that only the B/R-D group scored significantly worse than controls in light/dark adaptation, visual acuity, depth perception, peripheral vision, and visual processing speed, whereas B/R-D only scored worse in depth perception and light/dark adaptation compared to T-D, suggesting the influence of the type of initial symptoms on visuospatial processing [134]. Other authors have noticed an increased risk of developing VHs in rigid-akinetic patients [135], whereas patients with postural instability and gait difficulty performed worse than those with T-D on visuospatial measures [131].

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5. Gender Influence in Visuospatial Symptoms in Parkinson’s Disease

Several studies have focused on the analysis of gender differences in the manifestation of cognitive damage in patients with PD and the affected sub-processes. Gender differences have been observed on the Road Map Test of Direction Sense, a right–left discrimination task that requires egocentric mental rotation in space, with men being superior [136]. Davidsdottir et al. [132] examined spatial navigation and visuospatial functioning. Gender differences were found in the navigation task, egocentric midline test, line bisection, and motion perception. Oltra et al. [137] observed that female patients had lower scores than males in the JLO. In the same line, Pasotti et al. [138] analyzed 306 patients of both sexes, observing gender differences in the execution of cognitive tasks, with males superior in visuospatial tasks. These differences were more noticeable in the initial stages of the disease, with the presence of estrogens in dopaminergic neurons being a possible protective factor against cognitive deterioration in this disease. In this line, Crucian et al. [108] reported that men with PD demonstrated significantly lower scores on the Mental Rotations Test than men of similar age and education, whereas PD and control women performed at a similar low level.

Nevertheless, recent research has reported that there was no interaction between sex and Parkinson’s diagnosis (with or without mild cognitive impairment) for visuospatial function [139]. Other studies have reported no male–female differences, including Cronin-Golomb and Braun [140] on visuospatial problem solving using Raven’s Coloured Progressive Matrices; Amick et al. [110] on mental rotation; and Schendan et al. [133] on hierarchical pattern perception, a test of global and local visual pattern processing. The latter two studies reported LPD-RPD effects. Thus, several studies suggest that gender differences pertain to some, but not all, visuospatial abilities, and may interact with the side of disease onset [129].

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6. Neuroanatomical Correlates of Visuospatial and Visuoperceptive Deficits in Parkinson’s Disease

Visuospatial (VS) and visuoperceptive (VP) deficits in PD have been related to cortical thinning in the parieto-occipital and fronto-temporal networks, along with structural disturbances in antero-posterior white matter pathways (Table 1). Moreover, as brain degeneration progresses, there is a worsening in VS/VP performance, reaching its worst level with the onset of cognitive impairment [141,142] (Table 2).

Table 1

Neuroanatomical substrate of visuoperceptual, visuospatial, and visuoconstructive impairment in PD.

DomainTaskCortical Region

Visuoperceptual	Facial Recognition Test (FRT) [112,141] Visual Form Discrimination Test (VFDT) [112]	Fusiform Gyrus (BA 19, 36) Parahippocampal Middle Occipital Gyrus Inferior Frontal Gyrus (BA 47) Left Lateral Occipital Bilateral Superior Parietal (BA 7, 40) Superior Occipital (BA 19) Inferior Frontal Gyrus (BA 47)
Visuospatial	Judgement Line Orientation Task (JLO) [143]	Bilateral Superior Temporal Right Lateral Occipital
Visuoconstructive	Pentagon Copy Test (PCT) [143]	Right Supplementary Motor Left Rostral Middle Frontal Pars Triangularis Left Cuneus

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Table 2

Neuroanatomical substrate of cortical thinning as a possible biomarker of evolution to dementia [141].

DomainTaskCortical Region

Visuoperceptual	Facial Recognition Test (FRT) Symbol Digit Modalities Test (SDMT)	Left Lingual Gyrus Left Superior Temporal Left Parahippocampal Left Lingual Gyrus Right Parahippocampal
Visuospatial	Judgement Line Orientation Task (JLO)	Left Insula Inferior Temporal Superior Temporal Right Fusiform Gyrus
Visuoconstruction	Pentagon Copy Test (PCT)	Left Entorhinal Middle and Inferior Temporal Gyri Medial Temporal Pole Parahippocampal Fusiform Cortex Lingual Cortex Lateral Occipital Cortex

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Several structural magnetic resonance image (sMRI) studies have found a correlation between neuropsychological test scores and cortical thinning in PD. Pereira et al. [112] found that the gray matter density in the superior parietal and superior occipital gyrus correlated with visuospatial performance in PD, whereas reduced gray matter in the fusiform, the parahippocampus, and the middle occipital gyrus was associated with poor performance on visuoperceptual tests. These authors described the relationship between facial recognition deficits using the FRT and gray matter thinning in the fusiform gyrus (BA 19, 36), and clusters in the parahippocampal region, the middle occipital gyrus (BA 19), and the inferior frontal gyrus (BA 47). Using the same task, Garcia-Diaz et al. [141] described correlated patients’ performance with cortical thinning in the left lateral occipital area. On the other side, using the visual form discrimination test (VFDT), Pereira et al. [112] observed that performance correlated with gray matter reductions in the bilateral superior parietal lobes (BA 7, 40) and the superior occipital (BA 19), the middle frontal (BA 9), and the inferior frontal gyrus (BA 47). These results would indicate different patterns in gray matter reduction in visual associative areas for facial recognition and visual form discrimination. Facial recognition would be related to gray matter reductions in areas of the ventral occipitotemporal cortex, whereas visual form discrimination would in volve dorsal parietal areas. According to the authors, facial recognition impairmentwould correlate with the medial temporal lobe, and impaired visual form discrimination with superior parietal regions. Occipitotemporal and occipitoparietal pathways send projections to the prefrontal cortex, where spatial information is maintained “online”.

Filoteo et al. [143] have reported a decreased volume bilaterally in the superior temporal cortex and the right lateral occipital cortex (both in the object-based system), with poorer performance in the JLO.

Poorer visuoconstructive performance on the Pentagon Copy test (PCT) has been correlated with decreased volume in the frontal regions (right Supplementary Motor Area, left rostral middle frontal cortex, and pars triangularis), as well as in the object-based system (left Cuneus) [143].

Cognitively impaired PD patients show a more pronounced and extended pattern of cortical atrophy. Rektorova et al. [144] observed gray matter volume reductions in the hippocampus, amygdala, and neocortical temporal regions related to impairment in visuospatial abilities in demented PD patients. More recently, Garcia-Diaz et al. [141] found that MCI-PD patients exhibit significantly greater progressive cortical thinning in left lateral occipital and inferior parietal regions, and in right medial temporal regions. The authors reported that scores on the PCT correlated with a cluster in the left entorhinal region that included the middle and inferior temporal gyri, the medial temporal pole, and the parahippocampal, fusiform, lingual, and lateral occipital cortices. Their performance on the JLO was related to cortical atrophy in clusters in the left insula, inferior, and superior temporal areas, and the right fusiform gyrus, which extended to the left temporal pole, entorhinal, fusiform, and lingual cortices. FRT scores correlated with cortical thinning in the left lingual gyrus. Similarly, the Symbol Digit Modalities Test (SDMT) correlated with a diminution in the left superior temporal, parahippocampal, and lingual, and the right parahippocampal cortices. According to the authors, this anatomical pattern of cortical atrophy was valid when only patients with sustained cognitive impairment at the 4-year follow-up were included. Lee et al. [145] have also described different patterns between amnestic MCI-PD patients (the most common MCI subtype) and non-amnestic MCI-PD patients. The authors reported that visuospatial impairment was more severe in the amnestic group, and a direct comparison between both groups showed a decreased gray matter density in the bilateral precuneus, left primary motor, and right parietal areas.

The side-of-onset also influences the gray matter density pattern in relation to visuospatial alterations. Thus, left PD patients have greater visuospatial impairments compared to right PD patients and lower gray matter volume in the right Dorsolateral Prefrontal Cortex, regardless of other variables, such as age or premorbid cognitive status [146].

Another marker of cortical degeneration in PD related to VS/VP impairment is brain asymmetry. Segura et al. [101] described an initial deterioration of right temporo-parietal regions, followed by a progressive bilateral hemisphere degeneration in PD. As mentioned above, lateralized motor symptoms onset has been studied as a variable that modulates visuospatial skills deterioration.

White matter loss also contributes to worsening visuospatial performance in PD [147]. In the study by García-Díaz et al. [7] on a sample of PD patients, with and without cognitive impairment, the authors reported correlations between VS/VP scores and white matter fractional anisotropy values in the corpus callosum, bilateral forceps minor, uncinate fasciculus, inferior frontooccipital fasciculus, forceps major, and inferior longitudinal fasciculus. All the VS/VP tests studied showed significant correlations between their scored and fractional anisometropia values, but it was larger for the SDMT.

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7. Functional Neuroimage Correlates of VS/VP Deficits in Parkinsons’ Disease

Most functional neuroimaging studies (functional MRI, positron emission tomography PET, and single-photon emission computerized tomography SPECT) have reported altered activation, altered blood flow, or reduced metabolism in both dorsal and ventral visual pathways, which probably indicates an alteration in the normal bottom–top visual processing and the presence of aberrant top–down visual processing [148].

Early studies based on PET imaging by Eberling et al. [149] and Bohnen et al. [150] showed a clear reduction of cerebral glucose metabolism in visual association, primary visual, and right parietal cortices in non-cognitively affected patients. Furthermore, lower performance in visuospatial tasks has been associated with fluoro-deoxyglucose hypometabolism or hypoperfusion in occipital and frontal cortices of non-cognitively impaired subjects [151], along with lower levels of 123I-iodoamphetamine single-photon emission computed tomography-based assessment in right hemisphere posterior-frontal cortices [152].

With disease progression, incident dementia in idiopathic PD is announced by metabolic changes within visual association (BA 18) and posterior cingulate cortices. Findings indicate that incident dementia preferentially involves BA18, whereas reductions in the primary visual cortex (BA 17) can be seen in PD without dementia. The Benton Visual Retention test, which assesses visuospatial perception, visuomotor and visuoconstructive abilities, and visual memory, best correlates with BA 18 metabolism [153]. Extension of occipital hypometabolism from the primary to the visual association cortices, together with precuneus hypometabolism, may be the early cortical metabolic “signature” of incident dementia in PD, with visual association cortex hypometabolism linked to the presence of VHs in more advanced PD [153].

Using functional magnetic resonance (fMRI), Caproni et al. [154] found that PD patients had reduced activation of the right insula, left putamen, bilateral caudate (in particular, in the right hemisphere), and right hippocampus, together with greater activation of the right dorso-lateral prefrontal cortex (DLPFC) and bilateral posterior parietal cortex (PPC) during visuospatial judgement. The authors propose that the DLPFC activation reflex is a compensatory mechanism, through continuous control by the top–down visual processing system.

Alteration in PD brain function related to VS/VP deficits has also been observed via non-invasive resting state fMRI, supporting the idea of a marked fronto-occipital-parietal dysfunction in the right hemisphere [155], which is more noticeable as the disease progresses [156], and is responsible for VS/VP impairments [157]. Although such reduction of right-hemisphere functional connectivity is observable in early stages, it progresses and affects bilateral prefrontal and frontoparietal networks when mild cognitive impairment (MCI) and/or dementia appear [158], accompanied by increased Regional Homogeneity (ReHo) in the medial-superior occipital gyrus compared to healthy controls [159].

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8. Neuroanatomical Correlates of Parkinson’s Disease Visual Hallucinations

Imaging studies of VHs in PD to date have been based on relatively small samples and have used different designs to control for the degree of cognitive decline, stage of PD, and dopamine medication, showing little consistency across studies [160].

A number of structural neuroimaging studies have analyzed the cerebral basis of VHs in PD. Gray matter atrophy has been described in multiple regions of the brain, such as the primary visual and visual association cortices; limbic regions, and cholinergic structures, such as the pedunculopontine nucleus and substantia innominata, which are involved in visuospatial-perception, attention control, and memory [148].

Widespread reductions in the cortical thickness of PD patients with hallucinations have been identified in the occipital, parietal, temporal, frontal, and limbic lobes of PD patients with VHs. However, not all regions are equally affected, and, notably, there appears to be a posterior asymmetry, with relative sparing of the left ventral visual stream (ventral occipitotemporal cortex) compared to the homologous region in the right hemisphere [161]. Of these, the cuneus and superior frontal gyrus bilaterally emerged as the dominant components.

Ramírez-Ruiz et al. [162], in a VBM study, reported reduced GM volume in the left lingual gyrus and bilateral superior parietal lobule in PD patients with VHs compared to those without VHs. Pagonabarraga et al. [163] have proposed a progressive volume reduction from the unilateral left superior parietal in minor hallucinations (mHs) to the bilateral superior parietal in VHs. A significant reduction of GM volume has been observed in the right inferior frontal, left temporal, and thalamic areas [164]; the left opercula frontal gyrus and left superior frontal gyrus [165]; the cingulate [166]; and in the bilateral dorsolateral prefrontal cortex, rostral part of prefrontal cortex, bilateral primary visual cortex, and regions corresponding to the secondary visual cortex, such as the left inferior occipital gyrus, right lingual cortex, right supramarginal gyrus, and left fusiform gyrus [167]. Vignando et al. [161] report that regions with reduced thickness for higher severity scores (negative correlation) were found in the posterior parietal, posterior cingulate, and superior temporal cortex. It has also been observed a degenerative process in the head of the hippocampus in hallucinating PD patients [168] that would involve the whole hippocampus and cause dementia in hallucinating PD patients. The atrophy of cholinergic structures, such as the pedunculopontine nucleus (PPN) and its thalamic projections, has also been associated with VHs in PD [169], the substantia innominata (SI) [164]. Finally, reduced volume in cerebellar lobules VIII, IX/VII, and Crus 1 has been associated with VHs in PD [170].

Goldman et al. [166] observed that gray matter atrophy occurred both in the ventral (what) and dorsal (where) pathways, responsible for object and facial recognition and identification of the spatial locations of objects. Thus, those patients who experienced VHs exhibited gray matter atrophy in the cuneus, lingual, and fusiform gyri; middle occipital lobe; inferior parietal lobe; and cingulate, paracentral, and precentral gyri. These structural changes were not related to the presence of dementia.

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9. Functional Neuroimage Correlates of Parkinsons’ Disease Visual Hallucinations

A seminal fMRI study by Stebbins et al. [171] reported that VHs in PD patients reflect an abnormally increased activation in anterior cortical regions, such as the inferior frontal cortex and caudate nucleus, accompanied with reduced activation in the parietal lobe and cingulate gyrus, which has been explained as a shifting of attentional visual circuitry from posterior (down–top) to anterior (top–down) regions. More recently, Yao et al. [83] reported an increased functional connectivity in the default mode network (DMN) in the right middle frontal gyrus and bilateral posterior cingulate gyrus/precuneus of VH PD patients in comparison to non-VH patients. These data support the hypothesis of an excessive and aberrant top–down processing.

PET studies have reported an increased glucose metabolism in frontal regions (the left superior frontal gyrus) in hallucinating PD patients [172], and a decreased metabolism in the occipito-parieto-temporal region (sparing the occipital pole) that included both dorsal and ventral visual streams [173]. Park et al. [174] has reported a reduced metabolism in both visual streams of hallucinating PD patients, but predominantly in the ventral one. In the same line, a SPECT study by Oishi et al. [175] reported increased perfusion in the right superior and middle temporal gyri in hallucinating PD patients. The increased perfusion of the superior temporal gyrus is in line with the hypothesis of preval‎ent top–down visual processing. Also, the fronto–striatal circuit involved in the dorsal attention network has been involved in the pathogenesis of VHs in PD [176]. Kiferle et al. [177] have suggested that frontal impairment observed in PD patients with VHs may be due to a right caudate dysfunction, reflecting an impairment of cortico–subcortical circuits.

Dysregulation of the ventral attentional network (VAN), dorsal attentional network (DAN), and default mode network (DMN) have been implicated in models of VHs in PD [178] with reduced activity in the DAN [82]. PD patients with hallucinations show widespread disruption in structural connections, which particularly affect highly connected brain regions or “hubs” required for brain transitions between different cognitive states [179,180] (See Figure 2). Zarkali et al. [181] found that PD patients with hallucinations show impaired temporal dynamics, with a predisposition towards a predominantly segregated state of functional connectivity, and require less energy to transition from the integrated to the segregated state. Moreover, the thalamus and regions within the DMN are critically involved in the network imbalance in PD hallucinations.

Figure 2

The van Ommen [182] model of visual hallucinations. Complex VHs are the result of a dissociation of higher-order visual processing areas from the primary visual cortex. Simultaneously, a looping of brain activity across the outside-world-focused DAN, the inner-world-focused and memory-related DMN, and the saliency-focused VAN, bias conscious visual perception away from information coming from the outer world and towards internally generated percepts. Visual network (VIS). Modified from “Human Brain sketch with eyes and cerebrellum.svg”, work released into the public domain by Hankem. https://commons.wikimedia.org/wiki/File:Human_Brain_sketch_with_eyes_and_cerebrellum.svg (accessed on 11 July 2023).

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10. Neurochemistry of VS/VP Deficits in Parkinsons’ Disease

Dopaminergic neurons are diffusely present within the retina. Dopamine is present in the retina in amacrine cells of the inner border of the inner nuclear layer and in interplexiform cells, influencing the activity of photoreceptors, ganglion cells, and bipolar cell receptive fields [183,184]. The role of dopaminergic reduction in the basal ganglia and the impact on the arterial walls of the frontal cortex would explain the appearance of problems in eye movements and in pupil reactivity in PD [27,185]. Reductions in dopamine levels in the basal ganglia and frontal cortex may also deplete levels in the superior colliculus and, therefore, could be a factor in the production of defective saccades [186]. Furthermore, it has been observed that dopaminergic medication adversely impacts visuoperceptual performance, with it worse in the on compared to the off medication state. Thus, dopaminergic medication can ameliorate movement deficits, but reduces visuoperceptual accuracy because of overdosing [187]. Finally, it must be also noted that although all dopaminergic drugs have been associated with incident VHs, this association has been shown contradictory [163], and other factors could be considered responsible [188].

The imbalance of the pedunculopontine cholinergic projections on the visual cortex produces aberrant visuospatial processing [123]. Moreover, the appearance of abnormal VP phenomena, such as hallucinations in people with PD, has been associated with degeneration of the nucleus basalis of Meynert and its cholinergic projections between the brainstem and the frontal cortex, also affecting the execution of visuospatial tasks [189,190].

In addition to the dopaminergic–cholinergic balance, VHs have been associated with decreased occipital GABA in PD [191]. Whether this system is affected in the retina and visual cortex of all PD patients, and from early stages, remains to be determined.

Regarding other neurotransmission systems, the serotonergic system also seems to play a key role in the manifestation of VHs and visuoperceptual function in PD. Thus, VHs and other visual-perceptual dysfunctions have been observed in PD associated with a decrease of the serotonergic receptor 5-HT2A in the right insula, bilateral dorsolateral prefrontal cortex, right orbitofrontal cortex, right middle temporal gyrus, and right fusiform gyrus (Cho et al., 2017). Vignando et al. [161] reported an association for 5-HT2A and 5-HT1A confined to regions linked to VHs, rather than the cortex as a whole, suggesting the neurotransmitter effects were specific to VHs, suggesting the possibility that degeneration in these neurotransmitter systems in PD underlies synaptic loss and cortical thinning. Cho et al. [192] have also reported that 5-HT2A correlated with visuoperceptual function. In particular, the FRT score was related to receptor binding in the right anterior cingulate cortex (ACC), left DLPFC, and inferior temporal gyrus. A reduced binding in ACC and other prefrontal regions was related to a lower VOSP total score. The author also described negative correlations between 5-HT2A in the middle/inferior temporal gyrus and Rey Complex Figure copy scores, as well as between the occipital BA 18 and JLO scores. These regions are part of the ventral visual system within the bottom–up network.

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11. Genetic Factors of VS/VP Deficits in Parkinsons’ Disease

Although genetic factors are only implicated in a minority of cases of PD, genetic approaches could contribute to understanding the etiology and manifestation of VS/VP impairments in PD. Mutations in the leucine-rich repeat kinase 2 (LRRK2) and parkin RBR E3 ubiquitin protein ligase genes are thought to be protective against cognitive impairment and ocular disturbances [193]. Moreover, lysosomal enzyme glucocerebrosidase (GBA) mutation carriers show poor visuospatial performance [194,195] and a higher frequency of hallucinations [196]. Moreover, it has been reported that microtubule-associated protein tau (MAPT) H1 haplotype carriers present a higher risk of dementia, are less accurate with difficult spatial rotations, and show lower activity in the parietal cortex and caudate nuclei [197].

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12. Conclusions

Numerous visual and perceptual problems have been observed in patients with PD. However, these problems are usually under-recognized and poorly understood, leading to a lack of appropriate treatment.

Different structures and networks have been involved in visual deficits in PD, from lower-level structures, such as the retina, to visual pathways involved in higher-level visual cognition. Thus, low-level visual dysfunction, like visual acuity and contrast sensitivity impairment, has been extensively observed in PD. And, as most of the standard neuropsychological measures rely on visual input for assessing cognitive functions, it is observed that lower mechanisms’ deficits also affect higher cognitive capabilities. Moreover, visual scenery generation and perception are also limited by cognitive processes that are deteriorated in PD, such as attention [99]. As result, the incoming visual information is constantly regulated and tuned by top–down processes [154].

One of the main approaches to study the causes of VS/VP deficits in PD has been the structural neuroimage. Studies have shown alterations in the cortical thickness of bilateral temporo–parietal–occipital areas and widespread posterior–anterior white matter microstructure alterations [7,198]. In addition, bilateral degeneration of posterior cortical regions is associated with a progressive worsening in VS/VP performance. And, it is also observed a progressive cortical volume reduction in posterior parieto-temporal regions of PD-MCI in comparison with cognitively unimpaired PD subjects [141].

Functional alterations in both cortical areas and subcortical regions involved in VS/VP have been also observed in PD. In particular, a reduced activation in the right insula, left putamen, bilateral caudate, and right hippocampus, as well as an over-activation of the right dorso-lateral prefrontal and the posterior parietal cortices, particularly in the right hemisphere, have been observed in PD. Moreover, a loss of cortical efficiency and compensatory mechanisms during visual processing have also been observed in PD patients [143,154]. It has been observed that PD patients showed greater activations of right DLPFC and bilateral PPC. Both of these regions, together with frontal–striatal circuits, are known to be part of the “Top–Down” visual processing system, which is involved in the selection and organization of complex visual information. DLPFC greater activation has been considered a compensatory response in PD patients, involving a continuous control by “Top–Down” mechanisms associated with visual working memory. Moreover, the greater activation of bilateral PPC could be necessary to overcome the initial impairment of the network [154].

Some authors have also suggested a role for lateralization of the basal ganglia circuits in stimuli perception, associated with a different clinical manifestation in left- and right-side PD onset [133]. Thus, right-sided onset PD is characterized by impairment in local-level processing, a consequence of left frontal and parietal deficits, whereas left-sided onset PD is characterized by an alteration of global-level processing, due to right parietal dysfunction [110,133].

Research about neurochemical alterations associated with VS/VP disturbances in PD is scarce, and many times the results have been contradictory. Different studies have reported the dopaminergic basis of retinal and other neurodegenerative pathologies in PD related to visual and ocular alterations [27,185], whereas visuoperceptive and visuospatial impairments correlate with abnormalities in several interrelated neurotransmitter systems, mainly the dopaminergic, noradrenergic, serotonergic, and cholinergic systems [189,199].

Visuoperceptual and visuospatial deficits in PD correlate to disease progression and have been proposed as an early marker of cognitive deterioration [132]. For instance, cross-sectional correlational studies have established a relationship between the degree of atrophy in posterior brain regions and VS/VP impairment [141]. Moreover, visuoperceptual deficits have also been associated with VHs, which are also considered predictive of disease progression.

VHs are part of the syndrome of PD psychosis, and one of the most debilitating symptoms for patients’ quality of life. Although, the precise pathophysiology of VHs in PD remains unclear, neuropathological and structural imaging studies have provided some insight [199]. Thus, neuroimaging studies have revealed gray matter atrophy in multiple regions, with the most prominent changes across areas involved in visual perception (including the ventral ‘what’ and dorsal ‘where’ pathways), the hippocampus, and several cholinergic brain structures, such as the substantia innominata and pedunculopontine nucleus [164,166]. It has been considered that dentification of brain structures associated with VHs in PD may permit earlier detection of at-risk patients and ultimately development of new therapies for targeting hallucinations and visuoperceptive functions [166].

As detailed above, the etiology and neurobiological bases of visual and perceptual impairments in PD are complex and multifaceted, and the same could be said with regard to their association with motor symptoms. Ocular, visual, visuoperceptive, and visuospatial deficits can occur independently in PD and may depend on multiple factors. Thus, each individual with PD may present a unique combination of these impairments, with variable symptomatology. Moreover, these alterations are hypothesized to play a role in the etiology of the main motor signs of PD, such as the freezing of gait [200]. Therefore, to reveal the particular mechanisms behind visual symptomatology and the potential therapeutic strategies poses a tremendous challenge for future research. The combination of genetics and functional neuroimaging can provide a promising strategy for classification and identification of potential biomarkers, which can be tested in future clinical trials designed to fight and prevent PD.

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Funding Statement

This research received no external funding.

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Author Contributions

Conceptualization, F.N.-E. and I.C.-P.; writing—original draft preparation, F.N.-E. and E.O.-G.; writing—review and editing, F.N.-E., E.O.-G. and I.C.-P. All authors have read and agreed to the published version of the manuscript.

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Institutional Review Board Statement

Not applicable.

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Informed Consent Statement

Not applicable.

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Data Availability Statement

No new data were created.

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Conflicts of Interest

The authors declare no conflict of interest.

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Footnotes

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