There may be an analogy with dsyphagia. A medical event such as stroke may block the primary neural pathway for swallowing. There are fewer myofibrils per motor unit of the laryngeal muscles relative to larger muscles (4-6 vs 4,000), and there are numerous small numerous of this type that participate in the oropharyngeal phase of swallow, In addition, the motor units within each laryngeal muscle tend to fire asynchronously during a normal swallow, contrasting with the more synchronous firing of larger muscles designed for strength. Under this model, even a few days without the typical 600-2,400 normal swallows per day could lead to long-term dysphagia. Though this design of small muscles might make them more susceptible to failure from lack of use, it is possible that this design can respond more fully to ES. perhaps this areason why ES of the neck restores effective swallow with fewer treatments than required for restoration of appropriate function by ES of other muscles of the body. Alternatively, fewer treatments might be associated with stimulating a reflex, since swallowing is a complex action that is usually initiated voluntarily but is always completed as a reflex involving afferent and efferent cranial nerves and primary and secondary swallow centers in the cortex. These muscle tone and reflex hypotheses also pertain to the success of ES in treating urinary incontinence. Much research is required to determine whether ES, applied at a sensory level in our study, works via a peripheral nerve, a direct effect on the small muscles, the central nervous system, or a combination of these factors.
Our data directly address issues of safety. ES of the head and neck, discussed in the recent third edition of Charles Darwin's The Expression of the Emotions in Man and Animals, has been the subject of major recent debate about safely. possible risks include arrhythmia, hypotension, interference with pacemaker, laryngospasm, glottic closure, burns, and tumor growth. However, one successful study that applied extemal ES to a nerve of the neck had no complications. Other studies also observed no change in vital signs, electrocadiograph, or other adverse effects in patients who received implantable recurrent laryngeal and vagal nerve stimulators used to treat spastic dysphonia or control epilepsy. External application of ES with a muscle stimulator within the settings used in our study appears safe, at the sensory level of application. Standard electrode placement in our study purposely avoids the carotid body. In addition, the voltage and current used in our device are lower than is delivered by a standard neuromuscular stimulator, assumed by other authors concerned over the safety of ES.
The most important theoretical risk of ES is laryngospasm. In an animal study, laryngospasm was achieved with repetitive suprathreshold ES, but not with singleshock excitation of the superior laryngeal nerve. As stimulus frequency went above 32-64Hz, there was a decrease in adductor after-discharge and glottic pressure. In our study, suprathreshold levels of stimulation of the superior laryngeal nerve did not occur, because of the level of therapeutic current, limits on the maximum current of the stimulator, and attenuation by soft tissues of the neck. The high-frequency stimulation of ES for dysphagia exceeded 64 Hz and may be one of the factors protecting against laryngospasm. In addition, the constant current stimulator automatically dropped the voltage to maintain a constant current dose in the event of decreased electrode or tissue resistance. With these safeguards, a device as configured for our study is apparently safe. The hypothetical concerns about safety are not supported by our data.
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