Polysomnography is important in the investigation of patients with excessive daytime somnolence, disorders of initiating and maintaining sleep, disorders of the sleep-wake cycle, and disorders associated with certain sleep stages (parasomnias).
It is important in confirming the existence of insomnia and characterizing its nature by determining, for example, whether it is associated with nocturnal myoclonus or periodic leg movements. Some patients complain of insomnia but, in fact, have a normal amount of sleep. Patients with complaints of excessive daytime somnolence may have sleep apnea, which can be diagnosed by polysomnography. Apnea is defined in this context as the cessation of air flow at the mouth and nostrils for at least 10 seconds, whereas hypopnea refers to a reduction in respiratory air flow to one third of its basal value, with an associated reduction of abdominal and thoracic respiratory movements and a decline in oxygen saturation. The number of respiratory irregularities per hour of sleep can be calculated by dividing the number of apneic and hypopneic episodes by the total sleep time in minutes and multiplying the result by 60. A value of 5 or less is regarded as within the normal range. y Polysomnography further indicates whether sleep apnea syndrome relates to a central disturbance, an obstructive disturbance in the oropharynx ( .Fig...24-11 ), or a mixed disorder. Sleep apnea is dangerous because various cardiac abnormalities may occur during the apneic episodes, sometimes resulting in death. Polysomnography permits the severity of the syndrome and the presence of cardiac abnormalities to be determined, thereby indicating the need for prompt therapeutic intervention.
Another important cause of excessive daytime sleepiness is narcolepsy, which is diagnosed by the multiple sleep latency test. This quantifies the time required to fall asleep and allows the recognition of abnormally short latencies for going into REM sleep.  , y Patients are monitored for five periods of 20 minutes at intervals of 2 hours, during which they are allowed to fall asleep; between these periods, they are kept awake and alert. A mean daily score of less than 5 minutes indicates a pathological level of daytime sleepiness. The number of sleep-onset REM periods (with REM sleep occurring within 15 minutes of sleep onset) is determined. Normally, REM sleep does not occur during these periods; two sleep-onset REM episodes strongly support a diagnosis of narcolepsy. The sensitivity and specificity of the test, however, are not clearly defined. y
Certain complex behavioral disorders may disrupt sleep. These parasomnias are best characterized also by polysomnography. They tend to occur particularly during slow-wave sleep, and precise diagnosis facilitates appropriate treatment. In other patients, seizures may occur during sleep and these can be recognized by the study. Further discussion of sleep disorders is provided in Chapt§L54. .
Stimulation of certain sensory systems leads to the generation of cerebral potentials that can be recorded over the scalp with surface electrodes. The latency and morphology of these evoked potentials depend on the eliciting stimulus. Evoked potentials are generally of low voltage and are
Figure 24-11 Polysomnogram of a patient with obstructive sleep apnea. An obstructive apneic episode is seen during REM sleep and is characterized by cessation of air flow associated with oxygen desaturation, accompanied by attempted respiratory movements of the thoracic and abdominal musculature.
intermixed with background EEG activity. For this reason, they are not easily seen without computer averaging of a number of responses so that signals that are time locked to the stimulus are enhanced whereas other electrocerebral activity is averaged out.
Evoked potential recordings provide a means of detecting lesions in the afferent pathways under study. They assess the functional integrity of these pathways, whereas imaging techniques such as magnetic resonance imaging evaluate their anatomical basis. Thus, evoked potential studies sometimes reveal abnormalities missed by magnetic resonance imaging, and vice versa. The findings may be important for diagnostic purposes, in following the course of certain neurological disorders, or for determining the extent of pathological involvement. y In patients with known pathological processes involving the CNS, evoked potential studies help to detect and localize lesions. Subclinical abnormalities may be detected in a variety of disorders, and multifocal abnormalities occur not only in multiple sclerosis but also in a variety of different settings. The electrophysiological findings must therefore be interpreted in the clinical context in which they are obtained. Evoked potential studies are also helpful in the evaluation of ill-defined complaints to categorize more precisely the functional integrity of any afferent pathways that may be responsible for the symptoms in question.y
The clinical utility of the various evoked potential studies in widespread use depends on the context in which they are obtained. Although some investigators have attempted to localize lesions on the basis of the electrophysiological findings, precise localization by this approach may not be possible. The generators of many of the recorded components are not known with confidence, and some components may have multiple generators.
Evoked potentials have been used to monitor neural structures at risk during surgical procedures, such as correction of scoliosis, in an attempt to minimize or prevent neurological damage. When an evoked potential abnormality occurs during the surgical procedure, it is hoped that alteration or reversal of the procedure may prevent or minimize damage. However, the utility of intraoperative monitoring is not well established in most circumstances. Moreover, even if an electrophysiological change occurs intraoperatively, it sometimes remains unclear whether this simply predicts an adverse outcome rather than actually permitting that outcome to be prevented if the surgical technique is modified. Nevertheless, in limited contexts, it is clear that intraoperative monitoring is beneficial. During posterior fossa surgery, for example, recordings of brain stem auditory evoked potentials are helpful in monitoring the status of the eighth cranial nerve to prevent its damage, and stimulation of the seventh cranial nerve aids in identifying this nerve and ensuring its functional continuity. Similarly, somatosensory evoked potential studies to monitor spinal cord function seem to reduce the morbidity associated with the surgical correction of scoliosis.
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