Laryngeal Dysfunction in Sleep

Sleep is an essential behavior in humans and many other species. Although it may look externally very simple, sleep is in fact an elaborate state resulting from a complex interplay of different central nervous system areas. There are two types of sleep: rapid eye movement (REM) and non–rapid eye movement (NREM) sleep, which are differentiated mainly by the presence or absence of rapid eye movements, muscle atonia, and characteristic electroencephalogram (EEG) activity. There are many changes in physiologic function during sleep that result in significant modifications in respiration, blood pressure, heart rate, muscle tone, and brain metabolism. Respiration, for example, is significantly altered, and the clinical relevance of this is illustrated in obstructive sleep apnea/hypopnea syndrome in which patients breathe normally while awake but experience repeated upper airway occlusions during sleep, most likely precipitated by relaxation of the upper airway muscles. The larynx also modifies its function during sleep. In a normal subject, the vocal cords abduct on inspiration, increasing the glottic area and reducing airway resistance, and adduct on expiration, both during wakefulness and sleep. Animal experiments have shown that during all stages of sleep, and particularly during REM sleep, there is a marked reduction in activity of the intrinsic muscles of the larynx, including the posterior cricoarytenoid muscle, the lone vocal cord abductor. As a result, the glottic caliber is narrowed and upper airway resistance increases, resulting in a more negative laryngeal pressure.1,2 If this physiologic change occurs in combination with any underlying abnormality in the laryngeal function (e.g., vocal cord abductor paralysis) or anatomy (e.g., laryngomalacia) that reduces the glottic area, further increases in airway resistance may reach the critical level for significant glottic obstruction during sleep. This explains why some patients with laryngeal obstruction only show clinical signs of dysfunction (such as nocturnal stridor or obstructive sleep apnea) during sleep. Laryngeal dysfunction during sleep occurs in several neurologic diseases that affect supranuclear structures, the nucleus ambiguus, or the recurrent laryngeal nerves, and it has been described, for example, in several neurodegenerative diseases, sporadic or inherited, such as multiple system atrophy, spinocerebellar ataxia, and Parkinson disease. In some of these conditions, laryngeal dysfunction during sleep may be the presenting symptom of the disease and may be associated with sudden death often during sleep.

Laryngeal function during sleep may be evaluated by different methods. Fiberoptic laryngoscopy enables visualizing the vocal cord movements during fully inspiration, expiration, and phonation. This procedure is usually performed in the awake patient, but it can be achieved under drug-induced sleep with intravenous diazepam.³ This is particularly interesting because laryngeal narrowing is exacerbated by sleep, and therefore some subjects with normal vocal cord movements during wakefulness may present vocal cord abductor restriction or paradoxical movements of the vocal cords only during sleep, both leading to nocturnal stridor. On the other hand, nocturnal polysomnography with synchronized audiovisual recording (videopolysomnography [VPSG]) allows for identification of the presence and severity of stridor, apneas, and oxyhemoglobin desaturations. Although laryngoscopy during natural sleep is the ideal procedure to define the mechanism, characteristics, and severity of laryngeal dysfunction during sleep, it is bothersome and difficult to tolerate. Because nocturnal stridor may be mistaken for soft-palate snoring or other respiratory noises, a tape recording while the patient sleeps at home may be useful in distinguishing these sounds. Laryngeal electromyography, although technically very difficult, may help to evaluate the presence of denervation or dystonia. This chapter briefly reviews the characteristics of the laryngeal dysfunction in sleep that occurs in the setting of several conditions, using multiple system atrophy as the prototypical disorder.

Multiple System Atrophy

Multiple system atrophy (MSA) is a sporadic neurodegenerative disease characterized by parkinsonism, cerebellar syndrome, and autonomic failure in any combination. Mean life expectancy is less than 10 years, and the most frequent causes of death are bronchopneumonia and sudden death.4 There are two different causes of sleep-disordered breathing (SDB) in MSA. One is central alveolar hypoventilation leading to central sleep apnea (CSA) due to degeneration of the pontomedullary respiratory centers. The other is obstructive sleep apnea/hypopnea (OSAH) and nocturnal stridor, which result from upper airway obstruction at the pharyngeal and laryngeal levels.5

Importance and Characteristics of Upper Airway Obstruction and Stridor

Detection of stridor in MSA is very important because it is associated with life-threatening episodes of respiratory failure, nocturnal choking episodes, sudden death during sleep, and short survival.5,6 Nocturnal stridor occurs in all clinical stages of MSA, and it may be the initial symptom of the disease. Patients with stridor have similar age, gender, body mass index, duration and severity of the disease, and MSA subtype (cerebellar or parkinsonian) than those without stridor, but have a higher number of apneas and hypopneas during sleep, oxyhemoglobin desaturations, and vocal cord abnormalities on laryngoscopy. Severity of nocturnal stridor gradually worsens with disease progression. Patients are usually unaware themselves of their nocturnal stridor, and unexpected stridor may be found in routine VPSG studies. Stridor during wakefulness appears in patients who already have nocturnal stridor and reflects a marked laryngeal obstruction that may herald respiratory failure.⁷

Fiberoptic Laryngoscopy Findings 3,6,7

In patients without stridor, laryngoscopy may show asymptomatic partial vocal cord abduction restriction. In patients with stridor, laryngoscopy during wakefulness usually detects normal adduction of the vocal cords in phonation, and partial or complete abduction restriction of the vocal cords during inspiration. In addition, peculiar twisting-like dystonic movements may also be observed in vocal cords with impaired abduction. Abduction restriction may be unilateral or bilateral. Subjects with complete unilateral vocal cord abduction restriction do not present severe dyspnea because the glottic space is relatively wide during inspiration. Subjects with complete bilateral abduction restriction usually have diurnal and nocturnal stridor. These subjects may not experience dyspnea because their vital capacity is so low and their activity is so limited that the narrowed airway is sufficient, although they are at a high risk of developing episodes of respiratory insufficiency. Normal movements of the vocal cords during wakefulness, however, may be seen in a few subjects with mild nocturnal stridor. In these subjects, laryngoscopy during diazepam-induced sleep discloses either paradoxical movements of the vocal cords or partial vocal cord abduction.³ Vocal abductor restriction is exacerbated during sleep, and partial abduction limitation during wakefulness may become total during sleep. Isozaki et al³ classified vocal cord abnormalities in MSA as follows: stage 1, normal movement during wakefulness and paradoxical movements during sleep; stage 2, abduction restriction during wakefulness and paradoxical movements during sleep; stage 3, almost midline position during both wakefulness and sleep.