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Sleep‐disordered breathing in neuromuscular disorders: A condition in search of recognition

Sleep-disordered breathing (SDB) is commonly associated with insidiously developing chronic respiratory failure in neuromuscular disorders, especially in the advanced stages, but is often unrecognized and untreated. It is sometimes the initial presentation of certain disorders, such as motor neuron disease and acid maltase deficiency. SDB associated with respiratory infection causing acute respiratory failure is the commonest cause of death in progressive neuromuscular disorders. Effective treatment is available that improves the quality of life without always altering the natural history of the illness. Disordered breathing in neuromuscular disorders often manifests in sleep. Physiologically, all humans are vulnerable to respiratory impairment in sleep, particularly rapid-eye-movement (REM) sleep. Control of respiration during sleep and wakefulness depends on two anatomically and functionally independent systems: the metabolic (automatic) and behavioral (voluntary) systems. Both metabolic and voluntary systems are active during wakefulness, whereas only the metabolic system participates during sleep. Respiratory homeostasis is unprotected during sleep. Minute ventilation and alveolar ventilation decrease due to a reduction of tidal volume. Sleep-related alveolar hypoventilation results from a combination of the following factors: loss of wakefulness stimuli, increased upper airway resistance, impaired chemoresponsiveness of the respiratory neurons, and falling metabolic rate (reduced Vo2 and Vco2). The increased upper airway resistance results from decreased activity of the upper airway dilator muscles and reduced output from the medullary respiratory neurons during sleep. The upper airway dilating and intercostal muscles show mild hypotonia during non-REM sleep but marked hypotonia or atonia during REM sleep. The diaphragm maintains phasic activities, but tonic activity is reduced in REM sleep. The functional residual capacity decreases in the supine position due to hypotonia of the intercostal muscles. As a result of mild alveolar hypoventilation during sleep in normal individuals, arterial oxygen tension falls, causing less than 2% decrement of oxygen saturation, and arterial carbon dioxide tension rises slightly. Hypoxic and hypercapnic ventilatory responses decrease mildly during non-REM sleep, with a more marked decrement during REM sleep. These sleep-related ventilatory changes do not have any significant effects in normal persons. They become critical, transforming physiological nocturnal hypoventilation into pathological sleep-related hypoventilation, and may trigger lifethreatening hypoxemia, abnormal breathing patterns, and respiratory failure in patients with neuromuscular disorders associated with respiratory muscle weakness. The term SDB generally includes apnea, hypopnea, upper airway resistance syndrome or respiratory effort-related repeated arousals, paradoxical breathing, periodic respiration, and sleep hypoventilation. Based on an analysis of the breathing patterns, three types of sleep-related apneas have been defined: central, obstructive, and mixed apneas. During central apnea, both the respiratory effort and airflow (air exchange) through the nose and mouth are absent. In obstructive apnea, the effort persists but the airflow ceases. An initial central followed by obstructive apnea is called mixed apnea. Hypopnea is generally defined as a decrease in effort and airflow to 50% or less of the preceding or following respiratory cycle. Sleep apneas and hypopneas are usually accompanied by O2 desaturation and terminated by an arousal, causing sleep fragmentation. Recurrent apCorrespondence to: schok@worldnet.att.net