Sleep dysfunction in neuromuscular disorders

Abstract

There is an increasing awareness of sleep dysfunction in neuromuscular disorders. Most of the sleep disturbances in neuromuscular disorders are secondary to sleep disordered breathing. Sleep-disordered breathing in neuromuscular disorders is commonly associated with a slowly developing chronic respiratory failure, particularly in the advanced stages, but the condition often remains unrecognized and untreated. The most common sleep-disordered breathing in neuromuscular disorders is sleep-related hypoventilation which initially manifests during REM sleep and later as the disease advances, it is also noted during non-REM sleep and even during daytime. In addition to the hypoventilation, central and upper airway obstructive apneas as well as hypopneas occur. Hypoventilation during sleep gives rise to hypoxemia and hypercapnea causing chronic respiratory failure. The abnormal blood gases may later persist during the daytime. Some patients may, however, have sleep onset or maintenance insomnia as a result of associated pain, muscle immobility, contractures, joint pains and muscle cramps as well as anxiety and depression.The commonest complaint in patients with neuromuscular disorders associated with sleep-disordered breathing is excessive daytime somnolence as a result of repeated arousals and sleep fragmentation due to sleep hypoventilation and transient nocturnal hypoxemia. Sleep-disordered breathing causing sleep disturbance is well known in patients with poliomyelitis, postpolio syndrome, amyotrophic lateral sclerosis, also known as motor neuron disease, primary muscle disorders including muscular dystrophies and myotonic dystrophy, congenital or acquired myopathies, neuromuscular junctional disorders and polyneuropathies. All of these conditions may cause weakness of the diaphragm, the intercostal and accessory muscles of respiration causing breathlessness and other respiratory dysrhythmias. As a result of the respiratory and upper airway muscle weakness, the normal sleep-related respiratory physiologic vulnerability becomes pathological in these patients with neuro-muscular disorders causing hypoventilation or central and upper airway obstructive apneas during sleep. Multiple factors are responsible for sleep-disordered breathing in neuromuscular disorders causing sleep hypoventilation and other respiratory dysrhythmias, and these include: impaired chest bellows, increased work of breathing, hypo-responsive respiratory chemoreceptors, increased upper airway resistance, decreased minute and alveolar ventilation, REM-related marked hypotonia or atonia of the respiratory muscles except the diaphragm, respiratory muscle fatigue and kyphoscoliosis secondary to neuromuscular disorders causing extrapulmonary restriction of the lungs. Nocturnal hypoventilation and chronic respiratory failure in neuromuscular disorders may present insidiously and initially may remain asymptomatic. A high index of clinical suspicion is needed. Clinical clues suggesting sleep-disordered breathing include daytime hypersomnolence, breathlessness, disturbed nocturnal sleep and unexplained leg edema. If the clinical clues strongly suggest sleep-disordered breathing, a physical examination must be directed to uncover bulbar and respiratory muscle weakness. Patients with neuromuscular disorders showing these clinical features must be investigated to uncover nocturnal hypoventilation to prevent serious consequences of chronic.