Respiratory muscle involvement in neurological disease may be underestimated because of the reduced mobility that may occur in these conditions. As the respiratory muscles are vital for life, formal tests of respiratory muscle function have an important role in neurological disorders. Moderate degrees of impairment may be associated with nocturnal hypoventilation, whereas more severe involvement leads to respiratory failure. If acute this can lead to respiratory arrest, and if of gradual onset may be associated with signs of cor pulmonale. Weakness of the expiratory muscles may impair coughing and bulbar weakness may lead to aspiration.

Respiratory muscle weakness may be produced by lesions affecting the central nervous system, the peripheral nerves, the neuromuscular junction or the muscles themselves.

Central nervous system

Respiratory dysfunction is seen in patients after hemiplegia secondary to cerebrovascular disease (De Troyer, Zegers de Beyl & Thirion, 1981). Patients with a unilateral corticospinal lesion demonstrate reduced activity of both the intercostal muscles and the diaphragm on the side of the paresis during inspiration.

Parkinson's disease may lead to low vital capacity and a reduction in maximum inspiratory effort (Bogaard et al., 1989). Bruin and colleagues recently concluded that anti-parkinsonian medication improved the co-ordination of upper airways and chest wall musculature, particularly during rapid events such as coughing and swallowing, and therefore might reduce the incidence of aspiration (De Bruin et al., 1993). Chorea may impose involuntary efforts on the normal respiratory pattern (Newsom-Davis, 1970).