Neuromuscular diseases are a diverse group of disorders that vary markedly in etiology, rate of progression, pattern of respiratory involvement, prognosis, and therapy. Neuromuscular disorders impair the respiratory system as a vital pump; however, depending on the particular disease, the respiratory pump may be impaired at the level of the central nervous system (e.g., cerebral cortex or brain stem), spinal cord, peripheral nerve, neuromuscular junction, or respiratory muscle (Table 84-1).
TABLE 84-1Levels of Respiratory System Dysfunction Induced by Neuromuscular Diseases and Conditions ||Download (.pdf) TABLE 84-1 Levels of Respiratory System Dysfunction Induced by Neuromuscular Diseases and Conditions
|Disease or Condition
|Upper motor neuron
| Spinal cord
|Lower motor neuron
| Anterior horn cells
Spinal muscle atrophy
Amyotrophic lateral sclerosis
| Motor nerves
| Neuromuscular junction
Critical illness polyneuropathy
Acid maltase deficiency
The pattern of ventilatory impairment among these disorders is highly dependent on the specific neuromuscular disease. For example, some disorders may impair ventilation at only one level (e.g., isolated diaphragm paralysis) or simultaneously affect it at different levels (e.g., multiple sclerosis). Additionally, the severity of impairment may be minimal and totally resolve with time and proper treatment (e.g., Guillain-Barré syndrome) or is characterized by relentless progression to eventual respiratory death (e.g., amyotrophic lateral sclerosis). Moreover, some neuromuscular diseases concomitantly affect several structures (e.g., swallowing dysfunction in poliomyelitis, interstitial lung disease in polymyositis), increasing ventilatory workload in patients who already have diminished ventilatory reserve.
In this chapter we describe the etiology, pathophysiology, and treatment of ventilatory dysfunction in neuromuscular diseases.
Substantial information exists concerning the ventilatory function of patients with neuromuscular disease at rest and during sleep, as well as the effects on maximum static inspiratory and expiratory efforts and responses associated with these disorders to hypoxic and hypercapnic challenges. In general, the response of the respiratory system to moderate or severe neuromuscular disease is relatively stereotyped. The typical features are a reduced forced vital capacity, reduced respiratory muscle strength, and, in some cases, malfunction of the neurons that control breathing.
The breathing pattern is often abnormal in patients with neuromuscular disease. In comparison with healthy subjects, patients with respiratory muscle weakness have a low tidal volume and a high respiratory rate that persists in response even to hypoxic or hypercapnic challenge. Moreover, this rapid, shallow breathing pattern is not due to abnormalities in gas exchange (i.e., hypoxemia or hypercapnia) but is more likely to be due to severe muscle weakness and/or disordered afferent and efferent output in motor neurons impaired by the underlying neuromuscular ...