Assessment of ventilatory function in patients with neuromuscular disease

In early phases of neuromuscular disease, patients are either free of respiratory symptoms or have exertional dyspnea not explained by obvious obstructive or restrictive lung disease. Physical examination may be negative because generalized muscle weakness does not correlate with the degree of respiratory muscle involvement. When the diaphragm is involved, one may detect the absence of outward excursion during inspiration or even paradoxic inward inspiratory movement of the abdomen on one side. A substantial loss of respiratory muscle strength is typically accompanied by little or no change in spirometry or arterial blood gas composition. Other characteristics are moderate loss of maximal voluntary ventilation and an increase in residual volume, yet PImax and PEmax may be as low as 50% of the predicted value. In more advanced neuromuscular disease, patients may have severe symptoms if the onset is acute or subacute; however, patients with chronic advanced generalized muscle weakness do not exercise and, therefore, may not be breathless. Many patients with advanced neuromuscular disease present with daytime somnolence as a manifestation of a sleep-related breathing disorder. Physical examination may reveal generalized muscle weakness and difficulty with speech or swallowing. Signs specific to respiratory involvement include tachypnea, use of neck inspiratory muscles and abdominal expiratory muscles, and loss of chest-abdomen synchrony. Sometimes paradoxic bilateral inward movement of the abdomen with inspiration is overt. Patients may be unable to cough effectively, have scoliosis, and lack a gag reflex. At this advanced stage, PImax and PEmax are lower than 50% of the predicted value, and the vital capacity is reduced. Maximal voluntary ventilation increases, and residual volume increases further. Patients may not yet exhibit CO2 retention during the day and may even have a low PaCO3. A sleep study may reveal significant hypopneas with severe desaturation and hypercapnia, especially during REM sleep. It is important to be aware that overt ventilatory failure can occur abruptly and that measurement of arterial blood gas composition is not a reliable indicator of this danger. Therefore, it is critically important to heed clinical phenomena, such as increasing dyspnea and tachypnea, and symptoms of sleep disturbance, such as morning headache and daytime somnolence. Physicians should make serial measurements of VC and respiratory muscle strength in patients considered to be at risk for further deterioration.(ABSTRACT TRUNCATED AT 400 WORDS)