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Demyelinating diseases: PFTs

Respiratory muscle weakness is common among patients who have neuromuscular disease. It can be acute (e.g., guillain-barré syndrome), chronic and relapsing (e.g., multiple sclerosis, myasthenia gravis), or relentlessly progressive (e.g., amyotrophic lateral sclerosis [als]).

It is estimated that 15-28% of patients with myasthenia gravis and 20-30% of patients with Guillain-Barré syndrome will require invasive mechanical ventilation. Respiratory muscle (inspiratory, expiratory, upper airway) weakness due to neuromuscular disease can cause insufficient ventilation, nocturnal hypoventilation, or ineffective cough. It can also be associated with bulbar dysfunction.

Respiratory muscle weakness is confirmed by pulmonary function testing. Typical abnormalities include :

A pattern of restriction:

  • reduced forced expiratory volume in one second (FEV1, less than 80% of predicted)
  • reduced forced vital capacity (FVC, less than 80% of predicted), normal FEV1/FVC ratio (greater than 70% of predicted)
  • reduced total lung capacity (TLC, less than 80% of predicted).

Patients with predominantly expiratory muscle weakness also demonstrate an increased residual volume (RV).

FVC measured in the supine position is more than 10% lower than that measured in the upright position. Vital capacity (VC) measured in the supine position is >10% lower than that measured in the upright position. The VC is usually higher than the FVC, with the difference being directly related to the degree of obstruction. The VC may be a more reliable indicator of respiratory muscle weakness because it is not affected by coexisting obstructive airway disease.

Diffusing capacity for carbon monoxide (DLCO) is normal.