Neuromuscular and Chest Wall Disorders: Failing the Bellows
Recognize the restrictive pattern
Neuromuscular disease produces a restrictive defect: low TLC and vital capacity with a preserved or high FEV1/FVC ratio and normal DLCO. Respiratory muscle strength falls before routine spirometry is dramatic, so measure maximal inspiratory pressure, maximal expiratory pressure, and vital capacity in both upright and supine positions. A supine vital capacity drop greater than 20% (some sources use 25%) suggests diaphragm weakness, and nocturnal hypercapnia typically precedes daytime hypercapnia.
Specific diseases
Amyotrophic lateral sclerosis produces progressive weakness, often with bulbar dysfunction; noninvasive ventilation improves survival and quality of life, particularly with orthopnea or a forced vital capacity below 50% predicted. Guillain-Barré syndrome can cause rapid respiratory failure: intubate for vital capacity below about 15–20 mL/kg, or a decline of more than 30% from baseline, or inability to handle secretions. Myasthenia gravis crisis requires airway assessment and treatment of the underlying exacerbation. Kyphoscoliosis and post-polio syndrome cause chronic alveolar hypoventilation treated with nocturnal noninvasive ventilation. Obesity hypoventilation overlaps here when the chest wall load is dominant.
Complications and traps
The first clue to nocturnal hypoventilation may be morning headache, daytime somnolence, or unexplained polycythemia. Patients with neuromuscular disease may have normal oxygen saturation until late, so oximetry alone is inadequate screening. Secretion clearance, cough assistance, and aspiration prevention matter as much as ventilation. Do not attribute dyspnea to deconditioning when vital capacity is falling.
Worked rule
In a patient with ALS and FVC 45% predicted plus orthopnea: start nocturnal noninvasive ventilation rather than waiting for daytime hypercapnia.