What pulmonary function pattern is typical in MG due to respiratory muscle weakness?

• A. Decreasing tidal volume, vital capacity, and maximal inspiratory pressure
• B. Increased tidal volume with normal capacities
• C. Increased FEV1 and FVC
• D. Normal volumes with restrictive pattern only on imaging

Answer: (A)

Respiratory muscle weakness from myasthenia gravis reduces the ability to generate strong, sustained inspiratory effort, so the pattern you see on pulmonary function testing is smaller breaths and overall lower lung volumes. Tidal volume falls because quiet inspiration becomes shallow when the diaphragm and accessory inspiratory muscles are weak. Vital capacity drops as the muscles can’t sustain a full, deep inspiration. Maximal inspiratory pressure, which directly reflects the strength of the inspiratory muscles, is decreased.

Airway flow measurements like FEV1 and FVC are typically normal or near normal in MG because the problem isn’t with the airways or lung parenchyma, but with the muscles that move air. The FEV1/FVC ratio stays normal unless there’s superimposed disease. In more fatigued states, volumes can decline further, but the hallmark pattern due to respiratory muscle weakness is reduced tidal volume, reduced vital capacity, and reduced maximal inspiratory pressure.