Q: In Myasthenia Gravis (MG), to assess the muscle strength - Vital Capacity can be measured both in sitting and supine position. Diaphragmatic weakness is more apparent in which position?


Answer:  Supine

Vital Capacity (VC) is the most commonly and easily used quick assessment at bedside to assess the mechanical function of both inspiratory and expiratory muscle strength. Diaphragmatic weakness is more apparent on the supine VC measurement. But generally sitting position measurement is well accepted as a cut off point to transfer patient from floor/ward to ICU. 

Important clinical lessons learned from pitfalls of VC measurement are:

1. It is not the absolute number at VC but the "speed" of a downward trend in VC. It should be measured frequently, as often as every hour as patients  may precipitously fatigue and go in rapid development of respiratory failure before a downward trend in VC is evident.

2. Many patients with facial weakness caused by myasthenia gravis may have a falsely low VC. 

3. Hypoxemia and hypercarbia are insensitive measures of respiratory muscle weakness, and ABG should not be relied on too much but progressive hypercarbia indicates quickly failing respiratory system.



Reference:

 Rabinstein AA, Wijdicks EF. Warning signs of imminent respiratory failure in neurological patients. Semin Neurol 2003; 23:97.