Muscle strength in myasthenia gravis

Myasthenia gravis (MG) is characterized by fatigue and fluctuating muscle weakness as a result of impaired neuromuscular transmission (NMT). Although MG is a prototypic fatiguing disorder, little is known about how the condition affects fixed weakness, and if present, whether weakness is related to disease duration or gender. The aim of this study was to quantify the strength of patients with MG and investigate whether it is related to disease duration.

Eight muscle groups were tested by manual muscle testing and with a hand-held dynamometer in 38 patients with generalized MG and 37 healthy age- and gender-matched controls. The disease duration was recorded and compared with strength measures.

On average, muscle strength was decreased by 28% compared with controls (P<0.01). Repeated strength measures in individual patients did not differ, suggesting that the muscle force reported was not subject to fatigue, but reflected fixed weakness. The male patients showed a greater reduction in muscle force in all eight muscle groups than women with MG (60% vs 77% of normal, P<0.05). In both men and women with MG, strength in shoulder abductors was most affected (51% vs 62% of normal). Muscle strength and disease duration were not related.

These findings show that patients with generalized MG have (i) a significant generalized, fixed muscle weakness, (ii) that male patients with MG have a more severe muscle weakness than women, (iii) that proximal upper limb muscles are most affected, and (iv) that disease duration alone is not a predictor of loss of muscle strength.