Diurnal and day-to-day variation of isometric muscle strength in myasthenia gravis.

INTRODUCTION

In patients with myasthenia gravis (MG), muscle strength is expected to decrease gradually during the day due to physical activities.

METHODS

Isometric muscle strength at the shoulder, knee, and ankle was determined in 10 MG patients (MGFA class II-IV) who were receiving usual medical treatment and in 10 control subjects. To determine diurnal and day-to-day variation, muscle strength was measured 4 times during day 1 and once at day 2.

RESULTS

Knee extension strength decreased during the day in both patients and controls. Neither diurnal nor day-to-day variation of muscle strength was higher in patients compared with controls.

CONCLUSIONS

Patients with mild to moderate MG did not have increased variation of isometric muscle strength during the day or from day-to-day compared with controls. This suggests that isometric muscle performance can be determined with high reproducibility in similar groups of MG patients without regard to time of day.