Atypical motor unit potentials in Emery-Dreifuss muscular dystrophy (EDMD)

The aim of the study was to analyse electromyographic changes in Emery-Dreifuss muscular dystrophy (EDMD) that are atypical for myopathy. Our special interest was focused on high amplitude polyphasic motor unit potentials (MUPs), also termed irregular MUPs.

We studied 21 EDMD patients with the diagnosis based on clinical data, DNA analysis and immunohistochemical muscle studies. Rectus femoris muscle biopsies were investigated in all affected patients. Electrophysiological investigations involved quantitative concentric needle electromyography (CNEMG) of biceps brachii (BB) and rectus femoris (RF) muscles. Simulation studies were performed to approximate the number, diameter and distribution of muscle fibers, which contribute to irregular MUPs.

The EMG data in EDMD were compatible with myopathy. Irregular MUPs showed longer duration, larger area, size index and higher amplitude then simple ones (P < 0.05). The approximation of features of muscle fibers contributing to irregular MUP also indicated smaller (<45 microm) and larger (>55 microm) diameters than normal (50 +/- 5 microm). Muscle biopsy specimens revealed the variable muscle fiber size due to atrophy, hypertrophy, and muscle fiber splitting.

Irregular MUPs recorded in EDMD are due to hypertrophied and atrophied fibers as well as increased fiber density. They reflect reorganization of the motor unit in a slow progression myopathic process (muscle fiber hypertrophy and splitting).

Irregular MUPs in EDMD most probably reflect increased variability of the muscle fiber size.