Altered motor neuron excitability in myotonic dystrophy

Using standard electrophysiologic techniques motor conduction velocity (MCV), motor action potential amplitude (Mmax), minimum F latency (Flat), maximum F amplitude (Fmax), F chronodispersion (Fdisp), Fmax/Mmax, and F persistence (Fpers) were measured in median, ulnar, peroneal, and tibial nerves (N = 60) in 15 patients with myotonic muscular dystrophy (DM). The results were compared to data from 80 nerves in 20 healthy controls. The DM group (mean age = 39.7 yrs, mean duration of symptoms = 20.8 yrs) had diminished or absent biceps, triceps, knee, and ankle reflexes but retained 4- or better strength (modified MRC Scale) in the respective muscles. Thirteen nerves in 6 patients had absent F waves. For the remaining 47 nerves, no statistically significant difference was noted in parameters primarily related to conduction through peripheral nerve fibers (MVC, Flat, Fdisp) or Fmax/Mmax. Fmax, Fpers and Mmax were significant (p less than or equal to .02) diminished in all nerve subgroups in the DM patients. The results indicate that in DM there is altered excitability of the motorneuron pool. This may underlie the early loss of DTR's and produce the characteristic distal pattern weakness.