Muscle excitability abnormalities in Machado-Joseph disease

To estimate the frequency of and to characterize muscle excitability abnormalities in Machado-Joseph disease (MJD).

Machado-Joseph disease is a common autosomal dominant cerebellar ataxia caused by an unstable CAG trinucleotide repeat expansion. Muscle cramps and fasciculations are frequent and sometimes disabling manifestations. However, their frequency and pathophysiological mechanisms remain largely unknown. Symptomatic patients with MJD (hereinafter MJD patients) with molecular confirmation were assessed prospectively. A standard questionnaire addressing clinical features of muscle cramps and fasciculations was used. The Cramps Disability Scale was used to quantify cramps-related disability. Patients underwent neurophysiological testing with routine techniques. F waves of the right median nerves were obtained, and persistence indexes were calculated. Four muscles (deltoid, first dorsal interossei, tibialis anterior, and vastus lateralis) were examined by needle electromyography. A semiquantitative scale (from 0 [no activity] to 4 [continuous activity]) was used to determine the frequency of rest fasciculations in each muscle.

Fifty MJD patients (29 men) were included in the study. Their mean age at examination was 46.3 years, their mean age at onset of the disease was 35 years, and the mean duration of disease was 11.2 years. Abnormal CAG(n) varied from 59 to 75 repeats. Forty-one patients presented with muscle cramps; in 10, this was their first symptom. The frequency of cramps varied between 1 and 90 episodes a week. For 15 patients, cramps were the chief complaint, frequently disturbing sleep or work (Cramps Disability Scale score, 2 or 3). Lower limbs were affected in 37 individuals, but unusual regions, such as the face and abdominal muscles, were also involved. Fasciculations were found in 25 individuals; in 8 patients, they included facial muscles. However, fasciculations were not a significant complaint for any of these patients. The clinical and neurophysiological profile of MJD patients with and without cramps was not significantly different. However, MJD patients with fasciculations had more severe damage to their peripheral nerves.

Muscle excitability abnormalities were found in 41 MJD patients (82%), and they were the presenting complaint in 10 (20%). They are related to altered excitability of peripheral motor axons, but mechanisms underlying cramps and fasciculations are possibly distinct in MJD patients.