Sympathetic skin response in monomelic amyotrophy

Monomelic amyotrophy (MMA) a variant of motor neuron disease, has the characteristic features of wasting and weakness usually confined to a single upper or lower limb occurring predominantly in young males and a benign outcome. Symptoms of increased sweating, coldness and cyanosis have been observed in a few patients. The objective was to evaluate the involvement of the sympathetic nervous system in MMA by measuring sympathetic skin response.

Electromyography, motor and sensory nerve conduction studies were done in all the four limbs of 9 patients with atrophy of one upper limb. Stimulation at Erb's point, and above and below elbow was done to look for evidence of conduction block. The sympathetic skin response (SSR) was recorded in all the limbs of these patients. Wasting and weakness of right upper limb in 7 patients and left upper limb in 2 patients was seen. The mean age was 28.3+/-10.1 years. Twenty-five age matched (24.8+/-4.8 years) healthy subjects served as controls.

The mean SSR latency in the affected upper limbs of 9 patients was prolonged compared to the 25 control subjects (1.51+/-0.07 s vs 1.42+/-0.19 s, P=0.03). The mean value of SSR latency in 18 upper limbs of the 9 patients which included atrophied and unatrophied limbs was also prolonged compared to the controls (1.50+/-0.08 s vs 1.42+/-0.19 s, P=0.05). There was no significant difference of the mean latency of SSR between the atrophied upper limbs and the clinically normal upper limbs (1.51+/-0.07 s vs 1.49+/-0.09 s, P=0.51). The mean SSR latency in the lower limbs of the patients (2.09+/-0.09 s) did not significantly differ from the control subjects (1.97+/-0.28 s, P=0.09). Motor and sensory nerve conduction was normal and there was no evidence of conduction block.

In MMA the sympathetic nervous system is involved in the atrophic upper limb and also in the clinically unaffected upper limb but not in the lower limbs.