Electrophysiological study in juvenile asymmetric segmental spinal muscular atrophy (Hirayama disease)

Hirayama disease (HD) is characterized by slowly progressive distal amyotrophy of upper limb, usually monolateral, occurring predominantly in young Asian men. The aetiology of HD is not well understood, various aetiologic factors have been suggested such as microcirculatory disturbances of spinal cord or transient cord compression during neck flexion as well as autoimmune, atopic, and genetic factors. We report on a 21-year-old man who developed a slowly progressive weakness and atrophy with cold paresis of his right hand and forearm within four years. Neurological examination revealed slight muscular atrophy of the right hypothenar and forearm, distinct paresis of right abductor digiti minimi (ADM), adductor pollicis brevis, and interossei muscles. He complained diffuse numbness in digiti IV and V. Biceps, radioperiosteal, triceps, and Trömner jerks were equally weakened bilaterally. Pyramidal signs were not present. First clinical suspicion of sulcus ulnaris syndrome could not be maintained as EMG studies identified neurogenic changes in bilateral triceps, right ADM, and right extensor and flexor carpi ulnaris muscles. Denervation activity (fibrillations and occasional fasciculations) was present in right ADM and right triceps muscles. Fasciculations could also be observed by neurosonology in right ADM and triceps muscles. Sulcus ulnaris and cubital tunnel showed normal anatomy. Median and ulnar motor and sensory conduction studies were normal as well as central motor conduction time to upper and lower limbs. Conduction blocks were not present. Routine blood and cerebrospinal fluid examination was normal. Cervical MR images demonstrated right pronounced focal atrophy of the cervical cord at the C4-C7 vertebral levels. Distinct cold paresis was confirmed clinically. The patient was diagnosed to have Hirayama disease.

Because flexion myelopathy is one of the suggested aetiologic mechanisms of HD, ulnar nerve SEP was performed in standard condition and during neck flexion showing no significant difference of amplitude or latency of N13. F waves recorded from ADM by stimulating ulnar nerve did not significantly change during neck flexion as well. Right sided F latency was prolonged already in neutral position (35.6ms) with decline in persistence, left sided ulnar F wave was unrecordable. There is still conflicting evidence concerning the aetiology of HD and previously published electrophysiological studies are heterogenous. However, our patient showed no significant changes in SEP or F wave parameters during neck flexion. If flexion myelopathy is claimed, neck flexion should be associated with symptoms regressing in neutral position. Furthermore spinal flexion does not explain the natural history of the disease which stabilizes after a few years. Our results argue against a flexion-induced cervical myelopathy and support the view that HD is an intrinsic motor neuron disease.