[Juvenile muscular atrophy of unilateral upper extremity (Hirayama disease)--half-century progress and establishment since its discovery].

This disease occurs in adolescence, predominatly in male. The main clinical features include predominantly unilateral muscular weakness and wasting of the hand and forearm (the brachioradialis is spared: oblique amyotrophy). The clinical course is incidious onset and slowly progressive, followed by a spontaneous arrest within several years. Twelve patients with this disease were first reported by Hirayama and his associates in 1959, who clinically distinguished this disorder from previously known degenerative and progressive motor neuron diseases. The clinical features had been further clarified by the report on 20 patients in 1963, and completed in the report on 38 patients in 1972. A quarter of a century had passed without pathological confirmation, primarily due to the benign course of the disease. The first autopsy case was obtained in 1982. The neuropathological findings were reported by Hirayama and his associates in 1985 in Japanese, then in 1987 in English. The spinal cord showed anteroposterior flattening and ischemic necrotic changes of the anterior horns of the cervical cord at C5-T1, mostly severe at C7 and C8, predominantly on the left (the patient had bilateral muscular atrophy, predominantly on the left). These findings suggested a circulatory insufficiency of the lower cerivical cord, but the intra- and extra-medullary vessels were normal. The pathologic evidence prompted neuroradiologic (CT, MRI) studies in the late 1980s. Our studies of 73 patients revealed that dynamic compression of the lower cervical cord due to forward displacement of the cervical dural sac (especially posterior segment) and spinal cord on neck flexion was confined to an early and progressive stage of the disease. An absence of forward displacement in a later and non-progressive stage of the disease suggested that the dynamic compression had pathogenic significance. The pathologic findings and results of radiological studies suggest that sustained or repeated neck flexion might cause an anterior shift of the cervical dural sac, then the compressed cervical cord at the segments induce an increased intramedullary pressure, resulting in microcirculatory disturbance in the anterior horn, the most vulnerable structure to ischemia in the spinal cord. Based on this hypothesis, we tried cervical collar therapy for patients when they may have sustained or repeated neck flexion, and reported these data in 1991, 1992 and 2001. No one showed further progression of signs and symptoms. This favorable effect supports our pathogenic hypothesis described above. The author proposes that the etiology of this disease is disproportionate growth between the vertebral column and the contents of the spinal canal especially the dural sac during the juvenile growth. The nationwide epidemiological study in Japan was carried out from 1996 to 1998, identified 333 cases of the disease. There were fewer case reports from other countries than from Japan. As the number of patients is exceedingly large in Japan, there might be an ethnic factor in this disorder.