MRI findings in Hirayama's disease: flexion-induced cervical myelopathy orintrinsic motor neuron disease?

Hirayama's disease is a benign juvenile form of focal amyotrophy affecting the upper limbs. Previous studies have suggested that the disorder is a nec k flexion induced cervical myelopathy. We report clinical and magnetic reso nance imaging findings in nine patients with Hirayama's disease. Cervical i maging of seven patients revealed spinal cord changes consisting of focal a trophy and foci of signal alterations. On neck flexion a forward movement a nd mild reduction in the anteroposterior diameter of the lower cervical cor d against the vertebral bodies was noted in affected individuals as well as in five normal controls. In contrast to earlier reports, none of our patie nts showed complete obliteration of the posterior subarachnoid space. Measu rement of the anteroposterior spinal cord diameter in each vertebral segmen t (C4-C7) revealed no significant differences in the degree of spinal cord flattening between the two groups. Furthermore, two of our patients had sig nificant degenerative changes in the cervical spine (disc herniation, retro spondylosis) contralateral to the clinically affected side. These degenerat ive changes resulted in a marked cord compression on neck flexion but were not associated with ipsilateral clinical abnormalities or spinal cord alter ations. Our results argue against a flexion-induced cervical myelopathy and support the view that Hirayama's disease is an intrinsic motor neuron dise ase.