Cervical myelopathy due to a "tight dural canal in flexion" with a posterior epidural cavity.

A 41-year-old man noticed weakness and atrophy in his right hand and forearm resembling the non-progressive juvenile muscular atrophy of unilateral upper extremity (Hirayama's disease). MRI showed an abnormal cavity in the posterior epidural space which appeared on neck flexion communicating with the subarachnoid space in addition to the flattening of the lower cervical spinal cord on neck flexion. When evaluating atypical cases of Hirayama's disease, the pathomechanism demonstrated in the present case should be taken into consideration.