[Pathomechanism of juvenile muscular atrophy of unilateral upper extremity (Hirayama's disease)--extensibility and asymmetry of the cervical posterior dural wall].

We performed myelography in chin-up position during neck flexion in 20 patients with juvenile muscular atrophy of unilateral upper extremity, and we also performed CT-myelography in rotational position during neck flexion in 15 of them. In this disease, in which posterior lower cervical dural wall shifts anteriorly and compresses the spinal cord during neck flexion, we found that the anterior dural shift and cord compression became lessened by making chin-up without changing the position of neck flexion. By measuring the posterior dural length from foramen magnum to C6 vertebra on the profile of myelogram in neutral and neck flexion posture, we found that the posterior lower cervical dural wall of this disease was less extensible during neck flexion than that of control cases. On CT-myelogram the spinal cord compression of muscularly atrophic side increased by the neck rotation to the non-atrophic side during neck flexion, which is the position of maximum extension of posterior dural wall. The spinal cord compression decreased by the rotation to the atrophic side. We think that the low extensibility and asymmetry of posterior lower cervical dural wall may be the cause of this disease and its laterality may be relevant to unilaterality of this disease. These findings could also explain the efficacy of cervical immobilization by using cervical coller.