[A case of idiopathic herniation of the spinal cord associated with duplicated dura mater and with an arachnoid cyst].

Idiopathic herniation of the spinal cord is a very rare disease, only 2 cases being so far reported in the literature. A 61-year-old man with a gradual loss of power in his lower extremities was diagnosed as intradural arachnoid cyst, and underwent operation. During operation, not only an arachnoid cyst was observed dorsally, but was also duplicated the dura mater ventrally. In the latter lesion, a ventral part of the spinal cord was herniated from a defected area of the dural inner layer. Although this dural inner layer as well as the arachnoid cyst was resected, his clinical symptoms remained unchanged after operation. When a shift of the spinal cord is detected in an image, we should precisely determine the relationship between the spinal cord and the dura mater during operation with consideration of a possibility of the presence of idiopathic herniation of the spinal cord.