[Extra dural arachnoid cyst: case report].

Extradural arachnoid cysts are uncommon expanding lesions in the spinal canal, which may communicate with the subarachnoid space. Usually located in the lower thoracic spine, they may cause symptoms by compressing the spinal cord or nerve roots. We report a case of an extradural thoracic arachnoid cyst revealed by progressive spinal cord compression. CT myelography and MRI enabled diagnosis. Rapid neurological improvement was observed after surgical resection.