Intradural cyst with compression of the cauda equina in ankylosing spondylitis.

A cauda equina syndrome complicating long-standing ankylosing spondylitis was found to be due to a large multiloculated cyst. This differed from posterior dural diverticula identified in similar cases in that the cyst filled the lumbar canal with erosion of the upper lumbar vertebral bodies and chronic inflammatory changes were evident in the cyst wall. Cauda equina compromise in ankylosing spondylitis has several possible mechanisms including fibrosis following arachnoiditis, root damage by dural diverticula, and direct compression by arachnoid cyst.