Cerebrospinal fluid dynamics in the tardive cauda equina syndrome of ankylosing spondylitis. Typical cauda equina syndrome secondary to long-standing ankylosing spondylitis is reported in a 63-year-old man. Radionuclide cisternography demonstrated a resorption defect of cerebrospinal fluid in the enlarged lumbosacral dural sac. After transient symptomatic improvement with acetazolamide, a lumboperitoneal shunt was placed. The rate of cerebrospinal fluid, isotope resorption became normal. In the 5 years of follow-up, partial remission has been observed. 