Just the facts: Risk stratifying nontraumatic back pain for Cauda Equina Syndrome in the emergency department.

A 43-year-old male, with a history of chronic back pain, presents to the emergency department (ED) with acute onset chronic pain. He states he "tweaked something" and has been debilitated by back pain, radiating down both his legs, for 24 hours. He has not had a bowel movement but denies noticing any "saddle anesthesia." His clinical exam is limited by pain, and it is difficult to determine if he has objective weakness. His perineal sensation is intact, as is his sensation upon digital rectal examination. The patient has a post-void residual of 250 mL, but you are unsure how to interpret this value. As an emergency physician, when should you suspect, and how should you evaluate cauda equina syndrome?