A prospective, observational cohort study.The aim of this study was to determine the role of pre and postvoid bladder scan in predicting cauda equina syndrome (CES). The thesis was that bladder scanning [specifically postvoid residual (PVR) volume] would have higher diagnostic accuracy than physical examination alone.CES is an ill-defined condition with a spectrum of presenting symptoms. There is neither a combination of clinical symptoms and/or signs that reliably predicts cauda equina compression nor single defining clinical criterion that has 100% predictive value to confirm or exclude CES.Patients with suspected CES admitted over a 6-month period at a single institution were prospectively assessed by physical examination (including digital rectal examination and pin prick perianal sensation) and bladder ultrasound scanning (recording pre- and PVR volume). These results were compared with the subsequent magnetic resonance imaging (MRI) scans and those patients who had emergent surgery for CES.Ninety-two patients were included in the study (52 women) with a mean age of 44.9 years.An MRI scan demonstrating causing compression of the cauda equina was present in only 18% (17/92).The sensitivity of anal tone to predict CES was 52.9%. Peri-anal numbness (either unilateral or bilateral) had sensitivity of 82.3% and negative predictive value of 92%.For nonoperated group (without CES), mean PVR was 199 mL (95% confidence interval ± 59 mL). On the basis of receiver operating curves, the optimal bladder volume cut-off for predicting CES was ≥200 mL for PVR volume. A PVR of <200 mL gave CES probability of 3.6%. If >200 mL, then the probability of having CES is 43% (P < 0.000003). A PVR <200 mL had a negative predictive value of 97%.Bladder scanning was a useful adjunct in the diagnosis of CES. It had a better negative predictive value than physical examination.3.