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STUDY DESIGN A prospective, observational cohort study. OBJECTIVE 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. SUMMARY OF BACKGROUND DATA 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. METHODS 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. RESULTS 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%. CONCLUSION Bladder scanning was a useful adjunct in the diagnosis of CES. It had a better negative predictive value than physical examination. LEVEL OF EVIDENCE 3.