Assessment of factors affecting the difficulty of caudal epidural injections in adults using ultrasound

BACKGROUND: In unaided caudal epidural block (CEB), incorrect needle insertion has been reported to occur in 15% to 38% of attempts. OBJECTIVE: To statistically analyze the anatomical variables affecting difficult CEB using ultrasonographic measurement. METHODS: Preprocedural ultrasonography was performed and the following measurements were obtained in 146 patients: the distance from the skin to the apex of the sacral hiatus; the depth of the sacral canal at the apex of the sacral hiatus; the length of the sacrococcygeal ligament between the apex of the sacral hiatus and sacral base; the distance from the skin to the apex of the sacral cornu; and the distance between the apexes of bilateral cornua. One clinician, unaware of the ultrasonographic findings, performed the injections using the landmark technique. The procedures were videotaped and were subsequently reviewed by an independent investigator. RESULTS: The means (+/- SDs) of the abovementioned measurements were 12.1 +/- 3.7 mm, 6.1 +/- 2.1 mm, 25.9 +/- 7.4 mm, 10.0 +/- 4.0 mm and 16.4 +/- 3.2 mm, respectively. Injections failed in 16 (11%) patients and were defined as difficult in 21 (14.4%) patients. The depth of the sacral canal at the apex of sacral hiatus (P<0.001) and the length of the sacrococcygeal ligament between the apex of the sacral hiatus and sacral base (P=0.001) were significant predictors of difficult CEB. Of all patients, 85.7% and 75.2% were correctly classified as difficult or not difficult, respectively. The cutoff values of the depth of the sacral canal at the apex of the sacral hiatus and the length of the sacrococcygeal ligament between the apex of the sacral hiatus and the sacral base to predict a difficult CEB were 3.7 mm and 17.6 mm, respectively. CONCLUSIONS: Both the depth of the sacral canal at the apex of the sacral hiatus and the length of the sacrococcygeal ligament between the apex of the sacral hiatus and sacral base are significant variables affecting the difficulty of the CEB.