Transverse plane ultrasound-guided caudal epidural injections: sonographic anatomy and stepwise technique

The present study aims to provide a step-by-step procedural and anatomical familiarization guide for transverse plane ultrasound (US)-guided caudal epidural (CE) injection.

The study cohort consisted of 23 chronic low back pain patients (23-67 years old) previously unresponsive to conservative management. A transverse plane US-guided CE injection was performed, with each procedure step documenting and emphasizing sonographic anatomy. Several Thiel's method fixed cadaveric specimen dissections were also performed to demonstrate relevant CE injection-related anatomy.

The sacral hiatus location can be estimated by visually forming an equilateral triangle between the posterior superior iliac spines and the sacral apex (trigonum sacrale). Follow-up palpation locates the sacral cornua, guiding transducer placement visualizing over the paired cornua 'bull frog's eye's appearance, with the epidural space visualized as a hypoechoic line, between the eyes. Then, 2-3 ml of 1% lidocaine is injected subcutaneously at the mid-point between the sacral cornua and superficial to the posterior sacrococcygeal ligament (SCL). Although keeping the cornua, superficial posterior SCL and epidural space in view, the needle is slowly advanced to the epidural space at around a 20 degree cephalad angle till the tip becomes visible. Expansion of the epidural space is monitored under the transverse sacral ligament as the injectant is slowly introduced.

The present study demonstrated the anatomical landmarks necessary for the transverse ultrasound caudal epidural technique and that the cornua, superficial posterior SCL, CE space, and other relevant sacral hiatal anatomy are well visualized with this technique.