Ultrasound as a screening tool for proceeding with caudal epidural injections

To study the anatomical structure of the sacral hiatus using ultrasound. Based on the sonographic images of the sacral hiatus, the feasibility of caudal epidural injection can then be assessed.

Case-controlled study.

Rehabilitation outpatient clinic in a tertiary medical center.

Patients (N=47; 20 women, 27 men) with low back pain and sciatica who were to receive caudal epidural injection treatments were recruited into this study.

Sonographic images of the sacral hiatus were obtained from all the patients. An ultrasound machine capable of examining musculoskeletal tissues with real-time linear-array ultrasound transducer was used to measure the distance between the anterior wall and posterior wall of the sacral hiatus (diameter of the sacral hiatus) and the distance between bilateral cornua.

Diameter of the sacral canal and distance between bilateral cornua measured in millimeters.

The mean diameter of the sacral canal was measured to be 5.3+/-2.0 mm in our recruited patients. The mean distance between bilateral cornua was measured to be 9.7+/-1.9 mm. Caudal epidural injections failed in 7 patients. In these 7 patients, 4 had very small diameter of the sacral canal (1.6, 1.2, 1.4, and 1.5 mm). In 1 man, sonographic images revealed a closed sacral hiatus (no sacral canal diameter can be measured). Two patients revealed flow of fresh blood into the syringe while checking for the escape of cerebrospinal fluid after the needles were inserted into the sacral canal. For safety reasons, steroid injections were not performed in these 2 patients.

Ultrasound may be used as an effective screening tool for caudal epidural injections. Anatomic variations of the sacral hiatus can be clearly observed using ultrasound. Sonographic images indicating a closed sacral canal and sacral diameters ranging from 1.2 to 1.6mm may suggest a higher failure rate in caudal epidural injection.