Ultrasound predictors of difficult spinal anesthesia: a prospective single-blind observational study

Ultrasound showed to improve the precision and efficacy of spinal anesthesia (SA) through the identification of specific structures surrounding the intrathecal space, such as the anterior and posterior complex of dura mater (DM). The aim of this study was to verify the efficacy of ultrasonography in predicting difficult SA trough the analysis of different ultrasound patterns.

This prospective single-blind observational study involved 100 patients undergoing orthopedic or urological surgery. A first operator chose by landmarks the intervertebral space where he wanted to perform SA. Then a second operator recorded the visibility of DM complexes at ultrasound. Subsequently, the first operator, blinded to the ultrasound evaluation, performed SA, defined as "difficult" in case of failure, change of intervertebral space, operator exchange, duration >400 seconds or more than 10 needle passes.

The ultrasound visualization of only posterior complex or the failure in visualization of both complexes showed a positive predictive value of 76% and 100%, respectively, towards difficult SA vs. 6% when both complexes were visible; P<0.001. A negative correlation was found between the number of visible complexes and both patients' age and BMI. Landmark-guided evaluation underestimated the intervertebral level in 30% of cases.

Ultrasound showed a high accuracy in detecting difficult spinal anesthesia and its use should be recommended in the daily clinical practice in order to increase success rate and minimize patient discomfort. The absence of both DM complexes at ultrasound should lead the anesthetist to evaluate other intervertebral levels or consider alternative techniques.