Utilization of Thecal Sac Area to Dose Spinal Anesthesia Reduces Risk of Anesthetic Failure in Lumbar Surgery

Introduction: Greater thecal sac volumes are associated with an increased risk of spinal anesthesia (SA) failure. Given the thecal sac cross-sectional area accurately predicts thecal sac volume, the thecal sac area may be used to adjust the dose of spinal anesthesia to prevent anesthetic failure. We aim to assess the rate of SA failure in a prospective cohort of lumbar surgery patients who receive an individualized dose of bupivacaine based on preoperative measurement of their thecal sac area.

Methods: A total of 80 patients prospectively received lumbar spine surgery under SA at a single academic center (2022-2023). Prior to surgery, the cross-sectional area of the thecal sac was measured at the planned level of SA injection using T2-weighted MRI. Patients with an area < 175 mm2, equal to or between 175 and 225 mm2, and >225 mm2 received an SA injection of 15 mg, 20 mg, or 25 mg of 0.5% isobaric bupivacaine, respectively. Instances of anesthetic failure or adverse outcomes were noted. Patient characteristics and incidence of SA failure were compared to a historical cohort (2019-2022) at the same institution.

Results: No patients in the individualized dose cohort experienced failure of SA compared to 14 patients (5.6%) in the control cohort that experienced failure of SA (p = 0.0259). The average thecal sac area was 187.49 mm2 in the prospective cohort. A total of 28 patients received 15 mg of bupivacaine, 42 patients received 20 mg of bupivacaine, and 10 patients received 25 mg of bupivacaine. None of the patients experienced any adverse outcomes associated with SA. Patients in the prospective and historic cohort were comparable, with both groups having a similar distribution of lumbar procedures and comorbidities.

Conclusion : Adjusting the dose of SA based on thecal sac area reduces the rate of SA failure in patients undergoing lumbar spine surgery.