Conus medullaris termination: Assessing safety of spinal anesthesia in the L2–L3 interspace

BACKGROUND: Classic teaching is that spinal anesthesia is safe at or below the L2-L3 interspace. To evaluate this, we sought to determine the percentage of individuals with a conus medullaris termination (CMT) level at or below the L1-L2 interspace. Further, the relationship of CMT level to age, sex, body mass index (BMI), and spinal pathology was examined, as was the reliability of using Tuffier's line (TL) as an anatomical landmark. METHODS: This retrospective study evaluated magnetic resonance images of 944 adult patients to determine the CMT level. The relationship between age, sex, height, BMI, and spinal pathology and CMT level was explored by logistic regression. The correspondence of the TL line to the L4-L5 interspace and the presence of overlap with the CMT were examined using 720 lumbar x-rays of the same patient cohort. RESULTS: Of 944 patients (mean age, 57.8 years; 49% male), 18.9% had CMT at or below the L1-L2 interspace, and spinal anesthesia at the L2-L3 interspace was found to carry a 0.7% incidence of neuraxial risk. Only the presence of congenital spinal abnormalities was found to be significantly predictive of having a CMT at or below the L1-L2 interspace. TL was found to correspond to the L4-L5 interspace in 99.8% of patients with lumbar x-rays. CONCLUSIONS: Spinal anesthesia at the L2-L3 interspace, using TL as an anatomical landmark, is safe in >99% of patients. However, caution must be exercised in all patients as demographic variables were found to be limited in predicting a low CMT level. EDITORIAL COMMENT: Unlike previous smaller studies, this retrospective study included MRI data from a total of 944 patients. The present study confirms that spinal anesthesia at the L2-L3 interspace or below can be considered safe. The findings indicate that Tuffier's line can be used as a reliable anatomical landmark.