Nerve monitoring changes related to iliac artery compression during anterior lumbar spine surgery.

BACKGROUND CONTEXT

There are no studies in the literature that correlate compression of the iliac vessels resulting in obstruction of blood flow with changes in nerve monitoring parameters during anterior lumbar surgery.

PURPOSE

To determine whether there is significant compression of the iliac vessels that can cause temporary nerve root ischemia or limb ischemia that could be responsible for loss of somatosensory evoked potentials (SSEP) while retractors are in place for exposure during anterior lumbar interbody fusion (ALIF).

SETTING

Patients coming to the operating room for ALIF from levels L2-L3 to L5-S1 would be studied for nerve monitoring changes during the procedure with particular attention to the intervals just before placement of a retractor, while the retractor was in place and immediately after removal of the retractor.

PATIENT SAMPLE

Forty-five consecutive patients were studied for changes in SSEP and oxygen saturation (SaO(2)) while undergoing ALIF.

OUTCOME MEASURES

Patients were considered to have lost saturation if the SaO(2) decreased to below 90%. Patients were considered to have abnormal SSEP with any increase in latency and decrease in amplitude.

METHODS

SSEP and SaO(2) were monitored continuously and simultaneously before exposure of the disc spaces, during exposure with retractors in place and after removal of the retractors.

RESULTS

Thirteen of 23 patients with exposure at L4-L5 had both loss of SSEP signals and loss of SaO(2) with exposure. All 13 patients had return to normal saturation and recovery of the SSEP signals within 15 minutes of removal of the retractors. Both of these are significant correlations (p<.001).

CONCLUSION

This study showed that the majority (57%) of patients undergoing ALIF at the L4-L5 level are subject to compression of the left iliac vessels enough to cause desaturation distally as measured by pulse oxymetry. This vascular compromise, as well as the return to normal saturation, correlates with changes noted in SSEP soon after both deployment and removal of the retractors used for exposure. The mechanism appears to be a transient ischemic response. Failure of the SSEP signals to recover may be diagnostic of left iliac artery thrombosis.