Ischemia changes and tolerance ratio of evoked potential monitoring in intracranial aneurysm surgery.

OBJECTIVE

We assessed the relationship between cerebral ischemia-induced changes in evoked potentials and the degree of ischemia tolerance.

METHODS

47 patients underwent somatosensory evoked potential (SEP) and motor evoked potential (MEP) monitoring in intracranial aneurysm surgery. Three duration parameters (time) were recorded: Time 1, from the starting of temporary occlusion unavoidable in aneurysm surgery to the time the evoked potentials decrease from basal level to reaching the warning criterion; Time 2, from evoked potentials reaching the warning criterion to the time the blood flow was resumed; Time 3, after resuming the blood flow, the time it took the evoked potentials to recover to baseline. All three times can be reliably calculated in the SEP recording, but not in the MEP recording which consisted of either unchanged amplitudes or abruptly changing amplitudes, making it impossible to obtain Time 1. The ischemic tolerance ratio (ITR) was calculated as ITR=time 2/time 1×100%. New decreasing myodynamia and fresh infarction after the surgery were employed for evaluating neurological deficits postoperatively, and their correlations with the ischemia-induced changes of evoked potentials recorded during the surgery were analyzed.

RESULTS

We found a change in SEPs in 12 patients whose cerebral ischemia was induced by temporary occlusion of the aneurysm's parent artery. We also found the development of postoperative neurological deficits in 4 patients whose ischemic tolerance ratio (ITR) reached over 80%, while no deficits were found in the other 8 patients whose ITR was less than 50%. MEP changes were seen in 4 patients whose cerebral ischemia was caused by accidentally clamping the perforating branches, causing the development of postoperative neurological deficits but not necessarily leading to significant SEP changes.

CONCLUSION

The Ischemia tolerance ratio (ITR) in SEP recordings is valuable to predicting postoperative neurological deficits caused by temporary occlusion of aneurysm's parent artery. Maintaining the ITR under 50% during operation can effectively avoid postoperative neurological deficits, while an ITR above 80% reliably forecasts postoperative neurological deficits. Complementary to SEPs, MEP recordings are particularly valuable in monitoring ischemic effects caused by accidentally clamping perforating branches. Taken together, this system of monitoring makes it possible to promptly adjust surgery procedures and minimize postoperative neurological deficits.