Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring.

This investigation sought the optimal (highest amplitude) derivation for monitoring the posterior tibial P37 for each side in each individual, and determined whether this may change intraoperatively. Fifty monitored patients were studied using a partial P37 map consisting of FPz, Fz, Cz, Cz', Pz, POz, C4', and C3' to a noncephalic reference. From this, the highest amplitude scalp derivation was determined for each side. Of 100 tibial nerves, the initial optimal input 1 was Cz' in 52%, Pz in 28%, and Cz or iC' in 10%, and optimal input 2 was cC' in 69% and FPz in 31%. The optimal derivation was the same for each side in 34% of patients and different in 66%. Of 31 patients with at least one subsequent trial later during surgery, P37 topography changed in 14 and affected optimal inputs in 12. This occurred regularly during sitting-position posterior fossa surgery because of intracranial air, but sometimes occurred during other surgeries as well. The most common change consisted of FPz replacing cC' as optimal input 2. Input 1 changes were predominantly in an anterior or posterior sagittal direction. The results demonstrate great inter- and intraindividual P37 variability, and document intraoperative topographic changes. Both phenomena can be addressed by a practical method to refine intraoperative monitoring by individually optimizing scalp derivations and identifying topographic P37 changes during surgery.