This investigation sought the optimal (highest amplitude) derivation for mo
nitoring the posterior tibial P37 for each side in each individual, and det
ermined whether this may change intraoperatively. Fifty monitored patients
were studied using a partial P37 map consisting of FPz, Fz, Cz, Cz', Pz, PO
z, C4', and CT to a noncephalic reference. From this, the highest amplitude
scalp derivation was determined for each side. Of 100 tibial nerves, the i
nitial optimal input I 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 patie
nts with at least one subsequent trial later during surgery, P37 topography
changed in 14 and affected optimal inputs in 12. This occurred regularly d
uring sitting-position posterior fossa surgery because of intracranial air,
but sometimes occurred during other surgeries as well. The most common cha
nge consisted of FPz replacing cC' as optimal input 2. Input I 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 pr
actical method to refine intraoperative monitoring by individually optimizi
ng scalp derivations and identifying topographic P37 changes during surgery
.