A comparison between derivation optimization and Cz'-FPz for posterior tibial P37 somatosensory evoked potential intraoperative monitoring.

Objective: To compare P37 derivation optimization to Cz'-FPz.

Methods: After induction in 120 patients, monitoring derivations optimized by mapping FPz, Cz, Cz', Pz, C4', C2', C1' and C3'-mastoid to determine the P37 and N37 maximums for use as inputs 1 and 2 were compared to Cz'-FPz. This was repeated later in 35 surgeries.

Results: Eleven optimal derivations occurred and usually differed between sides. Input 1 was Cz', Pz, Cz, iCi', or Ci' and input 2 was Cc', FPz, Ci' or Pz. Even the most frequent Cz'-Cc' derivation was optimal for both sides of an individual in only 17% and this was true for Cz'-FPz in only 4%. Optimization produced higher amplitudes than Cz'-FPz (P<0.001). The ratio was [squareroot of 2] : 1 in 61% of patients and > or =2:1 in 28%, approximately halving or quartering averaging times. Optimization assessed decussation, disclosing non-decussation in one patient while Cz'-FPz did not. Alterations of P37 topography that reduced initially optimal derivation amplitude and made a different derivation optimal were demonstrated by repeat optimization in 13 of 35 patients, preventing misinterpretation in one. While also affected, Cz'-FPz neither detected nor adjusted for potentially misleading topographic changes.

Conclusions: Higher amplitudes, decussation assessment and topographic adjustment make P37 derivation optimization superior to Cz'-FPz for monitoring this highly variable potential.