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 in 61% of patients and ≥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.