Tibial somatosensory evoked potential intraoperative monitoring: recommendations based on signal to noise ratio analysis of popliteal fossa, optimized P37, standard P37, and P31 potentials.

OBJECTIVE

To compare the intraoperative signal-to-noise ratio (SNR), reproducibility and rapidity of popliteal fossa (PF), optimized P37, standard P37 and P31 potentials.

METHODS

Raw sweeps and 11 averages doubling sweep number from 2 to 2048 were compared in 37 patients undergoing scoliosis surgery. Optimized (highest amplitude or SNR) P37 derivations were Cz-CPc (22), CPz-CPc (27), Pz-CPc (7), iCPi-CPc (8), CPi-CPc (1), Cz-Pz (2) or Pz-FPz (3), and in two patients with non-decussation, Cz-CPi (1) or CPz-CPi (3). Standard P37 and P31 derivations were CPz-FPz and FPz-C5S. Signal amplitude was measured in 2048-sweep averages; peak noise was measured in raw sweeps and +/- averages; SNR was amplitude/noise. Visual superimposability and < 20-30% amplitude variation determined reproducibility. Sweeps to reproducibility determined rapidity.

RESULTS

The SNR order was PF >> optimized P37 > standard P37 > P31. Mean optimized P37 SNR advantages over the standard P37 and P31 were 2.1:1 and 4.9:1. SNR had powerful non-linear correlations to reproducibility and rapidity. Median sweeps to reproducibility were PF: 2, optimized P37: 128, standard P37: 512 and P31: 1024. EEG noise was greatest in FPz derivations. Burst-suppression increased scalp potential SNR and rapidity.

CONCLUSIONS

Optimized P37 and PF recordings are most rapidly reproducible due to superior SNRs and are recommended. FPz should be avoided. Burst-suppression may be desirable.

SIGNIFICANCE

CPz-FPz and FPz-C5S should no longer be standard.