T88. Optimizing lower limb Somatosensory Evoked Potentials (SEPs) with additional derivations

Abstract

SEPs recorded over the scalp serve as surrogate markers of dorsal column proprioceptive pathway integrity during surgery. Changes in SEP amplitude and/or latency meeting “alarm criteria” can indicate neural injury and prompt the surgical team to take action. Alarm criteria (e.g. 50% drop in amplitude) lose specificity when SEP measurement variability is high (e.g. ⩾25%). Lower limb scalp SEPs has vary dipole vectors between individuals. Standard recording electrode derivations of SEPs have been shown to be suboptimal when compared to a derivation individually optimized for each patient. Scalp lower limb SEPs were monitored from a consecutive sample of 51 patients during non-emergent procedures at Mayo Clinic Rochester. Two additional scalp electrodes at Cz and Pz were setup beside the standard scalp electrodes (CPz, Fz, CPi and CPc). Two standard derivations (CPz-Fz and CPi-CPc) and five expanded derivations (CPz-CPc, Cz-CPc, Pz-CPc, CPz-CPi and Pz-Fz) were recorded. P38 amplitudes were measured for all derivations; the derivation with the lowest coefficient of variation (CV) and highest amplitude from each group (standard vs. expanded montage) were determined. The P38 with highest amplitude and lowest CV were found in the expanded montage in 78 (76%) and 71 (70%) out of 102 limbs from 51 patients. The expanded montage significantly lowered the minimum CV by 1.75% (P < 0.0001, linear regression) and increased the maximal amplitude by 0.13 μV (P < 0.0001, linear regression) as compared to the standard montage. The CPz-CPc was the most frequent derivation with lowest CV and highest amplitude among all seven derivations. The effort to implement an expanded scalp montage should be balanced with the potential improvements, which by our data, are only incremental. An expanded scalp montage may be highest yield in patients where recordings from a conventional montage are of poor quality.