Scalp, earlobe and nasopharyngeal recordings of the median nerve somatosensory evoked P14 potential in coma and brain death

Median nerve somatosensory evoked potentials (SEPs) were recorded in a total of 181 patients in coma and brain death. Special attention was paid to derivation of P14 (the positive potential occurring approximately 14 ms after median nerve stimulation) with different electrode montages, using midfrontal scalp (Fz), linked earlobe (A1/2), median nasopharyngeal (Pgz) and non-cephalic reference (NC) electrodes. The P14 amplitude (and, to a lesser extent, latency) were invariably lower in brain death than in coma. The potential was preserved in coma in all patients, but lost in brain death in 9.8% in Fz-NC and Pgz-NC recordings, in 23.2% in Fz-A1/2, and in 100% in Fz-Pgz. Thus, Fz-Pgz was the derivation yielding the most reliable results with respect to the distinction between coma and brain death and is therefore recommended as a confirmatory test, when other diseases interrupting the lemniscal pathway (isolated brainstem death, high cervical transverse cord lesion and focal bilateral lemniscal lesion) are excluded. Theoretical considerations lead to the hypothesis of different (rostral and caudal) segments of the P14 generator dipole being recorded by the different electrode montages. It is assumed that Fz-Pgz picks up the most rostral part of P14 (rP14) that is invariably lost in brain death and preserved in coma.