[Non-cephalic reference makes it possible to avoid errors in the interpretation of somatosensory evoked potentials in...

The validity of clinical tests with somatosensory evoked potentials (SEPs) relies on the montages chosen and on adequate interpretation making use of recent pathophysiological data. Only non-cephalic reference recordings can reliably identify the actual neural generators involved. Early SEPs include near-field (neck N11 and N13; parietal N20-P27-P45 and prerolandic P22-N30) and far-field (P9-P11-P14) components. Far fields diffuse in the volume conductor through a solid angle of 180 degrees downstream from the propagated dipole generator. P9 reflects the afferent volley at the brachial plexus, P11 the volley in the dorsal column of the spinal cord, and P14 reflects the volley in the medial lemniscus. Far fields have stationary latencies, but their voltage varies and is maximum downstream in the axis of the generator dipole. The spinal N11 shows a latency shift of 0.9 msec along the neck which allows dorsal column conduction to be estimated. Its dipole has a longitudinal axis. The N13 dipole is fixed and has a horizontal axis: the N13 phase reverses into a P13 at oesophageal (prevertebral) derivations. This N13-P13 spinal dipole produces no far field at the scalp. The cortical pre- and post-rolandic components can be studied with an earlobe reference, but not with a frontal reference which confounds the distinct generators. For spinal potentials, the cephalic (often frontal) reference still used in recent publications introduces major distortions. The algebraic addition of scalp far fields picked up at the front fabricates illegitimate components in such traces: the neck P9 becomes an "N9" of variable amplitude, the "N11" augmented by the (inverted) frontal P11 loses its latency shift characteristics, and an "N14" (whose generator is actually above the foramen magnum) superimposes on the legitimate spinal components. In general, the clinical use of hybrid montages mixing distinct components that can be distorted or delayed by a neurological lesion leads to severe errors of interpretation.