Inadequacy of the forehead reference montage for detecting abnormalities of the spinal N13 SEP in cervical cord lesions.

Cervical somatosensory evoked potentials (SEPs) recorded using forehead and anterior cervical reference montages were compared in 6 patients whose MRI showed a cervical syrinx. All patients presented with a segmental loss of pain and temperature sensation in upper limbs, but no clinical evidence of dorsal column system dysfunction. Cervical SEPs recorded using the forehead reference montage were normal in all cases, while the N13 potential recorded using an anterior cervical reference was reduced, or absent, in 11 median nerve SEPs out of 12. This discrepancy results from persisting scalp P13-P14 far-field potentials, which were picked up by the forehead, but not by the anterior cervical, reference. It is concluded that the forehead reference montage is inadequate for assessing selectively the spinal N13 potential and should be abandoned for cervical SEP recording.