The dissociation of early SEP components in lesions of the cervico-medullary junction: a cue for routine interpretation of abnormal cervical responses to median nerve stimulation.

In non-cephalic reference records the lesions of upper cervical cord and medulla dissociate SEPs to median nerve stimulation, cervical N11 and N13 potentials being preserved whereas later components, generated above the foramen magnum, are absent or desynchronized and delayed. This was observed in 4 patients with space occupying lesions of the cervico-medullary junction. In three of them serial postoperative SEP records demonstrated a progressive normalization of the responses following decompression. During normalization delayed cortical components could reappear on the scalp before the far-field P14 positivity. Before surgery the widespread N18 potential was absent, at least on one side, in the 4 patients and was never found to be dissociated from the P14 component. The reversibility of early SEP dissociations after surgery allowed a documented study of the abnormal patterns of cervical to Fz responses that may be observed with various lesions of the cervical cord, including demyelination. When the P14 component is delayed because of conduction slowing it is injected as an abnormal 'N14' in the Cv6-Fz response; in this situation the use of a non-cephalic reference is necessary to make the distinction between the cervical N13 potential and brain-stem 'N14' negativity.