Subcortical and cervical somato-sensory evoked potentials in cervical spondylotic myelopathy

To determine whether the diagnostic yield of SEPs in cervical spondylotic myelopathy can be improved by assessing separately dorsal column and dorsal horn responses to stimulation of the median nerve in patients with cervical spondylotic myelopathy. The study was carried out on 20 patients with cervical spondylotic myelopathy and 20 healthy controls. Somatosensory evoked potentials to median nerve stimulation were studied. The following montages were used: Erb's point ipsilateral to stimulation, Erb's point contralateral to stimulation [N9], spinous process of 5[th] cervical vertebra, anterior cervical [N13], parietal scalp contralateral to stimulation, parietal scalp ipsilateral to stimulation [N20], parietal scalp contralateral to stimulation and shoulder contralateral to stimulation [P9, P14]. Abnormal N13 potential, P14 and N20 was found in 17 [85%], 10 [50%] and 6 [30%] of cases respectively. Thus normal N20 coexist with a abnormal P14 in four patients .Abnormalities of scalp and cervical SEPs, defined respectively as abnormal N20 or P14 on the one hand and abnormal N13 on the other were combined in three different patterns, and distributed as follows: 9 patients [45%] had normal scalp SEPs with abnormal cervical responses; 8 patients [40%] showed abnormalities of both scalp and cervical SEPs; in two patients [10%] normal cervical responses were associated with abnormal scalp SEPs on both sides. Thus, about half of the patients with an isolated abnormality of the cervical N13 response would have been considered as having normal upper limb SEPs if recorded with a conventional frontal montage, which does not allow a selective assessment of this variable. In nine patients there was an increased signal on T2-weighted MRI scans of the cord at the cervical level. The cervical potential N13 and the subcortical potential P14 are reliable in diagnosing cervical spondylotic myelopathy. They are more frequently abnormal than the cortical N20 potential