The contribution of median nerve SEPs in the functional assessment of the cervical spinal cord in syringomyelia. A study of 24 patients.

Since the early study of Halliday and Wakefield (1963) it has generally been agreed that scalp somatosensory evoked potentials (SEPs) are normal in patients with dissociated loss of pain and temperature sensation. Up to now a few patients with abnormal spinal N13 and preserved scalp P14 and N20 have been reported in the literature, but there is no firm evidence, based on group data, that this dissociation can be related to any form of dissociated sensory loss. We studied median nerve SEPs in 24 patients with syringomyelia documented by CT scan or MRI. For the recording of the cervical N13 we used a Cv6 anterior cervical montage, which cancels the potentials generated above the foramen magnum and enhances the amplitude of N13. Scalp far-field and early cortical SEPs were recorded using a noncephalic reference electrode. Since N13 pathological changes are mainly changes in amplitude we measured the N13/P9 amplitude ratio in normal subjects and patients and found that it was a reliable index to quantify the amplitude decrease of N13. Absent or reduced N13 was observed in 40 median nerve SEPs (83%) in conjunction with normal P14 and N20 in 30 SEPs. Thus the dissociated loss of the cervical N13 was identified as the most conspicuous SEP feature in syringomyelia. A significant correlation was found between abnormal N13 and loss of pain and temperature sensations, whereas P14 abnormalities correlated well only with loss of joint and touch sensations. In the median nerve territory, sensation was either normal (6 cases) or lost only for pain and temperature (24 cases) when SEPs showed abnormal N13 and normal P14. Although it does not directly reflect the postsynaptic activity of spinal cells receiving their inputs from A delta and C fibres the N13 potential proved to be a reliable index of spinal cord grey matter dysfunction in syringomyelia.