Selective abnormality of the N13 spinal SEP to dermatomal stimulation in patients with cervical monoradiculopathy.

Scalp somatosensory evoked potentials (SEP) to dermatomal stimulation have so far proved to be only partially useful in the diagnosis of monoradiculopathy, mostly in cases without motor impairment. The aim of our study was to test the sensitivity of the spinal N13 potential in uncovering lesions of single cervical roots. We studied five patients suffering from cervical monoradiculopathy, using a recording technique allowing specific recording of the genuine N13 potential which is probably generated by dorsal horn cells. No patient showed signs of muscle impairment and needle EMG was always normal. In four patients, the N13 SEP was absent following stimulation of the dermatome corresponding to the damaged root, while both the lemniscal P14 and the cortical N20 components were normal. SEP recorded after stimulation of upper limb troncular nerves showed no abnormality in all patients. Our findings suggest that the N13 potential, the loss of which after dermatomal stimulation could be due to deafferentation of dorsal horn neurones, is particularly sensitive to initial root compression. Therefore, our montage allowing analysis of the genuine N13 SEP can improve the sensitivity of dermatomal SEP recording in patients with cervical monoradiculopathies.