Spinal N13 versus cortical N20 and dermatomal somatosensory evoked potential studies in patients with cervical radiculopathy

Most studies on somatosensory evoked potentials [SEPs] in cases of cervical radiculopathy routinely analyze scalp [cortical] responses [mixed or dermatomal SEPs], depending mainly on evaluation of N20 whose origin is the primary somatosensory cortex. It was suggested that selective study of the N13 potential, might be a useful technique to improve both accuracy and sensitivity of the diagnosis of cervical radiculopathy. The aim of our study was to test the sensitivity of the spinal N13 potential in uncovering lesions of cervical nerve root; and to compare it to dermatomal and mixed nerve SEPs. Forty patients with clinical suspicion of cervical nerve root lesion had been selected. Twenty normal subjects constituted the control group. Magnetic resonance imaging [MRI] of the cervical spine was performed for all patients to reveal the presence of any cervical nerve root compression. The following electrophysiological studies were done [sensory and motor conduction studies of the median and ulnar nerves, mixed SEP of median nerve, dermatomal SEP and spinal N13 SEP study of the median nerve. There was a positive correlation between the mixed and the dermatomal SEPs and between the mixed SEP and N13. On the other hand there was a positive correlation between the dermatomal SEPs and N13. There was a positive correlation between the MRI and the mixed SEP, the dermatomal SEP and N13, respectively. Sensitivity and specificity of N13, dermatomal somatosensory evoked potentials [DSEP] and mixed SEP were determined using MRI as a gold standard. N13 is the most sensitive followed by DSEP and the least was the mixed somatosensory evoked potentials [MSEP]. They were all specific with a high positive predictive value. N13 had the highest negative predictive value followed by DSEP and the least was MSEP and we concluded from this study that N13 is a sensitive technique which is suitable for detecting early and mild symptomatic cervical nerve root lesion