Dorsal horn and dorsal column dysfunction in intramedullary cervical cord tumours.  A somatosensory evoked potential study. 
Median and tibial nerves somatosensory evoked potentials (SEPs) were recorded in 20 patients with intramedullary cervical spinal cord tumours.  The longitudinal extent of the tumour was determined by magnetic resonance imaging (MRI) and the surgeon's intraoperative observations.  Somatosensory evoked potentials to median or tibial nerve stimulation were abnormal in 85% of patients.  Three groups were identified on the basis of the median nerve SEP findings: Group I (20% of cases) with normal SEPs responses; Group II (30% of cases) with impaired cervical dorsal horn postsynaptic activity (abnormal N13 potential) but preserved dorsal columns transmission up to the cortex (normal P14 and N20 potentials); Group III (50% of cases) where the dorsal horn activity and dorsal columns transmission were both impaired.  The isolated abolition of the N13 potential observed in Group II reflects a functional dissociation between segmental dorsal horn and dorsal columns structures and should prompt myelographic investigations.  Postoperative follow-up suggests that absent N13, P14 and N20 potentials before surgery carry an ominous functional prognosis.  However, postoperative recovery of the N13 potential was found to occur in a few cases, suggesting that these tumours do not necessarily produce irreversible damage to the cervical dorsal horn neurons.  Thus SEPs proved to have a high sensitivity in detecting cervical spinal cord dysfunction in intramedullary tumours, provided that a selective recording of the N13 potential is performed using a cervical-supraglottal derivation.