Central somatosensory conduction time and short latency somatosensory evoked potentials in post-traumatic coma

Short latency evoked potentials (SEPs) were elicited by stimulation of the median nerve at the wrist and recorded simultaneously from the neck and the contralateral scalp in 44 comatose patients with signs of brain stem impairment due to head injury. Thirty-four patients were studied in acute coma on day 1 or 2 after brain injury. Twenty-three patients were studied in prolonged coma during days 3-12 after trauma. Six patients were examined in brain death. Brain stem involvement was divided clinically and by CT scan into secondary lesions due to supratentorial mass displacement and primary lesions due to direct violence to the brain stem. The central somatosensory conduction time (CCT) was measured by subtracting the peak latency of the major response from the neck (N14) from that of the primary scalp response (N20). The amplitude ratios (ARs) N20/N14 were calculated for each trace. Further asymmetries and absence of SEP over one or both hemispheres were noted. In cases in coma due to supratentorial lesions CCT and AR were close to normal in patients with good outcome. CCT increased and AR decreased with the worsening of outcome both in acute and prolonged coma. Asymmetries of SEPs indicated moderate or severe final disability. Patients with absent SEPs over one or both hemispheres due to supratentorial lesions died or survived severely disabled (1 case). In patients suffering from primary brain stem dysfunction, confirmed by a normal or slightly abnormal CT scan, prolonged CCT, asymmetric but also absent SEPs were also found in patients with good outcome both in acute and prolonged coma. AR was generally low in these cases. Early appearance of SEPs or early recovery of initially distorted SEPs and decrease of CCT in prolonged coma or during recovery was a favourable prognostic sign. Therefore even absent or severely distorted SEPs should be interpreted cautiously in patients who may suffer from primary brain stem involvement. Scalp SEPs were totally absent in patients with brain death.