Short-latency somatosensory evoked potentials in patients with brain stem tumor: study of N20 and N18 potentials

Short-latency somatosensory evoked potentials (SSEPs) elicited by median nerve stimulation were studied in 17 patients with brain stem tumor. A total of 35 SSEP records were obtained and classified into five groups: 13 in Group I, central conduction time (CCT) < 7.56 msec (within the normal limits); eight in Group II, CCT > or = 7.56 msec; nine in Group III, N20 potential was abolished; four in Group IV, N20 and N18 potentials were abolished; and one in Group V, N20, N18, and P14 potentials were abolished. These SSEP groups were correlated with the size and location of the brain stem tumor on magnetic resonance images. N20 potentials were unchanged in latency in patients with small localized gadolinium (Gd)-enhanced lesions. In contrast, the N20 potentials were abolished in patients with tumors extending to the dorsal pons and the upper medulla oblongata. The extent of nonenhanced low-intensity lesion did not correlate with the changes of the N20 potentials. The degree of the impairment of the N20 potentials reflected the severity of the clinical symptoms. The N20 potential can evaluate brain stem dysfunction caused by brain stem tumor. The N18 potentials were abolished in four patients, in whom the tumor extension (one Gd-enhanced lesion, three low-intensity lesions) involved not only the pons but also the medulla oblongata. Therefore, the N18 potentials are probably generated from the medulla oblongata.