Median nerve SEP after a high medullary lesion: preserved N18 and absent P14 components (case report)

Abstract

-Median nerve SEPs recorded from a patient with a high medullary lesion are described. The lesion involved the anteromedial and anterolateral right upper third of the medulla, as documented by MRI. Forty one days after the lesion, left median nerve SEP showed preserved N18 and absent P14 and N20 components; stimulation of the right median nerve evoked normal responses. These findings agree with the proposition that low medullary levels are involved in the generation of the N18 component of the median nerve SEP.KEY WORDS: median nerve, somatosensory evoked potential, medulla, subcortical.PESS do nervo mediano após lesão bulbar alta: componente N18 preservado e P14 ausente. Relato de caso RESUMO -Descrevemos os potenciais evocados somatossensitivos obtidos por estimulação dos nervos medianos em um paciente apresentando uma lesão localizada, envolvendo as porções anteromedial e anterolateral do terço superior do bulbo, documentada por ressonância nuclear magnética. Quarenta e um dias após o estabelecimento da lesão os potenciais evocados por estimulação do nervo mediano esquerdo evidenciaram ausência dos componentes P14 e N20 e preservação do componente N18; após estimulação do nervo mediano direito as respostas apresentaram-se normais. Estes achados estão de acordo com a sugestão de que as porções baixas do bulbo estão envolvidas na geração do componente N18. PALAVRAS-CHAVE: nervo mediano, potencial evocado somatossensitivo, bulbo, subcortical.The N18 component of the median nerve somatosensory evoked potential (SEP) was described in 1981 1 . Since then substantial controversy has developed in relation to its generator structure. While during the eighties, generator structures were suggested to be located from the frontal cortex 2 to the midbrain 3 , during the nineties discussions have been focused from the midbrain 4-6 to the medulla 7-14 . While the first discussion was solved by the clear documentation of persistence of the N18 component in cases with hemispherectomies 3 , the second discussion (i.e., from the midbrain to the medulla) is still going on. The recent descriptions of selected cases with well defined lesions at the high medulla, showing preservation of the N18 component 9,13 , suggest that indeed the lower medulla is capable of generate the N18 component. However, some contradictions still persists 15 and this point does not seem to be universally accepted 16 . In view of this picture we believe that it is pertinent to report the SEP findings of another case with a lesion at high medullary level.