Central and peripheral SEP defects in neurologically symptomatic and asymptomatic subjects with low vitamin B12 levels.

Somatosensory evoked potentials (SEPs) following median nerve stimulation were abnormal in 7 patients with sensory impairment due to vitamin B12 deficiency. Extensor plantar reflexes indicated a central sensory pathway lesion in 4 cases and absent tendon jerks suggested peripheral neuropathy in 4, but median nerve SEPs indicated a predominantly central lesion without marked peripheral nerve involvement in 6 and an axonal neuropathy without CNS involvement in 1. The latter had evidence of central slowing of conduction in SEPs following posterior tibial nerve stimulation. Consequently, it is suggested that the brunt of sensory pathway involvement usually falls on the CNS, although peripheral neuropathy may occur as the major abnormality in some cases. In 2 patients SEPs showed a marked improvement following treatment with vitamin B12 injections, one consistent with restored central conduction and the other with recovery from peripheral neuropathy. No peripheral or central SEP abnormalities were seen in 18 dairy-produce eating vegetarians with low vitamin B12 levels, although 6 reported mild sensory symptoms suggestive of peripheral neuropathy and 3 had corroborative clinical signs.