Neurological associations of absent P60 component of the posterior tibial nerve somatosensory evoked potential

Abstract. The cortically generated P60 component of the posterior tibial nerve somatosensory evoked potential (PTSEP) is occasionally found to be absent in neurological patients,while the preceding P40 is preserved (Absent P60 pattern). A retrospective analysis of 24 such cases showed them to represent a different clinical population from that represented by 24 age- and sex-matched but otherwise unselected patients with entirely normal PTSEPs. The most frequent diagnoses of the patients with normal PTSEPs (conversion disorder and definite or suspected multiple sclerosis) were significantly less prevalent in the patients with the Absent P60 pattern, while miscellaneous other diseases affecting the peripheral and/or central sensory pathways were more frequent. In comparison with a second matched patient group with abnormal P40 in addition to P60, the patients with the Absent P60 pattern had a significantly lower incidence of large fibre sensory deficits (impaired vibration and/or joint-position sense). The incidence of small fibre deficits (impaired pain and/or temperature sensation) was similar in both groups with PTSEP changes. In conjunction with previously published findings in normal subjects, the data suggest that the P60 is a late response of the primary sensorimotor cortex due to activation of large diameter myelinated sensory fibres, but which is also tonically influenced by small fibre input. The Absent P60 pattern may be recognized as a distinct PTSEP abnormality, although its occurrence in some normal individuals should be noted.