Somatosensory tibial nerve evoked potentials with parasagittal tumours: a contribution to the problem of generators.

Somatosensory evoked potentials following stimulation of the median nerve at the wrist and the tibial nerve at the ankle were recorded in 5 patients with parasagittal tumours, 4 in the fronto-parietal and 1 in the frontal region. Three had intracerebral tumours and 2 parasagittal meningiomas. The extent of each lesion was determined by CT scan, showing a unilateral process involving predominantly the paracentral lobule and adjacent parts of the post- and precentral gyri ("Mantelkante') in 4 patients and a more frontal location of one meningioma affecting mainly the superior frontal gyrus. SEPs recorded from the patients with fronto-parietal tumours showed a uniform pattern with a complete absence of the wave N70 of the tibial SEP elicited by stimulating the nerve contralateral to the lesion. All other waves, including the P40-N50 complex on both sides and the median SEP, were in the normal range. In contrast, the frontal meningioma led to an only slightly altered from of the waves immediately following the initial P40-N50 complex. It is concluded that the tumour abolished the generators of the wave N70 which are apparently located in the cortical somatosensory leg area. Therefore unilateral loss of the wave N70 is indicative of parasagittal lesions. The results may give good evidence that the P40-N50 complex is generated in the thalamus or thalamo-cortical connections.