Early trigeminal evoked potentials in tumours of the base of the skull and trigeminal neuralgia.

Early scalp responses evoked by stimulation of the infraorbital nerve (W1, W2, W3) have been investigated in 23 patients affected by tumours of the base of the skull (parasellar area and cerebello-pontine angle) and in 38 patients suffering from 'idiopathic' trigeminal neuralgia. Differences in conduction times between healthy and affected side were evaluated and confronted with data obtained from 30 normal volunteers. Alterations of the response were found in all the patients with tumours of the base of the skull who had clinical signs in the trigeminal area and in 7 of the 12 cases without such signs. The usual pattern of alteration in cases with tumours of the parasellar area was a parallel involvement of W2 and W3 (both absent or delayed to the same extent), whereas in tumours of the cerebello-pontine angle W3 was more seriously affected than W2. Wave W1 was never altered. Pre- and post-operative recording sessions in 2 patients showed definite improvement of the responses after removal of the tumour. In 9 patients suffering from 'idiopathic' trigeminal neuralgia delays of conduction were found on the painful side, suggesting that damage to the trigeminal root, possibly at its entry zone into the pons, had taken place. Retrogasserian injection of glycerol was performed in 12 of the 38 patients with trigeminal neuralgia. Stimulation of the operated side showed disappearance of W2 and W3 in 9 cases, prolonged W1-W3 interval in 2 cases and no alterations in 1 case. The extent of response alteration usually paralleled the clinical results.