Tonic Mesencephalic Seizure with Posterior Fossa Lesion

Abstract

We have examined two patients with posterior fossa lesions. Although they showed a similar tonic posture, one was thought to be epileptic and the other not.The first case was a two-year-old boy. He was admitted to Nagasaki University Hospital because o€ a large head and he was diagnosed as a Dandy-Walker cyst. The boy was stuporous and showed an opisthotonic posture all day long and the deep reflexes were hyperactive.The attacks occurred 2-3 times a day. He suddenly developed a decerebrated posture superimposed upon opisthotonus. In each attack, the hands were pronated and the neck and back were hyperextended which lasted two or three minutes. During the attacks, he was thought to fall into unconsciousness and a gradual increase of the respiratory rate was also observed at that time. The EEG was characteristic. In the interictal stage, slow wave bursts, slow spikes and slow spike-and-waves were observed at all leads. In contrast, during each attack, the EEG mainly consisted of diffuse low voltage fast activity (desynchronization), suggesting that the discharging focus was in the midbrain reticular formation.After an operation of the ventriculoperitoneal shunt, he regained consciousness and the attacks increased markedly, probably because of an occurrence of the transtentori-a1 upward herniation.During the attacks, the EEG also revealed a low voltage fast pattern. Then, by adding the cyst-peritoneal shunt, such decerebrated attacks disappeared and the opisthotonic posture also improved. The EEG recorded at that time showed mainly slow wave activity but neither spikes nor low voltage fast activities.The second case was a 10-year-old girl with Lt-cerebellopontine glioma. Three months after the operation, she possessed the opisthotonic posture, and had often the general tonic spasm. But she clearly remained in a consciousness state. The EEG showed almost normal activity even at the time of increased tonic spasm.In conclusion, the terms decerebrate, opisthotonic, tonic mesencephalic as well as cerebellar seizure (fit) have been dropped from the international classification of epilepsy because of the fact that they had nothing to do with epileptic discharges. In our first case, however, the EEG strongly suggested that his decerebrate attacks had been caused by the mesencephalic discharging lesion. Included were two different mechanisms under the name of tonic mesencephalic seizure ( Penfield and Jasper), the first true in epileptic nature and the other not epileptic.