Tentorial Herniation, the Midbrain and the Pupil

Abstract

ANY syndromes associated with mass lesions above the tentorlum are now ascribed to the effects of transtentorial herniation of the temporal lobe. Among those already explored to some extent experimentally are ipsilateral pupillary dilatation and cardiorespiratory variations. Less attention has been given to the mechanisms underlying impairment of consciousness and loss of upward gaze. This paper reports the first stage of an attempt to elucidate the various effects of tentorial herniation dynamically, by animal experimentation. The pattern of midbrain deformity consequent on temporal lobe herniation has been of particular concern on account of the association of one of us (WBJ) with Sir Geoffrey Jefferson's clinic in Manchester where the clinicopathological correlatives of this phenomenon have been studied for some years (Jefferson, 4 Johnson and Yates, 6'~ JohnsonS). Fixing in situ with formalin the brains of patients dying of cerebral compression these workers found consistent patterns of distortion related to the site of the hernia, which was in turn a reflection of the situation of the primary compressing lesion. Attention was drawn to the association of loss of upward gaze with compression of the dorsal midbrain, caused by a posterior, as distinct from a lateral, hernia. There have been relatively few attempts to produce hernias experimentally. The classical work of Reid and Cone 1~ consisted in the introduction of fluid into the extradural space in acute experiments, whilst hygroscopic material was relied on for chronic compressions. In monkeys these workers succeeded in producing an ipsilateral dilated pupil, and at autopsy there was invariably a hernia directly distorting the third nerve. Perret 9 injected coloured, melted paraffin into the white matter of the parietal lobes of cats and sacrificed the animals after varying periods of time. Those allowed to survive only half an hour showed little evidence of herniation of brain tissue, but this was increasingly more apparent over the next !4 hours. Similar amounts of herniation were found over the following 4 days, after which the hernias became less apparent. This experimental lesion was a static one,