Contralateral temporal horn widening in unilateral supratentorial mass lesions: a diagnostic sign indicating tentorial herniation.

Tentorial herniation secondary to supratentorial mass lesions will cause aqueductal compression and raised intraventricular pressure. Under ordinary circumstances, a raised intraventricular pressure will result in the development of hydrocephalus, but, in the presence of a large supratentorial mass, it is impossible for a generalized hydrocephalus to develop; only those parts of the ventricular system that are somehow shielded from the pressure effect of the mass lesion will be able to dilate. This dilatation will most frequently involve the contralateral temporal horn. Often, the atrium of the ventricle and the occipital horn will also be involved and, occasionally, the entire ventricle on the side opposite the mass will be dilated. The association of a large supratentorial mass with considerable shift of midline structures and computed tomography findings of a dilated contralateral temporal horn should be considered indicative of tentorial herniation, unless there are signs of preexisting atrophy or other preexisting disease processes that could have caused widening of the temporal horn.