Temporal Lobe Cerebritis with Midbrain Compression

This is 52 year old diabetic with pan sinusitis and altered signal intensity and inhomogenous enhancement in the temporal lobe which rapidly progressed on day 2 and patient developed ipsilateral III nerve palsy and contralateral weakness, which was explained on the basis of cerebral peduncle compression. 

Learning points: Uncal (transtentorial) herniation is herniation of the medial temporal lobe from the middle into the posterior fossa, across the tentorial opening. The uncus of the temporal lobe is forced into the gap between the midbrain and the edge of the tentorium. This compresses the ipsilateral oculomotor nerve, causing a fixed and dilated pupil, and collapses the ipsilateral posterior cerebral artery, causing an infarct in its distribution. Cortical blindness resulting from this infarct is a false localizing sign because it gives the erroneous impression that the primary lesion is in the occipital lobe. As the herniating uncus displaces the midbrain laterally, the contralateral cerebral peduncle is compressed against the edge of the tentorium, causing paralysis on the same side as the primary lesion, another false localizing sign.