MEDizzy - A patient suffers a large intracranial hemorrhage. On

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Neurocritical Care

A patient suffers a large intracranial hemorrhage. On examination, he is comatose with a right fixed dilated pupil and seems to be hemiparetic on the left side. On subsequent MRI, the hemorrhage is visualized causing mass effect, but there is also an ischemic infarct in the right posterior cerebral artery distribution. Which type of herniation does this patient most likely have?

ExplanationA. Uncal herniation produces mass effect and pressure over the ipsilateral midbrain, affecting the ipsilateral cranial nerve III nucleus and nerve. The mass effect compresses parasympathetic fibers that mediate miosis, resulting in mydriasis. A fixed dilated pupil localizes the side of the uncal herniation. These patients have impaired consciousness secondary to distortion of the ascending arousal system in the brainstem. Hemiparesis also occurs secondary to compromise of the corticospinal tract. Since the corticospinal tract has not decussated at the level of the midbrain, if the uncus compresses the ipsilateral cerebral peduncle, the patient will have a contralateral hemiparesis. Occasionally, the uncal herniation will lead to displacement of the midbrain against the contralateral Kernohan’s notch, resulting in a contralateral compression of the corticospinal tract, and therefore an ipsilateral hemiparesis. Uncal herniation may also cause compression of the posterior cerebral artery in the tentorial notch, causing infarction in this territory. The other types of herniation syndromes do not cause an ipsilateral dilated pupil with contralateral hemiparesis, and posterior cerebral artery infarction.