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Question:

A 55-year-old, right-handed woman is brought to the emergency department due to acute-onset headache and difficulty with vision.  En route to the hospital, she becomes unconscious.  Medical history is significant for hypertension.  Blood pressure is 150/90 mm Hg and pulse is 90/min and regular.  CT scan of the head without contrast demonstrates an acute hemorrhage in the left temporal lobe with compression of the anterior medial temporal lobe against the free margin of the tentorium cerebelli.  Which of the following cranial nerves is most likely to be compromised in this patient?

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Explanation:

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The cranial vault is formed by the cranial bones and divided into compartments by the dural folds (ie, falx cerebri, tentorium cerebelli).  Because the cranial vault is rigid, there is minimal space for brain expansion if a rapidly enlarging mass (eg, hemorrhage, tumor) or severe, generalized edema (eg, traumatic brain injury) develops.  As a result, portions of the brain may protrude through the openings in the dural folds, often according to the following herniation patterns:

  • Subfalcine: herniation of the cingulate gyrus under the falx cerebri
  • Uncal: herniation of the medial temporal lobe (ie, uncus) through the tentorial notch
  • Tonsillar: herniation of the cerebellar tonsils through the foramen magnum

This patient with compression of the anterior medial temporal lobe against the free margin of the tentorium cerebelli has uncal herniation.  As uncal herniation progresses, the following may occur:

  • Ipsilateral oculomotor nerve (CN III) compression leads to an ipsilateral fixed and dilated pupil.  Paralysis of the oculomotor muscles occurs later and leads to ptosis and a down-and-out position of the ipsilateral eye.

  • Ipsilateral posterior cerebral artery compression leads to contralateral homonymous hemianopsia with macular sparing.

  • Compression of the ipsilateral cerebral peduncle of the midbrain against the tentorium may occur, damaging descending corticospinal tracts and causing contralateral hemiparesis.

  • As the midbrain is pushed toward the contralateral side, the contralateral cerebral peduncle may also become compressed against the tentorium, causing ipsilateral hemiparesis (false localizing sign).

Without prompt treatment, further pressure on the midbrain can cause downward brainstem displacement (eg, central herniation) and brainstem hemorrhages (ie, Duret hemorrhages) from stretching and rupture of paramedian branches of the basilar artery.  This is usually fatal.

(Choices A, B, D, E, F, and G)  None of the other cranial nerves become compressed due to uncal herniation.

Educational objective:
Uncal herniation is a possible complication of an expanding ipsilateral mass lesion (eg, hemorrhage, tumor).  The first sign of uncal herniation is a fixed, dilated pupil on the side of the lesion.  Contralateral or ipsilateral hemiparesis and contralateral homonymous hemianopsia with macular sparing may also occur.