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 The third nerve is often stretched
and damaged when there is transtentorial or
UNCAL herniation. A rapidly expanding
mass lesion (like an intracerebral clot, epidural
hematoma or brain swelling following trauma) will
result in the ventral medial portion of the
temporal lobe (uncus and parahippocampal gyrus)
ipsilateral to the mass being squeezed or
herniating (L. to rupture) medial to and under the
free edge of the tentorium cerebelli. This area
(called the tentorial notch) is normally occupied
by the midbrain (approximately level #10). When the
cortical structures are squeezed into the tentorial
notch, usually the IPSILATERAL third nerve
is stretched and damaged, resulting in a dilated
pupil of the IPSILATERAL PUPIL (to the mass
lesion). In addition, the contralateral cerebral
peduncle is pushed against the free edge of the
contralateral tentorium. This causes damage to the
corticospinal axons. Thus, the hemiplegia,
spasticity and Babinski are on the SAME side
as the dilated pupil (IPSI to the cortical
mass). Patients with such herniations are usually
in a coma due to damage to other brain stem
centers.
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