Summary:
Transtentorial, or uncal, herniation is a well-described neurologic
phenomenon caused by an expanding supratentorial mass. The medial
aspect of the temporal lobe is forced downward over the tentorium,
compressing the neighboring oculomotor nerve and resulting in
ipsilateral pupillary dilatation, which is of
Discussion
Transtentorial, or uncal, herniation is a well-described neurologic
phenomenon caused by an expanding supratentorial mass. The medial
aspect of the temporal lobe is forced downward over the tentorium,
compressing the neighboring oculomotor nerve and resulting in
ipsilateral pupillary dilatation, which is often followed by
oculomotor ophthalmoplegia. With time, the integrity of the crus
cerebri and its descending corticospinal tracts is disturbed, and a
contralateral motor deficit is produced with a deteriorating level
of consciousness. Lateral pressure on the midbrain can, however,
compress the opposite crus cerebri against the free edge of the
tentorium, forming an indentation in the crus known as Kernohan's
notch. This may produce a hemiparesis ipsilateral to the expanding
mass known as Kernohan's phenomenon, which is a false localizing
sign. This was first described by Kernohan et al
1 in
1929 after an autopsy study revealed a notched cerebral peduncle
from a contralateral herniation syndrome. Because of streak
artifacts from the petrous bone, CT is usually unable to detect
these changes. MRI can be useful in revealing a deformity or injury
of the cerebral peduncle that results from ongoing or past
transtentorial herniation.
2-6
CONCLUSION
Kernohan's phenomenon should be suspected when a motor deficit
develops ipsilateral to a space-occupying supratentorial mass. MRI
is the study of choice to confirm injury to the contralateral crus
cerebri.
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