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Expert
Note Case 3.
Brisk Reflexes
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The patient had suffered
a lacunar stroke in the base of the pons caused by occlusion of a
small medial penetrating branch of the basilar artery. The damage
interrupted the corticospinal tract (located in the base of the pons)
which produced contralateral upper motor neuron paralysis of the arm
and leg. The same clinical findings may be seen after a lacunar stroke
in the internal capsule due to occlusion of a lenticulostriate artery.
However, in pontine lesions, the face is usually spared because the
corticobulbar fibers have separated from the corticospinal tract at
this level and are located much farther dorsal. By contrast,
the corticobulbar and corticospinal tracts run close to each other
in the posterior limb of the internal capsule and therefore can be
damaged together, producing contralateral paralysis that involves
the lower face as well.
If the pontine lesion had been less severe, producing a milder motor
problem (e.g., hemiparesis instead of hemiplegia), then there may
have been ataxia of the weakened limbs. Ataxia cannot be seen
if there is complete limb paralysis--to detect the presence of ataxia
(incoordination of voluntary limb movements), the patient must be
able to move the limb! Ataxia, indicative of a problem with cerebellar
function, presumably reflects damage to the pontine nuclei (griseum
pontis) that project their axons to the cerebellum via the middle
cerebellar peduncle. |
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