Strokes in Middle Cerebral Artery Territory
Infarctions in the distribution of the MCA are by far the most common strokes
that are seen in clinical practice.
Unlike the situation
with the extracranial internal carotid, there are limited ways to establish
anastamotic blood flow that can "rescue" ischemic tissue if the MCA
or its branches are blocked.
Question:
Can
you think of a possible place where anastaomses could form that would
rescue brain tissue in MCA territory?
Answer:
anastomoses
between cortical branches in the border zones between MCA, ACA and PCA.
Occlusion of the
MCA stem is most often caused by embolism from a cardiac source or artery-to-artery
embolism from the extracranial ICA. In a complete occlusion, blood flow is blocked
both to the 10-15 deep penetrating lenticulostriate branches, which supply the
internal capsule and basal ganglia, and to the more lateral superior and inferior
MCA branches, which supply most of the lateral cerebral cortex. As a result,
much of the hemisphere is infarcted.
After such a large stroke, the resulting brain swelling may cause sufficient
side-to-side and downward herniation of the brain that the resulting midbrain
compression kills the patient. Those who survive the initial period will have
a contralateral hemiplegia affecting the lower face, arm and (to a lesser extent)
leg. Damage to the frontal lobe motor cortex and its projections results in
brisk reflexes and a dorsiflexor Babinski response. There is also contralateral
hemianesthesia produced by damage to the anterior parietal lobe somatosensory
cortex, and a complete contralateral homonymous hemianopsia resulting from damage
to the visual radiations as they travel from the lateral geniculate nucleus
to the primary visual cortex in the white matter of the posterior temporal and
parietal lobes.
In addition, damage to frontal, parietal and temporal lobes near the sylvian
fissure in the language-dominant hemisphere (almost always the left one) produces
global aphasia. Although fully conscious, these patients understand nothing
that is said and are unable to utter more than a few stereotyped words.
Damage to the posterior parietal and temporal lobes (and, most likely, the frontal
lobes as well) in the non-language, right hemisphere produces a severe defect
in visuospatial conceptualization. These patients neglect the left side of their
own body and of their world. They may deny that their left limbs are paralyzed
or even deny that their left arm and leg belong to them. They may shave only
the right side of their face or draw only the right side of a symmetrical object
such as a clock. They may have major problems in putting on clothes, an
activity that requires understanding 3-D visuospatial relationships. They often
appear emotionally blunted, dull, inattentive, apathetic or confused.
