Interactive Transcript
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This is an MRI scan of the brain
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in a two-and-a half-month-old child
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with severe seizures,
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predominantly affecting the right side of the body.
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Immediately, we see several abnormalities.
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First of all,
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we see an asymmetry in the size
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of the left hemicrania.
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We can see here the left hemicrania
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is larger than the right.
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The left cerebral hemisphere
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looks larger than the right.
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The left lateral ventricle,
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especially posteriorly,
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looks larger than the right.
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Anytime you have a hemispheric asymmetry,
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the first thing to figure out is whether one
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side is too big or one side is too small.
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In most cases,
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the side that is too small is
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the abnormal hemisphere.
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But there's one case where the size that is
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side that is larger is the abnormal side,
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and that's this case.
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This is called hemimegalencephaly.
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Hemi meaning half.
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Megalo, meaning big.
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Encephaly meaning head or brain.
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So, half of the brain is too big.
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And we can see several different abnormalities.
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First of all, posteriorly,
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we see thickened cortex,
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which is a featureless surface.
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So this is a lissencephaly-like appearance.
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We're seeing white matter signal abnormalities
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subjacent to that.
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We're seeing ipsilateral enlargement of
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the right lateral ventricle.
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Now,
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I mentioned in hemispheric size asymmetry,
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trying to figure out which side is abnormal.
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In general, a rule of thumb is
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the side with the larger
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ventricle is the abnormal side.
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So here, even though the left side is abnormal,
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it also has a bigger ventricle.
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Now, anteriorly, the brain is not normal either.
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We have areas where we don't
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see the sulcation as well.
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And this is actually a focal area of
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polymicrogyria in the left middle frontal gyrus.
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And then, if we look at the white matter,
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the white matter in the left cerebral hemisphere
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is darker than the white matter in the right
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cerebral hemisphere.
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In hemimegalencephaly,
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it is not uncommon to have
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accelerated myelination.
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Some of these features can be seen
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on T1-weighted imaging,
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where you have a subtle, hazy,
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intermediate bright signal in the white matter
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compared to the normally expected
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hypomyelination at this age,
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seen in the right cerebral hemisphere.
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So, this is likely a sign of accelerated
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myelination in the abnormal hemisphere
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in the setting of hemimegalencephaly.
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One of the things that can be done
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to treat hemimegalencephaly,
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given that it is often refractory
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to medical therapy,
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is a procedure called a functional hemispherectomy.
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A functional hemispherectomy has to do with
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either resection or disconnection of nearly the
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entirety of the left cerebral hemisphere.
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We see here on this early postoperative image,
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there has been resection of
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the left temporal lobe.
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There has been resection of
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the left insular cortex,
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but the frontal pole in the occipital pole
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remains present.
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Instead of removing it,
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they disconnect.
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So, we can see this transection go
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from the margin of the temporal lobectomy
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to the posterior aspect of
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the intrahemispheric fissure.
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So, this posterior cortex remains.
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But even if a seizure arose from this area,
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it will not propagate to the rest of the brain
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because it has been completely disconnected.
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Similar the frontal lobe has been disconnected.
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There's some parenchyma near the vertex
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also in the left cerebral hemisphere,
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the posterior frontal lobe,
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as well as the parietal lobe that was present.
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The way this is disconnected is they do a corpus
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callosotomy at the time of the procedure.
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So here, we see a transection going in towards
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the interhemispheric fissure and we see the
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corpus callosum has been transected.
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So the remaining portions of the left frontal lobe
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and left occipital lobe,
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as well as up near the vertex, the left parietal lobe.
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While they remain present,
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they do not have the ability to have seizures
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propagate to the right cerebral hemisphere
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or the working cerebral hemisphere.
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There's a few things to be aware of in this.
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First of all,
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this procedure of a functional hemispherectomy
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works well and works best
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in very young children,
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often performed by the first year of life.
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Why is that?
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Because normally the right cerebral hemisphere
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controls the left side of the body
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from motor and sensory,
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and the left cerebral hemisphere controls
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the right side of the body.
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If a functional hemispherectomy takes place
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prior to full maturation of the brain,
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it is possible for the right cerebral hemisphere
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to also retain the ability to control the
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right side of the body.
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So therefore,
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a patient with a functional hemispherectomy
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will not be hemiporetic.
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The other issue is language.
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In a majority of individuals,
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the left cerebral hemisphere is
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where language is centered.
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In this individual,
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where there was left sided hemimegalencephaly,
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there was a chance if the functional
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hemispherectomy was performed later
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after development of language,
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that the patient would have severe impairment.
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But performing some of these surgeries prior to
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full maturation of motor and language functions
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gives the chance for the remaining cerebral
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hemisphere to take over some of those functions.
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