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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.