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OVERVIEW

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Disorders of neurogenesis, neuronal maturation, and proliferation are responsible for many severe forms of childhood epilepsy. They may arise in discrete cortical areas or involve widespread brain regions in one or both cerebral hemispheres. While extensive unilateral involvement of one cerebral hemisphere is a relatively uncommon condition, the rare occurrence of a hemispheric malformation is associated with a high rate of morbidity and mortality. Affected children typically manifest cognitive delay, intractable epileptic seizures, and lateralized neurological deficits.

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An awareness of the clinical presentations and management (Fig. 36–1) of this unusual group of disorders is an important part of pediatric epilepsy as the frequent association of unrelenting partial seizures and clinical deterioration may prove catastrophic in the first year of life. Prompt recognition of the various underlying causes and aggressive treatment, frequently surgical, are therefore essential.

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Figure 36–1.

Management algorhythm: hemispheric disorders associated with cortical malformation.

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HEMIMEGALENCEPHALY

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Hemimegalencephaly is a rare but clinically significant brain malformation characterized by the excessive growth of one cerebral hemisphere. Hemispheric asymmetry may present at birth but typically increases during the first year of life due to progressive hemispheric expansion. Affected infants often appear normal and their head circumference and shape appear unremarkable as the hemisphere enlarges. The classical signs of raised intracranial pressure, bulging fontanel, widening of skull sutures, and the "setting sun sign" are notably absent. Either cerebral hemisphere may be affected.

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Three subtypes of hemimegalencephaly are now recognized.

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ISOLATED HEMIMEGALENCEPHALY

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In the isolated form, unilateral hemispheric enlargement is not associated with somatic hemicorporeal hypertrophy, cutaneous manifestations or other signs of systemic involvement.1 Isolated hemimegalencephaly accounts for approximately half of all known hemimegalencephaly cases.2 Its occurrence is typically sporadic with no known genetic pattern of inheritance.

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SYNDROMIC HEMIMEGALENCEPHALY

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The second form, syndromic hemimegalencephaly, is associated with one of several well-recognized developmental syndromes, mostly neurocutaneous disorders. All have known patterns of Mendelian inheritance, and systemic involvement is common. Syndromic disorders with hemimegalencephaly include the following disorders.

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EPIDERMAL NEVUS SYNDROME

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Affected patients manifest cutaneous epidermal nevi that are often unilateral. The linear nevus sebaceous of Jadassohn is commonly associated. Ocular and skeletal abnormalities are frequently noted.

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HYPOMELANOSIS OF ITO
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This neurocutaneous disorder presents with a linear area of depigmentaion variably associated with hemimegalencephaly ipsilateral or contralateral to the skin lesion. Both macrocephaly and callosal agenesis have been observed in affected children.

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INCONTINENTIA PIGMENTI
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A disorder characterized by irregular marbled or wavy linear cutaneous pigmentation caused by excess melanin deposition. Cavitary lesions in subcortical white matter and loss of cerebellar cortical neurons are common. Hemimegalencephaly has been described in a portion ...

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