Hemimegalencephaly

What it is

From: Orphanet

Hemimegalencephaly is a rare cerebral malformation characterized by overgrowth of all or part of a cerebral hemisphere, often with ipsilateral severe cortical dysplasia or dysgenesis, white matter hypertrophy and dilated lateral ventricle, presenting in early infancy with progressive hemiparesis, severe psychomotor retardation and intractable seizures. Hemimegalencephaly may be an isolated finding or associated with other syndromes such as angioosteohypertrophic syndrome, epidermal nevus syndrome and Ito hypomelanosis. Management includes seizure control by antiepileptic medications and early hemispherectomy.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Hyperintensity of cerebral white matter on MRI · Very frequent (99-80%)

A brighter than expected signal on magnetic resonance imaging emanating from the cerebral white matter.

Interictal EEG abnormality · Very frequent (99-80%)

Interictal refers to a period of time between epileptic seizures. Electroencephalographic (EEG) patterns are important in the differential diagnosis of epilepsy, and the EEG is almost always abnormal during a seizure. Some persons with seizures may show EEG abnormalities between seizures, while others do not. In some cases, multiple interictal EEGs must be recorded before an abnormality is observed. In most cases the electrographic pattern of seizure onset is completely different from the activity recorded during interictal discharge.

Seizure · Very frequent (99-80%)

A seizure is an intermittent abnormality of nervous system physiology characterized by a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain.

Abnormal skull morphology · Frequent (79-30%)

An abnormality of the skull, the bony framework of the head which is comprised of the neurocranium (with eight cranial bones) and the viscerocranium (facial skeleton) that comprises fourteen facial bones with the mandible as its largest bone.

Cranial asymmetry · Frequent (79-30%)

Asymmetry of the bones of the skull.

EEG with burst suppression · Frequent (79-30%)

The burst suppression pattern in electroencephalography refers to a characteristic periodic pattern of low voltage (<10 microvolts) suppressed background and a relatively shorter pattern of higher amplitude slow, sharp, and spiking complexes.

EEG with focal sharp slow waves · Frequent (79-30%)

EEG with focal sharp transient waves of a duration between 80 and 200 msec followed by a slow wave.

EEG with focal spikes · Frequent (79-30%)

EEG with focal sharp transient waves of a duration less than 80 msec.

EEG with polyspike wave complexes · Frequent (79-30%)

The presence of complexes of repetitive spikes and waves in EEG.

Focal cortical dysplasia · Frequent (79-30%)

A type of malformation of cortical development that primarily affects areas of neocortex. It can be identified on conventional magnetic resonance imaging as focal cortical thickening, abnormal gyration, and blurring between gray and white matter, often associated with clusters of heterotopic neurons.

Focal motor seizure · Frequent (79-30%)

A type of focal-onset seizure characterized by a motor sign as its initial semiological manifestation.

Global developmental delay · Frequent (79-30%)

A delay in the achievement of motor or mental milestones in the domains of development of a child, including motor skills, speech and language, cognitive skills, and social and emotional skills. This term should only be used to describe children younger than five years of age.

Hemihypsarrhythmia · Frequent (79-30%)

Hypsarrhythmia occurring in one hemisphere.

Hemimegalencephaly · Frequent (79-30%)

Enlargement of all or parts of one cerebral hemisphere.

Other findings in the same source

From: Orphanet

Additional reported features include Polymicrogyria (Frequent (79-30%)); Ventriculomegaly (Frequent (79-30%)); Abnormal neuron morphology (Occasional (29-5%)); Atonic seizure (Occasional (29-5%)); Epileptic spasm (Occasional (29-5%)); Focal tonic seizure (Occasional (29-5%)); Gliosis (Occasional (29-5%)); Gray matter heterotopia (Occasional (29-5%)); Hemianopia (Occasional (29-5%)); Hemiparesis (Occasional (29-5%)). This is a selected summary, not a complete description of the condition.

When it may begin

From: Orphanet

Infancy

Inheritance in the source

From: Orphanet

Not applicable

Frequency and the population described

From: Orphanet

Point prevalence: Unknown; Worldwide; Class only.

Which doctor should you see?

The suggested department for discussing Hemimegalencephaly is Clinical Genetics, with a clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with clinical geneticist referral.

Additional services that may be relevant, depending on the findings, include: Neurology.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Does the exact genetic or chromosome finding explain the observed features?
• Would a genetic counsellor help the family understand the result?
• Which organ-specific assessments are appropriate for this particular diagnosis?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Hemimegalencephaly. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.