Fact-sheet: Unilateral megalencephaly


Updated on 09/01/2022 at 4:04 PM

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Definition

Cortical dysgenesis with hamartomatous overgrowth of all or part of a cerebral hemisphere. Secondary to one or both of the following mechanisms: increased cell proliferation. Defective apoptosis (mTOR pathway dysfunction) of developing neurons.

Clinical features

Rare prevalence: < 5% of malformations seen on imaging and ~0.2% of pediatric epilepsy cases.

Psychomotor developmental delay, epilepsy (infantile spasms) in 90% of cases

Occasionally macrocephaly, hemiparesis, hemianopsia, total or partial body hemihypertrophy (syndromic form).

Most often isolated form

Syndromic form: Proteus syndrome, Klippel-Trenaunay-Weber syndrome, Neurofibromatosis type 1, Ito hypomelanosis, epidermal nevus syndrome, TSC, McCune-Albright syndrome, CLOVES syndrome.

Laboratory findings

Pathology:

Defective cellular organization and neuronal migration resulting in excessive hamartomatous growth of a hemisphere

the affected hemisphere shows, in addition to the overgrowth, abnormal cortical development with areas of lissencephaly, agyria, pachygyria, and polymicrogyria in variable proportions

CT

Total form if the ipsilateral cerebellum and brainstem are involved

Atypical form if partial, localized, or lobar involvement

  • Hypertrophy of a cerebral hemisphere
  • Hypertrophy of the ipsilateral hemicranium with calvarial thickening
  • Ventricular enlargement
  • White matter calcifications
  • White matter asymmetry due to unilateral myelination abnormalities

MRI

Also allows better visualization of:

  • Cortical abnormalities: pachygyria, lissencephaly, agyria, or polymicrogyria
  • Gray matter heterotopia
  • The heterogeneous and T2-hyperintense "gliosis-like" appearance of white matter, sometimes with cysts
  • Sometimes developmental venous anomalies

Differential diagnosis

Enlarged hemisphere:

  • Cerebral gliomatosis

Atrophy of one hemisphere making the normal hemisphere appear large:

  • Rasmussen encephalitis
  • Dyke-Davidoff-Masson syndrome
  • Sturge-Weber-Dimitri syndrome

Focal forms may be mistaken for neuronal migration abnormalities, but body hemihypertrophy points to the diagnosis:

  • polymicrogyria / lissencephaly / agyria / pachygyria