Purpose or Case Report: Case Presentation: We will show two cases, of patients aged 16 and 24 years, at 24 and 30 weeks of gestation, who were referred for a second-trimester ultrasound.
Findings: The ultrasound revealed an enlarged lateral ventricle associated with an increased size of the ipsilateral cerebral hemisphere. A fetal MRI was performed revealing subependymal gray matter nodules, pachygyria, and areas of gliosis in the white matter.
Discussion: Hemimegalencephaly is a rare congenital malformation initially described by Sims in 1835. It involves the partial or total overgrowth of a cerebral hemisphere with varying degrees of pachygyria, polymicrogyria, neuronal heterotopia, and white matter gliosis due to a disorder in germ cell proliferation, migration, and differentiation.
Its etiology remains unknown, and it does not exhibit a gender preference. Clinically, 90% of patients present with focal or generalized epileptic seizures. They may also exhibit hemiparesis and mental retardation.
Hemimegalencephaly can present in an isolated manner or be associated with various syndromes, including hypertrophy of the ipsilateral hemibody or total involvement, which affects the cerebellum and brainstem.
Prenatal diagnosis is suspected through ultrasound, identifying ventricular asymmetry and unilateral ventricular dilation. Fetal MRI confirms these findings and allows the assessment of gray and white matter disorders.
Conclusion: Hemimegalencephaly is a rare and severe condition that should be considered in prenatal assessment by ultrasound. Its diagnosis is facilitated by advances in fetal MRI, where the findings and their severity correlate with the clinical presentation.
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