- 1 in 100,000 births.
- There is cerebral overgrowth and ventriculomegaly of one hemisphere resulting in shift in the midline in the standard transverse view of the fetal head. The diagnosis is usually made >26 weeks' gestation.
- There are always abnormalities of sulcation, including agyria, pachygyria, or polymicrogyria.
- Chromosomal abnormalities: no increased risk.
- Genetic syndromes are very common:
- Proteus syndrome: sporadic; hemimegalencephaly, disproportional overgrowth of hands and feet.
- Klippel–Trenaunay syndrome: sporadic; hemimegalencephaly, hemangiomatosis of one or more limbs, heart defects, renal agenesis, laryngeal atresia.
- Detailed ultrasound examination, including neurosonography.
- Fetal brain MRI especially for the differential diagnosis with brain tumors.
- Ultrasound scans every 4 weeks to monitor the evolution of head circumference.
- Standard obstetric care and delivery.
- Cesarean section if the head circumference is >40 cm.
- Very poor: severe neurodevelopmental delay, hemiplegia and intractable seizures.
- No increased risk of recurrence.