Prevalence:

• 1 in 100,000 births.

Ultrasound diagnosis:

• 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.

Associated abnormalities:

• 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.

Investigations:

• Detailed ultrasound examination, including neurosonography.
• Fetal brain MRI especially for the differential diagnosis with brain tumors.

Follow up:

• Ultrasound scans every 4 weeks to monitor the evolution of head circumference.

Delivery:

• Standard obstetric care and delivery.
• Cesarean section if the head circumference is >40 cm.

Prognosis:

• Very poor: severe neurodevelopmental delay, hemiplegia and intractable seizures.

Recurrence:

• No increased risk of recurrence.