Hemimegalencephaly 

is a rare congenital disorder of cortical formation characterized by hamartomatous overgrowth of all or part of a cerebral hemisphere. This overgrowth results from either increased proliferation or decreased apoptosis (or both) of developing neurons  .

Epidemiology

Hemimegalencephaly is a cryptogenic congenital disorder and does not have a recognized racial or gender predilection  . It is rare, accounting for only ~0.2% of cases of childhood epilepsy  .

Clinical presentation

The majority (90%) of patients present with focal and generalized infantile spasms. Developmental delay, hemiparesis, and hemianopia are usually also evident 

Although normocephaly is often present, macrocephaly may be pronounced, and normal vaginal delivery may be impossible. Despite the markedly enlarged head, there are no symptoms of raised intracranial pressure  .

• syndromic:
• associated with a variety of syndromes typically including hemihypertrophy of the ipsilateral part of the body
  • epidermal nevus syndrome
  • Klippel–Trenaunay Syndrome
  • McCune-Albright syndrome
  • Proteus syndrome
  • unilateral hypomelanosis of Ito
  • neurofibromatosis type 1 (NF1): rare
  • tuberous sclerosis: rare
  • CLOVES syndrome
• total hemimegalencephaly: hemihypertrophy also involves the brainstem and cerebellum