Occipitofrontal circumference > 2 SD above the mean for age and sex.

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Aetiology

Megalencephaly:

• Benign familial
• Neurocutaneous syndromes:
  • Neurofibromatosis, tuberous sclerosis, Sturge-Weber, Klippel-Trenaunay-Weber, linear sebaceous nevus
• Others:
  • Sotos, Fragile X syndrome
• Leukodystrophies:
  • Alexander, Canavan, megalencephalic leukoencephalopathy
• Lysosomal storage diseases:
  • Tay-Sachs, mucopolysaccharidosis, gangliosidosis

Increased cerebrospinal fluid:

• Hydrocephalus
• Benign enlargement of subarachnoid space
• Hydranencephaly, choroid plexus papilloma

Enlarged vascular compartment:

• Arteriovenous malformation
• Subdural, epidural, subarachnoid or intraventricular hemorrhage

Increase in bony compartment:

• Bone disease:
  • Achondroplasia, osteogenesis imperfecta, osteopetrosis, hyperphosphatasia, cleidocranial dysostosis
• Bone marrow expansion:
  • Thalassemia major

Miscellaneous causes:

• Intracranial mass lesions:
  • Cyst, abscess or tumor
• Raised intracranial pressure:
  • Idiopathic pseudotumor cerebri, lead poisoning, hypervitarninosis A, galactosemia