Alexander disease- cause of Megalencephaly. Here, a 9 month infant has HC=39cm, so he/she has microcephaly, (Normal head circumference at bih = 33-35 cm) Rate of increase in HC 1st 3 months 2cm/month Next 3 months 1cm/month Next 6 months 0.5cm/month Next 2 years 0.2cm/month Megalencephaly causes:- Benign familial megalencephaly (Most common cause) Amino acid disorders Maple syrup urine disease glutaric aciduria Lysosomal storage disorder Weaver syndrome Achondroplasia Neurofibromatosis soto's syndrome Alexander disease Canavan disease Galactosemia The other 3 are causes of microcephaly. At 9 months ; ideally HC should be 43.5-45.5 cm.