The Aromatic L-Amino Acid Decarboxylase Deficiency (AADC) is a rare metabolic disorder characterised by the lack of the AADC enzyme.

This causes flaws in the brain's neurotransmitters and results in movement abnormalities.

The head of the National University Hospital's paediatric neurology division, Associate Professor Stacey Tay, said symptoms of the disorder include periods of weakness or fluctuating motor ability, dystonia (involuntary movements and extended muscle contractions), developmental delay, drooling, sleep difficulties and irritability.

AADC is known as an autosomal recessive disorder, where both carrier parents have a one-in-four chance of having an affected child.

AADC affects both sexes.

Prof Tay, who has been treating the Chang sisters since 2005, added that this disorder may be diagnosed as global developmental delay or cerebral palsy.