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Aromatic Amino Acid Decarboxylase Deficiency

Aromatic Amino Acid Decarboxylase (AADC) Deficiency is a rare disorder of neurotransmitter metabolism that affects both the biosynthesis of dopamine and serotonin, since the enzyme responsible for both the decarboxylation of 5-hydroxy-tryptophan into serotonin and the conversion of L-dopa into dopamine is deficient. This deficiency results in a usually severe condition with features of mental and motor retardation. The age of onset ranges from newborn to infancy or early childhood. The most predominant clinical characteristics are hypotonia, oculogyric crises, hypokinesia, choreoathetosis, dystonia and bulbar dysfunction (speech and feeding difficulties). AADC deficiency is frequently misclassified as cerebral palsy syndrome.
Characteristic neurochemical findings include:
- Low concentrations in Cerebrospinal Fluid (CSF) of the neurotransmnitter metabolites HVA, 5-HIAA and MHPG, with an accumulation of the precursors 5-HTP, L-dopa and it latter derivative, 3-methoxy-tyrosine.
- AADC enzyme activity, for both substrates L-dopa and 5-HTP, in plasma is very low.
- In urine, decreased concentrations of HVA, 5-HIAA and VMA are found, but paradoxically normal-to-elevated dopamine levels are commonly observed. In addition, the endproduct of L-dopa breakdown, vanillactic acid (VLA), is increased.