A deficiency of hepatic dihydropteridine reductase (DHPR) activity was found in a neurologically impaired infant with mild hyperphenylalaninemia and normal levels of hepatic phenylalanine hydroxylase. DHPR is required for the regeneration of tetrahydrobiopterin, an essential cofactor in aromatic amino acid hydroxylation, a necessary step in the biosynthesis of the neurotransmitters, dopamine and serotonin. Evidence for decreased synthesis of these transmitters in this patient was provided by the finding of reduced levels of homovanillic acid and 5-hydroxyindole acetic acid, metabolites of dopamine and serotonin, respectively, in the cerebrospinal fluid and urine. Treatment with dopamine and serotonin precursors, l-3,4 dihydroxyphenylalanine and 5-hydroxytryptophan, respectively, was associated with improvement in temperament and motor tone and less frequent seizures. However, there was no improvement in gross motor function or language development.

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