Phenylketonuria (PKU): Not always 'PKU'

Neonatal screening aims to detect serious but treatable diseases in neonates. A positive screening for phenylketonuria (PKU) does not necessarily mean hyperphenylalaninemia as a consequence of classic PKU. We report on a female patient diagnosed with dihydropteridin reductase (DHPR) deficiency through neonatal screening. This is a rare inborn error of the biopterin metabolism, characterized by a more severe neurological involvement compared to classic PKU. A diagnostic oral tetrahydrobiopterin (BH4) loading test was performed in our patient followed by detailed metabolic investigations of the pterin metabolism to confirm the correct diagnosis of DHPR deficiency. Despite adequate treatment by suppletion of BH4, folinic acid, L-dopa/carbidopa (Sinemet®) and L-5-hydroxytryptophan (Oxitriptan®) the patient deceased suddenly at the age of 10 months.