Phenylketonuria (PKU) due to Guanine triphosphate cyclohydrolase (GTPCH) deficiency.
Incidence
Very rare condition. AR inheritance.
Clinical Characteristics
This condition is a very rare defect, where both neopterine and biopterine in urine are low and neopterine-biopterine ratio is normal. The clinical symptoms are progressive mental and physical retardation despite treatment for PKU, that is especially seen in infancy and childhood. There are also feeding difficulties present in the neonatal period. Central nervous symptoms include a variable tone with marked hypotonia to opistotonus and spasticity. These changes in tone are present since the neonatal period. Also, temperature instability. There may be seizures of myoclonic type in infancy and childhood. Microcephaly is present since birth. Hypersalivation is also present. Mental retardation is observed in infancy and childhood. Laboratory findings include elevated phenylalanine in infancy and childhood. In the neonate, it can be normal but usually elevated. Abnormal urinary pterines are present since the neonatal period. Tetrahydrobiopterine (BH4) loading test (20 mg/kg) is abnormal since the neonatal period until childhood. While awating diagnostic confirmation by pterines and enzymatic analysis, these patients should be treated with BH4 (2 to 5 mg./K/day). Low phenylalanine diet is not beneficial. The treatment should be similar to biopterine dependent PKU due to 6PTS deficiency type.
Precipitants
no
Provocation Tests
Tetrahydrobiopterine (BH4) loading test (20 mg/kg) is abnormal since the neonatal period until childhood.
Diagnostic Procedures
EB-W,L,B,K. There is elevated phenylalanine in infancy and childhood, but not as high as classical PKU. Both neopterine and biopterine in urine are low and neopterine-biopterine ratio is normal.