Medical approach of the neonates screened with HPA
The objective is to specify the diagnosis and to apply the adequate treatment as early as possible after birth (5,6).
The plasma level at screening indicates the degree of severity. Those infants with blood phenylalanine levels between 150 and 300 μmol/L are retested; those with levels between 300 and 600 μmol/L are controlled, investigated, and then monitored at the outpatient clinic. The neonates with blood phenylalanine levels >600 μmol/L are hospitalized for further investigations, treatment, and parental information and education.
Most of the screened newborns have primary HPA. However, secondary HPA due to various liver diseases should be excluded. Premature or small-for-gestation-age babies may have transient HPA, especially when they are fed a parenteral nutrition containing amino acids.
Disorders of BH4 metabolism are systematically excluded by analysis of the pteridin profile in urine and measurement of dihydropteridine reductase activity in erythrocytes. In a few clinics, an oral BH4 loading test (20 mg/kg) is used to rapidly screen for BH4 synthesis defects and eventually to test BH4 responsiveness in some atypical PKU patients.
The phenotypes of the phenylalanine hydroxylase defect are determined by measuring blood phenylalanine and tyrosine levels while the infants are fed a normal diet containing at least 500 mg/d of phenylalanine for 5 consecutive days. The patients with the classical or typical form of PKU have phenylalanine levels above 1200 μmol/L due to hepatic phenylalanine hydroxylase residual activity <1%. Atypical PKU patients have levels between 600 and 1200 μmol/L and 1–5% residual activity. Permanent mild HPA patients have levels <600 μmol/L and >5% residual activity.
Patients affected with both the classical and atypical PKU require a (lifelong) dietary phenylalanine restriction, whereas patients affected with mild permanent HPA will develop normally without treatment. Moreover, treating such children with a phenylalanine-restricted diet can lead to undesirable nutritional problems. Recently, some BH4 responsiveness has been described in patients affected with atypical PKU and BH4 supplementation may offer them an alternative to a burdensome diet (7).
From a practical point of view, all the measurements required to define the disorder can be performed within 24 h for those patients whose blood phenylalanine levels are above 600 μmol/L. Those patients with blood phenylalanine levels between 300 and 600 μmol/L may require a phenylalanine load for 5 consecutive d if they do not receive a sufficient amount of phenylalanine in their usual diet. This phenylalanine load could be followed by a BH4 loading test. The general protocols used in our unit are summarized in Figure 2.
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