Atypical phenylketonuria. An approach to diagnosis and management

Intelligence in mild atypical phenylketonuria

In 82 children with mild phenylketonuria (PKU) (blood phenylalanine (Phe) concentrations consistently below 900 mumol/l throughout follow up) the relationship between intelligence at age 4 (IQ by Stanford-Binet) and average blood Phe concentrations from birth to 4 years was examined. Of the 82 children 24 had received no treatment. In the group as a whole, and in the 24 untreated subjects alone, mean IQs were significantly below population norms, with deficits of approximately 4.5 points and 9 points respectively. After allowing for social class IQ fell progressively by approximately 6 points for each 100 mumol/l rise in mean Phe concentrations in both the treated and untreated subjects. This relationship resembled that previously reported in early treated children with more severe forms of PKU, except that the scale of the relationship was even greater. We conclude that all children whose blood Phe concentrations reach 400 mumol/l or above should receive a low Phe diet, at least during the preschool years, and that the aim should be to control blood Phe levels below 400 mumol/l throughout early childhood in all forms of PKU.

Maternal phenylketonuria (PKU)--a review

This review points out the very high incidence of damage to the fetus in untreated maternal phenylketonuria (PKU). In classical cases, 92% of the offspring are mentally retarded, 73% have microcephaly, 40% are growth retarded at birth, and 12% have congenital anomalies. Less severe types of PKU and its variants and patients treated with a low phenylalanine diet during pregnancy have a much lower incidence of these defects in their offspring. Very promising results have been obtained in a small number of preconception and early first trimester treated patients under very strict dietary control. Nutrition of the mother and fetus is a major concern during the application of this restrictive diet and must be monitored closely to avoid fetal damage from malnutrition. A 7-year collaborative study of maternal PKU began in November 1984 in the US and Canada, but even in this well publicized study, many patients are presenting late for treatment. It is suggested that premarital and/or prenatal screening for maternal PKU should be initiated for the next generation.

Phenylketonuria and its variants: observations on intellectual functioning

The age at diagnosis, dietary treatment and intelligence quotient (IQ) of 119 patients with phenylketonuria (PKU) and its variants who were under long-term observation were studied. In 27 of the 79 patients with classic PKU the diagnosis had been made and treatment begun late (after 2 months of age). The mean IQ of these 27 patients was 57.6 when they were in their early 20s (although 2 had normal IQs). In contrast, among the 52 patients with classic PKU who were not treated late the mean IQ was 93.6 for the 27 who were still receiving dietary therapy. The mean IQs were 99.3 and 92.7 at ages 5 (when the diet was discontinued) and 15 years respectively for the 12 who had been treated "adequately". It was 76.0 for the 13 who were "over-treated" (malnourished) in the first 6 months of life. Among the patients with atypical PKU, who were treated early, the mean IQs were 110.0 for the 7 who were still receiving dietary therapy and 102.7 for the 12 who were not. The 21 patients with persistent benign hyperphenylalaninemia, who were not treated, had a mean IQ of 104.2. The most important factor in the ultimate IQ of patients with classic PKU is very early diagnosis (by 2 weeks of age) along with immediate initiation of dietary therapy.

The offspring of a phenylketonuric couple

Biochemical studies carried out in seven members of a family with five phenylketonuric subjects are described. The concentration of phenylalanine and the phenylalanine/tyrosine ratios in the blood, together with the values obtained from the L-phenylalanine loading tests, were the criteria used to establish the diagnosis. The results suggest that the non-consanguineous parents, the children and their maternal aunt are homozygous for classical phenylketonuria; the paternal grandmother, as expected, was found to be heterozygous for this metabolic disorder. The mother, diagnosed in the fifth month of her last pregnancy, was treated with a low phenylalanine diet until delivery. Nevertheless, the girl who was born from that pregnancy showed some symptoms resulting from "maternal phenylketonuria", being diagnosed as phenylketonuric at one week of age and treated with a low phenylalanine diet thereafter.

Different phenotypes for phenylalanine hydroxylase deficiency

Abnormalities related to the hydroxylation of phenylalanine to tyrosine are classified into three phenotypes based on the amount of dietary phenylalanine tolerated to keep serum phenylalanine within therapeutic levels, i.e., 180-425 micronmol/l(3-7 mg/100 ml) : (1) Classical phenylketonuria (PKU) tolerating approximately 17% of a normal intake of phenylalanine or approximately 0.12 mmol per kg body weight; (2) mild PKU with a tolerance some 50% higher; and (3) persistent hyperphenylalaninaemia (HPA) with serum phenylalanine values within therapeutic levels on a daily intake of greater than or equal to 0.7 mmol phenylalanine per kg body weight. Oral phenylalanine loading was performed on 100 heterozygotes for these abnormalities and 33 normal homozygotes. The slope of the rise in serum tyrosine multiplied by the maximum serum tyrosine concentration over the maximum phenylalanine concentration was the most powerful discriminant (D/-s, 3.54; overlapping 2.4%). Three heterozygous phenotypes were distinguished by this discriminant, and a significant correlation was observed between the phenotypic combination of the parents and the phenotype of their affected child. In particular, parents of children with classical PKU were clearly distinguished from heterozygotes for the other two abnormalities.

[Urinary phenylalanine metabolites in hyperphenylalaninemia (author's transl)]

Urinary phenylalanine metabolites after phenylalanine load of 4 healthy controls, 15 patients with classical phenylketonuria and 8 patients with hyperphenylalaninemic variants are measured quantitatively by gas chromatography. Statistically significant differences are found for a number of metabolites, e.g. phenylpyruvate, phenyllactate, ortho-hydroxy-phenylacetate, para-hydroxy-phenylpyruvate and para-hydroxy-phenyllactate. The results are discussed with respect to discrimination of phenylketonuria patients and patients with hyperphenylalaninemic variants and in respect to pathophysiological aspects.

Phenylketonuria variants in Ontario

Since mass screening of the newborn population for phenylketonuria (PKU) by the Guthrie test was begun in Ontario in July 1965 many variants of PKU have been recognized in the 96 to 97% screened. Seventy-one cases of classic PKU were detected (four were missed). Of 48 cases of persistent hyperphenylalaninemia discovered, 18 were classified as atypical PKU and 30 as persistent benign hyperphenylalaninemia. Numerous infants with transient hyperphenylalaninemia (initial values over 10 mg/dl in 12), in many instances the result of transient neonatal tyrosinemia, were discovered. There was a slight predominance of males. Serum phenylalanine values of up to 15 mg/dl seemed to be harmless to the developing brain. A survey of 67 247 adults in the general population revealed 1 person with PKU and 1 with persistent benign hyperphenylalaninemia; both had normal intelligence quotients. Of 1548 mothers of retarded children tested, none had hyperphenylalaninemia.