How is phenylketonuria diagnosed?

Written by Hu Qi Feng

Pediatrics

Updated on September 04, 2024

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Phenylketonuria screening is routinely conducted in China for all newborns. This involves pricking the heel to collect peripheral blood, which is then dropped on specialized filter paper and dried. The samples are sent to a screening laboratory for the measurement of phenylalanine concentration. If the concentration exceeds the threshold, further testing for phenylalanine is conducted. Normally, the concentration should be less than 120 micromoles per liter. Additionally, an analysis of the urinary purine profile can be performed.

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Written by Tong Peng

Pediatrics

1min 5sec

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Does phenylketonuria have a distinctive facial appearance?

Phenylketonuria is characterized by a specific facial appearance, primarily presenting with pale skin and yellow hair. Some affected children may develop eczema due to a disruption in melanin synthesis, which results in a distinctive skin color. Additionally, there are neuropsychiatric abnormalities, such as intellectual disability, increased excitability, and heightened muscle tone. The urine of patients has a plastic-like smell, largely because phenylketonuria is a hereditary metabolic disease. Newborns exhibit hyperphenylalaninemia, but as harmful metabolites of phenylalanine are not present in high concentrations before feeding begins, initial conditions at birth are often normal. However, as diet is introduced and metabolite concentrations increase, specific clinical symptoms, including severe cases of epileptic seizures manifested as infantile spasms, nodding seizures, or other forms, become evident. Therefore, dietary management is essential in treatment.

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Written by Yan Xin Liang

Pediatrics

1min 18sec

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Phenylketonuria is caused by how?

Phenylketonuria is caused by how? It is an autosomal recessive genetic disease, mainly due to a mutation in the phenylalanine hydroxylase gene, which leads to a reduction in the activity of phenylalanine hydroxylase. Phenylalanine is an essential amino acid in the human body. The phenylalanine ingested is partly used for protein synthesis, and the rest is converted into tyrosine by the action of phenylalanine hydroxylase, with only a small amount of phenylalanine being converted into phenylpyruvic acid through a minor metabolic pathway under the action of transaminase. Due to the reduced activity of phenylalanine hydroxylase, phenylalanine cannot be converted into tyrosine, resulting in an extremely high concentration of phenylalanine in the blood, cerebrospinal fluid, and tissues. Through a bypass metabolism, a large amount of phenylpyruvic acid, phenylacetic acid, phenyllactic acid, and p-hydroxyphenylacetic acid are produced. High concentrations of phenylalanine and its metabolites thus lead to brain damage. Due to the synthesis disorder of tyrosine, this also leads to symptoms such as light skin and hair pigmentation.

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Written by Zeng Hai Jiang

Pediatrics

38sec

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Phenylketonuria treatment duration before discontinuation

Phenylketonuria is the first genetic metabolic disease that can be treated through dietary control. Once diagnosed with phenylketonuria, active treatment must be given, ceasing a natural diet and implementing a low-phenylalanine diet. Generally, a low-phenylalanine milk formula should be administered at least until the age of three. When the concentration of phenylalanine in the blood is adjusted to an ideal range, a small amount of natural diet can gradually be reintroduced. The reintroduced diet should still adhere to low-protein and low-phenylalanine standards.

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Written by Zeng Hai Jiang

Pediatrics

1min 2sec

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Does phenylketonuria require lifelong dietary control?

Theoretically, phenylketonuria requires lifelong dietary control. Phenylketonuria is caused by a deficiency in phenylalanine hydroxylase in the metabolic pathway of phenylalanine, leading to disordered metabolism of phenylalanine in the liver. Phenylketonuria is the first genetic metabolic disease that can be managed through diet. Since natural foods all contain certain amounts of phenylalanine, once diagnosed, affected children should cease consuming natural diets and should be treated with a low-phenylalanine diet. Treatment with a low-phenylalanine formula should continue at least until the age of 12. When the concentration of phenylalanine in the blood is controlled at an ideal level, natural foods can gradually be reintroduced in small amounts. The reintroduction of food should still follow the principles of low protein and low phenylalanine. The types and amounts of food that each child can add vary individually and are related to the severity of the enzyme deficiency.

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Written by Yan Xin Liang

Pediatrics

1min 1sec

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How is phenylketonuria diagnosed?

How is phenylketonuria diagnosed? Firstly, we start with screening, which is usually done when the newborn is 3 to 7 days old by heel prick to collect peripheral blood, which is then dropped onto specialized filter paper for the determination of phenylalanine concentration. If the concentration of phenylalanine is above the cutoff value, further determination of phenylalanine concentration is conducted. The normal concentration is less than 120 umol/L. If the concentration of phenylalanine is greater than 1200 umol/L, it suggests severe phenylketonuria. Moderate levels are above 360 umol/L, and mild levels are above 120 umol/L but less than 360 umol/L. Furthermore, diagnosis can be confirmed through DNA analysis, currently available tests include screening for mutations in the phenylalanine hydroxylase gene.