Swati Kolpuru
Consultant Pediatrician, USA
First Created: 02/28/2001 


Phenylketonuria is an inborn error of metabolism resulting from a deficiency of phenylalanine hydroxylase that is responsible for processing the essential amino acid phenylalanine and leads to elevated levels of phenylalanine and phenylalanine metabolites. PKU is inherited as an autosomal recessive trait. The mutation that causes PKU is located on chromosome 12. The specific type of mutation varies, resulting in variable severity in the clinical course of the disorder.

Newborn screening is very important for the diagnosis of this disease and has largely eliminated mental retardation caused by this disease.


Symptoms in Newborn:

  • Symptoms are usually absent

  • May be abnormally drowsy or listless

  • Feeding difficulty

  • Light hair, skin, and eyes

  • Eczema

  • Severe mental retardation if not treated

Symptoms in Children:

  • Seizures

  • Nausea

  • Vomiting

  • Hyperactivity

  • Aggressive behavior

  • Clumsy (Poor coordination)

  • Abnormal gait

  • Abnormal posturing

  • Self-injurious behavior

  • Mousy body odor

Pku in the Mother and Pregnancy

Elevated levels of phenylalanine in the mother during pregnancy are teratogenic. It can result in

  • Spontaneous miscarriage

  • IUGR

  • Microcephaly

  • Psychomotor retardation

  • Congenital heart defects

  • Postnatal growth retardation

  • Abnormal neurological findings

  • Mild craniofacial dysmorphic features

Higher the levels of phenylalanine in the mother, the greater are the adverse effects seen in the developing fetus. Also, the abnormalities are more likely to occur if there is no control of phenylalanine levels during the period of embryogenesis and organogenesis in pregnancy.


Early diagnosis is made when a high phenylalanine level and a low tyrosine level are detected during the screening of a newborn. If phenylketonuria runs in the family and DNA is available from the affected family member, amniocentesis or chorionic villus sampling with DNA analysis can be performed to determine whether the fetus has the disorder.

Newborn screening:
Screening for phenylketonuria by measurement of phenylalanine level on a dried-blood spot specimen, collected by heel stick and adsorbed onto filter paper is recommended for all newborns. There are two screening methods that are widely used

  • Guthrie inhibition assay

  • McCamon-Robins fluorometric test

The blood specimen for screening should be obtained at least 12 hours after birth. If done before 12 hours the test needs to be repeated regardless of the result by 2 weeks of age. Premature and sick infants should be tested at or near 7 days of age. Repeat testing is also required if the test results are positive. False-positive in the initial screening may be the result of:

  • Improperly prepared sample

  • Liver immaturity

  • Protein overload in newborns who are fed cow's milk

All parents should be informed regarding the indication for testing, interpretation of test results, and adequate counseling and referral if the test is positive. Dietary treatment should commence as soon as the diagnosis is confirmed

Carrier status identification: Carrier status for phenylalanine deficiency can be determined in at-risk family members with a positive family history. Methods used are:

  • Plasma phenylalanine concentration and phenylalanine/tyrosine ratio

  • DNA analysis if the disease-causing PAH mutation in the family is known

  • Linkage analysis


The only treatment for this disease is a diet restricted in phenylalanine. Phenylalanine must be restricted starting in the first few weeks of life. Infants with PKU should be breastfed along with special formula which is phenylalanine free. A diet should be planned with adequate calories and amino acids to ensure adequate growth and development.

Certain vegetables, fruits, grains, and other low phenylalanine foods should be added later. Regular milk, cheese, eggs, meat fish should never be allowed. Individuals with PKU must remain on a restricted diet throughout life. A restricted diet, started early and well maintained, makes normal development possible, and prevents mental retardation.

Follow Up

Individuals with phenylketonuria should be adequately followed so that phenylalanine blood levels are monitored and the importance of dietary restriction emphasized.

Blood levels are monitored weekly during infancy. After that, they can be done every 3 months. Plasma amino acid concentrations are determined once or twice a year to ensure adequate tyrosine levels.

Phenylketonuria Phenylketonuria 2001-02-28
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