Definition and epidemiology
Phenylketonuria (PKU) is an autosomal recessive disorder caused by mutations in the PAH gene encoding phenylalanine hydroxylase, the hepatic enzyme that converts phenylalanine to tyrosine. Loss of this activity causes phenylalanine to accumulate in blood and tissues, which is neurotoxic to the developing brain.
- Incidence is approximately 1 in 10,000-15,000 births in most Western populations, with wide variation by ethnicity and region.
- It is one of the most common inborn errors of amino acid metabolism and one of the first conditions targeted by universal newborn screening programs worldwide.
- Both sexes are affected equally, consistent with autosomal recessive inheritance.
- A minority of cases (roughly 1-3%) are due to defects in tetrahydrobiopterin (BH4) synthesis or regeneration rather than the PAH enzyme itself ("malignant" or BH4-responsive/non-responsive variant PKU).
- Autosomal recessive deficiency of phenylalanine hydroxylase causes PKU.
- Detected by universal newborn screening via tandem mass spectrometry.
- Untreated PKU causes intellectual disability, musty odor, fair skin/hair, and seizures.
- Treatment is lifelong dietary phenylalanine restriction plus tyrosine supplementation, started early.
- Sapropterin helps BH4-responsive patients; pegvaliase is used in uncontrolled adult disease.
- Maternal PKU with poor control causes fetal microcephaly and congenital heart defects.
- BH4 deficiency mimics PKU biochemically but requires neurotransmitter precursor and BH4 replacement, not just diet.
- Brain damage from hyperphenylalaninemia is preventable but irreversible once established.
Etiology and risk factors
- Classic PKU: biallelic loss-of-function mutations in PAH, resulting in minimal or absent enzyme activity.
- Variant/mild hyperphenylalaninemia: partial PAH activity due to less severe mutations, producing milder phenotypes.
- BH4 deficiency (non-PAH PKU): mutations in genes for BH4 synthesis or regeneration (e.g., PTS, GCH1, QDPR); BH4 is a required cofactor for phenylalanine hydroxylase as well as tyrosine and tryptophan hydroxylases, so deficiency also impairs neurotransmitter synthesis.
- Risk factors: consanguinity, family history of PKU, and being born in a population with higher carrier frequency.
- Maternal PKU: women with PKU who are poorly controlled during pregnancy expose the fetus to high phenylalanine levels, causing a distinct teratogenic syndrome (not the infant inheriting PKU itself, but a toxic in-utero exposure).
Pathophysiology
Phenylalanine hydroxylase normally converts phenylalanine to tyrosine in the liver, using BH4 as a cofactor. When this enzyme (or its cofactor) is deficient, phenylalanine accumulates and is shunted into alternative pathways, producing phenylpyruvate and phenylacetate, which are excreted in urine and give it a characteristic musty or mousy odor.
- High phenylalanine competes with other large neutral amino acids (tyrosine, tryptophan) for transport across the blood-brain barrier via the LAT1 transporter, reducing their brain levels.
- Reduced tyrosine and tryptophan impair synthesis of dopamine, norepinephrine, and serotonin, contributing to neurocognitive dysfunction.
- Elevated phenylalanine also disrupts myelin formation and protein synthesis in the developing brain, which is why the first few years of life are the critical window for neurologic injury.
- Tyrosine deficiency additionally reduces melanin synthesis, producing the classic fair skin, light hair, and blue eyes seen in untreated patients.
Clinical features
Infants are normal at birth because maternal PAH clears phenylalanine in utero; features emerge over the first months if untreated.
- Musty or mousy body and urine odor
- Fair skin, light hair, and blue eyes (due to decreased melanin from tyrosine deficiency)
- Progressive intellectual disability and developmental delay if untreated
- Microcephaly
- Seizures
- Eczema-like skin rash
- Behavioral abnormalities: hyperactivity, autistic features, self-injurious behavior
- Mild tremor, hypertonia, and gait abnormalities later in untreated or poorly controlled disease
Maternal PKU syndrome in offspring of poorly controlled pregnant women with PKU includes microcephaly, congenital heart defects, intrauterine growth restriction, and intellectual disability, independent of the infant's own genotype.
Diagnosis
Diagnosis in developed countries is almost always made through universal newborn screening, typically using tandem mass spectrometry on a heel-prick blood sample obtained 24-72 hours after birth.
- Screening detects elevated blood phenylalanine, usually with an elevated phenylalanine-to-tyrosine ratio to improve specificity.
- A positive screen requires confirmatory quantitative plasma amino acid analysis.
- Diagnostic threshold is generally a persistently elevated plasma phenylalanine above the normal range (typically greater than 120 micromol/L on repeat testing, with classic PKU usually well above 1000 micromol/L).
- BH4 loading test or molecular/genetic testing differentiates classic PAH-deficient PKU from BH4-responsive or BH4-deficient variants, which changes management.
- Urine organic acids may show elevated phenylpyruvate and phenylacetate but are not used as the primary diagnostic test.
- Prenatal diagnosis via genetic testing is possible in families with a known pathogenic variant.
Differential diagnosis
| Condition | Distinguishing feature |
|---|---|
| Transient/benign hyperphenylalaninemia | Mild elevation, normalizes, no PAH mutation |
| Tyrosinemia | Elevated tyrosine, hepatic/renal involvement, succinylacetone positive |
| Maple syrup urine disease | Elevated branched-chain amino acids, sweet odor, acute encephalopathy |
| Homocystinuria | Elevated homocysteine/methionine, lens dislocation, thromboembolism |
| BH4 (cofactor) deficiency | Elevated phenylalanine but poor response to phenylalanine-only diet, neurologic deterioration despite dietary control |
| Galactosemia | Presents with jaundice, hepatomegaly, cataracts, not elevated phenylalanine |
Management
The cornerstone of treatment is lifelong dietary phenylalanine restriction started as early as possible, ideally within the first days to weeks of life, since neurologic damage from hyperphenylalaninemia is largely preventable but not reversible once established.
- Low-phenylalanine diet: restriction of high-protein natural foods (meat, dairy, eggs, nuts) with a phenylalanine-free medical formula to supply other amino acids, calories, vitamins, and minerals.
- Tyrosine supplementation since tyrosine becomes conditionally essential.
- Regular monitoring of plasma phenylalanine levels, especially frequent in infancy and childhood, with lifelong follow-up even in adulthood ("diet for life" is now the consensus across US and European guidelines, moving away from older practice of relaxing diet after childhood).
- Sapropterin (synthetic BH4): used in BH4-responsive patients to lower phenylalanine levels and allow some dietary liberalization.
- Pegvaliase (PEG-conjugated phenylalanine ammonia lyase): an enzyme substitution therapy approved for adults with uncontrolled phenylalanine levels despite dietary management.
- Large neutral amino acid (LNAA) supplementation: sometimes used as an adjunct, particularly in adults with difficulty maintaining strict dietary control, to competitively reduce phenylalanine transport into the brain.
- Maternal PKU management: strict phenylalanine control before conception and throughout pregnancy is essential to prevent maternal PKU syndrome in the fetus.
- Genetic counseling should be offered to affected individuals and families given autosomal recessive inheritance.
Complications and prognosis
- Untreated classic PKU leads to severe, irreversible intellectual disability, microcephaly, seizures, and behavioral disturbance.
- With early diagnosis via newborn screening and consistent dietary control, cognitive and developmental outcomes are generally normal or near-normal.
- Even treated patients may have subtler issues with executive function, processing speed, and mood/anxiety disorders if phenylalanine control is suboptimal.
- Discontinuing dietary restriction in adolescence or adulthood (previously common practice) is now discouraged, as it is associated with decline in neurocognitive and psychiatric functioning; lifelong dietary adherence is the current consensus recommendation.
- Poor dietary control during pregnancy in a woman with PKU causes maternal PKU syndrome in the fetus, including congenital heart disease and intellectual disability, independent of the fetus's own PAH genotype.
PKU = "Phenylalanine Kept Up" — think of a baby with fair hair, musty smell, and a special formula: the enzyme (PAH) is down, so Phenylalanine stays Up in the blood.
Practice questions
1. A 2-week-old infant has a positive newborn screen showing markedly elevated phenylalanine with a normal tyrosine level. The parents are first cousins. Which of the following is the most appropriate next step?
2. A 6-year-old girl with known classic PKU has been following a strict low-phenylalalanine diet since infancy and has normal development. Her parents ask if she can stop the special diet now that she is older and doing well. What is the most appropriate response?
3. A 24-year-old woman with PKU who has been off her strict diet since adolescence is planning a pregnancy. She asks about risks to her future child. Which statement is most accurate?
Frequently asked questions
What causes phenylketonuria?
PKU is caused by autosomal recessive mutations in the PAH gene, which encodes phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine. Without this enzyme, phenylalanine builds up in the blood and brain. A small subset of cases result from defects in BH4, the cofactor the enzyme requires, rather than the enzyme gene itself.
What are the first symptoms of PKU?
Infants with PKU appear normal at birth because maternal metabolism clears phenylalanine before delivery. Without treatment, symptoms develop over the following months, including a musty body odor, fair skin and hair, developmental delay, and eventually seizures and intellectual disability. This delayed onset is exactly why newborn screening, not waiting for symptoms, is essential.
How is PKU diagnosed?
PKU is diagnosed through universal newborn screening, which measures blood phenylalanine (and the phenylalanine-to-tyrosine ratio) from a heel-prick sample taken a day or two after birth. A positive screen is confirmed with quantitative plasma amino acid testing, and further genetic or BH4-loading testing distinguishes classic PAH-deficient PKU from BH4-related variants.
Is PKU curable?
PKU is not curable but it is highly manageable. Lifelong dietary phenylalanine restriction, started shortly after birth, allows normal or near-normal cognitive development. Newer therapies like sapropterin and pegvaliase can help some patients control phenylalanine levels, but dietary management remains the lifelong foundation of treatment rather than a cure.
PKU vs tyrosinemia — what is the difference?
PKU involves elevated phenylalanine due to phenylalanine hydroxylase deficiency, causing neurologic and cognitive problems if untreated. Tyrosinemia involves elevated tyrosine (and often succinylacetone) due to defects further down the tyrosine degradation pathway, causing liver and kidney damage rather than primarily neurologic disease. Newborn screening amino acid panels distinguish them by which amino acid is elevated.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: E70.0
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.