Fenilketonuria

01Pathophysiology

Phenylketonuria results from a deficiency of phenylalanine hydroxylase (PAH), the hepatic enzyme responsible for converting phenylalanine to tyrosine. This is the rate-limiting step in phenylalanine catabolism, and it requires the cofactor tetrahydrobiopterin (BH4) to function. When PAH is absent or severely reduced, phenylalanine accumulates in the blood and crosses the blood-brain barrier, where it exerts direct neurotoxic effects on the developing brain.

The accumulation of phenylalanine leads to competitive inhibition of large neutral amino acid transport across the blood-brain barrier. This reduces the availability of tyrosine, tryptophan, and other amino acids needed for neurotransmitter synthesis (dopamine, serotonin, norepinephrine). The deficit in dopamine and melanin (both derived from tyrosine) explains two hallmark findings: the neurological deterioration and the fair complexion (light skin, blonde hair, blue eyes) seen in untreated patients. Melanin production depends on tyrosine as its precursor, so tyrosine deficiency leads to hypopigmentation.

The musty or mousy body odor is caused by the accumulation of phenylacetic acid, a byproduct of phenylalanine metabolism through alternative transaminase pathways. When the primary hydroxylation pathway is blocked, phenylalanine is shunted into transamination and decarboxylation, producing phenylpyruvate, phenylacetate, and phenyllactate. These are the "phenylketones" excreted in urine, giving the disease its name.

A critical teaching point: the baby is normal at birth because maternal enzymes clear phenylalanine during fetal life via placental transfer. Damage only begins postnatally once the infant starts consuming phenylalanine through breast milk or formula. This is precisely why newborn screening is essential and why early dietary intervention prevents intellectual disability entirely.

In a small subset of cases (approximately 2%), the defect is not in PAH itself but in the synthesis or recycling of BH4. This distinction matters because BH4 is also required for tyrosine hydroxylase and tryptophan hydroxylase, meaning BH4-deficient patients will have more severe neurotransmitter deficiencies that a phenylalanine-restricted diet alone cannot correct.

02Classification and Clinical Manifestation

Maternal PKU is another high-yield concept. A woman with PKU who does not maintain strict dietary control before and during pregnancy exposes the fetus to high phenylalanine levels. The fetus itself may not have PKU, but the teratogenic effect of maternal hyperphenylalaninemia causes microcephaly, congenital heart defects, intrauterine growth restriction, and intellectual disability in the offspring. The vignette typically presents a woman with a history of PKU who stopped her diet and gives birth to an affected infant.

03Diagnostic Workup

The diagnostic approach begins with newborn screening, which is mandatory in virtually all countries. The original Guthrie bacterial inhibition assay has been largely replaced by tandem mass spectrometry (MS/MS), which simultaneously measures phenylalanine and tyrosine, providing both the absolute level and the ratio. Screening must occur after the infant has been fed protein (ideally 24-48 hours of life), because a sample taken too early (before phenylalanine accumulates from dietary intake) can yield a false negative.

When the screen is positive, the next step is a quantitative serum amino acid analysis to confirm the phenylalanine level and classify the severity. The phenylalanine-to-tyrosine ratio being elevated helps differentiate PAH deficiency from transient or benign hyperphenylalaninemia.

Every confirmed case should undergo a BH4 loading test to rule out cofactor deficiency. This is done by giving an oral dose of BH4 and measuring the phenylalanine response over 24 hours. If phenylalanine drops substantially (typically > 30%), the patient is considered BH4-responsive. This has direct treatment implications: BH4-responsive patients can use sapropterin and may tolerate a less restricted diet. If BH4 deficiency is suspected, urine pterin profiling and erythrocyte DHPR enzyme assay are performed to identify the exact metabolic block.

04Management and Treatment

The cornerstone of PKU management is a lifelong phenylalanine-restricted diet initiated as early as possible, ideally within the first 7-10 days of life. The goal is to maintain blood phenylalanine between 2-6 mg/dL (120-360 micromol/L). This requires severe restriction of high-protein foods (meat, dairy, eggs, nuts, legumes) and use of a phenylalanine-free medical formula that supplies adequate protein and calories.

Tyrosine becomes a conditionally essential amino acid in PKU because the conversion pathway from phenylalanine is blocked. Therefore, tyrosine supplementation through medical formula is standard.

For patients identified as BH4-responsive on loading test, sapropterin (synthetic BH4) at 10-20 mg/kg/day can increase residual PAH activity enough to allow a more liberalized diet. A formal trial period of 1-4 weeks with phenylalanine monitoring is needed to confirm responsiveness before committing to long-term therapy.

In BH4 deficiency, diet alone fails because the cofactor is also required for catecholamine and serotonin synthesis. These patients need BH4 replacement along with L-DOPA/carbidopa (for dopamine) and 5-hydroxytryptophan (for serotonin), plus folinic acid if the defect involves dihydropteridine reductase (which also affects folate metabolism).

For maternal PKU, the critical teaching point is that dietary control must begin before conception. If a woman with PKU becomes pregnant with uncontrolled phenylalanine levels, the fetal damage (cardiac defects, microcephaly) occurs during the first trimester and is irreversible.

Pegvaliase is a newer enzyme substitution therapy approved for adults (> 16 years) with uncontrolled PKU. It is administered subcutaneously and works by degrading phenylalanine through an alternative enzymatic pathway (phenylalanine ammonia lyase). It carries a risk of anaphylaxis and requires an observation protocol during initiation.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common trap on this topic involves timing of the newborn screen. A vignette may describe a baby screened within the first few hours of life whose test comes back normal, then later presents with developmental delay and a musty odor. Students who do not recognize that early screening before adequate protein intake can produce a false-negative result will miss the diagnosis. The test must be performed after the infant has been feeding for at least 24-48 hours.

Another frequently tested trap is the distinction between classic PKU and BH4 deficiency. Both produce elevated phenylalanine on screening, but BH4 deficiency will not respond to dietary restriction alone. If a vignette describes a child on a phenylalanine-restricted diet who continues to have seizures and progressive neurological decline, the answer is BH4 deficiency, and the next step is a BH4 loading test or urine pterin analysis.

Maternal PKU is a perennial favorite. The vignette will describe a woman with known PKU who discontinued her diet, presenting with a newborn who has microcephaly and congenital heart defects. The infant's own phenylalanine level may be normal (the child may not carry two copies of the mutation). The damage was caused by in utero exposure to the mother's elevated phenylalanine, not by the infant's own metabolism.

Finally, students sometimes confuse the biochemical pathway. Phenylalanine hydroxylase converts phenylalanine to tyrosine, not the other way around. The downstream consequences (reduced melanin, reduced catecholamines) all flow from tyrosine deficiency, and remembering this single enzymatic block connects every clinical feature: the fair complexion, the neurological damage, and the rationale for tyrosine supplementation.