Phenylketonuria

Phenylketonuria (PKU) is a rare inborn metabolic disease where body is unable to break down phenylalanine, an amino acid that is one of the building blocks of proteins. In classical phenylketonuria, there is a deficiency of the enzyme phenylalanine hydroxylase and the metabolism of phenylalanine (phe) is disrupted, so it accumulates in tissues and organs, with developing brain being the most vulnerable to excess phenylalanine. In order to prevent the accumulation of phenylalanine in the body, it is necessary to implement a diet without or with as little as possible phenylalanine, which means a low-protein diet.

As the symptoms of untreated phenylketonuria occur very early, a low-protein diet should be started in the first month of a baby's life. The diet is most rigorous in the first years of a child's life, because the growth and development of the child's brain are intense at that time. Any potential increase of phenylalanine blood level must be corrected immediately so that the brain continues to develop properly, which is necessary for a child in order to learn to sit, walk, get out of diapers and speak.

How to get a diagnosis?

Neonatal screening for phenylketonuria is carried out throughout Serbia. Tests are performed at all maternity hospitals in the country and samples are sent to two centers – the Institute for Child and Youth Healthcare of Vojvodina in Novi Sad and the Mother and Child Healthcare Institute of Serbia “Dr Vukan Cupic” in Belgrade.

The test is called the Guthrie test, because in 1960 American microbiologist Robert Guthrie developed an easy way to diagnose phenylketonuria. The Guthrie test is performed by pricking the baby's heel (or toddler’s finger) and putting a few drops of blood on a special filter paper cards, called Guthrie cards.

Owing to neonatal screening and early diagnosis, prompt implementation of treatment is enabled and therefore manifestation of this metabolic disease is avoided. If left undetected and untreated, phenylketonuria has a severe clinical picture. Untreated phenylketonuria is accompanied by numerous nervous system damages, seizures and severe mental retardation.

Many countries, including Serbia, provide patients with the most basic food (special flour, special milk and egg substitutes), as well as protein supplements, replacing all the forbidden food (meet, eggs, fish, rice, etc.).

Types of phenylketonuria:

• Classic phenylketonuria (blood phenylalanine level is over 20 mg/dl or 1200 µmol/l – a strict diet is required)
• Moderate (mild) phenylketonuria (blood phenylalanine level is over 6 mg/dl and under 20 mg/dl or 600–1200 µmol/l – a diet is required, but not as rigorous)
• Hyperphenylalaninemia (blood phenylalanine level is up to 6 mg/dl or 600 µmol/l – in some countries, patients are initially put on a diet which is later discontinued. In some countries, patients are only monitored by regular blood analysis and if there is a high value, they are put on a diet).

In Serbia patients with hyperphenylalaninemia are also put on a diet, but their daily intake of phenylalnine is higher than that of patients with classic and mild phenylketonuria.