Inborn errors of amino acid metabolism are the commonest of all metabolism related disorders. Phenylketonuria (PKU) is one of them. It is a rare inherited condition which is characterized by abnormal rise of phenylalanine, a type of amino acid that you usually obtain from the food that you eat. Physiologically amino acids are not only building block of protein; they have many other functions as well.
Phenylalanine is present in protein food and in artificial sweeteners. In your body, phenylalanine is converted into tyrosine a type of amino acid with the help of an enzyme called phenylalanine hydroxylase.Tyrosine is essential for production of neurotransmitters in brain such as dopamine, epinephrine and norepinephrine. PKU develops when phenylalanine cannot be converted into tyrosine due to deficiency of this enzyme.
Phenylketonuria is an important cause of mental retardation in children. Early diagnosis and treatment is the key in preventing damage to the brain and constellation of other symptoms.
Causes Of Phenylketonuria
Phenylketonuria is a rare genetic inherited disorder caused due to a defect in certain gene called PAH. This defect leads to absence of an enzyme called phenylalanine hydroxylase which is needed to break phenylalanine.Since phenylalanine is present in protein containing food, it can build to dangerous level if a person is deficient in the enzyme.
Phenylketonuria develops in a child when both the parents carry a copy of defective gene. The child may be a carrier if only one parent has defective gene.
Signs And Symptoms Of Phenylketonuria
Phenylketonuria symptoms can be mild or severe. The severe form of this condition is also called classic PKU. For first few months, infant with PKU may be apparently normal, the child can suffer from constellation of symptoms; mental deficiency being the leading symptom which may be permanent.
Majority of affected children are blue eyed or blonde with fair skin. Other symptoms are:
- Mental retardation
- Hyperactive child
- Seborrhea and skin eczema.
- Mousy odor from skin, urine, and breath of the child due to excessive accumulation of phenylalanine.
- The size of head is abnormally small.
- Low birth weight and physical growth retardation.
If a woman who is having phenylketonuria becomes pregnant and is not following a diet meant for PKU patients, the fetus inside her womb is exposed to high phenylalanine. Hence there is increased risk of mental disability, congenital deformities, small sized head and neuro-developmental delay.
Diagnosis Of Phenylketonuria
It has become a routine practice in United Sates to test the blood of newborn child for phenylketoneuria who is born in a hospital. Blood test is performed 2 to 3 days after the birth of child. If the tests indicate the child having more phenylalanine in his blood sample, additional specialized tests will be recommended to confirm the diagnosis. These tests may include alteration in gene PAH which causes phenylketonuria.
If the initial test is not performed, and the symptoms indicate phenylketoneuria such as delay in development, hyperactivity, mental disability, blood sample may be taken to analyze the enzyme that is responsible for conversion of phenylalanine into tyrosine.
Dietary Treatment For Phenylketonuria
Once the patient is diagnosed with phenylketoneuria, he is recommended a special diet that is low in protein as phenylalanine is present in all protein rich food. The diet is supplemented with tyrosine.
Patient is fed with the diet that is low in protein, but at the same time it has to be rich in other nutrients that are necessary for growth and normal health. The amount is adjusted according to the age and the sensitivity to phelylalanine.
The child cannot be breast fed; instead a special formula product is given to the child. Once the child starts eating solid food, certain foods are avoided in his diet. This includes foods such as egg, fish, pulses, milk, chicken, meat. Supplementary foods are given to the child that contains all amino acids except phenylalanine.
Children and adults suffering from phenylketonuria should avoid beverages and soft drinks that contain artificial sweeteners, because they release phenylalanine during the process of digestion.