Phenylketonuria (PKU)

An inborn error of metabolism happens when an infant is born with a defect in one of the enzyme systems that metabolize each of the 8-10 essential amino acids (Stanfield and Hui, 2010).  This defect causes products to accumulate in the blood or urine.  The type of defect will determine which products accumulate and which side effects the accumulation will have on the child.

Phenylketonuria (PKU) is an inborn error of metabolism involving phenylalanine and tyrosine.  Both of these essential amino acids utilize the enzyme phenylalanine hydroxylase, and when this enzyme is absent or low-activity, the body cannot change phenylalanine to tyrosine and phenylalanine and phenylpyruvic acid start to accumulate in the blood and urine (Stanfield and Hui, 2010).

The Guthrie Test is a heel prick done when the infant is 2-5 days old.  The doctor uses this test to measure the phenylalanine levels in the infant’s blood.  Normal blood contains 1-2mg phenylalanine/100ml of plasma; PKU blood contains 15-30mg of phenylalanine/100m of plasma (Stanfield and Hui, 2010).  The test results are more accurate if the doctor waits until the infant has been drinking breast milk or formula for at least 2-3 days; there could be an incorrect result if the test is done within 24 hours of birth (WebMD, 2010).   A second, and sometimes a third, test is done at approximately 2 weeks and 4 weeks of age to monitor

phenylalanine levels in the blood, especially if the first test results were positive for PKU. 

It is important to diagnose and treat PKU early, which is why a doctor will perform 2 or 3 tests in the first 3 months of life.  Once PKU is diagnosed, the doctor will continue to monitor the phenylalanine levels in the blood to ensure the modified diet is effective.  If phenylalanine levels are allowed to build up in the blood, the child can suffer irreversible brain damage, seizure, and intellectual disability (WebMD, 2010).  The symptoms of untreated PKU are as follows: 

·      The infant loses interest in his surroundings by 3-6 months.

·      The child is obviously developmentally delayed by age 1.

·      The child is irritable.

·      The child displays behavior problems.

·      The child may smell musty.

·      The child may have dry skin or rashes.

·      The child will suffer seizures.

·      The child will be physically well developed.

·      The child will have blonder hair than the rest of the family.

(March of Dimes, 2008). 

The lighter hair and eyes is due to the lack of ability to convert phenylalanine to tyrosine.  Tyrosine is responsible for making pigments, and if the child cannot create tyrosine or cannot create enough tyrosine, she will have less pigmentation than a person who does (Stanfield and Hui, 2010).

PKU infants are put on a strict diet limiting phenylalanine intake; however, some phenylalanine is necessary for normal growth and development (Stanfield and Hui, 2010).  As phenylalanine is found in most animal products, a PKU child can have an allotted amount of milk.  For healthy developmental development, the child must continue with low-protein from food sources and use a special high-protein, low-phenylalanine formula while developing (WebMD, 2008). 

Once a child is eating whole foods, the phenylalanine, protein, and calorie contents of each food introduced must be known in order to group foods into exchange lists (Stanfield and Hui, 2010).  The age a PKU child is allowed to relax the diet restrictions varies from source to source.  Normal diet may resume at age 5 with a chance of developmental problems in the teenage years (Stanfield and Hui, 2010).  However, other studies have found that children who followed the low-phenylalanine diet until at least 10 had less loss of IQ than those who reverted to a normal diet at age 5 or 8 (Smith, et al, 1991).  The best recommendation would be to continue with the PKU restrictions throughout the teenage years.

 

References:

March of Dimes (2008)  PKU (Phenylketonuria).  Retrieved on February 11, 2012, from www.marchof dimes.com/baby/birthdefects_pku.html

Smith, M; Beasley, G; Ades, AE (1991)  Effect on intelligence of relaxing the low phenylalanine diet in phenylketonuria.  Retrieved on February 11, 2012, from www.ncbi.nlm.ih.gov/pmc/articles/PMC1792859

Stanfield, Peggy and Hui, YH (2010)  Nutrition and Diet Therapy, 5^th Edition.  Jones and Bartlet

WebMD (2010) Phenylketonuria (PKU) Test.  Retrieved on February 11, 2012, from www.webmd.com/parenting/baby/phenylketonuria-pku-test