WHAT IS PKU?
PKU (phenylketonuria), in its ìclassicî
form, is a rare, inherited metabolic disease that results in mental
retardation and other neurological problems when treatment is
not started within the first few weeks of life. When a very strict
diet is begun early and well-maintained, effected children can
expect normal development and a normal life span. (Not all elevations
of blood phenylalanine require treatment; any child with a level
less than 6 mg/dl does not need to be on a special diet and is
not risk for mental retardation.)
The Enzyme Deficiency
The disease arises from the absence of a single
enzyme (phenylalanine hydroxylase). This enzyme normally converts
the essential amino acid, phenylalanine, to another amino acid,
tyrosine. Failure of the conversion to take place results in a
buildup of phenylalanine. Through a mechanism that is not well
understood, the excess phenylalanine is toxic to the central nervous
system and causes the severe problems normally associated with
PKU. Not every child has the same degree of enzyme deficiency,
however; some have enough enzyme activity that the diet can be
quite liberal, while others must have the very strict diet. The
nature of the diet for an individual child must be determined
by an experienced PKU treatment program.
Genetics
PKU is carried through an ìautosomal
recessiveî gene. This means that two people who conceive
a child must both be ìsilent carriersî of the gene
in order for there to be a chance that the baby will have PKU.
When two carriers conceive a child, there is a one in four (or
25%) chance for each pregnancy that the baby will have
PKU. The incidence of carriers in the general population is approximately
one in fifty people, but the chance that two carriers will mate
is only one in 2500. Carrier tests are available only through
PKU treatment programs.
Screening
Because of the very positive outcome when children
are treated early and well, newborn screening for PKU is carried
out in every state in the US and in many other countries. Such
screening started in the mid-1960ís. Although PKU affects
only one out of every approximately 10,000 babies born in the
US, there are several hundred babies diagnosed and put on the
diet each year. Children with the severe form of PKU once were
destined to become mentally retarded and spend their lives in
institutions. Children with the disease are now growing up normally.
They are attending college and becoming productive adults as doctors,
lawyers, teachers and engineers because of early diagnosis and
strict adherence to the diet.
Diet Composition
The diet for the most severe form of PKU eliminates
all of the very high protein foods since all protein contains
phenylalanine. This means that all concentrated sources of protein
must be eliminated from the diet in order to limit the amount
of phenylalanine. Except in rare circumstances, the diet does
not allow consumption of meat, fish, poultry, milk, eggs, cheese,
ice cream, legumes, nuts, or many products containing regular
flour. A synthetic formula is used as a nutritional substitute
for the eliminated foods. This formula is very expensive. Fortunately,
a number of states have mandated insurance coverage for the cost,
or in some instances the state health department may provide the
formula to families of affected children. The diet is supplemented
with special low protein foods and weighed or measured amounts
of fruits, vegetables and some grain products. These foods are
allowed in quantities that suit the individual childís
tolerance for phenylalanine; some can have fairly liberal diets
and still maintain good control of blood phe, while others must
have a very strict diet. Some states now have laws mandating insurance
coverage of the expensive special low protein foods.
Diet Maintenance
In the early days of treating PKU, clinicians
believed that the diet could be stopped at an early age (6-10
years). However, it is now known that stopping the diet can result
in a variety of serious problems. These include drops in IQ,
learning disabilities, behavior problems such as hyperactivity
and irritability, neurological problems such as tremors, eczema
(a skin disorder) and personality disorders (including schizophrenia,
panic attacks and agoraphobia). As a result of the problems that
have developed in young people who have discontinued the diet,
it is believed that the diet should be maintained for a lifetime.
It is also now generally believed that keeping blood phenylalanine
levels in the range of 2-6 mg/dl is the safest, especially in
infancy and early childhood. Frequent blood monitoring to achieve
this goal is critical. All individuals, male or female, who have
stopped the diet would be wise to return to the diet under the
supervision of a PKU treatment program.
Maternal PKU
Returning to the diet is especially important
for young women with PKU who want to become pregnant. Women with
PKU who have high levels of phenylalanine in their blood have
a very large probability of harming their unborn baby. ìMaternal
PKUî has become a significant concern as young women who
were once taken off the diet are now reaching childbearing ages
in ever-increasing numbers. A large collaborative study has shown
that returning the woman to diet before conception and keeping
blood phenylalanine levels below 6 mg/dl results in the best outcome
for the baby. Many children born to mothers with PKU are developing
normally because of early and strict treatment.
PKU Treatment Programs
There is at least one specialized PKU treatment
program in most US states. In some less-populated states, families
may need to travel to another nearby state, or the treatment may
be coordinated through a state health department. All children
with PKU ideally should be followed by a specialized PKU program
with adequate monitoring facilities. Information about the nearest
PKU treatment program can be obtained through the state newborn
screening program.
By Virginia E. Schuett, MS, RD
Director, National PKU News
6869 Woodlawn Ave. NE #116
Seattle, WA 98115-5469
tel: 206-525-8140
fax: 206-525-5023