From the 6/01 Juvenile HD Handbook states: (all taken directly from this book)
For reasons that only became clear after the gene responsible for HD was
discovered in 1993, individuals with a very early onset of HD are far less
likely to have an affected father than an affected mother. It is very
unlikely for HD to appear in a child whose parent was not also affected
with HD. If this situation appears to be present, the physician should consider
diagnosis other than HD.
Occasionally an HD-affected child without an affected parent can be
explained by the early death of a parent (before the parent's symptoms
were evident), misdiagnosis or lack of diagnosis in a parent who was
affected, non-paternity (a biological father who is not the same as the
apparent father), onset of symptoms in the child before the parent's onset,
Documenting the diagnosis of HD in other relatives can be helpful to the
physician as the child is evaluated for HD. A parent who suspects that his
or her child has juvenile HD can help the physician by assembling information
about the family history.
If the family history is missing because the child was adopted, it may still be
possible to obtain the missing information if it is important to the child's
diagnosis. Adoption agencies and county or regional social services departments,
when given an understanding of the serious nature of HD and its hereditary
pattern, may be able to contact the birth parents to obtain more information.
Similarly, a mother or family who becomes aware of the family history of HD
should be honest and open with the adoption agency, so the adopting parents are
aware of the child's genetic risks and are able to plan appropriately.
DIAGNOSTIC GENETIC TESTING IN CHILDREN
The ability to detect changes in the HD gene itself has made confirmation of the
diagnosis of HD much simpler. The HD gene can be isolated from a blood sample,
and examined chemically; the abnormality in the gene that causes HD is called a
"CAG repeat expansion." The HD gene normally has a variable number of
"CAG repeats" - any number up to 35 repeats is normal.
There is a relationship between the repeat numbers and the age that HD
symptoms begin, so that higher repeat numbers are associated with
younger ages of onset.
Most adults with HD have between 40-50 CAG repeats in their abnormal
HD gene. Usually, juvenile HD is associated with CAG repeat numbers
of 50 or higher, although it is not possible to define rigid boundaries.
Very young age of onset are associated with very high CAG repeat numbers;
children with HD onset at age 2-3 years and with over 100 CAG repeats
have been reported.
The gene test is close to 100% accurate. If the tests show two normal
HD genes, the child will never develop HD and is not at-risk for passing
HD on to his children.
If the test shows an abnormal HD gene, the child will someday develop HD.
The gene test, however, cannot predict when a particular person's
symptoms will begin.
Occasionally, individuals will have a CAG repeat length in the 36-39
range, which may or may not be associated with the development of HD
symptoms during a normal life span. Results in this "intermediate range"
are not usually a factor when testing children for possible symptomatic HD.
And rarely, very high CAG repeat numbers (over 100 CAG repeats) are
not detected by the standard gene test. If HD is strongly suspected in a
very young child who has a normal gene test results, the physician may
want to contact the laboratory or a genetic counselor to discuss the
possibility of a special analysis to look for very large CAG repeat numbers.
The potential risks of testing a child for the HD gene inappropriately or
prematurely cannot be emphasized enough. Individuals who develop HD
have the CAG repeat expansion in one of their HD genes from the moment
of conception - years or decades before their symptoms begin.
There are two ways in which a premature gene test can be misleading or
damaging to a child's care.
First, an abnormal gene test result may incorrectly be assumed to "explain" a
child's symptoms, when in fact the symptoms are not clearly related to HD.
For example, an adult whose gene test was felt to "explain" his blurry vision
and headaches was incorrectly diagnosed as having HD and, as a result, the
diagnosis of his pituitary tumor was delayed.
Secondly, it is possible that a gene test will show a small CAG repeat
expansion, one that is likely to be associated with adult-onset HD but not
juvenile onset HD. This is equivalent to a predictive gene test and does
not help to explain the child's current symptoms.
Due to the very sensitive nature of the gene test results, it is important for
counseling to occur before the results are given, so there are no misunderstandings
about their significance. If the physician is unable or does not have the time
to explain the gene test in details, a genetic counselor may be asked to help
with this part of the process.
TESTING CHILDREN WHO DO NOT HAVE SYMPTOMS
When one child has been diagnosed with HD, parents may want to have their
other children tested as well. Testing of a person who does not have symptoms
of HD is called predictive testing, in order to distinguish it from diagnostic
testing for a person who has symptoms suggestive of HD.
Although, at first thought, it may seem reassuring for parents to find out
that their other children do not carry the HD gene, it is very important to
consider the complex potential effects of the tests results on the entire
family, as well as on the individual or individuals being tested.
The risks of premature genetic diagnosis of HD have already been discussed
and genetic testing experts believe that predictive tests should be reserved
for individuals who are able to understand the potential risks and benefits
of the test and who are able to give informed consent.
Experience in the United States has shown that most adults at-risk for HD
do not choose to undergo predictive genetic testing, so a parent who requests
predictive testing for a child is most likely doing something that the child
would not want if he or she were able to make the choice.
In addition, at this time, there is no medical advantage to knowing that
someone carries the HD gene - treatments that prevent or delay the
disease have not been developed yet.
For all these reasons, most genetic professionals in North American
decline parental requests for predictive tests on their asymptomatic
children. Occasional exceptions might be made for adolescents in
adult situations, such as an "emancipated minor" or a married teenager.
In the United States, potential adoption is not usually felt to be an
appropriate indication for HD predictive testing, because of the potential
for social, financial, educational, insurance and employment discrimination
based on the test results and the lack of medical treatment or care to
balance the potential social harms. Other countries may have different