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Huntington Disease
Parts from eMedicine Journal, September
11 2001, Volume 2, Number 9. Very thorough workup on HD Authored by Fredy J Revilla, MD, Instructor,Department of Neurology,
Washington University in Saint Louis School of Medicine
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What about testing for HD before
symptoms appear?
In other cases, a family member may die of another cause before he or she begins to show signs of
HD. Sometimes, the cause of death for a relative may not be known, or the family is not aware of a relative's death.
Adopted children may not know their genetic heritage, or early symptoms in an individual may be too
slight to attract attention. These are among the many complicating factors that reflect the complexity of diagnosing HD.
It is now possible to accurately identify persons carrying the HD gene before symptoms appear. Discovery
of the HD gene has led to an accurate blood test that is applicable to the majority of people at risk for HD.
The genetic testing itself involves donating a small sample of blood that is screened in the laboratory
for the presence or absence of the HD gene mutation. Testing may require a sample of DNA from a closely related affected relative,
preferably a parent, for the purpose of confirming the diagnosis of HD in the family. This is especially important if
the family history for HD is unclear or unusual in some way.
Results of the test should be given only in person and only to the individual being tested. Test results
are confidential. Regardless of test results, follow-up visits are recommended.
In order to protect the interests of minors, including confidentiality, testing is not recommended
for those under the age of 18 unless there is a compelling medical reason (for example, the child is exhibiting symptoms). |
Can testing before birth (prenatal testing)
be done for HD?
Testing of a fetus (prenatal testing) presents special challenges and risks and some centers
do not perform genetic testing on fetuses. Because a positive test result using direct genetic testing means the at-risk parent
is also a gene carrier, at-risk individuals who are considering a pregnancy are advised to seek genetic counseling prior to
conception.
Some at-risk parents may wish to know the risk to their fetus but not their own. In this situation,
parents may opt for prenatal testing using linked DNA markers rather than direct gene testing. In this case, testing does
not look for the HD gene itself but instead indicates whether or not the fetus has inherited a chromosome 4 from the affected
grandparent or from the unaffected grandparent on the side of the family with HD.
If the test shows that the fetus has inherited a chromosome 4 from the affected grandparent,
the parents then learn that the fetus's risk is the same as the parent (50-50), but they learn nothing new about the parent's
risk. If the test shows that the fetus has inherited a chromosome 4 from the unaffected grand-parent, the risk to the fetus
is very low (less than 1%) in most cases.
Another option available to parents is in vitro fertilization (IVF) with pre-implantation screening.
In this procedure, embryos are screened to determine which ones carry the HD mutation. Embryos determined not to have the
HD gene mutation are then implanted in the woman's uterus.
In terms of emotional and practical consequences, not only for the individual taking the test but
for his or her entire family, testing is enormously complex and has been surrounded by considerable controversy. For example,
people with a positive test result may risk losing health and life insurance, suffer loss of employment, and other liabilities.
Persons undergoing testing may wish to cover the cost themselves, since coverage by an insurer may
lead to loss of health insurance in the event of a positive result, although this may change in the future.
With the participation of health professionals and people from families with HD, scientists have developed
testing guidelines. All individuals seeking a genetic test should obtain a copy of these guidelines, either from their
testing center or from the organizations listed below.
These organizations have information on sites that perform testing using the established procedures and they strongly recommend that individuals avoid testing that does not adhere
to these guidelines. |
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