What is Canavan Disease?
Newborns with Canavan disease appear healthy until between three and nine months of age when subtle changes are noticeable, such as visual inattentiveness or an inability to grasp objects, roll over and perform other motor tasks. The child eventually becomes blind, but hearing remains sharp, so that affected children continue to respond to the voices of their caregivers. As the child grows, the presentation and progression of Canavan symptoms vary from one child to the next. In general, difficulties that surface include weak muscles that keep children from sitting or crawling, seizures and eating problems. Children with Canavan disease usually die in infancy or early childhood, although some survive into adolescence.
Canavan disease (CD) belongs to a group of conditions known as leukodystrophies, characterized by defects in myelin, commonly known as the "white matter" in the brain. Myelin protects nerves and allows messages to be sent to and from the brain. All CD symptoms are explained by the progressive loss of myelin.
As with Tay-Sachs, children with CD have an enzyme deficiency. The enzyme, aspartoacylase, is responsible for the break-down of a particular chemical in the brain. Without this enzyme, the chemical accumulates and causes the destruction of myelin.
Why is a baby born with Canavan?
Canavan, like Tay-Sachs, is an inherited disease, transmitted from parent to child through genes. In the case of Canavan, the critical pair of genes controls production of the enzyme mentioned above, aspartoacylase enzyme. When both parents are Canavan carriers, each one may transmit the mutated gene to their offspring, in which case the child is born with Canavan disease. The inheritance pattern is identical to that of Tay-Sachs, so you may want to re-read the section "Why is a baby born with Tay-Sachs".
Who is at risk?
Canavan disease is most prevalent among folks of eastern european Jewish descent. the carrier rate in this population is one in 35. While Canavan does occur in other ethnic groups, it is rare and carrier frequency in the non-jewish population has not yet been measured.
How do you know if you're a Canavan carrier?
As with Tay-Sachs, the answer is a simple blood test.
While the common Tay-Sachs screening is a serum test that measures enzyme level in the blood, DNA testing is required in order to identify Canavan carriers.
Remember, Tay-Sachs and Canavan are caused by different alterations in different genes. The fact that you are, or are not, a Tay-Sachs carrier has no bearing on whether you are a Canavan carrier. You must be tested for both diseases to know your Tay-Sachs and Canavan carrier status.
There are many family planning options for carrier couples.
Couples who are both carriers of the same disease will want to explore their many options for a healthy family. It is best to consult a genetic counselor who can explain the various choices open to you when planning a family.
Prenatal diagnosis early in pregnancy will reveal if the fetus has Tay-Sachs or Canavan disease. At-risk couples can choose from two procedures: amniocentesis, done around the 16th week of pregnancy, and chorionic villus sampling (CVS), performed between the 10th and 13th weeks. In either case, if the fetus is affected with Tay-Sachs or Canavan, couples may elect to have a therapeutic abortion.
In addition to adoption, there are several medical procedures available to couples for whom abortion is not an option. Your genetic counselor can explain these alternatives so that you are prepared to meet with a medical specialist, should you choose this route.