Although Huntington's Disease is often called a midlife disorder, approximately seven to ten percent of Huntington's Disease patients are children or young people under 20.
Huntington's Disease is caused by an expansion of CAG repeats in a section of the gene which encodes for the huntingtin's protein. The HD version of the protein causes multiple problems in certain neurons in the brain. For some period of time the brain is able to cope with the challenges of the HD protein, but after a time the neurons become dysfunctional and eventually die. The age of onset is influenced by the CAG count. On average, the higher the count, the earlier the onset. However, there are other factors such as environmental factors and other modifier genes, which are only now being discovered. The age of onset for two individuals with the same CAG count can vary widely.
Juvenile Huntington's Disease is associated with higher CAG counts than in adult onset HD, especially for children under ten. The highest known expansion of the gene is 250 repeats. The literature often states that the CAG counts will be 60 or higher in JHD. However, a recent study of patients treated at the Saltpetriere Hospital in Paris showed that in nearly half of the cases, the CAG count was in the range of 45 to 58.
There are few parents of at risk children who haven't wondered if they might be seeing signs of juvenile Huntington's Disease. Developmental delays are common in childhood and development advances can be uneven in children without the HD gene. If a parent manages to get to their child's teen years without worrying, adolescence will make anyone wonder.
It is important to understand the symptoms of Juvenile Huntington's Disease to help deal with those worries when they are groundless and to recognize the disease when it is present. Links to research articles can be found on the left side of the page and family stories are on the right side. Books and resources can be downloaded from the HDSA website here: http://www.hdsa.org/living-with-huntingtons/family-care/juvenile-hd.html