The length of the trinucleotide repeat accounts for 60% of the variation in the age of onset and the rate of progression of symptoms. A longer repeat results in an earlier age of onset and a faster progression of symptoms. For example, individuals with a trinucleotide repeat greater than sixty repeats often develop the disease before twenty years of age, and those with less than forty repeats may not develop noticeable symptoms. The remaining variation is due to environmental factors and other genes that influence the mechanism of the disease.
Other associated risks include choking, physical injury from falls, and malnutrition. Other areas of high localization have been found in Tasmania and specific regions of Scotland, Wales and Sweden. Some of these carriers have been traced back hundreds of years using genealogical studies.
Until the the discovery of a genetic test, statistics could only include clinical diagnosis based on physical symptoms and a family history of HD, excluding those who died of other causes before diagnosis. These cases can now be included in statistics and as the test becomes more widely available, estimates of the prevalence and incidence of the disorder are likely to increase.
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