By: Emily Fisher and Alicia Semaka
There are two ways to answer the question “how many people have Huntington disease (HD)?” The first is by determining the prevalence (i.e., the proportion of a population that has a given condition at a particular point in time), and the second is by determining the incidence (i.e., the number of people who will develop a condition over a defined period of time) of HD. Prevalence answers the question of “how many people currently have HD?” and incidence answers the question of “how frequently are people being diagnosed with HD?” Research suggests that the answers to both of these questions are changing and perhaps increasing. These changes will have important biological, economic, and social implications for the future.
The prevalence of HD has been reported to vary with ethnicity and geographical location, confirming that demographic differences influence the number and composition of individuals with HD. Since HD is believed to have major origins in Northern Europe, 2 it is not surprising that populations of Northern European descent are recorded as having the highest prevalence of HD in theworld.3 In the 1970‘s and 1980’s, a number of studies around the globe examined the prevalence of HD. In Europe it was estimated that an average of 4-7 persons per 100,000 were affected with the disease.4,5,6,7 Canadian studies suggested 2.4-8.4 persons per 100,000 had HD.8,9 In the United States, it was estimated that 4.1 – 5.2 persons per 100,000 were living with thedisorder.10,11 Figure 1 reviews additional global prevalence estimates for HD. While a number of studies have examined the prevalence of HD, very few studies have investigated the disease’s incidence. Findings from Minnesota, USA suggested that 3 individuals per million were diagnosed with HD each year between 1950-1989.12 In British Columbia, Canada, 6.9 individuals per million were diagnosed each year between 1987-1999.13
In recent years, the accuracy of published prevalence estimates for HD has been called into question. The majority of prevalence estimates for HD were conducted prior to the identification of the genetic mutation underlying the disease; with a genetic test now available, diagnosing HD has become more precise. Patients with atypical symptoms, late disease onset, and negative family histories are more likely to be diagnosed, reflecting a likely increase in the number of persons living with the disease. However, fear of experiencing genetic discrimination from health insurers, among family members, or in social settings14 may prevent some individuals from disclosing that they are either at-risk or premanifest for HD, thus making it difficult to accurately identify every patient in a given population.
A brief review of the number of patients cared for by the Huntington Disease Association in the United Kingdom (UK) indicated that approximately 6,700 individuals in the UK are currently living with HD. This means that 12.4 per 100,000 persons or 1 out of every 8,065 individuals may be affected with HD15.Although these numbers at best provide a minimum prevalence estimate in the UK, since the Association does not care for all HD patients in England and Wales, the numbers do demonstrate that previous prevalence figures are likely underestimated. Fortunately, recent increases in HD awareness and access to information and support, developments of legislations dedicated to preventing genetic discrimination, and continuations of promising basic and clinical HD research have had impact on the willingness of individuals to involve themselves in the HD community. More patients and families are reaching out to medical service providers and becoming involved in HD research, and their increased involvement in the HD community provides an ideal opportunity to reassess the global prevalence of HD.
While the need to establish new baseline prevalence estimates for HD is apparent, these figures must be frequently revised to determine how the prevalence of HD changes over time. Factors such as the projected demographic shifts in median age and our increasing longevity will likely impact the prevalence of HD in the coming years. With an aging population that is expected to live longer, more individuals who have late onset HD will be diagnosed, thus increasing the number of affected individuals. Accurate prevalence estimates are vital to ensuring that there is appropriate access to clinical services and support for patients and families. Additionally, since medical and research funding is often allocated on the basis of the disease burden on society, it is imperative to know the true prevalence of the disease in order to secure proportional financial support for HD research and care. With a combined global effort from HD patients, families, researchers, and caregivers, we will finally be able to accurately answer the question: “How many people have Huntington disease?”
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