Huntington’s Disease (HD) is a progressive neurodegenerative disorder characterized by deficits of motor control, decline of cognitive ability, and emotional dysregulation (Albanese & Jankovic, 2012). The mean age of onset for HD is 40 and illness duration is typically 17 years, with men and women equally affected (Dickson & Weller, 2011). A variant known as Juvenile Huntington’s Disease has also been described, with symptoms differing from classic HD and onset of the condition occurring before age 21 (Nance, 1997). The leading causes of fatality in HD are pneumonia and suicide (Heemskerk & Roos, 2010; Roos, 2010).
Prevalence of HD in the Caucasian population is estimated at between 1 in 10,000 and 1 in 20,000 (Roos, 2010); this is significantly higher than the rate observed in other ethnic groups (Warby et al., 2011). A meta-analysis conducted by Pringsheim et al. (2012) estimated the worldwide prevalence of the disease at 2.71 incidents per 100,000. While exact statistics on the Republic of Ireland’s incidence of HD are unavailable at the time of writing, the minimum number of those affected in Northern Ireland has been calculated as 10.6 in 100,000 (Morrison, Harding-Lester, & Bradley, 2011); extrapolations made from this data estimate the number of HD cases in the Republic of Ireland at approximately 500 (Huntington’s Disease Association of Ireland, 2010).
Symptoms of preclinical HD include weight loss, mild cognitive decline, and problems with mood (e.g. irritability), as well as subtle problems with movement of the fingers, toes, face and eyes (Ruppr et al., 2010).
Presenting physical characteristics of HD include jerking, involuntary movements known as chorea, unstable stance, and speech/swallowing difficulties; slowed processing of movement and muscle rigidity often occur in later disease stages (Gil-Mohapel, 2012). Common cognitive deficits include difficulty organising thoughts; problems with working memory, visual memory, and verbal memory; and impaired concentration (King et al., 2011). These deficits can progress to levels similar to those observed in cortical dementias such as Alzheimer’s Disease (Albanese & Jankovic, 2012). Neuropsychiatric disturbances associated with HD include depression, obsessive compulsive disorder, anxiety, and psychoticism (Marshall et al., 2007). This can result in increased risk taking, erratic behaviour and mood swings. Significantly higher risk of suicide has also been observed in patients (Hubers et al., 2012).
The specific mechanisms which trigger HD are as of yet unknown (Spinelle, 2011), however it has been identified as an autosomal dominant disorder which causes cell death in the area of the brain area known as the basal ganglia (Martin, 2006). This cell death is a result of genetic mutation in chromosome 4 which causes accumulation of neurotoxic protein clumps, particularly in the striatum (Ribeiro et al., 2012). It has been observed that the rate of cerebral atrophy in HD is only partially related to innate sequencing factors; therefore other non-DNA related genetic factors and environmental factors may play a role in the disease progression (Rosas et al., 2011).
There are no treatments currently available to slow the disease onset or progression (Aron, Tsebetkov, & Finkbeiner, 2013), however recent research using genetically engineered mice has identified the efficacy of progenitor cells (which are similar to stem cells) in delaying the disease progression (Benraiss et al, 2013). Use of chemical compounds to inhibit a specific enzyme, kynurenine 3-monooxygenase (KMO), has also shown positive results in alleviating HD-relevant phenotypes in transgenic animal models of HD and Alzheimer’s Disease (Amaral et al., 2013). Specific antioxidants have been found to reduce HD related damage to DNA in a mouse model of the disease (Xun et al., 2012). While the results of these studies may be promising in terms of therapeutic treatment, caution should be taken in relation to the external validity of the studies and the application of such treatments to human samples. Clinical trials with human participants would be beneficial in considering the value of these findings.
While no treatments are currently available, therapeutic interventions such as psychopharmacological treatment, psychological support services, physical therapy, speech and language therapy, and occupational therapy are recommended to reduce associated symptoms and improve quality of life for individuals with this devastating neurodegenerative disease (Dickson & Weller, 2011).
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