The genetic basis of the disease is an abnormally increased number of CAG repeats in the huntingtin gene on the short arm of chromosome 4 (more than 38 repeats used to confirm the diagnosis), often with progressive increase in the length of the repeating sequences in successive generations, resulting in symptoms earlier in life (anticipation). The incidence is ~10/100,000, with average age of onset between 35 to 45 years and symptom progression until death within 15 to 20 years of onset [20]. Abnormal aggregates of the huntingtin protein accumulate in the brain and impair the function of a variety of transcription factors, ultimately leading to the loss of GABAergic medium spiny neurons, particularly in the striatum and cortex [20]. Genetic testing to determine the CAG repeat number for each allele is commercially available and the diagnostic test of choice. Patients with suspected HD should undergo genetic counseling and testing to exclude or confirm HD in concert with initial imaging, given that imaging may be normal early on in the disease course. CT Head CT is not the preferred imaging modality for suspected HD because of its limited soft-tissue characterization when compared with MRI. However, CT may be useful for excluding other etiologies of chorea, such as cerebrovascular disease or acute infectious or inflammatory processes. Contrast is typically not indicated when CT of the head is being utilized in the initial evaluation of chorea. As with MRI, CT imaging may be normal early in HD, with the progressive and disproportionate volume loss of the neostriatum only becoming apparent on later imaging studies. FDG-PET/CT Brain There is insufficient evidence to support the use of FDG-PET/CT of the brain in the initial evaluation of a patient with chorea or suspected HD. Among known HD gene carriers, FDG-PET may help demonstrate early neostriatal dysfunction (manifested as hypometabolism), which can precede neuronal loss detectable on structural imaging [21].
Movement Disorders and Neurodegenerative Diseases. The genetic basis of the disease is an abnormally increased number of CAG repeats in the huntingtin gene on the short arm of chromosome 4 (more than 38 repeats used to confirm the diagnosis), often with progressive increase in the length of the repeating sequences in successive generations, resulting in symptoms earlier in life (anticipation). The incidence is ~10/100,000, with average age of onset between 35 to 45 years and symptom progression until death within 15 to 20 years of onset [20]. Abnormal aggregates of the huntingtin protein accumulate in the brain and impair the function of a variety of transcription factors, ultimately leading to the loss of GABAergic medium spiny neurons, particularly in the striatum and cortex [20]. Genetic testing to determine the CAG repeat number for each allele is commercially available and the diagnostic test of choice. Patients with suspected HD should undergo genetic counseling and testing to exclude or confirm HD in concert with initial imaging, given that imaging may be normal early on in the disease course. CT Head CT is not the preferred imaging modality for suspected HD because of its limited soft-tissue characterization when compared with MRI. However, CT may be useful for excluding other etiologies of chorea, such as cerebrovascular disease or acute infectious or inflammatory processes. Contrast is typically not indicated when CT of the head is being utilized in the initial evaluation of chorea. As with MRI, CT imaging may be normal early in HD, with the progressive and disproportionate volume loss of the neostriatum only becoming apparent on later imaging studies. FDG-PET/CT Brain There is insufficient evidence to support the use of FDG-PET/CT of the brain in the initial evaluation of a patient with chorea or suspected HD. Among known HD gene carriers, FDG-PET may help demonstrate early neostriatal dysfunction (manifested as hypometabolism), which can precede neuronal loss detectable on structural imaging [21].