Epidemiology
The National Institute of Health reports that SCA1 has a prevalence of approximately 1 or 2 per 100,000 however a review of literature has shown that these estimates vary significantly from study to study and can be less than 1 per 100,000 or as high as 6 per 100,000. Among all types of SCA, SCA1 is among the most common and the portion accounted for by SCA1 varies between geographic regions, with percentages as high as 40% of all SCA diagnoses in populations in Russia and South Africa being SCA1. In the United States SCA1 accounts for 6% of SCA diagnoses. Overall, SCA1 accounts for 6-27% of all cases dominant ataxias. : 6  Because of its late onset, often appearing after reproductive age, SCA1 exerts low selection intensity, ranking about 0.19 on the Crows index, but intensity may vary with time within a population or family, as anticipation increases the number of CAG repeats. One implication of this is that SCA1 is unlikely to disappear from a population by natural selection alone.Prevalence of each type of SCA varies with geographic region and ethnicity, possibly because of founder effects and historic migration patterns. High prevalence regions include central Poland, where 68% of autosomal dominant cerebellar disorders are SCA1; communities in Tamil Nadu, where up to 7.2% of the population has SCA1 in some small villages; the Tohoku region on the northern portion of Honshu Island, with 24.8% of cases being SCA1; and among Yakut populations in eastern Siberia, with a prevalence 46 per 100,000 in rural populations.
Maculopathy has been reported in rare cases, and may be linked effects from the mutation on the ATXN1 locus on genes in neighboring loci. Task specific dystonias have been reported in individual cases, often in the form of writers cramps or cervical dystonia.SCAs can also be detected before serious atrophy with electrophysiologic techniques, using electrodes on the scalp to detect changes in electric potential within the brain in response to sensations or movements. Individuals with SCA1 often exhibit abnormal brainstem auditory evoked potential, including prolonged latency and absent or poorly defined waveforms, with one study reporting 73.3% of test subjects exhibiting abnormalities. The same study also found abnormalities in visual evoked potential and median somatosensory evoked potential in some SCA1 persons. These results were similar to those exhibited in other SCAs and differences between the SCAs were not statistically significant, so electrophysiologic techniques cannot replace genetic testing for specific diagnoses of SCAs.All SCAs cause atrophy in various neural tissues that are detectable using magnetic resonance imaging, computed tomography, or other imaging techniques. In SCA1 some degradation in the grey matter of the cerebellum and brain stem can sometimes be detected in presymptomatic individuals with the expansion in ATXN1. Typically, grey matter loss can be observed in cerebellar vermis in all lobules of the cerebellum and in the paramedian portions of both hemispheres. White matter loss can also be observed in the middle cerebellar peduncles.