All persons with MJD have the same disease gene mutation: a DNA repeat expansion in the ATXN3 gene. The wide range in symptoms among affected individuals led researchers to separate the disease into distinct types that are broadly distinguished by age of onset and range of symptoms.
Type I MJD is characterized by onset between about 10 and 30 years of age, with faster progression and more dystonia and rigidity than ataxia.
Type II, the most common type of MJD, generally begins between the ages of about 20 and 50 years, has an intermediate rate of progression, and causes various symptoms, including prominent ataxia, spastic gait, and enhanced reflex responses.
Individuals affected by Type III MJD have the latest onset of disease (beginning between approximately 40 and 70 years of age) which progresses relatively slowly and is characterized as much by peripheral neuromuscular involvement (muscle twitching, weakness, atrophy, and abnormal sensations such as numbness, tingling, cramps, and pain in the hands and feet) as by ataxia.
Most individuals with MJD, but especially those with types I and II, experience one or more problems with vision, including double vision or blurred vision, loss of ability to distinguish color and/or contrast, and inability to control eye movements. Some individuals also experience prominent Parkinson’s disease-like symptoms, such as slowness of movement, rigidity or stiffness of the limbs and trunk, and tremor or trembling in the hands.