Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) infantile-onset spinocerebellar ataxia ?
    ### Response:
    Infantile-onset spinocerebellar ataxia (IOSCA) is a progressive disorder that affects the nervous system. Babies with IOSCA develop normally during the first year of life. During early childhood, however, they begin experiencing difficulty coordinating movements (ataxia); very weak muscle tone (hypotonia); involuntary writhing movements of the limbs (athetosis); and decreased reflexes.
    