Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by familial restrictive cardiomyopathy ?
    ### Response:
    The prevalence of familial restrictive cardiomyopathy is unknown. Although cardiomyopathy is a relatively common condition, restrictive cardiomyopathy, in which relaxation of the heart muscle is impaired, is the least common type. Some other forms of cardiomyopathy involve a weak or enlarged heart muscle with impaired contraction.
    