Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Best vitelliform macular dystrophy ?
    ### Response:
    Best vitelliform macular dystrophy (BVMD) is a slowly progressive form of macular degeneration. It usually begins in childhood or adolescence, but age of onset and severity of vision loss can vary. Affected people first have normal vision, followed by decreased central visual acuity and distorted vision (metamorphopsia).
    