Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) congenital myasthenic syndrome ?
    ### Response:
    Congenital myasthenic syndrome is a group of conditions characterized by muscle weakness (myasthenia) that worsens with physical exertion. The muscle weakness typically begins in early childhood but can also appear in adolescence or adulthood. Facial muscles, including muscles that control the eyelids, muscles that move the eyes, and muscles used for chewing and swallowing, are most commonly affected.
    