Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) hereditary hyperekplexia ?
    ### Response:
    Hereditary hyperekplexia is a condition in which affected infants have increased muscle tone (hypertonia) and an exaggerated startle reaction to unexpected stimuli, especially loud noises. Following the startle reaction, infants experience a brief period in which they are very rigid and unable to move.
    