Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Hydranencephaly ?
    ### Response:
    Hydranencephaly is a rare condition in which the brain's cerebral hemispheres are absent and replaced by sacs filled with cerebrospinal fluid (CSF). Affected infants may appear and act normal at birth, but irritability and hypertonia often develop within a few weeks.
    