In 1880, a special supplemental census was recorded for all individuals who were considered either temporarlity or permanently disabled, including those who were "blind, deaf and dumb, idiotic, insane, maimed, crippled, bedridden or otherwise disabled." If your ancestor's 1880 Census listing has a mark in columns 15 through 20, additional detail pertaining to his or her condition should have been recorded on the supplemental schedule, often referred to as the "DDD (Defective, Dependent & Delinquent) Schedule."
Until recently, accessing these schedules has been somewhat challenging -- only a few state's records had been microfilmed by the National Archives. For other states, the records were retained in various repositories such as state archives, major libraries, and state historical societies. Access has recently become easier, as Ancestry.com has digitized and indexed the DDD schedules for the following states: California, Illinois, Iowa, Kansas, Maine, Massachusetts, Michigan, Nebraska, New Jersey, New York, South Carolina, Texas, Virginia and Washington Territory. Check out these very interesting resources at Ancestry.com!