Doctor Marage, who performed the autopsy on Ludwig Van on March 27, 1827, brought up Beethoven's deafness at the conference of the French Academy of Sciences on January 9th and 23rd 1928, as well as December 2nd 1929.
He thought that Beethoven's deafness was due to a labyrinthitis of intestinal origin, that is to say that he had a lesion of the inner ear. According to Doctor Marage, who has studied Beethoven's letters, buzzing noises and other sounds started at around 1796. Deafness broke out in 1798 and Beethoven had lost 60% of his hearing by 1801. In 1816 he was completely deaf. The above information is from Ludwig Van Beethoven's Biography. The supposition is that he was able to hear most of his life and, therefore, could recognize tones and especially disonances simply by the written musical notes. In fact, some, at the time, speculated that one reason for his brilliant compositions was that he didn't hear and this enabled him to construct symphonies without the distraction of hearing other composer's work...