Musicians are arguably all a bit mad in the head. This is certainly true for Ludwig van Beethoven. There are various reports of Beethoven becoming increasingly... unmanageable in his later life. He hardly had any friends, drank a lot, and didn't care about trivial things as basic hygene. One acquaintaince of Beethoven's reported that he saw Beethoven being arrested because he was dressed as a homeless person and peering into people's windows. 

Beethoven was deaf. Everyone knows that. But he wasn't the only one. The most famous other composer was probably Bedrich Smetana, a Czech composer who is most famous for his work Ma Vlast (My Country). 
Smetana went deaf and because of that also went insane in his later years. Ma vlast is one of the most popular classical works that exist today.