Having a piece of Voldemort's soul shoved into the front of his head, Harry simply wasn't normal. After all, there's only so much space in his head, now isn't there?

Somehow, the piece of Voldemort's soul affects Harry, both physically and mentally.

Required:

1. Harry must either be blind or have extremely bad vision, due to the soul placing pressure on the optic nerve.
2. Chronic headaches, worsened in the presence of Voldemort's magic. (This includes Dark Marks).
3. Damage to the frontal lobe of his brain (the frontal lobe does "involve the ability to recognize future consequences resulting from current actions, to choose between good and bad actions (or better and best), override and suppress socially unacceptable responses, and determine similarities and differences between things or events." It is also involved in long term memory not associated with tasks.) To what degree is your choice.
4. Severus is the first to notice the damage, and for whatever reason, is the one who leads the treatment. Thus, he is curious, confused, or disoriented around Harry--not hating him as the son of James.
5. The damage must begin when Harry gains the Horcrux, but can worsen with the revival of Voldemort.

Optional ideas:

• Slytherin!Harry
• Harry can hear the voice of Voldemort/Tom Riddle in his head
• Easier for Voldemort to possess Harry or access his mind
• Someone believes Harry is the next Dark Lord
• Harry has access to some of Voldemort's knowledge or memories

Apart from that, this idea is yours.