What was Sloth's handicap?

In The Goonies, Sloth was the unwanted son of Mama Fratelli and the hero at the end of the story.

He had an obvious facial deformity, but he was also really strong, could speak and capable of love.

He seemed to lack some intelligence; but that could have been from the way he was raised (secluded), or dropped as a child (Mama admits to doing this at least twice; but we get the impression it was more) or maybe both.

If Chunk and Chunk's mom took Sloth to the doctor the day after they were rescued, what diagnose would he receive?

In other words, was he mentally handicapped or just deformed? And what could cause that kind of deformation?