Since he was a turtle tot, Donnie had an excessive compulsion to grind his teeth, put things in his mouth, and chew things that weren't supposed to be chewed. Every kid goes through something of the sort, but while his brothers grew out of it, he did not.

He had other tendencies that pointed to something out of the norm that differentiated him from his brothers. All his symptoms aligned with what he's read about ADHD. Obviously, being a mutant turtle, visiting a doctor or attempting to get diagnosed wasn't possible.

As he sat at his desk in his lab, he sighed and instinctively stuck the end of his pencil in his mouth, gnawing on the eraser to focus better. The invention he was working on was beginning to stump his brain. He's been staring at the same paper for hours.