Often a visual disorder is misdiagnosed as ADD or ADHD. Children may test with perfect 20/20 vision on a routine eye exam and still be suffering from a convergence insufficiency or another visual disorder. Children with convergence insufficiency tend to lose focus, avoid reading and writing, get frustrated, angry, and seem restless like they cannot pay attention to the work in front of them. This almost matches symptom for symptom those of ADD/ADHD. They may even get headaches or be nauseous.
It is likely that it is not a problem with attention span but that they cannot see the world the way you or I might. It can be difficult to identify this as at home, symptoms often occur more frequently at school where visual tasks are more demanding. And many school eye exams are not comprehensive enough to diagnose a visual disability.
In a 2005 study, Dr. David Granet, a professor of ophthalmology and pediatrics at the University of California, San Diego, studied 266 patients with convergence insufficiency, and found 10% also had an ADD/ADHD diagnosis. He also studied 1,700 hospital records of ADD/ADHD kids and found a 16% convergence insufficiency rate. This is 3 times the national average.