According to the Mayo Clinic, convergence insufficiency, CI, occurs "when your eyes don't turn inward properly while you're focusing on a nearby object. When you read or look at a close object, your eyes should converge — turn inward together to focus — so that they provide binocular vision and you see a single image. But if you have convergence insufficiency, you won't be able to move your eyes inward to focus normally."
This disorder is not caught in standard eye exams given by opthamologists and needs to have an examination done by developmental optometrists.
Some people are more prone to CI than others. CI affects 3 percent to 5 percent of the population. For example, nearly 10 percent of subject with CI also had diagnoses of attention deficit hyperactivity disorder (ADD or ADHD). Many of the symptoms of CI overlap that of ADHD. In fact, it is possible that the "medications used to treat ADHD are aggravating CI. For some of these drugs, difficulties with accommodation and blurring have been reported (Bennett et al., 1999).”