Here is a great info-graphic on convergence insufficiency. It comes from the Vision Help blog, which is one of the world's best sources for information on vision development, developmental optometry, vision therapy and eye movement disorders.
Convergence insufficiency is one of the most common eye-movement disorders. It prevents the eyes from focusing properly when doing near work. It can interfere with school, learning and reading and it has signs and symptoms that are similar to attention deficit/hyperactivity disorder. That means that some kids who have been diagnosed with ADHD may actually have convergence insufficiency which is most effectively treated with vision therapy. Vision therapy for convergence insufficiency has been shown in studies to improve academic behaviours.