Vergencies direct the eyes from distant vision to near vision or upside down. Both eyes must be coordinated in order to point to the same object. This alignment of both eyes must be accurate. A phoria is defined as a latent deviation, that is, both eyes go simultaneously but they must perform an over-effort to be coordinated.
This overheating is what rules the typical symptoms: intermittent blurring, headache, tiredness, occasional double vision, worse at the end of the day, difficulty in concentrating, words move when it is read, drowsiness, low speed and poor understanding reader
The diagnoses that exist are: insufficiency of convergence and divergence, excess of convergence and divergence, basic esophoria and basic exoforia.
Treatment is through positive additions, visual therapy or prisms.