Exophoria is a type of heterophoria characterized by a tendency for outward deviation of the eyes from the standard visual axis. In the majority of cases, it arises due to convergence insufficiency or inability to maintain proper eye alignment at near vision and the cause is unknown.
Apart from visual observation of the tendency of the eyes to drift outward, symptoms are frequently absent , but when it is a constitutive feature of convergence insufficiency (CI), defined as inability to preserve normal eye alignment at near, it can present with symptoms such as eye fatigue, headaches, and double vision when attempting to perform activities that require near sight, for e.g. reading  . CI, apart from exophoria, can be accompanied by a remote near point of convergence or decreased positive fusional vergence  . Other symptoms that are reported in patients suffering from exophoria are the frequent loss of place when reading, print moving on the page, difficulty concentrating and a short attention span . In some cases, hyperphoria - a permanent upward deviation of the visual axis of the eye, was described as well . Some authors consider that severe exophoria progressively leads to intermittent exophoria, a type of strabismus and that it may develop as early as infancy .
Physicians can make a preliminary diagnosis just by close inspection of the eyes during the physical examination, while patient history can reveal signs and symptoms that indicate problems with near vision. Some tests, however, may be employed to confirm exophoria as the cause of symptoms. Firstly, simple cover testing and evaluation of accommodation can be performed, which will provoke the outward movement of the eye in virtually all cases. The Snellen chart should not be used as a sole diagnostic method, since it may reveal completely normal findings in patients suffering from exophoria. The severity of prism diopter (PD) deviation should be examined by skilled ophthalmologists, in order to evaluate the need for correction .