orthophoria

Orthophoria is the absence of heterophoria. Without an adequate fusion stimulus, the lines of sight still intersect at the point of reference, usually the point of binocular fixation.¹ In this ideal fusion-free position, the visual lines are parallel for distance fixation and properly converged for near vision.²

Orthophoria is rarely realized. Most people free of symptoms show some deviation of the visual axes once fusion is suspended, even if it is too small to measure clinically. Many clinicians therefore regard a certain amount of heterophoria as normal.² On cover testing, an orthophoric patient shows no movement of either eye with either the cover-uncover test or the alternating cover test.³

Orthophoria describes the eyes with fusion suspended, whereas orthotropia describes complete alignment of the visual axes on the object of regard.^1,2 Orthotropia is present if the cover test is negative on covering either eye and there is no amblyopia. The amblyopia condition matters because an amblyopic eye with eccentric fixation can show a negative cover test with its visual axis off the object of regard. Heterophoria is the rule in normal binocular vision, so orthophoria is a poor term for normal alignment, and orthotropia or orthoposition serves better.²