heterophoria

Also known as dissociated phoria, heterophoria is a latent condition where the eyes tend to deviate from the orthophoric position when binocular fusion is disrupted. It requires fusional vergence to maintain bifixation and alignment. Heterophoria can be exophoric (outward deviation) or esophoric (inward deviation). It is measured using dissociating tests that disrupt binocular fusion, like the cover test or Maddox rod. Excessive heterophoria can cause symptoms like eyestrain, diplopia, and asthenopia, but differs from strabismus, which is a manifest misalignment.