A pattern
An A pattern is a horizontal strabismus whose size changes between upgaze and downgaze, with more convergence in upgaze. In A esotropia the deviation increases in upgaze and decreases in downgaze, and in A exotropia the deviation increases in downgaze and decreases in upgaze.1
The deviation is measured with an accommodative target at 33 cm, the refractive error fully corrected, and the eyes in 25 degrees of elevation and 35 degrees of depression. An A pattern counts as significant at a horizontal difference of 10 prism diopters or more between those two positions.1 A V pattern needs a difference of 15 prism diopters, because divergence in upgaze and convergence in downgaze occur as normal variants, but an A pattern is never a normal variant.1
Abduction is a tertiary action of both oblique muscles, so overaction of the superior obliques or weakness of the inferior obliques can produce an A pattern.1 Orbital imaging also links A patterns to incyclotorsion of the pulley array and to displacement of individual muscle pulleys.2
More detail
The 10 prism diopter limit decides only whether vertical incomitance is present. Clinical significance depends on how much the pattern disturbs binocular function in primary position and in the downward reading position.1 An A exotropia of only 15 prism diopters in downgaze and 5 prism diopters in primary position may interfere with normal binocular vision and require treatment.1 In very young patients a reliable prism cover test in several gaze positions may be impossible, so an A pattern can be hard to establish.1
Sources (2)
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 19
- Yanoff and Duker, Ophthalmology (2023), ch. 11.8
Reviewed October 2026