A/V-Pattern Strabismus

Overview and Diagnostic Criteria

A- and V-pattern strabismus are horizontal deviations whose size changes meaningfully between upgaze and downgaze. The names come from the shape traced when the two gaze measurements are plotted: narrower at the top and wider at the bottom for an A pattern, the reverse for a V pattern. Both are defined by fixed thresholds rather than visual impression. An A pattern requires at least 10 prism diopters of difference between upgaze and downgaze, with less divergence (or more convergence) in upgaze than downgaze. A V pattern requires a larger threshold, at least 15 prism diopters, with more divergence in upgaze than downgaze.

Pattern strabismus can be superimposed on either an esotropia or an exotropia, giving four possible combinations: A-pattern esotropia, A-pattern exotropia, V-pattern esotropia, and V-pattern exotropia. In A-pattern esotropia, for instance, the eyes converge more in downgaze than in upgaze. The shared mechanism across all four is that the vertically-acting oblique muscles carry a horizontal secondary action, and overaction exaggerates that secondary pull enough to produce the pattern. The horizontal rectus muscles are not the problem.

Oblique Muscle Correlation

The direction of the pattern maps predictably onto which oblique muscle is overacting, and this pairing is one of the most testable facts in pediatric strabismus:

V-Pattern and Inferior Oblique Overaction

The inferior oblique's elevating action peaks when the eye is adducted, so overaction manifests as excessive elevation as the eye moves into adduction. That elevation widens the horizontal divergence measured in upgaze relative to downgaze, which is what produces a V pattern. Bilateral inferior oblique overaction, often asymmetric between the two eyes, is the most common cause of V-pattern exotropia and frequently comes with excyclotorsion visible on dilated fundus exam.

A-Pattern and Superior Oblique Overaction

The superior oblique's actions are depression and intorsion, both strongest with the eye adducted. Overaction increases depression and intorsion specifically in adduction, narrowing the horizontal deviation in downgaze relative to upgaze. Bilateral superior oblique overaction is the classic cause of A-pattern esotropia. The intorsion has its own sign on exam: dilated fundus exam shows excess incyclotorsion of the globe, a useful confirmatory finding alongside the horizontal gaze measurements.

Hering's Law and Asymmetric Overaction

Oblique muscle overaction is not always symmetric between the two eyes, and that asymmetry creates a diagnostic trap. Under Hering's law of equal innervation, yoke muscle pairs receive equal innervational signal. When one inferior oblique overacts more than its fellow, that equal signal can make the contralateral superior oblique look underactive even though it is entirely normal. The underaction is apparent, not true. Recognizing it as a Hering's-law artifact of asymmetric inferior oblique overaction, rather than a separate superior oblique palsy, keeps treatment aimed at the muscle that is actually overacting.

Surgical Management

Because V-pattern deviations are so often driven by inferior oblique overaction, the standard surgical approach weakens that muscle directly. Inferior oblique myectomy is most common, though recession or anterior transposition are also used. Weakening the muscle reduces the excessive elevation in adduction that was widening the deviation in upgaze, which corrects the V pattern along with the horizontal misalignment itself. A-pattern deviations driven by superior oblique overaction are managed the same way in principle: weakening the overacting muscle, or vertical muscle transposition.

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