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In the past decade there has been a growing interest in cases of horizontal strabismus in which the amount of deviation varies significantly between supraand infraversion. The cases exhibiting this vertical incomitance of horizontal squint have been graphically designated the "A" and "V" phenomena or syndromes. Those cases that are more divergent or less convergent in upward gaze demonstrate the V, whereas those that are more divergent or less convergent in downward gaze demonstrate the A.
Although there is general agreement about the existence of such cases, with estimates of their frequency varying between 10% and 50% of all horizontal strabismus, there is wide disagreement as to their etiology and consequently their treatment. There are three schools of thought. The first believes they are produced entirely by the action of the horizontal recti with the lateral recti causing the change in the amount of deviation in upward gaze and the
Knapp P. The "A" and "V" Syndromes. Arch Ophthalmol. 1962;67(3):277-278. doi:10.1001/archopht.1962.00960020279001