Binocular Vision Norms

A normal near point of convergence (NPC) is typically break at 5 cm or less with recovery at 7 cm or less, measured with a small accommodative target. A receded NPC (break beyond 10 cm or showing fatigue on repeated testing) is a key finding in convergence insufficiency. Repeat the measurement several times — a NPC that recedes with repetition suggests reduced vergence stamina.

Expected vergence ranges at near using prism bar or phoropter (Morgan's norms) are approximately: base-out blur 17, break 21, recovery 11 prism diopters; base-in blur 13, break 21, recovery 13 prism diopters. At distance, base-out is approximately blur 9, break 19, recovery 10 and base-in is approximately blur 7, break 11, recovery 7. Values vary by source, so compare your patient's results to the norms table above for the specific ranges used in your clinical setting.

A normal AC/A ratio is approximately 4:1 to 6:1 (4 to 6 prism diopters of vergence change per 1.00 D of accommodative stimulus). A high AC/A ratio (above 6:1) is associated with convergence excess and near esophoria that responds to plus lenses. A low AC/A ratio (below 4:1) may contribute to convergence insufficiency symptoms. The gradient method (measuring phoria change through a known lens power) is the most commonly used clinical technique.

Hofstetter's minimum expected amplitude formula is 15 − (0.25 × age). For example, a 20-year-old should have at least 10.00 D, a 30-year-old at least 7.50 D, and a 40-year-old at least 5.00 D. The average expected amplitude is 18.5 − (0.30 × age). Amplitudes significantly below the minimum formula warrant investigation for accommodative insufficiency, especially in younger patients presenting with near blur or asthenopia.

Normal accommodative facility with ±2.00 D flippers is approximately 11 cycles per minute (cpm) monocularly and 8 cpm binocularly for adults. Difficulty clearing minus lenses suggests accommodative insufficiency, while difficulty clearing plus lenses suggests accommodative excess. Record both the cycle count and which lens causes difficulty, as the pattern is often more informative than the number alone.

Blur is the point where fusional vergence is no longer sufficient and accommodation begins to compensate, causing the image to defocus. Break is the point where the patient can no longer maintain single vision and reports diplopia. Recovery is the point at which single vision is regained as prism demand is reduced. Not all patients report a blur point, particularly with base-in testing.

The gradient method is commonly used: measure the near phoria at baseline, then remeasure through a +1.00 or −1.00 D lens. The AC/A ratio equals the change in phoria divided by the lens power. Example: if a patient is 2 XP at near and becomes 2 EP with −1.00 D, the phoria changes by 4 prism diopters over 1.00 D, giving an AC/A ratio of 4:1.

Sheard's criterion states that the compensating fusional vergence reserve at the blur point should be at least twice the magnitude of the phoria to support comfortable binocular vision. For exophoria, the compensating reserve is base-out vergence, so a practical expression is BO blur ≥ 2 × exophoria. If the criterion is not met, symptoms are more likely and prism or therapy may be indicated.

Convergence insufficiency (CI) is a binocular vision disorder characterized by a receded NPC (typically beyond 10 cm), reduced base-out vergence ranges at near, exophoria greater at near than distance, and reduced vergence facility. Patients commonly report headaches, eye strain, blurred vision, and loss of place during sustained near work. CI is one of the most common binocular vision disorders and often responds well to office-based vision therapy.