5.2 Stereoacuity, Near Point of Accommodation, Near Point of Convergence & the AC/A Ratio

Stereoacuity

Stereopsis is binocular depth perception derived from horizontal retinal disparity. It is measured in seconds of arc — and smaller numbers mean better stereopsis.

Technique: test at 40 cm with the patient's habitual near correction plus the polarised or red-green glasses, hold the plate parallel to the face (tilting introduces monocular cues), and turn the booklet 180 degrees intermittently to confirm the patient is genuinely seeing depth rather than guessing.

Interpretation. Normal is about 40 seconds of arc or better. Reduced stereoacuity suggests amblyopia, a constant strabismus or unequal image quality. A manifest constant strabismus abolishes fine stereopsis, so a patient claiming a large monocular acuity loss but demonstrating 40 arcsec stereopsis is internally inconsistent.

Near point of accommodation and amplitude

Amplitude of accommodation is the total dioptric change the eye can make from far point to near point.

Push-up method. With the distance correction in place, occlude one eye, bring a near target (a single line of small print or an accommodative target) slowly toward the eye and record the distance at which it first blurs sustainably. Convert:

Amplitude (dioptres) = 100 ÷ near point (centimetres)

A near point of 10 cm gives 10.00 D; 25 cm gives 4.00 D; 50 cm gives 2.00 D.

Minus lens method. With the target fixed at 40 cm, add minus power in −0.25 D steps until sustained blur, then add +2.50 D to account for the 40 cm working distance. Minus-lens amplitudes usually run about 2 D lower than push-up values because push-up gains apparent amplitude from the target's increasing angular size.

Expected values (Donders/Duane).

Hofstetter's formulas approximate the same: minimum amplitude = 15 − 0.25 × age; average = 18.5 − 0.30 × age; maximum = 25 − 0.40 × age. Accommodative amplitude below the age-expected minimum, with normal refraction, suggests accommodative insufficiency.

Near point of convergence

Near point of convergence (NPC) is the closest point at which the eyes can maintain binocular single vision on a target moved slowly toward the nose.

Technique: binocular, with habitual correction, bring an accommodative target toward the bridge of the nose along the midline. Record two values:

• Break — the distance at which the patient reports diplopia or you observe one eye drift out.
• Recovery — the distance at which fusion is regained as the target is moved back out.

Normal: break within about 10 cm (many references use 6 to 10 cm) with recovery within about 15 cm. Record as "NPC 8/12 cm." Repeat the measurement several times — convergence insufficiency characteristically fatigues, so the fifth repetition is often markedly worse than the first, and that deterioration is itself the diagnostic finding.

A receded NPC with exophoria greater at near than distance, reduced positive fusional vergence and asthenopia on reading is the classic convergence insufficiency pattern.

The AC/A ratio

The accommodative convergence to accommodation (AC/A) ratio expresses how many prism dioptres of convergence accompany each dioptre of accommodation. Normal is roughly 3:1 to 5:1. It is the measurement that explains why some esotropias are larger at near and some exotropias larger at distance, and it drives bifocal prescribing in accommodative esotropia.

Gradient method (the more commonly examined). Measure the deviation at a fixed distance, normally 33 cm, then repeat with the same lens power over both eyes and divide the change in deviation by the magnitude of the lens power:

AC/A = |deviation with lens − deviation without lens| ÷ |lens power in dioptres|

Use either a minus lens, which stimulates accommodation and therefore drives an esodeviation larger, or a plus lens, which relaxes accommodation and drives an esodeviation smaller. Either direction yields the same ratio.

Worked example with a minus lens. A child measures 8 prism dioptres of esophoria at 33 cm. With −3.00 D over both eyes the esophoria rises to 26 prism dioptres. AC/A = (26 − 8) ÷ 3 = 6:1 — a high AC/A, the finding that supports a bifocal add in high-AC/A accommodative esotropia.

Worked example with a plus lens. A child measures 10 prism dioptres of esotropia at 33 cm. With +3.00 D over both eyes the deviation falls to 1 prism dioptre. AC/A = (10 − 1) ÷ 3 = 3:1 — a normal ratio, so the near deviation is not accommodatively driven and a bifocal add is unlikely to help.

Heterophoria method:

AC/A = interpupillary distance in centimetres + [(near deviation − distance deviation) ÷ dioptric demand at near]

The gradient method is generally preferred because it holds proximal convergence constant.

Clinical patterns.