5.3 NPC, stereopsis & binocular function tests

Near Point of Convergence and Binocular Function

Binocular function tests assess how well the two eyes work together for single, comfortable, three-dimensional vision. The COT routinely performs near point of convergence, near point of accommodation, stereopsis testing, and the Worth 4-dot test.

NPC technique and normal values

The near point of convergence (NPC) is the closest point at which the eyes maintain single binocular vision (bifoveal fixation) as a target is moved toward the nose. Technique: seat the patient, hold a small accommodative target (a fixation stick with a letter, or a penlight) at about 40 cm in the midline at eye level, and slowly advance it toward the bridge of the nose. Ask the patient to report when the target doubles (subjective diplopia); at the same time, watch the eyes and note when one eye drifts outward and loses fixation (objective break).

• Break point: the distance at which fusion is lost (one eye diverges / patient sees double).
• Recovery point: the distance at which single vision returns as the target is withdrawn.

A normal NPC break is roughly 5–8 cm from the nose (some references accept up to ~10 cm). A receded NPC (> ~10 cm) suggests convergence insufficiency, a common cause of asthenopia (eyestrain), blurred near vision, and headache during reading. Record both values, for example "NPC 6/9 cm."

Near point of accommodation (NPA)

The near point of accommodation is the closest point at which a target stays clear, reflecting the eye's focusing amplitude. It is measured monocularly by bringing near print toward the eye until it first blurs. NPA recedes with age as the crystalline lens stiffens — this age-related loss is presbyopia, typically symptomatic in the early-to-mid forties. Accommodative amplitude (in diopters) ≈ 100 / near point in centimeters.

Stereopsis testing

Stereopsis is the finest degree of binocular depth perception, produced when slightly disparate retinal images are fused. It is measured in seconds of arc (arc-seconds) — the SMALLER the number, the BETTER the stereoacuity.

• Titmus (Wirt) test: a Polaroid-glasses booklet with the "housefly," animals, and graded circles; the fly gives a gross ~3,000 arc-second check for young children, and the circles grade fine stereopsis down to about 40 arc-seconds.
• Randot test: uses random-dot patterns that eliminate monocular contour cues, so a child cannot "cheat"; it grades from roughly 250 down to about 20 arc-seconds.
• Normal fine stereoacuity is about 40 arc-seconds or better. Poor or absent stereopsis suggests strabismus, amblyopia, or suppression.

Worth 4-dot test

The Worth 4-dot test separates fusion, suppression, and diplopia. The patient wears red/green glasses (conventionally red over the right eye, green over the left) and views a flashlight showing four lights: one red (top), two green (sides), and one white (bottom). The white dot is visible to both eyes.

Because the target subtends a larger angle up close, testing at near stimulates peripheral fusion, whereas testing at distance (6 m) probes central fusion; a patient may fuse at near yet suppress at distance. When five dots are seen, their crossed or uncrossed arrangement shows whether the deviation is eso or exo.

The 4-prism-diopter base-out test

The 4Δ base-out test screens for a small central suppression scotoma, as in microtropia or monofixation syndrome, when stereopsis is reduced but cover testing looks normal. A 4Δ base-out prism is placed suddenly before one eye while the examiner watches for refixation movements. In a normal binocular patient, both eyes shift and then the uncovered eye makes a fusional (recovery) movement. When the prism is placed over an eye with a central scotoma, NO refixation movement occurs — the absent response reveals the suppression.

Fusional vergence amplitudes and clinical use

Beyond detecting fusion, the technician may measure fusional vergence amplitudes — how much prism the patient can overcome while keeping a single image. Base-out prisms test convergence (positive fusional vergence); base-in prisms test divergence (negative fusional vergence). Reduced convergence amplitudes accompany convergence insufficiency and correlate with the receded NPC described above. Bagolini striated lenses provide a more natural test of retinal correspondence and subtle suppression than the dissociating Worth 4-dot, because they barely disrupt fusion.

The three grades of binocular vision

Worth described three ascending grades: (1) simultaneous perception — each fovea perceives its own image at the same time; (2) fusion — the two images blend into one, with a range of motor fusion (fusional vergence amplitude) that keeps them single; and (3) stereopsis — the highest grade, giving true depth. A patient must possess the lower grades to achieve the higher, so stereoacuity is the most demanding proof of intact binocularity. Together these tests build a clinical picture: a receded NPC with normal stereopsis points to convergence insufficiency, while reduced stereopsis plus a suppression response on Worth 4-dot points to strabismic amblyopia.