Binocular Vision Assessment Clinical Guide

Purpose: Detects a manifest deviation (tropia). If a tropia is present, there is no need to perform the alternating cover test first — document the tropia.

1. Ask patient to fixate the distance target.
2. Cover one eye rapidly with the occluder. Observe the uncovered eye for any refixation movement.
3. If the uncovered eye moves to pick up fixation → strabismus (tropia) is present:
   • Moves inward (nasally) → exotropia (outward turn of the covered eye)
   • Moves outward (temporally) → esotropia (inward turn of the covered eye)
   • Moves upward → hypotropia of the uncovered eye (or hypertropia of the covered eye)
   • Moves downward → hypertropia of the uncovered eye (or hypotropia of the covered eye)
4. Now cover the other eye and observe the first (previously covered) eye for movement. This determines which eye is the deviated eye.
5. Remove the cover and observe whether the eye returns to its original position (indicates some fusional potential) or remains in the deviated position (constant tropia).
6. Repeat for near fixation at 40 cm.

Purpose: Breaks down binocular fusion to reveal the full latent deviation (phoria) plus any tropia. The ACT maximally dissociates the eyes and reveals the total deviation.

1. Ask patient to fixate the distance target.
2. Cover one eye for 2–3 seconds, then rapidly transfer the cover to the fellow eye. Continue alternating smoothly and rhythmically at approximately 1-second intervals.
3. After 3–4 alternations, observe the eye that has just had the cover removed. Look for a movement as the eye returns to — or moves towards — the fixation target.
4. Direction of movement of the uncovered eye:
   • Moves inward (nasally) → exophoria / exotropia (eye was drifting out under cover)
   • Moves outward (temporally) → esophoria / esotropia (eye was drifting in under cover)
   • Moves downward → hyperphoria (eye drifted up under cover)
   • No movement → orthophoria
5. Repeat for near fixation at 40 cm.
6. After the ACT, perform the cover-uncover test to check recovery (the speed and smoothness with which fusion is re-established when the cover is removed).

Purpose: Quantifies the magnitude of any detected deviation in prism dioptres (Δ). The gold standard for strabismus measurement in clinical practice.

1. Perform the ACT first to confirm the direction and presence of a deviation.
2. Place a prism (from a prism bar or loose prisms) in front of the deviating eye, apex in the direction of the deviation:
   • Esophoria/esotropia → Base-Out (BO) prism
   • Exophoria/exotropia → Base-In (BI) prism
   • Right hyperphoria → Base-Down (BD) prism over right eye (or Base-Up over left)
3. Continue the ACT while increasing prism power until no movement is observed (neutralisation point).
4. The prism power at neutralisation = the angle of deviation in prism dioptres.
5. Measure at both distance (6 m) and near (40 cm) to detect divergence excess, convergence insufficiency, and basic deviations.

Purpose: Estimates angle of strabismus by observing the position of the corneal light reflex relative to the pupil centre. Useful in uncooperative children and patients who cannot maintain fixation.

1. Shine a penlight from ~33 cm directly in front of the patient.
2. Ask patient (or child) to look at the light.
3. Observe the position of the corneal light reflex in each eye.
4. Interpreting reflex position:
   • Centred in both pupils = orthophoria / orthotropia
   • Reflex displaced temporally (towards the ear) = esotropia (~15Δ per mm of displacement)
   • Reflex displaced nasally (towards the nose) = exotropia
   • Reflex displaced inferiorly = hypertropia; superiorly = hypotropia

Krimsky test: Adds prisms over the fixating eye until the reflexes are centred in both eyes, giving a quantified measurement of the deviation.

The ocular motility exam is a simple yet profoundly important part of the ophthalmic examination. Six extraocular muscles act to move the eye up/down, left/right, and intort/extort in a complex combination of agonist and antagonist cooperation. Extraocular muscles have the densest ratio of motor neurons to muscle fibres of any muscle in the body, facilitating the fine motor control required for precise binocular alignment.

Purpose: Assesses conjugate movement of both eyes simultaneously in all positions of gaze. The H-pattern targets each cardinal position, where one muscle primarily drives the movement, making underactions and overactions easier to isolate and identify.

Step-by-Step Procedure:

1. Assess primary gaze alignment first. Before any movement, observe the eyes in the primary position (straight ahead). Consider using the Hirschberg corneal reflex test to document baseline alignment.
2. Instruct the patient to hold their head still. Gently stabilise the head with your hand on the forehead if needed, especially in children or patients with head tremor.
3. Select a target: Use your finger, a penlight, or a small toy (for children). Hold the target approximately 40–50 cm from the patient's face at eye level.
4. Trace the H-pattern. Move the target slowly and smoothly through the six cardinal positions in the shape of an “H”:
   • Move right (right lateral rectus / left medial rectus)
   • From right gaze, move up (right superior rectus)
   • From right gaze, move down (right inferior rectus)
   • Return to centre, then move left (left lateral rectus / right medial rectus)
   • From left gaze, move up (left superior rectus)
   • From left gaze, move down (left superior oblique)
5. Observe each eye separately in every position. Alternate your gaze between the left and right eye at each cardinal position — do not watch both simultaneously. Assess each eye's movement independently for full and equal excursion.
6. Ask about diplopia in each position of gaze. Even a subtle palsy may only produce diplopia in the field of action of the paretic muscle without a visible deviation.
7. Observe for:
   • Underaction: Reduced movement of one eye in a particular direction (muscle paresis or mechanical restriction).
   • Overaction: Excessive movement — often a secondary change per Hering's Law in the context of a paretic deviation.
   • Incomitance: Deviation magnitude changes in different gaze positions. Comitant deviations remain equal in all positions.
   • Nystagmus: Rhythmic oscillation in any gaze position — document direction, type, and worst position.
   • Lid changes: Ptosis in downgaze (CN III); lid retraction in upgaze (thyroid eye disease).
8. Grade each muscle −4 (complete underaction) to +4 (overaction), with 0 = full and normal. Document findings for each of the six muscles in each eye.

Purpose: Tests movement of each eye independently with the fellow eye occluded. Differentiates between a true muscle paresis (weakness) and mechanical restriction, and confirms the affected muscle when versions suggest an abnormality.

1. Cover the fellow eye.
2. Move the fixation target through all positions of gaze for the uncovered eye only.
3. Compare duction amplitude to the normal expected range.
4. If ductions are full but versions show underaction → suggests a paretic strabismus (the paretic muscle can move the eye when innervation demand is not shared via Hering's law).
5. If ductions are also restricted → suggests mechanical restriction (e.g., thyroid eye disease, blow-out fracture with entrapment, Brown syndrome).

Purpose: Assesses the ability to maintain foveal fixation on a slowly moving target. Smooth pursuits are controlled by the parieto-occipital cortex, cerebellum, and brainstem. Deficits indicate neurological, cerebellar, or posterior cortical pathology.

1. Hold the fixation target (penlight or accommodative target) approximately 40–50 cm from the patient.
2. Instruct patient: "Follow the target with your eyes without moving your head."
3. Move the target slowly and smoothly in a large H-pattern (horizontal then vertical), then a circle, at approximately 20°/s. The entire movement should take 3–5 seconds per direction.
4. Observe eye movements. Normal pursuit is smooth and continuous — the eyes follow the target without lag, catch-up, or interruption.
5. Abnormal findings:
   • Saccadic pursuits (cogwheel): Eyes make small jerky saccades instead of smooth movement. May be seen in neurological disease (multiple sclerosis, Parkinson's disease, cerebellar ataxia), or as a normal finding in older patients and very young children.
   • Pursuit lag with corrective saccades: Eye falls behind the target and makes catch-up saccades. Indicates pursuit gain <1.
   • Asymmetric pursuits: Smooth in one direction but saccadic in the other. Cortical or cerebellar hemisphere disease.
6. Use a grading scale for documentation:
   Grade 4: Normal, smooth, full-range tracking. Grade 3: Mild saccadic intrusions. Grade 2: Moderate saccadic pursuit. Grade 1: Severely disrupted, mostly saccadic. Grade 0: Unable to pursue at all.

Purpose: Assesses voluntary gaze shifts. Saccades are rapid, high-velocity eye movements that shift gaze from one point to another. Saccadic assessment evaluates latency, velocity, accuracy, and conjugacy. Abnormalities suggest frontal lobe, brainstem, or cerebellar pathology.

1. Place two targets (e.g., two penlights, or use your two index fingers) approximately 40–50 cm from the patient, separated horizontally by ~30–40°.
2. Ask patient to look from one target to the other when you indicate (e.g., by pointing or tapping).
3. Alternate between the targets 5–10 times. Repeat for vertical saccades (two vertically separated targets).
4. Assess the following:
   • Latency: Time from stimulus to onset of saccade. Normal ~200 ms. Increased in frontal lobe lesions, Parkinson's disease, schizophrenia.
   • Velocity: Speed of movement. Slow (low-velocity) saccades suggest internuclear ophthalmoplegia (INO), progressive supranuclear palsy (PSP), myasthenia gravis.
   • Accuracy: Does the eye land on the target? Undershoot (hypometria) is common in cerebellar disease. Overshoot (hypermetria) followed by a corrective saccade.
   • Conjugacy: Do both eyes move equally? Disconjugate saccades (one eye undershoots) suggest INO — the adducting eye is slow/absent in horizontal gaze.

Nystagmus is rhythmic, involuntary oscillation of the eyes. It should be documented in terms of: direction (horizontal, vertical, torsional, mixed), type(jerk — fast and slow phase; pendular — equal velocity both directions), gaze positionin which it appears, null point (gaze position in which nystagmus is minimal), and effect on vision.

Purpose: Measures the closest point at which the eyes can maintain single binocular vision by converging. A receded NPC is a hallmark finding in convergence insufficiency (CI).

1. Ask patient to wear their habitual correction (including reading glasses if worn for near).
2. Hold a small target (RAF rule with an accommodative target, e.g., small letter or picture) at arm's length (~50 cm) in front of the patient's midline.
3. Instruct patient: "Keep the target as one single clear image. Tell me immediately when it doubles or becomes blurry."
4. Slowly move the target towards the patient's nose at a rate of approximately 1–2 cm per second.
5. Break point: Record the distance at which the patient reports diplopia, or at which you observe one eye deviate outward (objectively confirm by watching for outward drift).
6. Slowly move the target back away. Recovery point: Distance at which single vision is re-established.
7. Repeat 3 times and record the average break and recovery distances.

Purpose: Measures the minimum distance at which print can be held clearly with maximum accommodation. Used to calculate amplitude of accommodation and detect accommodative insufficiency.

1. Test each eye separately first, then binocularly. Patient wearing distance correction (not reading addition).
2. Use a near vision chart (N5 or equivalent) on a RAF rule or ruler.
3. Begin with the target at 50 cm. Slowly advance the target towards the patient's eye.
4. Subjective break point: Distance at which patient reports the smallest letters first become blurred.
5. Move away until clear again (recovery point).
6. Calculate amplitude of accommodation: A = 1 / NPA (in metres). e.g., NPA = 10 cm = 0.10 m → A = 10 D.

Purpose: Measures the speed and flexibility of accommodation — the ability to switch focus between distances. Complements the NPA assessment. Particularly relevant in patients with asthenopic symptoms related to near work.

1. Use ±2.00D flipper lenses (for binocular testing) or ±2.00D monocular flippers.
2. Patient fixates a near chart (N5 or equivalent) at 40 cm.
3. Flip the +2.00D lens in front of the patient. When clear, they report "clear" and you flip to −2.00D. Each flip = one cycle.
4. Record the number of cycles per minute (cpm) for 1 minute.
5. Norms:
   • Binocular: ≥8 cpm (adults); monocular: ≥11 cpm (adults)
   • Fails with plus (cannot clear +2.00): suggests convergence excess or accommodative excess (high lag)
   • Fails with minus (cannot clear −2.00): suggests accommodative insufficiency or reduced amplitude

Purpose: Assesses sensory fusion and detects suppression or diplopia. Uses two targets (distance and near versions) viewed through red/green dissociating glasses.

1. Patient wears red filter over the right eye and green filter over the left eye (or vice versa — the convention should be consistent within the practice).
2. Patient views the Worth 4-dot torch: one red light (top), two green lights (sides), one white light (bottom).
3. Responses:
   • 4 dots (fusion): Red eye sees red + white; green eye sees green + white. Both perceived together = fusion. Normal.
   • 2 dots only (right eye suppression): Patient sees only the red/white dots — left (green) eye is being suppressed.
   • 3 dots only (left eye suppression): Patient sees only the green dots — right (red) eye is being suppressed.
   • 5 dots (diplopia): Patient sees 2 red and 3 green = no fusion, diplopia present.
   • Alternating 2 then 3 dots: Alternating suppression — consistent with alternating strabismus.
4. Test at both distance (6 m) and near (40 cm) — suppression may be present at one distance and not the other.

Maddox rod: A red cylindrical lens that transforms a white point light into a red streak perpendicular to the cylinder axis. Placed before one eye to dissociate binocular vision. Used with a prism bar to measure heterophoria at distance. If the red streak passes through the light, the patient is orthophoric. If displaced nasally → exophoria; displaced temporally → esophoria. A vertical rod detects vertical phoria.

Maddox wing: A handheld instrument for near testing. Patient sees a white arrow (right eye) and a red arrow (left eye) via dissociating septum. Arrow position in the numbered scale gives horizontal phoria directly in prism dioptres. A second scale gives vertical phoria. Norms: Horizontal ±2Δ exophoria (ESO to 6Δ EXO); vertical ≤0.5Δ.

Vision therapy (VT) is a programme of supervised eye movement and binocular vision exercises aimed at improving oculomotor control, vergence, accommodation, and sensory fusion. It is indicated for:

• Convergence insufficiency: Strong Level 1 evidence (CITT trial) supports in-office VT as the most effective treatment. Programme typically 12 weeks, 1 hour/week in-office + daily home exercises.
• Accommodative insufficiency and excess: Accommodative facility training with ±2.00D flippers.
• Oculomotor dysfunction: Saccadic and pursuit training for reading-related eye movement deficits.
• Intermittent exotropia: Antisuppression therapy, convergence training, divergence control. Surgery reserved for large angles not controlled by VT.
• Decompensated heterophoria: Fusional vergence range expansion; symptom relief without long-term prism reliance.