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Workup of Strabismus: Motor and Sensory Evaluation

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Overview

1. Motor evaluation - quantifying and characterizing the ocular deviation
2. Sensory evaluation - assessing binocular function, fusion, suppression, and retinal correspondence

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I. MOTOR EVALUATION

1. History

• Age of onset (congenital vs. acquired)
• Constant vs. intermittent deviation
• Which eye deviates (unilateral vs. alternating)
• Associated diplopia, headaches, or ocular pain
• Family history of squint or amblyopia
• Birth history, developmental milestones
• Previous glasses, patching, or surgery

2. Visual Acuity Assessment

• Fixation and following behavior: A face or bright target is used to assess visual alertness. Occlusion of one eye and observation of the child's objection reveals unilateral preference (strong objection = poor vision in the unoccluded eye).
• Fixation grading: Fixation is promoted in the squinting eye and graded as: central/non-central, and steady/unsteady (CSM - central, steady, maintained). If fixation returns immediately to the uncovered dominant eye after un-occlusion, VA is likely impaired in the squinting eye. If fixation is maintained through a blink, VA is probably good.
• 10-prism diopter (Δ) vertical prism test: A 10Δ vertical prism is used to provoke diplopia. Alternation between targets suggests equal VA.
• Rotation test: The child is rotated briskly through 360°. With normal vision, rotational nystagmus is suppressed by fixation when rotation stops. With severely impaired vision, post-rotatory nystagmus persists.
• Preferential looking tests: Teller acuity cards, Keeler acuity cards, Cardiff acuity cards - based on the infant's preference to look at patterned vs. homogeneous stimuli.

• Snellen VA chart (6 m) or LogMAR chart
• Near VA (N-notation)
• Pinhole VA to distinguish refractive from pathological causes

3. Refraction

• Cyclopentolate 1% (atropine in infants) is used
• Identifies accommodative components of esotropia
• Anisometropia is a risk factor for amblyopia

4. Corneal Reflex Tests (Objective Assessment of Deviation)

Hirschberg Test

• Each 1 mm of reflex displacement ≈ 7° (approximately 15Δ)
• Reflex at pupil border ≈ 10-15°
• Reflex at mid-iris ≈ 20-25°
• Reflex at limbus ≈ 40-45°

Krimsky Test

Prism Reflection Test

5. Cover Testing

a. Cover Test (detects manifest deviation - heterotropia)

• The patient fixates on a target.
• One eye is covered and the uncovered (suspected deviating) eye is watched for a movement to take up fixation:
  • No movement = orthotropia or the covered eye was deviated
  • Adduction (inward movement) = exotropia of uncovered eye
  • Abduction (outward movement) = esotropia of uncovered eye
  • Downward movement = hypertropia; upward = hypotropia
• The test is repeated for the other eye.

b. Uncover Test (detects latent deviation - heterophoria)

• After the cover test, the occluder is removed and the previously covered eye is watched:
  • No movement = orthophoria
  • Adduction to fixation = exophoria
  • Abduction to fixation = esophoria

c. Alternate Cover Test (dissociation test - reveals total deviation)

• The occluder is rapidly alternated between the two eyes, preventing fusion and revealing the maximum total deviation (phoria + tropia).
• After removal of the cover, the speed and completeness of recovery toward fusion is noted.
• This test should always be performed after the cover-uncover test.

Key tip: The alternate cover test dissociates fusion and so cannot differentiate tropia from phoria alone - the cover-uncover test must be done first.

6. Prism Cover Test (Quantification of Deviation)

1. The alternate cover test is performed first to establish the direction and approximate size of deviation.
2. Prisms of increasing strength are placed in front of one eye, with the base opposite to the direction of deviation:
   • Esotropia → base-out prism
   • Exotropia → base-in prism
   • Right hypertropia → base-down prism in front of right eye
3. The test is repeated while the prism strength is increased until no movement is seen on alternate cover testing (neutralization point).
4. Performed in the primary position (distance and near) and in 8 diagnostic positions of gaze.
5. In patients with intermittent exotropia, also tested with far-distance fixation.

• Angle of deviation in Δ (primary position, near and distance)
• Comitance or incomitance of the deviation across gaze positions

7. Ocular Motility Examination

• Primary position
• Up-right, up, up-left
• Right, left
• Down-right, down, down-left

Relevant Laws of Ocular Motility (governing interpretation):

• Sherrington's law of reciprocal innervation: When an agonist contracts, its ipsilateral antagonist relaxes (e.g., right medial rectus contraction → right lateral rectus relaxation).
• Hering's law of equal innervation: In any conjugate (version) movement, equal innervation flows simultaneously to both yoke muscles. This means in a paretic squint, the angle of deviation varies with which eye fixates:
  • Primary deviation = deviation when the normal eye fixes
  • Secondary deviation = deviation when the paretic eye fixes (always larger than primary deviation in a fresh palsy)

1. Primary underaction (e.g., left superior oblique)
2. Secondary overaction of contralateral synergist/yoke muscle (right inferior rectus - Hering's law)
3. Secondary overaction and contracture of ipsilateral antagonist (left inferior oblique - Sherrington's law)
4. Secondary inhibition of contralateral antagonist (right superior rectus)

Head Posture Assessment:

• Face turn: adopted for horizontal deviations. The face turns toward the field of action of the weak muscle (e.g., left face turn in left lateral rectus palsy), directing the eyes away from the area of maximum diplopia.
• Head tilt: for torsional/vertical diplopia. In right superior oblique weakness, the head tilts to the left (toward the hypotropic eye) to minimize vertical and torsional disparity. The Bielschowsky head tilt test exploits this - tilting toward the paretic eye worsens a vertical deviation.
• Chin elevation/depression: for weakness of elevators/depressors or A/V pattern deviations.

8. Measurement of Angle: Near vs. Distance

• High AC/A: deviation much larger at near than distance → accommodative component significant
• Normal AC/A: equal deviation at near and distance

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II. SENSORY EVALUATION

Background: Sensory Adaptations to Strabismus

• Central suppression: foveal image of deviating eye inhibited (avoids confusion)
• Peripheral suppression: peripheral image of deviating eye inhibited (avoids diplopia)
• Monocular suppression: constant suppression of the same eye → leads to amblyopia
• Alternating suppression: switches between eyes → amblyopia less likely
• Facultative: present only when eyes are misaligned (e.g., intermittent exotropia)
• Obligatory: present even when eyes appear straight

• A positive sensory adaptation (vs. negative adaptation by suppression)
• Allows anomalous binocular vision in the presence of small-angle manifest squint
• Most common in microtropia (small-angle esotropia)
• Binocular responses in ARC are never as good as in normal bifoveal BSV

• Foveal suppression → avoids confusion
• Peripheral suppression → avoids diplopia
• Constant monocular suppression → strabismic amblyopia
• ARC → anomalous BSV in small deviations

Sensory Tests

1. Worth Four-Dot Test

2. Bagolini Striated Glasses Test

• Each lens has fine striations converting a point light source into a streak line.
• The two lenses are placed at 45° and 135°, so each eye sees a perpendicular streak.
• If BSV: a complete cross (X) is seen through the point source.
• If a gap in one streak: central suppression scotoma (microtropia).
• If only one streak: suppression of the other eye (no simultaneous perception).
• If two non-crossing lines: diplopia.

3. Maddox Rod Test

• A Maddox rod converts a white point light into a red streak perpendicular to the cylinders.
• One eye sees a white spot; the other sees a red streak.
• The relative position of the two images indicates the type and direction of phoria.
• Quantified by placing prisms in front of one eye until images are superimposed.

4. 4-Prism Diopter (4Δ) Base-Out Test

• In normal BSV: placing a 4Δ base-out prism in front of one eye moves the image off the fovea, eliciting a refixation movement in that eye and a subsequent fusional convergence movement in the fellow eye.
• In microtropia with CSS: placing the prism over the microtropic eye produces no movement (image falls within the suppression scotoma); placing it over the normal eye produces only a refixation movement without a fusional movement in the microtropic eye.

5. Stereoacuity Tests

• Titmus/Wirt Fly test: Near test (40 cm), detects gross stereopsis (fly = 3000 arc sec), rings/animals (400-100 arc sec), circles (40 arc sec)
• TNO test: Uses red-green anaglyph; random-dot pattern; detects down to 15-480 arc sec
• Lang stereotest: No glasses needed (lens grating system); useful in children; detects 550-1200 arc sec
• Frisby test: Real-depth stereoacuity (no glasses); detects 15-600 arc sec

6. Synoptophore (Major Amblyoscope)

• The objective angle is measured by positioning the arms until the corneal reflexes are centered (not relying on the patient's perception).
• The subjective angle is determined by asking the patient to superimpose the two slides.
• The difference between subjective and objective angles = angle of anomaly (AoA):
  • AoA = 0 → Normal retinal correspondence (NRC)
  • AoA = angle of squint → Harmonious ARC (HARC)
  • 0 < AoA < angle of squint → Unharmonious ARC

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III. ADDITIONAL INVESTIGATIONS

Fundus Examination

• Disc tilt, macular ectopia (as in certain forms of nystagmus or structural causes of sensory strabismus)
• Fundus torsion (elevated in superior oblique palsy)
• Cyclovertical deviations are assessed with fundoscopy using the Guyton disc rotation method

Cycloplegic Refraction

• Mandatory in all children with strabismus
• Reveals latent hyperopia (main refractive basis for accommodative esotropia)
• Prescribing the full cycloplegic correction often partially or fully corrects accommodative esotropia

Sensory (Visual) Evoked Potentials

• In infants and young children when behavioral testing is unreliable
• Detects amblyopia, cortical visual impairment, and binocular rivalry

Neuroimaging (CT/MRI)

• Indicated when a sixth nerve palsy, third nerve palsy, or vertical gaze palsy is suspected
• Restrictive vs. paralytic strabismus (e.g., thyroid eye disease, orbital fracture) may also require CT orbits

Forced Duction Test / Force Generation Test

• Forced duction test: passive rotation of the eye under local anesthesia using forceps; resistance indicates a mechanical/restrictive cause (e.g., Brown syndrome, Duane syndrome, thyroid myopathy, orbital floor fracture with entrapment).
• Force generation test: measures the active contractile force of a muscle; reduced in muscle palsy.

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Summary Table

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