9.2 Quantitative Strabismus Measurements: Alternate Cover Test, Prism Cover Test & Lancaster Red-Green / Hess Screen

Quantitative Strabismus Measurements: Alternate Cover Test, Prism Cover Test & Lancaster Red-Green / Hess Screen

Core Clinical Mandate: Accurate quantification of ocular deviations is the cornerstone of pediatric ophthalmology, neuro-ophthalmic monitoring, and strabismus surgical planning. The Certified Ophthalmic Medical Technologist (COMT) must possess mastery over both objective corneal reflex estimates (Hirschberg and Krimsky) and dissociative subjective/objective cover tests. Understanding how to differentiate a manifest tropia from a latent phoria, how to neutralize complex deviations without prismatic optical artifact, and how to interpret diagnostic tangent screens (Lancaster Red-Green and Hess Screen) is critical for diagnosing cranial neuropathies, orbital restrictions, and monofixation syndrome.

Objective Corneal Light Reflex Testing: Hirschberg & Krimsky

Corneal light reflex tests are indicated when a patient cannot cooperate with subjective cover testing (e.g., infants, toddlers, uncooperative or cognitively impaired adults) or when profound sensory amblyopia/dense suppression prevents foveal fixation.

1. The Hirschberg Test

• Principle: A penlight or muscle light is held at 33 cm directly in front of the patient's face along their midline while the patient looks at the light. The examiner observes the position of the First Purkinje-Sanson image (corneal reflection) relative to the center of the pupil in each eye.
• Physiological Angle Kappa: In normal eyes, the anatomical pupillary axis does not perfectly coincide with the visual axis. Most individuals exhibit a small positive angle kappa (corneal reflex displaced approximately 0.5 mm nasal to pupil center), which mimics a mild pseudo-exotropia. A negative angle kappa (reflex temporal to center) mimics pseudo-esotropia.
• Mathematical Calibration:
  • 1 mm of corneal reflex decentration $\approx$ 22 Prism Diopters ($\Delta$) $\approx$ 15° of ocular deviation.
  • 0.5 mm decentration: $\approx$ 10–11 $\Delta$ ($\approx$ 7°).
  • 2.0 mm decentration (pupillary border in a 4 mm pupil): $\approx$ 45 $\Delta$ ($\approx$ 30°).
  • 4.0 mm decentration (mid-iris stroma): $\approx$ 90 $\Delta$.
  • 6.0 mm decentration (corneal limbus): $\approx$ 120–135 $\Delta$ ($\approx$ 45°).

Hirschberg Corneal Reflex Displacement Matrix:
  [Temporal Displacement]  -->  ESOTROPIA (Reflex displaced outward)
  [Nasal Displacement]     -->  EXOTROPIA (Reflex displaced inward)
  [Inferior Displacement]  -->  HYPERTROPIA (Reflex displaced downward)
  [Superior Displacement]  -->  HYPOTROPIA (Reflex displaced upward)

2. The Krimsky Test & Modified Krimsky

• Classic Krimsky Method: Prisms are placed in front of the deviating eye until the corneal light reflex is centered identically to that of the fixating eye. Because viewing the corneal reflex through strong prisms introduces distortion and prism reflections, this technique can be technically challenging.
• Modified Krimsky Method: Prisms are placed in front of the FIXATING (sound) eye. By forcing the fixating eye to shift toward the prism apex, Hering's Law drives a conjugate version movement in the deviating eye, bringing its corneal reflex into symmetrical central alignment. Prisms are adjusted until the corneal reflections in both eyes are perfectly symmetrical.

3. The Brückner Test

• Technique: Performed using a direct ophthalmoscope at 1 meter distance in a darkened room, viewing both pupils simultaneously through the large aperture.
• Interpretation: In orthophoric, identical eyes, the retinal red reflexes are symmetrical in color, brightness, and hue. An eye that is strabismic, amblyopic, or has high anisometropia or media opacities displays an asymmetrical, brighter, lighter red-orange reflex due to light reflecting off non-foveal retinal tissue.

Diagnostic Cover Testing Hierarchy

Cover testing is the definitive standard for evaluating ocular alignment. It requires three clinical prerequisites: binocular vision potential, adequate visual acuity to fixate, and the presentation of an accommodative fixation target (e.g., a single Snellen letter 1–2 lines above threshold acuity) to control accommodation.

1. The Cover-Uncover Test (Detection of Tropias vs. Phorias)

• Purpose: Differentiates a tropia (manifest deviation) from a phoria (latent deviation held in check by binocular fusional vergence).
• Protocol:
  1. The patient fixes steadily on the accommodative target.
  2. The examiner covers one eye (e.g., OD) and carefully observes the UNCOVERED eye (OS).
     • If OS makes a refixation movement, a tropia is present in OS.
       • OS moves from temporal to center: Exotropia (XT).
       • OS moves from nasal to center: Esotropia (ET).
       • OS moves from downward to center: Hypotropia (HypoT).
       • OS moves from upward to center: Hypertropia (HyperT).
     • If OS remains completely stationary, OS does not have a manifest tropia.
  3. Uncover OD and allow binocular fusion to re-establish for several seconds.
  4. Repeat by covering OS while observing the uncovered OD.
  5. Finally, observe the eye being uncovered: if the covered eye drifted while occluded and rapidly shifts back to straight when uncovered, a phoria is present.

2. The Alternate (Cross) Cover Test

• Purpose: Measures the TOTAL deviation (latent phoria plus manifest tropia) by completely disrupting and suspending binocular fusion.
• Protocol:
  • The occluder is shifted rapidly back and forth between the right and left eyes, allowing 2 to 3 seconds of occlusion over each eye without permitting the patient even a fraction of a second of binocular viewing.
  • The eye being uncovered is observed for its refixation recovery shift.
  • Because binocular fusion is thoroughly broken, the magnitude of deviation revealed by the alternate cover test represents the total physiological dissociation.

3. The Alternate Prism Cover Test (APCT)

• Prism Neutralization: Prisms of progressive dioptric power are placed in front of one eye while the alternate cover test is performed.
• Neutralization Endpoint: Prisms are increased until all refixation movement of the eyes ceases completely. Continuing to increase prism power until an initial reversal of movement is observed (overcorrection) confirms the true neutral endpoint.

4. The Simultaneous Prism Cover Test (SPCT)

• The Microtropia / Monofixation Dilemma: In patients with monofixation syndrome or microtropia, a tiny manifest tropia (e.g., 4 $\Delta$ to 8 $\Delta$) is stabilized by peripheral binocular fusion, alongside a larger underlying latent phoria (e.g., 14 $\Delta$). If the technologist performs an alternate cover test, peripheral fusion is broken, revealing the total 14 $\Delta$ deviation, masking the true microtropia!
• SPCT Protocol: The neutralizing prism is placed over the deviating eye at the exact same split-second that the fixating eye is covered by the occluder. Because the eyes are never dissociated for prolonged periods, peripheral fusion is preserved, allowing pure measurement of the manifest tropia alone.

Prism Orientation & Neutralization Rules

A prism refracts light toward its base, which causes the perceived optical image to shift toward its apex. Therefore, to neutralize an ocular deviation, the prism must be oriented so that light is redirected precisely onto the displaced fovea:

$\mathbf{\text{Universal Rule:}} \quad \text{Apex points in the direction of the deviation; Base points OPPOSITE the deviation.}$

Prismatic Orientation Matrix

Prism Base Orientation Rule:
        [Base-OUT]                     [Base-IN]
   <-- B        A -->               <-- A        B -->
      Apex points IN                     Apex points OUT
  (Neutralizes ESOTROPIA)            (Neutralizes EXOTROPIA)

Prismatic Placement Artifacts: Frontal Plane vs. Prentice Position

• Plastic Prism Bars: Calibrated for use in the frontal plane position (anterior surface of the prism parallel to the patient's face). Tilting a plastic prism bar creates severe optical power errors.
• Glass Prisms: Calibrated in the Prentice position (posterior surface perpendicular to the visual axis / line of sight).
• Prism Splitting: In large deviations (>20 $\Delta$), placing total prism power over one eye introduces distortion and asymmetric chromatic dispersion. Prisms should be split equally between both eyes (e.g., a 40 $\Delta$ ET is split into 20 $\Delta$ BO OD and 20 $\Delta$ BO OS).

Tangent Screen Dissociation: Lancaster Red-Green & Hess Screen

The Lancaster Red-Green Test and the Hess Screen are sophisticated diagnostic tangent screen evaluations designed to plot ocular motility, diagnose paretic vs. restrictive strabismus, and quantify cyclotropia across nine diagnostic fields of gaze.

1. The Lancaster Red-Green Test: Setup and Physics

• Optical Dissociation: The patient wears red-green goggles (by standard convention, the Red filter is placed over the Right Eye [OD], and the Green filter is placed over the Left Eye [OS]).
• Screen: A white or gray tangent screen calibrated in 1° squares (or 2 $\Delta$ per centimeter grid) at a distance of 1.0 meter or 2.0 meters.
• Projectors:
  • The examiner holds a flashlight projecting a red streak.
  • The patient holds a flashlight projecting a green streak.
  • Optical Filter Property: An eye covered with a red filter can only see the red light; an eye covered with a green filter can only see the green light. Thus, the two eyes are 100% dissociated.

2. Examination Protocol

1. The examiner projects the red streak onto the central fixation target on the screen (the right eye, viewing through the red filter, fixates this streak).
2. The patient is instructed to project the green streak and overlap it perfectly with the red streak.
3. The patient's left eye (viewing through green) shifts to place the green streak onto its fovea. If the left eye is misaligned, the green streak will be separated from the red streak on the screen.
4. The examiner repeats this projection in all nine diagnostic positions of gaze.
5. Reversal of Filters: The red-green goggles are then reversed (Red over OS, Green over OD) and the test is repeated, allowing fixation to be tested with the contralateral eye.

3. Chart Interpretation Rules: Underactions vs. Overactions

When viewing the completed Lancaster or Hess chart, the technologist applies three strict neuro-ophthalmic rules:

1. The Smaller (Contracted) Field Represents the Paretic Eye: The eye with the underacting muscle cannot move fully into its field of action; therefore, its projected field on the chart is compressed and smaller than normal.
2. The Larger (Expanded) Field Represents the Contralateral Normal Eye: When the paretic eye fixates, Hering's Law demands excessive innervation to hold fixation, driving the contralateral yoke muscle into massive overaction. This produces an abnormally large, expanded plot for the non-paretic eye.
3. Torsional Deviations (Cyclotropia): Because the Lancaster test projects linear slits (streaks) of light rather than round dots, the patient can rotate their streak until it appears parallel to the examiner's streak. If the patient must tilt their green streak to make it appear straight, cyclotropia is directly measured in degrees.

Comparison: Lancaster Red-Green vs. Hess Screen

| Feature | Lancaster Red-Green Test | Hess Screen Test | | :--- | :--- | :--- | | | Target Geometry | Linear streaks (allows torsion measurement) | Circular dots or intersection points | | Working Distance | Typically 1.0 m or 2.0 m | Typically 0.5 m (50 cm) screen | | Testing Coordinates | Measures deviation in Prism Diopters / degrees | Measures deviation directly in degrees | | Primary Utility | In-depth cyclodeviation & incomitance | Rapid mapping of neurogenic paresis |