Optometry Clinical Procedures and Analysis Notes

Stereoacuity Tests

Titmus Stereotest

• Purpose: Assesses stereopsis using disparity targets.

• Sections: Three main sections for different types of disparity targets.

  • Gross Stereopsis: Presented as a housefly, with an acuity of approximately 3000\'' (seconds of arc).

  • Graded Stereopsis (Younger Child): Three rows of animal figures.

  • Graded Stereopsis (Older Patient): Nine diamonds, known as Wirt rings.

• Procedure:

  1. Place polarizing glasses on the patient over their best correction in full illumination.

  2. Present the test booklet at $40\text{cm}$.

  3. Direct the patient's attention to the housefly. If appreciated, gross stereopsis is present.

  4. For young patients, present animal targets and ask them to point to the animal that "pops out" or is "out of line."

     • Cat: 400\''

     • Rabbit: 200\''

     • Monkey: 100\''

  5. For older patients (or after animals for young patients), proceed to Wirt rings. This step can be used directly for adult patients.

  6. Direct attention to diamond 1 and describe that one of the circles should appear to be "floating out."

  7. Ask the patient to identify the floating circle by reporting its position (top, bottom, right, or left).

  8. Continue through all nine diamonds if responses are consistently correct.

  9. If a patient misses one but then continues with correct responses, recheck the missed diamond.

  10. If a patient misses several consecutively, the last correct response is recorded as the stereoacuity.

• Recording:

  • Record the presence or absence of gross stereopsis.

  • Record the quantitative level of stereopsis based on the best response from animals or Wirt rings.

  • Note any added lenses or prisms used during testing.

Lang Stereotest

• Purpose: A simple test for random dot stereopsis that does not require polarized filters.

• Procedure:

  1. Position the test at approximately $40\text{cm}$ under full room illumination.

  2. Ensure the test is held exactly at right angles to the visual axis.

  3. Ask the child to point to the location of the test targets or match them with a corresponding card.

  4. Lang II Specific: Includes a star figure with no disparity, which can be seen monocularly. This helps confirm the child understands the test instructions.

• Recording:

  • A positive result is indicated by correct localization and naming of objects, attempting to grasp them, or shifting fixation between objects.

  • Record the best stereoacuity level achieved.

Frisby Stereotest

• Purpose: Measures stereoacuity using random dot stereograms without the need for filters.

• Design: Consists of three Plexiglas plates with patterns printed on both sides. The stereo threshold is created by the thickness of the plates and the working distance.

• No Filters: Polarized filters are not necessary for this test.

• Procedure:

  1. Conduct the test with full room lighting, with the test plate aligned squarely to the patient, and against a plain white background.

  2. Hold the thickest plate ($6\text{mm}$) at a convenient distance for demonstration.

  3. Point out the "floating circle" or "hole."

  4. Ask the patient to locate the circle by verbally reporting or pointing. Once correctly located, rotate the plate to a new position.

  5. Ensure there is no head movement during the test.

  6. To assess stereoacuity, begin by presenting the $6\text{mm}$, $3\text{mm}$, and $1\text{mm}$ plates sequentially at a $40\text{cm}$ test distance.

  7. Present each plate at least four times, randomizing the circle's position with each presentation.

  8. If the patient correctly identifies the circle's position in each plate at $40\text{cm}$ at least three out of four times, this indicates a stereoacuity of at least 55\'' (seconds of arc).

  9. Continue testing using the $1\text{mm}$ plate at greater test distances to find the threshold.

• Recording:

  • Record test distances, plate thickness, and the number of correct responses out of the total presentations given.

    • Example: $60\text{cm}$, $1\text{mm}$ plate, 3 of 4 correct.

  • Record the resulting stereo threshold using a provided table.

Sensory Status in Strabismus and Anisometropia

Conditions Affecting Visual Development

• Strabismus: Misalignment of the eyes.

  • Can lead to diplopia (seeing the same object in two different spatial locations).

• Anisometropia/Deprivation: Significant difference in refractive error between the eyes, or an obstruction to vision (e.g., ptosis, congenital cataract).

  • Can lead to confusion (seeing different objects in the same spatial location).

Sensory Adaptations

Overview

• Sensory adaptations may occur if strabismus and/or anisometropia are present early in life.

Binocular Adaptations (Both Eyes Open)

• Suppression: The brain ignores the unwanted second image to avoid diplopia and confusion. While helpful binocularly, it can lead to amblyopia or eccentric fixation.

  • Developed to manage retinal image differences:

    • Differences in image clarity (anisometropia).

    • Differences in image size (aniseikonia).

  • Occurs in zones:

    • Foveal suppression avoids confusion.

    • Foveal suppression of the non-fixating eye occurs in strabismus.

    • Foveal suppression of the blurred image occurs in anisometropia.

  • Can be intermittent or constant.

• Anomalous Retinal Correspondence (ARC): An aberrant neural wiring of binocular information. A non-foveal area of the deviating eye acquires a common visual direction with the foveal area of the fixating eye.

  • In the central retina, normal retinal correspondence is more likely, otherwise suppression or amblyopia tends to occur.

  • In the periphery, binocular function is more resistant to loss, increasing the chance of ARC.

Monocular Consequences (Present When Fellow Eye Covered)

• Amblyopia: Degraded development of visual pathways, resulting in poor vision even with optimal refractive correction.

• Eccentric Fixation: Loss of foveal viewing preference, where fixation of an object occurs by a point other than the fovea, which then adopts the principal visual direction.

Evaluation of Anomalous Retinal Correspondence (ARC)

Clinical Tests for ARC

• Bagolini Striated Lenses

• Worth 4 Dot (W4D) Test

• After-image Test

• Comparison of subjective and objective angles of deviation (strabismus)

Bagolini Striated Lenses

• Principle: Lenses with fine parallel striations (at $45^\circ$ and $135^\circ$) are placed before both eyes. When viewing a spotlight, each lens produces a line image perpendicular to its striations.

• Testing Conditions: Performed under relatively natural conditions (normal room lighting) to allow fusion of surrounds.

• Patient Report: Patient reports the number of line images and spotlights seen, and the form the lines make.

• Interpretations:

  • A = Normal Fusion or ARC (with manifest strabismus): Two lines crossing through a single spotlight. If a cover test shows alignment, it's normal fusion. If strabismus is present with a cover test, it's ARC.

  • B = Normal Fusion or ARC with Central Suppression of Left Eye: A single spotlight with one continuous line (from the right eye) and a broken line (from the left eye, indicating central suppression).

  • C = Horizontal Diplopia (Left eye on right = Crossed Diplopia): Two spotlights and two lines, with the line from the left eye appearing to the right (crossed).

  • D = Horizontal Diplopia (Left eye on left = Uncrossed Diplopia): Two spotlights and two lines, with the line from the left eye appearing to the left (uncrossed).

  • E = Vertical Diplopia: Two spotlights and two lines, with one line appearing above or below the other.

  • F = Horizontal Uncrossed Diplopia (Large Angle): Similar to D, but implies a larger separation, often with the lines visible but widely separated.

  • G = Right Suppression: Only the line from the left eye is seen, indicating suppression of the right eye.

  • H = Left Suppression: Only the line from the right eye is seen, indicating suppression of the left eye.

Worth 4 Dot (W4D) Test

• Used to assess fusion, suppression, and ARC. A normal fusion response or ARC result (in the presence of strabismus) is indicated by seeing four dots.

After-image Test

• Principle: Uses a bright flash of light (e.g., electronic flash gun, synoptophore) to create after-images on the fovea/foveal projection of both eyes. It demonstrates the relationship between the foveal projections.

• Purpose: It is a test of retinal correspondence, comparing retinal areas that have the same visual direction during binocular single vision.

• Procedure:

  1. A horizontal line image is presented to the fixating eye first.

  2. A vertical line image is presented to the deviating eye.

• Interpretation:

  • Normal Retinal Correspondence (NRC): The horizontal and vertical after-images intersect centrally, forming a cross with the center at the fixation point.

  • Anomalous Retinal Correspondence (ARC): The horizontal and vertical after-images do not intersect centrally, or the intersection is displaced, indicating that the fovea of the deviating eye is not corresponding with the fovea of the fixating eye.

Subjective and Objective Angles of Strabismus

• Concept: A difference between the objective angle of strabismus (measured by a cover test) and the subjective angle of strabismus (measured by a Maddox rod or similar test) indicates ARC.

  • If the subjective angle is equal to the objective angle, then Normal Retinal Correspondence is present.

  • Formulaic Representation: \text{Objective Angle} - \text{Subjective Angle} > 0 \implies \text{ARC}

• Example:

  • Alternating cover test measures $20\Delta$ (prism diopters) esotropia (objective angle).

  • Maddox rod is aligned without prism (subjective angle = $0\Delta$).

  • 20\Delta - 0\Delta = 20\Delta > 0, therefore, Anomalous Retinal Correspondence is present.

Amblyopia

• Definition: A monocular consequence of binocular adaptation (suppression), characterized by poor vision due to abnormal visual development.

• Diagnosis Criteria:

  • Visual acuity (VA) deficit in one or both eyes, with optimum refractive correction in place (typically $\ge 2$ line difference between eyes).

  • Presence of an amblyogenic factor is essential for diagnosis:

    • Strabismus.

    • Significant refractive error (including anisometropia).

    • Deprivation (e.g., ptosis, congenital cataract).

  • Fixation preference with the non-amblyopic eye.

  • NO signs of other ocular pathology (e.g., retinal disease, optic nerve abnormalities).

• Clinical Importance: Cycloplegic refraction is crucial to confirm any refractive contribution to amblyopia and for a thorough examination through dilated pupils.

Eccentric Fixation

• Definition: A uniocular condition (present when the fellow eye is covered) in which an object is fixated by a point other than the fovea. This non-foveal point then adopts the principal visual direction.

• Characteristics:

  • Fixation is parafoveal (rather than central).

  • Can be steady (one preferred fixation location) or unsteady (fixation location changes over time, e.g., wandering).

  • The degree of eccentric fixation is defined by its distance from the fovea in degrees.

• Clinical Significance:

  • Strong association with strabismus, amblyopia, and ARC.

  • Reduces the prognosis for successful amblyopia treatment.

  • Can complicate the accurate measurement of the objective angle of strabismus, which affects ARC determination.

  • Markedly reduces best-corrected visual acuity (BCVA) due to the resolution limitations of para-foveal areas compared to the fovea.

• Clinical Assessment: Performed with visuoscopy.

Visuoscopy

• Instrument: A visuoscope is essentially an ophthalmoscope with a graticule (a four-pointed star with a clear central area) that serves as a fixation target. Some visuoscopes include a green filter.

• Procedure:

  1. Occlude the patient's other eye.

  2. Ask the patient to look at the central target (the graticule's center).

  3. Dim the room lights.

  4. Observe the position of the foveal reflex in relation to the graticule.

  5. Determine if the target is steadily on the fovea:

     • If the foveal reflex is steady and centered within the graticule, it indicates central, foveal fixation (not eccentric).

     • If not, determine the magnitude (in degrees) and direction (e.g., superior nasal, inferior temporal) of the eccentric fixation.

     • Note: Each target notch typically corresponds to $1\Delta$ (prism diopter) or approximately $0.5^\circ$ (degrees), though this can vary by ophthalmoscope brand.

  6. Assess the stability of the eccentric location over time (is it steadily maintained, unsteady, or wandering?).

• Recording: Assess and record in terms of:

  • Position: Central, parafoveal, macular, peripheral.

  • State of Fixation: Steady, unsteady, wandering, or eccentric.

• Example Visuoscopy Images:

  • A = Nasal EF: Fovea is temporal to the fixation target.

  • B = Temporal EF: Fovea is nasal to the fixation target.

  • C = Superior Nasal EF: Fovea is inferior temporal to the fixation target.

  • D = Inferior Nasal EF: Fovea is superior temporal to the fixation target.

  • A (Image Labeling) = Parafoveal: Fixation close to the fovea but not central.

  • B (Image Labeling) = Macular: Fixation within the macula but not necessarily foveal.

  • C+D = Peripheral Areas: Fixation outside the macular region, potentially near small or large blood vessels.

• Example Record: "R. $1\Delta$ Temporal Fixation, Parafoveal, Wandering. L. Central Fixation."

Graphical Analysis - Vergence and Accommodation Criteria

Case 1 Example

• Patient: 10yo, $60\text{mm}$ IPD

• Findings:

  • Distance (D): $4$ exophoria

  • Near (N): $12$ exophoria

  • D: PFV 4/10/8; NFV -/10/8

  • N: PFV 12/20/14; NFV 15/20/16

  • NRA: $+1.50 \text{D}$

  • PRA: $-4.00 \text{D}$

  • NPA: $8\text{cm}$

  • NPC: $8\text{cm}$

• Question: Are Percival's and Sheard's criteria met?

Case 2 Example

• Patient: 15yo, $65\text{mm}$ IPD

• Findings:

  • Distance (D): Ortho

  • Near (N): $2$ esophoria

  • D: PFV 4/20/16; NFV -/4/2

  • N: PFV 8/25/20; NFV 4/6/4

  • NRA: $+1.25 \text{D}$

  • PRA: $-1.00 \text{D}$

  • NPA: $12.5\text{cm}$

  • NPC: $4\text{cm}$

• Question: Are Percival's and Sheard's criteria met?

Fixation Disparity Curve (FDC) Analysis

Case 1 Example

• Patient: 20yo, $60\text{mm}$ IPD

• Fixation Disparity Data:

  • Baseline: $7$ exo (mins of arc)

  • $3\text{BI}$: $5$ exo

  • $3\text{BO}$: $10$ exo

  • $6\text{BI}$: $3$ exo

  • $6\text{BO}$: $14$ exo

  • $9\text{BI}$: $2$ exo

  • $9\text{BO}$: $19$ exo

  • $12\text{BI}$: $1$ exo

  • $12\text{BO}$: Diplopia

  • $15\text{BI}$: Relative $1$ exo

  • $15\text{BO}$: -

• Curve Type: Type III Curve.

• Characteristics of Type III:

  • Adapts well to Base-In (BI) prism.

  • More commonly observed in exophores.

  • Management often prioritizes prism as a first option, followed by vision therapy (VT).

Case 2 Example

• Patient: 30yo, $58\text{mm}$ IPD

• Fixation Disparity Data:

  • Baseline: $2$ eso (mins of arc)

  • $3\text{BI}$: $4.5$ eso

  • $3\text{BO}$: $1.5$ eso

  • $6\text{BI}$: $10$ eso

  • $6\text{BO}$: $1$ eso

  • $9\text{BI}$: $17$ eso

  • $9\text{BO}$: $1$ eso

  • $12\text{BI}$: Diplopia

  • $12\text{BO}$: Diplopia

  • $15\text{BI}$: -

  • $15\text{BO}$: -

• Curve Type: Type II Curve.

• Characteristics of Type II:

  • Adapts more to Base-Out (BO) prism.

  • More commonly observed in esophores.

  • Responds better to prism or plus lens addition.

Integrative Analysis - Exophoria Case Study

Case 1: University Student (22 years old)

• Symptoms: Headaches when doing near work (after >2 hours), diplopia at the end of the day when tired.

• Current Spectacles: Low hyperopic/astigmatic correction (SVN - single vision near).

• Clinical Findings:

  • Vergence Posture: Distance ($4\Delta$ exophoria), Near ($18\Delta$ exophoria).

  • Accommodation Posture: R $+0.50\text{D}$, L $+0.75\text{D}$.

  • Amplitude of Accommodation: R $10\text{D}$, L $11\text{D}$.

  • Vergence Range (Distance): Near Fusion Vergence (NFV): 20/35/25; Positive Fusion Vergence (PFV): -/30/20.

  • Vergence Range (Near): NFV: 8/10/6; PFV: 14/16/10.

  • Accommodation Range (Near): PRA: $-5.00\text{D}$; NRA: $+1.25\text{D}$.

  • Facility:

    • Vergence Facility: $16$ cpm (cycles per minute), prefers BI (base-in).

    • Accommodation Facility: $0$ cpm, cannot clear plus.

  • AC:A Ratio: Plus (+): 0.5:1; Minus (-): 3.5:1.

• Analysis for Exophoria Patient (Focus on PFV group data):

  • PFV (Smooth/Step): Reduced (consistent with exophoria).

  • PFV (Vergence Facility): Reduced when adapting to BO (Base-Out) prism.

  • NRA (Negative Relative Accommodation): Reduced.

  • BAF (Binocular Accommodation Facility): Reduced when clearing plus lenses.

  • NPC (Near Point of Convergence): Receded (farther than normal).

  • MEM/FCC (Monocular Estimation Method Retinoscopy/Fused Cross Cylinder): Lead of accommodation observed.

Integrative Analysis - Esophoria Case Study

Case 2: School Student (11 years old)

• Symptoms: "Feels cross eyed."

• Current Spectacles: Low hyperopic/astigmatic correction with $+1.75\text{D}$ near add (PALS - progressive add lenses).

• Clinical Findings:

  • Vergence Posture: Distance ($25\Delta$ esophoria), Near ($25\Delta$ esophoria).

  • Accommodation Posture: R $+0.50\text{D}$, L $+0.50\text{D}$.

  • Amplitude of Accommodation: R $10\text{D}$, L $11\text{D}$.

  • Vergence Range (Distance): NFV: 5/8/4; PFV: -/40/25.

  • Vergence Range (Near): NFV: 14/16/6; PFV: 24/30/28.

  • Accommodation Range (Near): PRA: $-1.75\text{D}$; NRA: $+2.00\text{D}$.

  • Facility:

    • Vergence Facility: $0$ cpm, cannot clear BI (Base-In) prism.

    • Accommodation Facility: $2$ cpm, prefers plus lenses.

  • AC:A Ratio: Plus (+): 1:1; Minus (-): 2:1.

• Analysis for Esophoria Patient (Focus on NFV group data):

  • NFV (Smooth/Step): Reduced (consistent with esophoria).

  • NFV (Vergence Facility): Reduced when adapting to BI (Base-In) prism.

  • PRA (Positive Relative Accommodation): Reduced.

  • BAF (Binocular Accommodation Facility): Reduced when clearing minus lenses.

  • MEM/FCC (Monocular Estimation Method Retinoscopy/Fused Cross Cylinder): Lag of accommodation observed.