Part II - Binocular Vision II | 6th Semester Bachelor of Optometry

Investigations in Binocular Vision – History

Introduction:

History taking is the first and one of the most crucial steps in investigating binocular vision disorders. A thorough case history not only provides insights into the nature and cause of the condition but also guides the clinician toward selecting appropriate diagnostic procedures and treatment strategies. In the evaluation of strabismus, amblyopia, diplopia, and neuromuscular anomalies, history plays a pivotal role in differentiating between congenital and acquired conditions, ruling out neurological associations, and identifying psychosocial impact.

Importance of History in Binocular Vision Assessment

Accurate history helps in:

Understanding the onset, duration, and progression of the binocular disorder
Identifying triggering or aggravating factors
Assessing the patient’s adaptation to the deviation (e.g., suppression, abnormal head posture)
Determining whether the deviation is congenital, acquired, intermittent, or constant
Guiding the examiner to appropriate tests and referrals (neurology, pediatrics, endocrinology, etc.)

Key Components of History

1. Chief Complaint:

The patient's primary reason for visiting should be documented in their own words. Common complaints include:

Eye deviation (inward/outward/vertical)
Double vision (diplopia)
Eye strain or discomfort during near work
Headaches or difficulty concentrating
Cosmetic concerns (especially in children or parents)
Poor depth perception or coordination

2. Onset of the Problem:

Understanding the time of onset is critical:

Congenital/infantile: Deviation observed shortly after birth or within the first 6 months
Acquired: Sudden or gradual appearance of symptoms in later life

Ask the patient or guardian if they noticed any triggering events like trauma, infection, fever, or stress before the onset.

3. Duration and Progression:

Is the condition stable, improving, or worsening?
Is the deviation constant or intermittent?
Did the symptoms begin suddenly or insidiously?
Was the diplopia initially intermittent and later became constant?

4. Frequency and Variability:

Note any patterns:

Worsening during fatigue, illness, stress, or late in the day
Variability with viewing distance (greater at near or distance)
Situational triggers (e.g., reading, watching TV, bright light)

5. Associated Symptoms:

Record the presence of any associated issues such as:

Blurred vision
Photophobia
Head tilt or abnormal posture
Difficulty with school or work tasks
Dizziness, balance issues (suggesting neurological involvement)

6. Past Ocular History:

Previous diagnosis of strabismus or amblyopia
Use of spectacles, patching therapy, prisms, or previous vision therapy
History of ocular trauma or surgery
Any previous squint surgery or complications

7. Birth and Developmental History (for children):

Full-term or preterm birth?
Any neonatal complications or delayed milestones?
Maternal infections, smoking, or alcohol during pregnancy?
Any history of seizures or birth-related trauma?

8. Family History:

Since many strabismic and refractive conditions have hereditary components:

Family members with squint, amblyopia, or high refractive errors?
Consanguinity in parents?

9. General Health and Systemic History:

History of neurological disorders (e.g., multiple sclerosis, cerebral palsy, stroke)
Thyroid dysfunction or autoimmune disorders
Diabetes, hypertension, or cardiovascular issues
Use of medications that may affect vision (e.g., antiepileptics, steroids)

10. Educational and Social History:

In school-age children, understand the academic impact:

Reading difficulties or skipping lines?
Comprehension issues?
Child avoiding near tasks or displaying poor attention?

In adults, consider occupation-related challenges, especially if the job requires prolonged visual tasks or good depth perception (e.g., drivers, pilots, surgeons).

11. Parental Observations (in pediatric cases):

Which eye deviates more often?
When does the deviation become most visible?
Is there a history of the child closing one eye in bright sunlight?
Any behavior suggesting poor vision in one eye?

Techniques for Effective History Taking

Build rapport with the patient or caregiver
Use simple language and age-appropriate questions
Allow the patient time to describe their experience
Maintain a structured sequence to ensure nothing is missed
Document clearly and concisely, focusing on relevance to binocular vision

Conclusion

History taking is the foundation of clinical diagnosis in binocular vision assessments. It provides a window into the etiology, impact, and progression of the visual condition. Whether the strabismus is congenital, accommodative, paralytic, or restrictive, an accurate history helps tailor the examination, interpret test results appropriately, and plan a patient-centric management approach. A well-documented and thoughtful history leads to faster diagnosis, better outcomes, and improved patient satisfaction.

Investigations in Binocular Vision – Symptoms

Introduction:

Recognizing and understanding the symptoms of binocular vision anomalies is a vital component of the diagnostic process. Symptoms provide essential clues about the type, severity, and progression of the condition. They help differentiate between comitant and incomitant strabismus, accommodative versus non-accommodative deviations, and even neurological versus mechanical causes. Proper interpretation of the patient’s visual complaints enables more efficient clinical testing and targeted management.

Role of Symptom Analysis in Binocular Vision Disorders

Symptoms are subjective experiences reported by the patient. In binocular vision evaluation, these symptoms not only indicate the presence of a disorder but also highlight its functional impact. Symptoms can affect reading, coordination, driving, school performance, and overall quality of life.

Common Symptoms in Binocular Vision Anomalies

1. Diplopia (Double Vision)

Diplopia

One of the most specific and significant symptoms
Occurs when the visual axes fail to align, leading to two images perceived for one object
Horizontal diplopia: Often associated with exotropia or esotropia
Vertical diplopia: Seen in vertical deviations like superior oblique or inferior rectus palsy
Binocular in nature and disappears on covering one eye
Diplopia may be constant or intermittent and worsens under fatigue or poor lighting

2. Asthenopia (Eye Strain):

General discomfort or tiredness of the eyes, especially during near tasks
Often reported as burning, heaviness, watering, or redness
Associated with convergence insufficiency, decompensated heterophorias, or latent squints
Patients may take frequent breaks while reading or working on digital devices

3. Blurred or Fluctuating Vision:

Blurring of vision, especially during prolonged near work, is common in patients with convergence or accommodative anomalies
Fluctuation of clarity between the two eyes or during changes in fixation is often a sign of binocular imbalance

4. Headaches:

Often frontal or occipital in location
May increase with tasks involving prolonged near focus
Result from excessive accommodative or vergence demand, especially in convergence insufficiency, fusional vergence dysfunction, or high phorias

5. Suppression or Eye Avoidance:

In children, chronic binocular imbalance may result in the brain suppressing input from one eye to avoid confusion or diplopia
Parents may report that the child prefers one eye or closes one eye under bright light
In acquired strabismus, suppression is less common, and the patient experiences diplopia instead

6. Poor Depth Perception:

Difficulty in judging distances, catching objects, or pouring liquids accurately
Occurs in individuals with long-standing strabismus or poorly developed stereopsis
Common complaint in professions requiring fine spatial judgments

7. Abnormal Head Posture:

Abnormal head posture

Patients may adopt a head turn, chin-up or chin-down position to maintain single vision
Often compensatory in paralytic strabismus to align eyes using unaffected gaze fields
Important to observe posture even if the patient does not report diplopia

8. Reading Difficulties (especially in children):

Skipping lines, losing place while reading, or poor reading comprehension
May report visual discomfort or blurry vision after prolonged study
Common in convergence insufficiency or latent strabismus

9. Double-vision only in specific gaze directions:

Suggests muscle palsy or restrictive strabismus
Helps localize the affected muscle and guide ocular motility testing

Symptoms in Specific Age Groups

Children:

May not verbalize symptoms clearly
Look for indirect signs like covering one eye, closing one eye outdoors, head tilting, or poor school performance
Parents may report eye deviation noticed occasionally or consistently

Adults:

Often report sudden onset diplopia or asthenopia
Symptoms may interfere with work, especially tasks requiring prolonged focus or coordination
Diplopia is more distressing as suppression mechanisms are less developed in adults

Diagnostic Value of Symptoms

Symptom pattern helps determine:

If the deviation is intermittent or constant
Whether fusion and binocularity are still present
Presence of compensatory adaptations like suppression or head posture
Likelihood of a neurological vs. mechanical vs. sensory etiology

Correlation with Clinical Testing

Once symptoms are identified, targeted investigations like cover tests, fusional amplitude measurement, motility testing, and stereopsis evaluation are carried out. Symptom analysis allows for efficient use of time and equipment during examination and helps prioritize further neurological or imaging workups if necessary.

Conclusion

Symptom analysis is a vital part of binocular vision assessment and helps differentiate between various forms of strabismus, accommodative dysfunctions, and ocular motility disorders. It offers valuable insight into the impact of the condition on daily life and guides the selection of appropriate diagnostic tests. Clinicians should always approach symptom reporting with empathy and patience, especially in children and non-verbal patients, to uncover hidden or subtle signs of binocular vision stress.

Investigations in Binocular Vision – Head Posture

Introduction:

Abnormal head posture (AHP) is a compensatory adaptation used by patients with binocular vision anomalies to maintain single, clear, and comfortable vision. It involves a consistent or habitual change in the position of the head—tilting, turning, or raising/lowering the chin—to minimize diplopia or maximize binocular fusion. Recognizing and analyzing head posture is a critical part of the investigative process, particularly in diagnosing paralytic, restrictive, or congenital strabismus.

Definition of Abnormal Head Posture

Abnormal head posture

An abnormal head posture refers to any deviation from the normal head position adopted persistently or intermittently by a patient to enhance visual function or reduce symptoms such as diplopia or eye strain. The posture may include:

Face turn (horizontal deviation) – head turned to left or right
Head tilt (torsional deviation) – head tilted toward one shoulder
Chin-up or chin-down (vertical deviation) – head tilted upward or downward

Clinical Importance of Evaluating Head Posture

Observing head posture helps the clinician to:

Identify the presence and type of incomitant deviation
Localize paretic or restricted muscles based on compensatory movement
Determine if the patient is suppressing or fusing
Evaluate the adaptation mechanism in both children and adults
Plan appropriate diagnostic tests and tailor surgical interventions

How to Observe Head Posture

The patient should be observed in a natural, relaxed state:

While seated, walking, or interacting with others
During conversation or watching a distant target
Without drawing attention to the observation to avoid forced postures

Types of Head Posture and Their Associations

1. Face Turn (Turn of Chin Left or Right):

Common in sixth nerve palsy (lateral rectus)
Patient turns face toward the affected side to align the eye in the field where fusion is possible
Seen in Duane retraction syndrome, where face turn compensates for limited abduction or adduction

2. Head Tilt (Tilt Toward Either Shoulder):

Classically associated with superior oblique palsy (fourth nerve)
Head tilt occurs away from the side of the affected muscle to reduce torsional diplopia
Evaluated with Bielschowsky’s head tilt test

3. Chin-Up or Chin-Down Posture:

Occurs in vertical deviations such as superior or inferior rectus palsy
Chin is raised in cases with hypotropia (limited elevation)
Chin lowered when there's hypertropia (limited depression)
Seen in Brown syndrome and double elevator palsy

4. Combined or Complex Postures:

In some cases, patients adopt a combination of tilt, turn, and chin adjustments
Common in complex nerve palsies or congenital fibrosis syndromes
Can also indicate restrictive strabismus such as thyroid myopathy or orbital fracture

Head Posture in Children

Children with congenital strabismus often develop a compensatory head posture early in life. Important clues include:

Facial asymmetry due to chronic face turn
Head tilt during reading, drawing, or watching TV
Photophobia and closing one eye in bright light
Reluctance to change habitual head position

Early diagnosis and intervention are essential to prevent long-term musculoskeletal complications.

Conditions Commonly Associated with Abnormal Head Posture

Sixth nerve palsy (face turn)
Fourth nerve palsy (head tilt)
Superior rectus or inferior rectus palsy (chin elevation or depression)
Brown syndrome (chin-up posture)
Duane retraction syndrome (face turn)
Thyroid ophthalmopathy (variable postures depending on muscle restriction)
Congenital fibrosis syndrome (complex postures)

Evaluation Methods

The following assessments are helpful when abnormal head posture is observed:

Ocular motility testing: Identify the restricted or underacting muscle
Cover test in different gazes: Quantify change in deviation with head movements
Prism bar cover test: Compare primary and secondary deviations
Bielschowsky’s head tilt test: Particularly for superior oblique palsy
Photography: Useful for documenting and monitoring posture over time

Clinical Significance

Persistent head posture may indicate a longstanding deviation and the patient’s adaptation to maintain single vision. It should be carefully documented and monitored, especially in growing children, as it can lead to skeletal deformities, torticollis, or facial asymmetry. In acquired cases, it may signal a recent neurological deficit requiring urgent referral.

Management Approach

Address underlying strabismus via optical correction, prism therapy, or surgery
Vision therapy may help improve fusional ranges and reduce need for compensation
In congenital cases, timely strabismus surgery often corrects both the deviation and head posture
Posture rarely resolves spontaneously in adults; thus, early intervention is advised

Conclusion

Head posture is a subtle yet powerful indicator of binocular visual dysfunction. Observing and interpreting abnormal head positions can lead to accurate localization of muscle or nerve involvement and guide clinical decision-making. It is especially important in pediatric patients, where early recognition prevents long-term musculoskeletal complications and improves quality of life through proper alignment and binocularity restoration.

Investigations in Binocular Vision – Diplopia Charting

Introduction:

Diplopia, or double vision, is a significant symptom in many binocular vision anomalies, especially paralytic and restrictive strabismus. It occurs when the two eyes are not properly aligned, leading to the perception of two distinct images of a single object. Diplopia charting is a diagnostic tool used to document and analyze the characteristics of double vision. It helps determine the affected muscle, the type of deviation, and the presence of associated sensory adaptations. Accurate diplopia charting is essential in planning management, particularly surgical correction in incomitant strabismus.

What is Diplopia Charting?

Diplopia Charting

Diplopia charting is the process of mapping a patient’s subjective experience of double vision across different directions of gaze. It identifies the positions where diplopia is most prominent, and whether the images are crossed (exotropia), uncrossed (esotropia), or vertically separated (hypertropia or hypotropia). This chart is especially helpful in localizing the paretic or restricted extraocular muscle and assessing the field of action involved.

Indications for Diplopia Charting

All patients with recent onset of double vision
Suspected cranial nerve palsy (III, IV, VI)
Diagnosis of incomitant strabismus
Post-trauma or post-surgical cases with motility restrictions
Pre-operative and post-operative documentation

Objectives of Diplopia Charting

To confirm the presence and nature of diplopia (horizontal, vertical, or torsional)
To map the fields of gaze where diplopia occurs
To identify the muscle involved in the deviation
To differentiate between paralytic and restrictive causes
To track progression or recovery over time

Materials Required

Diplopia chart template (usually a 9-position grid)
Red-green goggles or Maddox rod (optional)
Laser pointer or small light target
Dark room or shaded light source

Procedure of Diplopia Charting

The basic procedure involves the following steps:

Ask the patient to wear red-green goggles (red over the right eye, green over the left eye).
Seat the patient at a fixed distance (usually 1 meter) from a whiteboard or chart.
Hold a white spot light or laser in each of the nine cardinal positions of gaze (primary, up, down, left, right, up-right, up-left, down-right, down-left).
Instruct the patient to report whether they see one or two images at each position and note the relative position of the images (horizontal, vertical, or torsional separation).
The patient should also report which image appears more blurred or dim—this corresponds to the image from the paretic eye.

Interpreting the Diplopia Chart

1. Direction of Diplopia:

Horizontal diplopia: Suggests involvement of lateral or medial rectus (e.g., sixth nerve palsy)
Vertical diplopia: Seen in superior or inferior rectus or oblique palsy
Oblique or torsional diplopia: Indicates involvement of oblique muscles, especially superior oblique

2. Separation of Images:

Greater separation of images indicates greater deviation and underaction of the involved muscle
The larger the distance between images in a given gaze direction, the more it implicates that direction’s primary muscle

3. Image Characteristics:

The false image (from the affected eye) is usually dimmer and may be tilted
The patient may describe vertical and torsional components together in superior oblique palsy

4. Gaze Position with Maximum Diplopia:

This is often the field of action of the affected muscle. For example:

Right lateral rectus palsy → maximum diplopia in right gaze
Left superior oblique palsy → maximum diplopia in down and right gaze, with vertical and torsional components

Example Chart Findings

A patient with right sixth nerve palsy may show:

Diplopia in right gaze (especially right lateral gaze)
Horizontal separation of images
False image from the right eye, appearing more blurred or fainter
Minimal or no diplopia in left gaze

Advantages of Diplopia Charting

Non-invasive and inexpensive
Provides functional, patient-perceived information
Helpful for both diagnosis and monitoring of recovery
Visual representation is easy to document and communicate with colleagues

Limitations

Requires good patient cooperation and communication
Not suitable for young children or those with poor vision in one eye
Cannot differentiate between restrictive and paralytic causes alone (requires FDT or Hess chart)

Clinical Tips

Ensure consistent test distance and lighting
Explain clearly what the patient should look for
Always compare findings with ocular motility and cover tests
Use diplopia charting in conjunction with Hess or Lees screen for detailed interpretation

Conclusion

Diplopia charting is an essential investigative tool in the diagnosis and management of binocular vision anomalies, particularly paralytic and restrictive strabismus. It provides valuable insights into the direction, extent, and impact of double vision on the patient's visual function. Through careful interpretation and correlation with clinical findings, it assists the clinician in localizing the affected muscle and planning the most appropriate therapeutic strategy.

Investigations in Binocular Vision – Hess Chart

Introduction:

The Hess Chart (also known as the Hess Screen test) is a diagnostic tool used to evaluate ocular motility and detect muscle underaction or overaction, particularly in cases of incomitant strabismus. It is especially valuable in identifying paralytic strabismus, monitoring progression or recovery, and differentiating between restrictive and neurogenic causes of eye movement disorders. The Hess Chart provides a graphical representation of eye movements, allowing clinicians to localize the affected muscle(s) and assess the severity of deviation in different gaze directions.

Principle of the Hess Chart

The Hess Chart operates on the principle of dissociation of the two eyes using red-green anaglyphic filters. The patient is presented with a fixed grid and asked to align targets in various gaze positions, enabling the examiner to plot the field of action for each eye. The test maps both the paretic and yoke muscle activity, illustrating deviations from normal motility.

Hess Chart

Indications for Hess Chart

Diagnosis of incomitant strabismus (especially paralytic squint)
Localization of affected extraocular muscle(s)
Documentation of ocular motility over time
Monitoring progression or recovery of cranial nerve palsy
Differentiation between restrictive and neurogenic causes
Pre- and post-operative surgical evaluation

Materials Required

Hess screen or computerized Hess chart setup
Red-green goggles (red filter over the right eye)
Green and red light pointers (projected or manual)
Chart grid with reference lines representing primary and secondary gaze positions

Procedure

The patient is seated 50 cm to 1 meter away from the Hess screen/grid.
They wear red-green goggles (red over the right eye, green over the left).
One eye sees the red targets; the other sees the green lights projected by the examiner or screen.
The examiner sequentially projects lights in nine gaze positions (primary, up, down, left, right, and diagonals).
The patient is asked to superimpose their light (e.g., red spot) over the green target light.
The location of each attempted alignment is plotted on the chart.
The process is repeated for the opposite eye, and two fields are generated—one for each eye.

Interpretation of the Hess Chart

1. Smaller Field (Paretic Eye):

The eye with the smaller plotted field is the affected (weaker) eye.
The field shows underaction of the paretic muscle and its associated movements.
The maximum field reduction occurs in the direction of action of the involved muscle.

2. Larger Field (Yoke Eye):

The larger field belongs to the unaffected (or less affected) eye.
It may show apparent overaction of the yoke muscle to compensate for the paretic muscle.
This is not a true overaction but a reflection of increased innervation to maintain fusion (Hering’s Law).

3. Displacement and Shape:

Normal fields are square and symmetric
Displaced or contracted fields indicate muscle palsy or restriction
Rotated fields may be seen in cases with torsional involvement

Examples

Sixth Nerve Palsy:

Paretic field (e.g., right eye) is smaller
Maximum underaction seen in right lateral gaze (field compressed to the right)
Left field (yoke eye) shows apparent overaction of left medial rectus

Fourth Nerve Palsy:

Smaller field in the affected eye, compressed in down-and-in gaze
Torsional deviation may affect the symmetry of the field

Differentiating Paralytic vs Restrictive Strabismus

In paralysis, fields are asymmetric with muscle-specific underaction and yoke muscle overaction
In restriction, both eyes may show limitations in the same gaze direction without true innervation imbalance
Restriction shows less disparity between field sizes
Forced duction test is used to confirm mechanical restriction

Advantages of the Hess Chart

Objective and reproducible results
Visual representation for documentation and counseling
Quantifies degree and direction of muscle dysfunction
Useful in detecting subtle palsies or residual motility limitations
Assists in surgical planning

Limitations

Requires patient cooperation and spatial awareness
Not suitable for very young children or those with severe amblyopia
Cannot measure exact angle of deviation in prism diopters
Only evaluates central field of gaze, not extreme positions

Comparison with Diplopia Charting

Diplopia charting is subjective—based on patient's perception
Hess chart is semi-objective and plots both fields
Both are complementary and often used together

Conclusion

The Hess Chart is an invaluable diagnostic tool in the investigation of binocular vision disorders, especially those involving extraocular muscle dysfunction. By graphically representing ocular motility, it aids in localizing the affected muscles, assessing severity, differentiating etiologies, and guiding treatment. When used alongside clinical examination and other tests like forced duction and diplopia charting, it offers a comprehensive view of the patient's ocular motor status and helps achieve better clinical outcomes.

Investigations in Binocular Vision – Prism Bar Cover Test (PBCT)

Introduction:

Prism Bar Cover Test

The Prism Bar Cover Test (PBCT) is one of the most essential and widely used procedures in binocular vision assessment. It serves as a quantitative method to measure the angle of deviation in patients with manifest (tropia) or latent (phoria) strabismus. By combining the cover-uncover or alternate cover test with a series of calibrated prisms, the PBCT provides precise values for deviation at both distance and near. This test is especially useful in monitoring progression, evaluating surgical outcomes, and planning orthoptic or surgical treatment strategies.

Principle of PBCT

The PBCT is based on the principle of neutralizing the deviation by introducing a prism of equal and opposite power until the movement of the eye during the cover test is eliminated. The amount of prism required to stop this movement corresponds to the angle of deviation in prism diopters (PD).

Indications for PBCT

To measure the magnitude of horizontal and vertical deviations
To quantify both near and distance deviations
To evaluate comitancy (variation across gaze directions)
To differentiate between phoria and tropia
To compare deviations in different positions of gaze or head tilt
Pre-operative and post-operative strabismus assessment

Materials Required

Horizontal and vertical prism bars (usually ranging from 1 to 45 PD)
Occluder or cover paddle
Distant and near fixation targets (e.g., Snellen chart or accommodative target)
Good illumination and a relaxed, cooperative patient

Procedure for PBCT

1. Preparation:

Explain the procedure clearly to the patient
Ensure appropriate refractive correction is worn during the test
Have the patient fixate on an accommodative target (letter or symbol) at distance (6m) and near (33-40cm)

2. Alternate Cover Test:

Use the alternate cover test to break fusion completely
Observe the direction and speed of eye movement as the occluder is alternated between eyes
Note whether the deviation is esotropia, exotropia, hypertropia, or combined

3. Placing the Prism:

Start with a low prism value (e.g., 4 or 6 PD) with the base placed opposite the direction of deviation
For esotropia → base out
For exotropia → base in
For hypertropia → base down in front of the hypertropic eye or base up in front of the hypotropic eye

4. Neutralization:

Continue increasing prism power until no movement is seen during alternate cover test
Record the prism value at which the movement is neutralized
If movement reverses, reduce the prism until a neutral endpoint is observed

Types of Measurements

Distant PBCT: Measures deviation at 6 meters; often larger in divergence excess exotropia
Near PBCT: Measures deviation at 33–40 cm; useful in convergence insufficiency or high AC/A ratio
Primary gaze: Standard testing position (straight ahead)
Secondary gaze positions: To detect incomitancy and guide muscle localization

Interpretation of Results

The prism diopter value equals the angle of deviation
Compare deviations between distance and near—important in diagnosing divergence excess or convergence weakness
Vertical deviations >2 PD may require prism correction or surgery
Large angle deviations (>30 PD) suggest constant tropia or long-standing strabismus

Advantages of PBCT

Simple, quick, and highly reliable
Objective and repeatable measurement of deviation
Can be performed with any level of deviation (phoria or tropia)
Helps guide orthoptic therapy or surgical decisions
Monitors progression or improvement over time

Limitations of PBCT

Requires patient cooperation and clear fixation
Not effective if suppression is present
Fusion must be completely disrupted to ensure accurate measurement
In large deviations, lateral head posture may reduce accuracy

PBCT in Special Situations

1. Children:

May require special attention or use of engaging fixation targets
Use alternate prism cover technique instead of moving prism bar quickly

2. Vertical and Torsional Deviations:

Use vertical prism bar for hypertropia or hypotropia
Torsional deviations require additional tests like Maddox double rod or synoptophore

3. Decompensated Phoria:

PBCT helps determine whether latent deviation is becoming symptomatic
Used in conjunction with vergence and accommodative tests

Documentation Format

Deviations are typically recorded as:

“30 PD XT at distance, 25 PD XT at near”
“12 PD ET with 4 PD right hypertropia”
Include presence of head posture, abnormal correspondence, or suppression

Conclusion

The Prism Bar Cover Test is an indispensable component of binocular vision assessment. It offers precise measurement of ocular misalignment, aiding in diagnosis, documentation, and clinical decision-making. When combined with history, motility testing, and sensory evaluation, PBCT allows for a comprehensive understanding of the patient's binocular status and facilitates effective treatment planning for both functional and cosmetic improvement.

Investigations in Binocular Vision – Nine Directions of Gaze

Introduction:

The assessment of eye movements in the nine cardinal positions of gaze is a cornerstone in the clinical evaluation of binocular vision anomalies. This test allows the examiner to observe the function of each of the extraocular muscles and their corresponding cranial nerves, helping to detect underactions, overactions, and incomitancy. It is especially valuable in diagnosing paralytic and restrictive strabismus, guiding further investigations such as the Hess chart, diplopia charting, and forced duction testing.

What Are the Nine Directions of Gaze?

Nine direction of gaze

The nine cardinal positions of gaze are the principal eye movement directions where each extraocular muscle demonstrates its primary or secondary action. These directions include:

Primary gaze (straight ahead)
Right gaze (dextroversion)
Left gaze (levoversion)
Up and right (dextroelevation)
Up and left (levoelevation)
Down and right (dextrodepression)
Down and left (levodepression)
Up gaze (supraversion)
Down gaze (infraversion)

By evaluating ocular motility in these positions, the action of all six extraocular muscles in each eye can be assessed systematically.

Purpose of the Test

To check the full range of ocular motility
To identify muscle palsies or restrictions
To detect overaction or underaction of specific muscles
To determine comitancy (angle remains the same in all directions) or incomitancy (angle varies)
To help localize cranial nerve involvement
To plan surgical correction of strabismus

Method of Performing the Test

Seat the patient comfortably at a distance of about 50 cm.
Ask the patient to keep their head still and follow a small target (e.g., penlight or fixation stick) with their eyes only.
Move the target slowly in an H-pattern to evaluate all 9 directions.
Observe both eyes for:
- Smoothness of movement
- Fullness of range
- Speed and symmetry of eye movement
- Any jerky, limited, or absent movements
- Presence of diplopia reported by the patient
Compare movements of each eye and look for overaction or underaction.

Muscle Action in the Cardinal Positions

Each direction of gaze tests specific pairs of yoke muscles (muscles that work together in both eyes to produce conjugate eye movement):

Direction of Gaze	Right Eye Muscle	Left Eye Muscle
Right gaze	Lateral rectus	Medial rectus
Left gaze	Medial rectus	Lateral rectus
Up and right	Superior rectus	Inferior oblique
Up and left	Inferior oblique	Superior rectus
Down and right	Inferior rectus	Superior oblique
Down and left	Superior oblique	Inferior rectus

Interpreting the Findings

1. Underaction:

If one eye does not move fully into a direction of gaze, suspect a muscle palsy or mechanical restriction.
Example: Limited abduction in right gaze suggests right lateral rectus palsy (sixth nerve).

2. Overaction:

Often compensatory or secondary to a yoke muscle palsy
May indicate oblique muscle overaction in conditions like V-pattern strabismus

3. Incomitancy:

If deviation varies with direction of gaze, strabismus is incomitant
Seen in paralytic strabismus, mechanical restriction, or long-standing untreated cases

4. Symmetry and Smoothness:

Movements should be smooth, equal in speed, and symmetrical in both eyes
Any asymmetry should be documented and followed up with further tests like Hess chart or diplopia charting

Clinical Clues from Nine Gaze Testing

Limitation in elevation with intact Bell’s phenomenon → suspect mechanical restriction
Overaction of inferior oblique in adduction → common in congenital esotropia
Head posture may match gaze preference – for example, chin up to avoid diplopia in downgaze

Limitations

Not always quantifiable – relies on clinician observation
Subject to patient cooperation and fixation ability
Subtle limitations may require confirmation with Hess chart or synoptophore

Tips for Better Assessment

Use a fixation target with a letter or symbol for more accurate feedback
Ask the patient to report any diplopia or blurring during each gaze
Use penlight reflections (corneal reflexes) to confirm alignment
Observe both eyes simultaneously – asymmetry often indicates the affected side

Conclusion

Examination of the nine directions of gaze is a fundamental clinical skill in binocular vision assessment. It provides invaluable information about ocular motility, muscle balance, and neuromuscular coordination. When performed systematically, it aids in the early detection of strabismus, cranial nerve palsies, and mechanical restrictions. Combined with other tests such as PBCT, diplopia charting, and the Hess screen, it forms a complete foundation for diagnosing and managing complex binocular vision disorders.

Investigations in Binocular Vision – Binocular Field of Vision

Introduction:

The binocular field of vision refers to the area of space that can be simultaneously seen by both eyes when the eyes are fixating on a point. It represents the overlapping visual fields of the two eyes and is responsible for depth perception and three-dimensional vision. Understanding and evaluating the binocular field is important in the assessment of visual efficiency, fusion capacity, and binocular anomalies. In clinical practice, this field is especially relevant in neuro-ophthalmic, strabismic, and low vision patients.

Monocular vs Binocular Field of Vision

Binocular and Monocular FoV

Monocular Field: Each eye has a visual field extending approximately 60° nasally, 100° temporally, 60° superiorly, and 75° inferiorly.
Binocular Field: The overlapping region of both monocular fields, generally about 120° horizontally (central overlap zone), constitutes the binocular field of vision.
Binocular field is narrower than the total monocular field combined, but it is the portion responsible for stereopsis and fusion.

Importance of Binocular Field

Essential for depth perception and spatial orientation
Plays a critical role in tasks requiring coordination—like driving, walking, reading, and sports
Aids in compensating for visual field defects in one eye
Helps in determining the functional impact of strabismus and suppression
Important in evaluating visual function in neurogenic and retinal pathologies

Clinical Applications of Binocular Field Testing

Detection of suppression scotomas in strabismus and amblyopia
Evaluation of binocular visual field constriction in glaucoma or optic neuropathies
Assessment of overlapping central fields in visual field loss
Estimation of usable vision in low vision rehabilitation
Screening for eligibility in driving and occupational visual standards

Methods of Evaluating Binocular Field of Vision

1. Confrontation Test:

Simple, quick screening method
Patient sits opposite the examiner and fixates on examiner’s nose
Simultaneous presentation of fingers or targets in different quadrants
Helps detect gross peripheral field defects
Limited sensitivity and specificity

2. Tangent Screen Testing:

Patient fixates on a central point while stimuli are presented in the peripheral field
Useful in assessing suppression and scotomas
Not widely used in modern practice

3. Perimetry:

Humphery Perimeter (Ziess)

Automated (e.g., Humphrey Field Analyzer) or manual (e.g., Goldman perimetry)
Monocular testing is standard, but binocular perimetry is gaining popularity in functional vision assessment
Binocular Esterman test is used to assess functional visual field for driving and vocational purposes

4. Synoptophore:

Synoptophore

Used to map suppression zones and measure the extent of binocular single vision field
Helpful in assessing small-angle strabismus or intermittent suppression

5. Hess-Lancaster Test:

Maps both eyes' field responses simultaneously
Identifies deviation patterns, suppression, and scotomas

Suppression and Binocular Field

Suppression is a cortical adaptation to avoid diplopia or visual confusion in strabismus. This leads to:

Suppression scotomas: Functional blind zones in one eye’s field
Reduced binocular field: Especially in intermittent exotropia, constant esotropia, and amblyopia
Compensatory head posture: Sometimes adopted to maximize the usable binocular field

Binocular Field and Driving Standards

Many countries require a minimum binocular horizontal field of 120° for driving license eligibility
Binocular Esterman test is standardized for this purpose
Patients with hemianopia or severe peripheral field loss may fail to meet these criteria despite good visual acuity

Impact of Binocular Field Loss

Loss of depth perception and orientation
Increased risk of falls and mobility challenges
Visual fatigue and eye strain due to excessive monocular dependence
Psychological impact including reduced confidence, social withdrawal, and depression

Enhancing Binocular Field in Low Vision

Use of prism glasses to shift peripheral fields into central view
Visual scanning training to increase field awareness
Use of reverse telescopes or mirrors in severe field constrictions
Encouragement of head movements to compensate for static field loss

Limitations in Binocular Field Testing

Harder to isolate contributions of individual eyes
May mask subtle monocular defects
Requires good attention and fixation from the patient

Conclusion

The binocular field of vision plays a central role in visual function, especially for depth perception, coordination, and spatial awareness. Evaluation of the binocular field is vital in patients with binocular vision anomalies, field defects, or low vision. Clinicians must understand its implications not just anatomically but functionally in terms of everyday life activities. By integrating binocular field analysis into visual assessments, a more holistic understanding of a patient’s vision can be achieved, leading to better management and rehabilitation outcomes.

Investigations in Binocular Vision – Amblyopia and its Relation to Squint

Introduction:

Amblyopia, commonly referred to as "lazy eye," is a developmental visual disorder characterized by reduced visual acuity in one or, less commonly, both eyes, which cannot be attributed to any structural abnormality of the eye or visual pathway. One of the most significant risk factors for amblyopia is strabismus (squint), where misalignment of the eyes causes the brain to suppress the image from the deviated eye, leading to poor visual development. Understanding the complex relationship between amblyopia and squint is essential for early detection, appropriate intervention, and long-term visual rehabilitation.

What is Amblyopia?

Amblyopia

Amblyopia is a functional defect that develops during the critical period of visual maturation (from birth to approximately 7-8 years). The visual cortex fails to receive or process information correctly from one eye, causing persistent visual suppression and underdevelopment of visual acuity, even if the eye is anatomically normal.

Types of Amblyopia:

Strabismic Amblyopia: Caused by constant unilateral deviation leading to suppression of the deviated eye
Refractive Amblyopia: Due to uncorrected high refractive errors, especially anisometropia
Deprivation Amblyopia: Caused by obstruction of visual input (e.g., congenital cataract, ptosis)
Mixed Amblyopia: Combination of the above, such as strabismus with anisometropia

Understanding Strabismus (Squint)

Strabismus refers to the misalignment of the eyes, where both eyes do not fixate on the same point simultaneously. It can be constant or intermittent, unilateral or alternating, and can involve horizontal (esotropia, exotropia), vertical (hypertropia, hypotropia), or torsional deviations.

Key Causes of Squint:

Muscle imbalance (extraocular muscle overaction or underaction)
Uncorrected refractive error
Cranial nerve palsy
Developmental anomalies
Orbital or systemic pathology

How Squint Leads to Amblyopia

When strabismus occurs during the critical period of visual development, the brain receives two dissimilar images – one from each eye. To avoid confusion or double vision (diplopia), the brain begins to suppress the image from the deviating eye. Over time, this suppression becomes permanent, leading to amblyopia.

Mechanisms Involved:

Suppression: The brain actively ignores input from the misaligned eye to prevent diplopia
Cortical Adaptation: The visual cortex develops fewer synaptic connections for the deviated eye
Lack of Binocular Fusion: Absence of stereopsis and depth perception from early suppression

Signs and Symptoms

Poor visual acuity in one eye not correctable by glasses
No structural abnormality seen on slit-lamp or fundus examination
History of eye turn or patching in early childhood
Reduced or absent stereopsis on testing (e.g., Titmus, Randot, Lang)
Suppression scotoma observed during binocular field tests or synoptophore

Investigating Amblyopia Associated with Squint

Visual acuity testing: Monocular acuity with age-appropriate charts (e.g., Snellen, Lea symbols)
Cover tests: To detect the presence, frequency, and direction of squint
PBCT: Measure angle of deviation and comitancy
Stereoacuity tests: Identify loss of binocular fusion
Worth Four Dot test: To check for suppression under different illumination levels
Bagolini striated glasses: Assess retinal correspondence and suppression

Impact of Amblyopia in Strabismus Patients

Reduces the potential for binocular vision and stereopsis
Leads to reduced visual efficiency and poor spatial judgment
Decreases success rate of surgical alignment if untreated
Limits career options where normal binocular vision is required
Psychological impact due to cosmetic and functional concerns

Management of Amblyopia Related to Squint

1. Optical Correction:

Full cycloplegic refraction and prescription of appropriate glasses
May partially or completely align the eyes in accommodative esotropia

2. Occlusion Therapy:

Occlusion therapy

Patching the better-seeing eye to stimulate the amblyopic eye
Duration based on age, severity, and compliance (e.g., 2–6 hours/day)
Reverse amblyopia (amblyopia in the patched eye) is a potential risk if overdone

3. Penalization Therapy:

Using atropine drops or optical blur in the sound eye to promote use of amblyopic eye
Useful in patients with poor patching compliance

4. Vision Therapy:

Exercises to improve fusion ranges, accommodative skills, and binocular coordination
Can complement occlusion therapy in older children

5. Surgical Alignment:

Once visual acuity improves or stabilizes, strabismus surgery may be considered
Early alignment enhances chances of developing or restoring binocular fusion

Prognosis

Best outcomes when amblyopia is detected and treated before 7 years of age
Older children and even adults can show visual improvement with structured therapy
Maintenance therapy may be required to avoid regression

Conclusion

Amblyopia and strabismus are closely interlinked, particularly in pediatric populations. Strabismus often leads to cortical suppression of the deviated eye, resulting in amblyopia if not managed in time. Early detection, thorough investigation, and a combination of optical, occlusion, and therapeutic interventions can significantly improve visual prognosis. Understanding their relationship enables optometrists and vision therapists to formulate effective, individualized treatment plans that restore not just vision, but also quality of life.

For more parts of Binocular Vision II 👇

👉 Part I

👉 Part III