Strabismus

The normal state of the eye is to be aligned (straight); any misalignment is an abnormality.

Definition:

Misalignment of visual axes of the two eyes.

2% of the population

Infantile esotropia and exotropia are associated with an increased risk of amblyopia. www.aao.org

May be life/eye/esteem threatening.

Why does it matter? Psycho-social issues

• People with strabismus are not treated equally.
• They are discriminated against in a blatant way leading to chronic depression, negative self-esteem, and surgery is cheaper than psychotherapy in these patients¹.
• Less job opportunities¹, difficulties in acquiring and keeping employment.
• Dating problems².

¹ John W. Simon. Caring for strabismus: An important Job, Done Well. J AAPOS 2006; 10: 384

² Mojon A, Potnik, Mojon. Opinions of dating agents about strabismic subject’s ability to find a partner. BJO 2008; 92: 765-769

Development of Fusion & Stereopsis

Fusion and stereopsis are vital aspects of binocular vision in ophthalmology, contributing to depth perception and visual clarity. Understanding their development is crucial in clinical assessment and management of visual disorders.

Fusion Development:

• Fusion is the process by which the brain combines images from both eyes to create a single, coherent perception.
• In infants, fusion is initially absent, and the eyes may be misaligned. Proper alignment typically develops within the first few months of life.
• Binocular vision matures as eye coordination improves. The development of fusion continues into childhood, culminating in stable, simultaneous vision.

Stereopsis Development:

• Stereopsis refers to the brain's ability to perceive depth by processing disparities in images from each eye.
• It relies on the accurate alignment of the eyes and the brain's capacity to merge the slightly disparate images.
• Stereopsis development parallels fusion, with rudimentary depth perception emerging in early childhood and maturing through experience.

Clinical Implications:

• Early detection of fusion and stereopsis deficits is essential in diagnosing strabismus (misalignment) and amblyopia (lazy eye).
• Ophthalmologists employ various tests, such as the Titmus or Randot stereotests, to assess stereopsis.
• Interventions, including vision therapy, patching, or surgery, may be recommended to enhance fusion and stereopsis in cases of visual impairment.

• The clinical picture of 'congenital' infantile esotropia was not present at birth in 4,211 infants.
• The majority of newborns have an unstable ocular alignment biased toward an exodeviation, and this persists for the first few weeks of life.
• Alignment at birth and at 1 month is similar.
• Prolonged esotropia in the first weeks or months does not preclude normal binocularity later.
• Normal infants as well as infants who are destined to develop congenital esotropia may be indistinguishable in the first two months by clinic means.
• There is a steady progression toward longer periods of straight eyes during the first six months of life.
• Esodeviation persisting after two months of age is abnormal motility.
• Exodeviation persisting beyond six to eight months is abnormal motility.
• Duane can be diagnosed in the neonatal period.

Scenarios & Perspectives

• Young or Old: Age can play a significant role in assessing the seriousness of strabismus.
• Congenital, Infantile, Acquired: The timing of strabismus onset can provide insights into its potential causes and implications.
• Chronic or Acute Onset: The suddenness of strabismus development may indicate underlying issues.

Examples:

1. 2-month-old with esotropia - Not considered serious until around 6 months of age, unless associated with other ocular symptoms, which could be an emergency even at birth.
2. 4-month-old with exotropia - Typically not considered serious until around 6 months of age.
3. A day-old with esotropia and one eye not moving very well - May require immediate attention.
4. 3-year-old with esotropia - Could be indicative of another underlying issue.
5. 10-year-old with on-and-off exotropia - May also signal an underlying problem.
6. Sudden esotropia in a 55-year-old - Potentially indicative of a serious condition such as a space-occupying lesion.
7. Eye strain in a 21-year-old, with straight-looking eyes - Might suggest an underlying issue that needs investigation.

Fusion & Requirements for Fusion

• Sensory: Unhindered looking and seeing, i.e., clear media and transmission from lid to visual cortex
• Motor: Movements of eyes in response to stimuli, coordination coordinated by CNS and muscles

Versions: These are binocular conjugate movements where both eyes move in the same direction simultaneously.

Ductions: Ductions involve uniocular movements, where each eye moves independently.

Vergences: Vergences refer to binocular non-conjugate movements, which are typically associated with the coordination of both eyes.

Nine Diagnostic Positions of Gaze

In ophthalmology, the "nine diagnostic positions of gaze" are a set of cardinal eye movements used for evaluating the function of extraocular muscles and identifying ocular motility disorders. These positions aid in diagnosing conditions like strabismus (ocular misalignment) and nerve palsies.

The Nine Diagnostic Positions:

1. Primary Position: The patient gazes straight ahead, focusing on a distant object. This is the reference position for all eye movements.
2. Right Gaze: The patient looks as far as possible to the right without moving their head.
3. Left Gaze: The patient looks as far as possible to the left without moving their head.
4. Up Gaze: The patient looks upward as much as possible without tilting their head.
5. Down Gaze: The patient looks downward as much as possible without tilting their head.
6. Right Diagonal Up and Right: The patient gazes diagonally upward and to the right.
7. Right Diagonal Down and Right: The patient gazes diagonally downward and to the right.
8. Left Diagonal Up and Left: The patient gazes diagonally upward and to the left.
9. Left Diagonal Down and Left: The patient gazes diagonally downward and to the left.

Ocular Motility & Diagnostic Position of Gaze and Muscles Involved

Ocular motility refers to the ability of the eyes to move together smoothly and accurately. The diagnostic position of gaze is a critical part of assessing eye movement. It involves asking the patient to follow an object with their eyes in specific directions to evaluate the function of extraocular muscles.

The six extraocular muscles responsible for eye movement are:

• Medial Rectus Muscle: Responsible for inward eye movement (adduction).
• Lateral Rectus Muscle: Responsible for outward eye movement (abduction).
• Superior Rectus Muscle: Elevates the eye and helps with inward movement (adduction and elevation).
• Inferior Rectus Muscle: Depresses the eye and helps with inward movement (adduction and depression).
• Superior Oblique Muscle: Primarily responsible for outward and downward eye movement (abduction and depression).
• Inferior Oblique Muscle: Primarily responsible for outward and upward eye movement (abduction and elevation).

Binocular Single Vision

Binocular single vision refers to the ability of both eyes to work together, creating a single, unified image. This is crucial for depth perception, which allows us to perceive the world in three dimensions. The expansion of the field of vision provided by having two eyes enhances our ability to navigate and interact with our environment.

Concept of Plasticity and Critical Period for Optimal Visual Development

The concept of plasticity in visual development refers to the brain's ability to adapt and rewire itself based on sensory experiences. During the critical period for optimal visual development, which typically occurs in early childhood, the brain is highly responsive to visual input and can make lasting changes.

Factors influencing optimal visual development include proper eye alignment, clear visual input, and early detection and correction of visual disorders. Early intervention in cases of amblyopia (lazy eye) or strabismus (misaligned eyes) is crucial to ensure the best possible visual outcomes.

Strabismus, commonly known as "crossed eyes," is a condition characterized by misalignment of the eyes. Several factors can contribute to the development of strabismus, including:

1. Congenital Absence of Fusion Potential: In some cases, individuals are born with an inability to coordinate the movement of their eye muscles, which can lead to strabismus. This congenital condition may result from developmental issues in the brain's control of eye movement.
2. Sensory Participation (Enumerating from Lid to Retina): The process of vision involves several sensory components, from the eyelids to the retina at the back of the eye. Any disruption in these sensory pathways can potentially lead to strabismus. For example, abnormalities in the sensory feedback loop that helps coordinate eye movement can contribute to strabismus.
3. Extraocular Muscle Problems: Strabismus can also arise from issues with the extraocular muscles responsible for controlling eye movement. These problems can be categorized into several types:
   • Motor Problems: Conditions affecting the strength or coordination of the eye muscles can result in strabismus. This may include muscle weakness or muscle imbalances.
   • Neurogenic Problems: Neurological conditions that affect the nerves controlling eye movement can lead to strabismus. For instance, damage or dysfunction of the cranial nerves responsible for eye movement can result in misalignment.
   • Neuro-Muscular Problems: Conditions that involve both neurological and muscular components can contribute to strabismus. These complex disorders may affect the communication between the brain and eye muscles.
   • Mechanical Problems: Mechanical obstructions or abnormalities in the eye socket can interfere with the normal movement of the eye muscles, leading to strabismus. These issues may include orbital fractures or restrictive eye conditions.
4. Genetics: Strabismus can have a genetic component, meaning it may run in families. Certain genetic factors can predispose individuals to develop strabismus, but the exact inheritance patterns can vary.

Amblyopia in Brief

Amblyopia, commonly known as "lazy eye," is a developmental disorder of spatial vision. It occurs when the brain fails to properly process visual information from one eye. This condition typically arises when the affected eye is not used normally during the critical period of visual development.

The most common causes of amblyopia include:

1. Strabismus: Strabismus is a condition in which the eyes do not align correctly, leading to misalignment. This misalignment disrupts the brain's ability to merge visual input from both eyes into a single, coherent image.
2. Anisometropia: Anisometropia occurs when the two eyes have different refractive powers. For example, one eye may be nearsighted (myopic) while the other is farsighted (hyperopic). This difference in focus between the eyes can result in amblyopia.
3. Form Deprivation: Form deprivation is a condition in which the affected eye is unable to receive clear images. This can happen when there are obstructions to clear vision, such as congenital cataracts that cloud the lens of the eye. The lack of clear visual input during the critical period can lead to amblyopia.

Ocular & Systemic Associations

1. Inflammation (Meningitis, Encephalitis): Inflammatory conditions affecting the brain and its membranes, such as meningitis and encephalitis, can sometimes manifest with ocular symptoms. This may include changes in vision or eye movements due to the close anatomical proximity of the eyes to the brain.
2. Neoplastic (Intraocular, Orbital & Brain Tumors): Tumors within the eye, orbit, or brain can impact ocular health. Depending on their location and size, these tumors may exert pressure on the optic nerve or surrounding structures, causing visual disturbances.
3. Vascular (Diabetes Mellitus, Hypertension, Aneurysm, Carvenous Sinus Thrombosis): Systemic vascular conditions, like diabetes mellitus and hypertension, can lead to eye-related complications such as diabetic retinopathy and hypertensive retinopathy. Additionally, vascular issues like aneurysms or carvenous sinus thrombosis may affect blood flow to the eyes, resulting in visual symptoms.
4. Trauma (Orbital and Head): Head and orbital trauma can cause various ocular injuries, ranging from mild to severe. These injuries may include fractures of the orbital bones, damage to the eye globe, or optic nerve injury, all of which can have lasting effects on vision.
5. Toxicity (Botulism): Toxic substances, such as botulism toxins, can lead to muscle paralysis, including the muscles controlling eye movements. This can result in double vision (diplopia) and other visual problems.
6. Neuro/Muscular (Myasthenia Gravis, CPEO - Chronic Progressive External Ophthalmoplegia): Neurological and muscular disorders like myasthenia gravis and CPEO can affect the muscles responsible for eye movement. Patients with these conditions may experience weakness in the eye muscles, leading to ptosis (drooping eyelids) and ophthalmoplegia (limited eye movement).
7. Genetics: Genetic factors play a significant role in various ocular conditions. Inherited genetic mutations can lead to congenital eye disorders or increase the risk of developing certain eye diseases later in life.

Terminologies in Amblyopia & Strabismus

• Strabismus: Strabismus refers to the misalignment of the two eyes, where they do not point in the same direction. This condition can lead to double vision (diplopia) or visual suppression of one eye.
• Amblyopia: Amblyopia is often both a cause and an effect of strabismus. It occurs when the brain favors one eye over the other, leading to reduced vision in the weaker eye. It differs from vision loss due to organic defects like cataracts or glaucoma.
  • Strabismic Diplopia/Suppression: Strabismic diplopia is double vision resulting from misaligned eyes. To avoid this, the brain may suppress the image from one eye, leading to amblyopia.
  • Refractive Anisometropia/High Isometropia: Anisometropia is a condition where the two eyes have significantly different refractive errors. High isometropia refers to extreme refractive errors in both eyes and can contribute to amblyopia.
  • Form Deprivation (Amblyopia ex Anopsia) & Occlusion: Form deprivation occurs when the eye is deprived of clear images during critical periods of visual development. Occlusion involves covering or patching the stronger eye to encourage the weaker eye's use in amblyopia treatment.
• Comitant (Non-paralytic, Non-restrictive): Comitant strabismus refers to misaligned eyes where the angle of deviation remains the same in all directions of gaze. It is typically associated with normal extraocular muscle function.
• Incomitant (Paralytic, or Restrictive): Incomitant strabismus occurs when the degree of misalignment varies in different gaze directions. This can result from abnormal function of the extraocular muscles or their controlling nerves.
• Phoria: A phoria is a latent misalignment of the eyes, which becomes noticeable when binocular viewing is interrupted. It doesn't manifest as a constant deviation.
• Tropia: A tropia is a manifest misalignment of the eyes, meaning the deviation is present even during binocular viewing.
• Preference: In the context of amblyopia, preference refers to the brain favoring one eye over the other for visual processing. This preference can lead to amblyopia in the less-favored eye.

Continuing with essential terminology related to amblyopia and strabismus, here are more key terms to be familiar with:

• E Esophoria for Distance: Esophoria refers to an inward tendency of the eyes when they are relaxed and looking at distant objects. It is not a manifest misalignment but rather a latent deviation.
• ET Esotropia for Distance: ET (Esotropia for Distance) denotes an inward eye turn or cross-eyed condition that occurs when focusing on distant objects.
• E(T) Intermittent Esotropia for Distance: E(T) represents intermittent esotropia, where the eye occasionally turns inward while viewing distant objects.
• E’ or ET’ or E(T)’ Near: These terms indicate similar conditions as above but in the context of near vision.
• X, XT, X(T): These terms refer to exotropia, which is an outward deviation of the eyes. The 'X' represents the condition, 'XT' is exotropia for distance, and 'X(T)' represents intermittent exotropia for distance.
• X’ or XT’ or X(T)’: These terms denote exotropia in the context of near vision.
• H, HT, H(T) Hyperdeviation: Hyperdeviation refers to an upward deviation of one eye. 'H' represents hyperdeviation, 'HT' is hyperdeviation for distance, and 'H(T)' signifies intermittent hyperdeviation for distance.
• DVD (Dissociated Vertical Deviation), IPD (Interpupillary Distance), NPA (Near Point of Accommodation), NPC (Near Point of Convergence), OA (Overaction), UA (Underaction): These terms are important measurements and clinical observations used in assessing eye alignment and function.
• CSM (Central, Steady, Maintained): CSM refers to the criteria for fusion during binocular vision. Central means that the images from both eyes are aligned centrally, steady indicates that the images remain stable, and maintained implies that the fusion is sustainable over time.

Strabismus can be classified based on various factors, providing valuable information for diagnosis and treatment. Here are some common classifications:

Based on Direction of Eye Misalignment:

1. Esotropia: In esotropia, one eye turns inward toward the nose, resulting in a "crossed" appearance. This is the most common type of strabismus.
2. Exotropia: Exotropia involves one eye turning outward away from the nose, leading to a "wall-eyed" appearance.
3. Hypertropia: Hypertropia occurs when one eye deviates upward while the other maintains normal alignment.
4. Hypotropia: Hypotropia is characterized by one eye deviating downward while the other remains properly aligned.

Based on Age of Onset:

1. Congenital Strabismus: Strabismus that is present at birth or develops within the first few months of life.
2. Acquired Strabismus: Strabismus that develops later in childhood or adulthood due to various factors, such as trauma, illness, or refractive errors.

Based on Cause:

1. Accommodative Esotropia: Often related to uncorrected hyperopia (farsightedness), this type of strabismus occurs when the eyes cross due to the effort of focusing on near objects.
2. Non-Accommodative Esotropia: This form of esotropia is not related to focusing on near objects and may have different underlying causes.
3. Intermittent Strabismus: The eye misalignment occurs intermittently, often when the individual is tired or not focusing on a specific target.
4. Constant Strabismus: The eye misalignment is present continuously.

Based on Associated Conditions:

1. Strabismus with Amblyopia: Some individuals with strabismus may develop amblyopia (lazy eye) in the deviating eye due to suppressed visual input.
2. Paralytic Strabismus: This type of strabismus results from paralysis or weakness of the eye muscles, often caused by neurological conditions or trauma.
3. Non-Paralytic Strabismus: Strabismus that is not associated with muscle weakness or paralysis.

Esotropia

Esotropia is a type of strabismus characterized by a convergent misalignment of the visual axes, where one eye turns inward toward the nose. It is important to note that infantile esotropia and exotropia are associated with an increased risk of amblyopia.

Categories of Esotropia:

1. Infantile Esotropia: Infantile esotropia is a form of esotropia that typically manifests in the first few months of life. It is associated with a higher risk of amblyopia.
2. Acquired Esotropia (Develop After Age 6 Months): Acquired esotropia refers to esotropia that develops after the age of 6 months. It can be further categorized based on underlying causes, including:
   • Accommodative Esotropia: Accommodative esotropia is linked to the effort of focusing on near objects. It can be subclassified into:
     • Accommodative Refractive Esotropia: This type of esotropia is associated with refractive errors and occurs when the eyes cross due to the effort of accommodating (focusing) on near objects.
     • Accommodative Refractive Esotropia with a High AC/A Ratio (Accommodative Convergence to Accommodation Ratio): Some cases of accommodative esotropia have a higher AC/A ratio, leading to more pronounced eye misalignment during accommodation.
     • Accommodative Non-Refractive Esotropia with a High AC/A Ratio: This type of esotropia is similar to the previous category but is not primarily associated with refractive errors.
   • Partially Accommodative Esotropia: Partially accommodative esotropia refers to cases where there is a combination of accommodative and non-accommodative factors contributing to the eye misalignment.
   • Non-Accommodative Esotropia: Non-accommodative esotropia occurs without a significant contribution from accommodative factors.
3. Other Forms of Esotropia: This category encompasses less common forms of esotropia that may have distinct characteristics or underlying causes.

Infantile Esotropia

Infantile esotropia is a type of esotropia that typically presents between the ages of 3 and 6 months in infants. It is characterized by inward eye turning, and while intermittent esotropia during the first 3 months of life is common, it does not necessarily predict the development of constant strabismus.

Children with infantile esotropia are at risk for amblyopia, a condition in which the brain favors one eye over the other, resulting in reduced vision in the weaker eye.

Characteristics of Infantile Esotropia:

• Onset Before the Age of 6 Months Without Spontaneous Resolution: Infantile esotropia typically begins before the age of 6 months and does not resolve on its own.
• Nonaccommodative or Partially Accommodative Etiology: This type of esotropia is usually not related to focusing on near objects (nonaccommodative) or may have a combination of accommodative and non-accommodative factors (partially accommodative).
• Constant Angle of Deviation That May Increase With Time: The degree of eye misalignment in infantile esotropia remains relatively stable or may increase over time.
• Frequent Cross Fixation With the Fixing Eye Held in Adduction: Children with infantile esotropia often exhibit cross fixation, where they rely on one eye for fixation while the other eye is turned inward (adduction).
• Abnormal Binocular Visual Function: Infantile esotropia can lead to impaired binocular vision, which affects the ability to perceive depth and coordinate both eyes.

Features that may not be present at the time of diagnosis include latent nystagmus, dissociated vertical deviation, oblique muscle dysfunction with A or V patterns, and optokinetic nystagmus asymmetry for nasal vs. temporal pursuit.

Accommodative Esotropia

Accommodative esotropia is a specific type of esotropia that is characterized by an inward eye turn, often associated with hyperopia (farsightedness) and a strong reliance on accommodation (focusing) for near vision.

Characteristics of Accommodative Esotropia:

• An Accommodative Component, Usually Associated with Hyperopia: Accommodative esotropia typically involves an accommodative component, and it is commonly associated with hyperopia (farsightedness), especially in cases where the hyperopia is greater than 2.00 diopters (D).
• Typical Onset Between the Ages of 1 and 8 Years: Accommodative esotropia typically begins between the ages of 1 and 8 years, with an average age of onset at approximately 2 years. However, it may also appear in infancy or reappear as a sequel to surgically corrected infantile esotropia.
• May Be Precipitated by Illness, Fever, or Minor Trauma: Some cases of accommodative esotropia may be triggered or exacerbated by factors such as illness, fever, or minor trauma.
• Binocular Visual Function That May Be Normal at the Onset of Deviation: Initially, children with accommodative esotropia may have normal binocular vision. However, the presence of esotropia becomes more pronounced during near tasks that require increased accommodation.

Etiology:

The underlying cause of accommodative esotropia is usually excessive convergence in a child with bilateral hyperopia, often greater than 2.00 diopters (D). Correcting the hyperopia can eliminate the esotropia, a condition referred to as "accommodative refractive esotropia." In some cases, correcting the hyperopia results in normal alignment at distance fixation but a persistent esotropia at near (accommodative refractive esotropia with a high AC/A ratio). Less frequently, children have normal alignment at distance fixation with no significant hyperopia but develop a constant or intermittent esotropia with near fixation (accommodative non-refractive esotropia with a high AC/A ratio).

Partially Accommodative Esotropia

Children with acquired partially accommodative esotropia experience a partial improvement of their esotropia when they wear corrective lenses for their hyperopia (farsightedness). However, they still have a residual esotropia greater than 10 prism diopters at both distance and near fixation.

Nonaccommodative Esotropia

Children with nonaccommodative esotropia have an acquired esotropia that is approximately equal in size at both distance and near fixation. This type of esotropia does not improve with the correction of refractive error with eyeglasses, or the child may have no significant refractive error.

Other Forms of Esotropia

In addition to the previously mentioned types of esotropia, there are several other forms of esotropia that may require consideration in a differential diagnosis. These include:

• Cranial Nerve VI Palsy: Esotropia can be caused by dysfunction of the sixth cranial nerve, resulting in reduced lateral eye movement.
• Esotropic Duane Syndrome: Duane syndrome is a congenital eye movement disorder that can manifest with esotropia and limited abduction (outward movement) of one eye.
• Sensory Esotropia: Sensory esotropia occurs when a visual impairment in one eye leads to the development of esotropia in an attempt to avoid double vision.
• Restrictive Esotropia: Restrictive esotropia results from mechanical restrictions that limit eye movement and cause esotropia.
• Consecutive Esotropia: Consecutive esotropia may occur as a secondary deviation following eye surgery or treatment for an initial deviation.
• Nystagmus Blockage Esotropia: Esotropia can be induced or worsened when nystagmus (involuntary eye movement) is blocked, often by covering one eye.

For further understanding, you can watch a video on this topic: Basic Cover test Final from University of Iowa Ophthalmology.

Inspection

During the inspection phase of the eye examination, several key aspects are assessed to understand the patient's condition. These include:

• Mongoloid or Antimongoloid Lid Fissures, A and V Patterns in Downward Gaze: Observation of the eyelid fissures and the presence of A and V patterns during downward gaze can provide insights into the patient's eye alignment and possible conditions.
• Exophthalmos: The presence of exophthalmos, where the eyes protrude from their sockets, may be indicative of conditions such as Graves' disease or congenital craniofacial anomalies.
• Enophthalmos: Enophthalmos, where one or both eyes appear sunken, can be associated with conditions like Duane syndrome or old blowout fractures.
• Lagophthalmos: Lagophthalmos, the inability to fully close the eyelids, may result from a complete N III paralysis with aberrant regeneration.
• Facial Asymmetry: Facial asymmetry can be linked to inconstant strabismus, plagiocephaly (abnormal head shape), or superior oblique muscle paralysis.
• Closure of One Eye: Closing one eye may be due to lower binocular thresholds and photophobia, especially in cases of intermittent exotropia.
• Prominent Epicanthal Folds: Prominent epicanthal folds may give the appearance of pseudostrabismus, where the eyes appear misaligned, but it's due to the facial features.
• Alternation: Alternating eye preference can suggest reduced visual acuity (amblyopia) or organic causes for visual disturbances.
• Variable Angle of Strabismus: A variable angle of strabismus can result from uncorrected refractive errors (refractive accommodative esotropia), anisometropia (unequal refractive errors between the eyes), or in association with the nystagmus compensation syndrome.

Examination Tools

During eye examinations, various tools and equipment are used to assess and diagnose visual conditions. These tools include:

• Visual acuity charts
• Stereo tests (e.g., Titmus, Randot, Lang, TNO)
• Worth 4 dot test and red-green spectacles
• Loose prisms, ranging from 1/2 diopter to 30 diopters
• One horizontal and one vertical plastic prism bar (1 diopter to 25 diopters)
• Rotary prism
• Opaque occluder
• Near fixation targets, silent or noise-producing, and animated, with capabilities for maintaining attention or stimulating accommodation
• Animated distance fixation targets
• A trial lens set with plus cylinders and prisms (to fit in a trial frame)
• Adult and pediatric trial frames
• Red and white Maddox rods
• Handheld fixation light
• Pinhole
• Bagolini lenses
• Halberg clips
• Retinoscope
• Refraction lens bar with convex and concave lenses
• Portable biomicroscope
• Direct and indirect ophthalmoscopes
• Hertel exophthalmometer
• Near vision cards
• Optokinetic drum or tape
• Fine-tooth forceps for passive ductions and estimation of generated muscle force
• Afterimage tester (modified handheld camera flash)
• Reading comprehension charts and word lists
• Major amblyoscope*
• Deviometer
• Hess or Lees' screen
• Spielmann translucent occluders
• Contrast sensitivity acuity chart
• Neutral density filters
• Visuscope or similar device to test fixation pattern
• Perimeter to determine the field of single binocular vision
• Special documentation tools for research and medicolegal purposes
• Some items of "second-line" equipment for systematic recording schemes
• Drops for dilating and anesthetizing the eyes
• Fresnel press-on prisms in diopters ranging from 1 to 40
• *A major amblyoscope is essential for orthoptists
• Rotary prism or horizontal prism bar for fusional amplitudes

Assessment - Examination

Visual acuity assessment is a critical part of the examination process, as it helps determine whether the child can see, the quality of their vision, and whether there are any disparities between the eyes. This assessment is particularly important for detecting the presence of amblyopia (lazy eye) and selecting the appropriate objective assessment method.

Visual Acuity Test Used: [Insert the specific visual acuity test used, such as Snellen chart, Tumbling E chart, or another relevant test]

Unaided Visual Acuity:

• Right Eye (PH): [Insert visual acuity measurement for the right eye without correction]
• Left Eye (PH): [Insert visual acuity measurement for the left eye without correction]

Aided Visual Acuity:

• Right Eye: [Insert visual acuity measurement for the right eye with correction]
• Left Eye: [Insert visual acuity measurement for the left eye with correction]

Age	Test Used
Preverbal Children (0-6 months)	Localisation Acuity: visual perception of the location of an object or appreciation and fixation preference. Preferential looking test e.g. Teller acuity chart, LEA Gratings, Others including optokinetic, nystagmus, visually evoked response
7months -2years	Detection Acuity- Measures the simple detection of objects, not their identification or naming e.g. visually directed reachig for objects, picture naming e.g. Allen card or equivalents.
3-5 years	Recognition Acuity: Measures threshold at which an individual can discriminate the separation between critical elements of a stimulus pattern. Tumbling E with directional pointing, Matching test e.g. LEA symbols, Sheridan Gardiner, Letter identification e.g. "HOTV" with matching card, Snellen's alphabet and number.
Over 5 years	Recognition Acuity: Standard test for assessment ETDRS Chart, Snellen's test types, number. E-chart Landolt's broken ring chart

Stereopsis Tests Before Dissociation

As part of the sensory evaluation, stereopsis (depth perception) is assessed using various tests before dissociation. These tests include:

• Stereo Acuity
• TITMUS
• RANDOT
• WORTH FOUR LIGHTS
• TITMUS FLY

Binocular Function:

• TITMUS Test: Fly +─
• Animal: 3
• Circle: 9

Motility Exam

During the motility exam, it is essential to start with the least threatening tests and gradually progress to more dissociating and invasive assessments. This approach is especially important when examining children. Here's a recommended sequence:

1. Begin with the doll's head maneuver, which is suitable for younger children.
2. Start with the least dissociating tests and gradually progress to more dissociating tests. For example, assess stereo acuity before using the dissociation prism and cover test.
3. Keep forced duction testing and estimation of generated muscle force for older children and adults, and perform these towards the end of the examination.

Is There Strabismus or Not? – Examination

During the examination, it's crucial to determine whether strabismus (misalignment of the eyes) is present or not. Consider the following:

Pseudostrabismus:

• Epicanthal Folds: Observe for prominent epicanthal folds, which can give the appearance of strabismus but are related to facial features.
• Facial Asymmetry: Check for facial asymmetry, which may contribute to the appearance of strabismus.

Measurement of Vergences

Using Prism or Haploscope, the following parameters are measured:

• Blur Point
• Break Point
• Recovery Point (2-4 Prisms below Break Point)

What Type of Strabismus is Present?

The type of strabismus is determined by assessing:

• Deviation: ET (Esotropia), XT (Exotropia), Alphabet pattern
• Hirschberg / Modified Krimsky Test: Degrees/Prism Diopters (º/∆)
• Near / Far, Up Gaze / Down Gaze Deviation (Prism Cover Test):
• Without Glasses: [Insert measurement]
• With Glasses: [Insert measurement] with [Insert Add]
• A-V Pattern: A [Checkbox], V [Checkbox], None [Checkbox]
• Near Point of Accommodation (NPA)
• Near Point of Convergence (NPC)
• AC/A Ratio

Fixation:

[Insert details about fixation]

Special Tests:

[Insert details about any special tests conducted]

Type of Deviation

Based on the results of the cover test, the type of deviation can be categorized as follows:

• Cover Test: Manifest Tropia
• Cover-Uncover Test: Latent Deviation (Heterophoria)

Measurement of Deviation

During the examination, the deviation is measured using various methods:

• Cover Test: Manifest Tropia
• Cover-Uncover Test: Latent Deviation (Heterophoria)

How Much is the Deviation?

[Insert measurement or description of the deviation]

Corneal Reflection Tests:

• HB (Hirschberg Test)
• Krimsky Test

Prism Cover Test: Prism Cover Test (ACT - Alternate Cover Test)

This test measures the total deviation.

Amblyoscope:

[Insert information about the use of the Amblyoscope]

Hirschberg's Test (Corneal Reflections)

Hirschberg's Test, which has been in use for over 100 years, serves as a valuable method, especially in situations where it may be the only feasible way to assess eye alignment. It is particularly useful in the following scenarios:

• In infants
• In individuals with severe disabilities
• In patients with vision less than 6/120 in both eyes

This test is typically conducted for near fixation. Here's how it's performed:

• A small spotlight is held in front at a distance of approximately 0.33 meters.
• The examiner positions themselves directly behind the light.
• The position of the light reflection on each cornea is carefully checked.

Prerequisites for conducting this test include the patient's attention, useful visual acuity, the ability to fixate, and central fixation in both eyes.

Prism Orientation

When working with prisms, it's important to consider the following guidelines:

• Do not stack horizontal prisms.
• You can combine horizontal and vertical prisms as needed.
• Hold the prism in relation to the orbit for accurate assessment.

Krimsky's Test (Prism Reflex Test, Prism Reflection Test)

Krimsky's Test is similar to the HB (Hirschberg) test, with the key difference being that prisms are placed in front of the fixating eye to center the corneal reflection in the deviated eye.

Prism and Cover Test

The Prism and Cover Test employs the Heremietral Trigger Mechanism to measure deviation. It involves the following steps:

• Progressive prism neutralization
• Over-correction produced
• Image crosses the midline of the retina
• Triggers diplopia

This test is known for its reliability and sensitivity and is typically performed in all adults and children who are old enough to provide an accurate response.

The test is widely used for assessing eye deviation possesses the following characteristics:

• It can detect manifest, latent, or combined deviations.
• It is applicable at any distance.
• It can be performed in any direction of gaze.
• It can be conducted with either eye fixing.
• It is suitable for assessing both horizontal and vertical deviations.
• Its results depend on the movement of the eyes to refixate in the presence of a prism.

Limitations of Prism and Cover Test

While the Prism and Cover Test is a valuable assessment tool, it comes with certain limitations, including:

• It presupposes accurate fixation, which may not be achievable in all cases.
• It cannot be performed if the deviating eye has poor vision, is blind, or exhibits eccentric fixation.
• The accuracy of the test is limited by the optical qualities of the prism used.

Cyclo-Refraction:

In cases of esotropia, cyclo-refraction is an important consideration for assessment and management.

Patch Test

The patch test is utilized for cases of intermittent exotropia (XT). The procedure involves the following steps:

• Patch the eye for approximately 45 minutes to suspend tonic fusional convergence.
• During this time, do not allow re-fixation.
• After the patching period, check the deviation at both distance and near.

Forced Duction Test

The Forced Duction Test is used to assess mechanical factors that may affect eye movement. It helps identify conditions such as contracture or fibrosis of a muscle, tightness of a muscle following excessive resection, or shrinkage/scarring of the conjunctiva or Tenon's capsule. The procedure involves:

• Performing the test in the office under topical anesthesia.
• Asking the patient to look towards the side of gaze limitation.
• Using fixation forceps to hold the conjunctiva and Tenon's capsule at the limbus.

Superior Oblique Traction Test

Diagnosis & Clinical Impression

Based on the examination and assessment, the diagnosis and clinical impression should be established. This includes determining whether surgery is warranted, which eye muscles may be involved, and the extent of intervention needed.

Plan: Decide whether surgery is required or not and specify which muscles and how much of each.

Follow-up: Schedule follow-up appointments with dates and expected findings.

1. First follow-up: [Insert date and expected findings]
2. Second follow-up: [Insert date and expected findings]

Questions for Decision Making

When making decisions regarding the patient's care, consider the following questions:

• Why is this patient here?
• What do I see? Is there strabismus or not?
• What type of strabismus is it, and how long has it been present? What is the underlying cause?
• What previous treatments or interventions have been attempted, and how effective were they? What was the outcome?
• Are there any ocular or systemic issues present that need consideration?
• What treatment options can I offer to the patient?
• What treatment plan will the patient be willing to accept?
• What are the potential outcomes of the proposed treatment?
• How should I modify my subsequent care based on the patient's response to treatment?

Why Treat Strabismus?

Treating strabismus is essential for several reasons:

• Strabismus is considered an abnormality because the normal position of the eyes is to be aligned.
• Strabismus care is crucial from both medical and psychological perspectives, as well as its social impact.
• It is often a medical necessity, especially in cases where it causes diplopia (double vision) in adults.
• In esotropia, it can lead to a constricted field of vision.
• Strabismus could indicate the presence of an intracranial pathology that needs attention.
• Treatment not only improves the appearance but also restores some degree of stereopsis (depth perception) and binocular single vision (BSV).
• It alleviates diplopia, expands peripheral vision, and provides solutions to associated handicaps.

The following findings and recommendations are important in the care of strabismus:

• Infantile esotropia and exotropia are associated with an increased risk of amblyopia (good evidence).
• Strabismus in children under 4 months of age sometimes resolves, especially if it's intermittent, variable, or measures less than 40 prism diopters (good evidence).
• Repeat cycloplegic refraction is indicated when esotropia does not respond to the initial prescription of hyperopic refraction or when esotropia recurs after surgery (strong recommendation, moderate evidence).
• Youthful children with intermittent exotropia and good fusional control can be followed without surgery (strong recommendation, moderate evidence).
• In patients with exotropia and a high accommodative convergence to accommodation ratio (AC/A), treatment with eyeglasses is generally preferred over surgery due to the risk of consecutive esotropia and diplopia after surgery (discretionary recommendation, moderate evidence).
• Esotropia that persists after exotropia surgery may place the patient at risk for amblyopia, diplopia, and loss of stereoacuity (moderate evidence).
• Children with untreated strabismus can experience reduced binocular potential, impaired social interactions, and negative perceptions from others, affecting their psychosocial quality of life (good evidence).

Note: When surgical realignment of the eye is indicated, it is considered reconstructive surgery rather than cosmetic surgery in all age groups.

Treatment Modalities

The following treatment modalities are used alone or in combination as required to achieve the therapeutic goal in strabismus cases:

• Correction of refractive errors
• Bifocals
• Prism therapy
• Amblyopia treatment
• Extraocular muscle surgery
• Botulinum toxin A injection
• Other methods

Treatment plans are formulated in consultation with the parent/caregiver and the patient, if appropriate. These plans should be responsive to the expectations and preferences of the parent/caregiver and patient. This includes their perception of the existing alignment, which may differ from the ophthalmologist's, and what they hope to achieve with treatment. It is crucial that the family/caregiver and ophthalmologist agree on the goals of treatment before surgery is performed. For patients for whom the potential for binocularity (binocular vision) is poor, surgery to restore normal appearance may be an appropriate treatment.

The surgery: Move/Cut/Adjust the EOM

Why Treat Strabismus?

Treating strabismus is crucial for several reasons:

• It is often a medical necessity.
• Strabismus can be a contributing factor to delayed milestones.
• Case Study: There are instances, such as in Congenital Fibrosis of the Extraocular Muscles (CFEOM), where strabismus can result in delayed walking. Surgery to correct strabismus in such cases can lead to the resolution of delayed milestones.
• Strabismus can be comitant, meaning the misalignment remains relatively constant across different gaze directions.

Strabismus - Incomitant

Incomitant strabismus refers to a condition where the misalignment of the eyes varies with different gaze directions. One specific example of incomitant strabismus is categorized as Duane's Classification 2.

Strabismus - Incomitant

Incomitant strabismus can also occur as a result of traumatic sixth nerve palsy. In this condition, the misalignment of the eyes varies with different gaze directions due to damage or injury to the sixth cranial nerve.

Post-Operative Challenges

Following strabismus surgery, there are several post-operative challenges and considerations:

• Improvement in stereopsis (depth perception) may be a challenge and requires monitoring.
• Continued monitoring for amblyopia (lazy eye) is essential even after surgery.
• Correction of refractive errors may still be necessary post-surgery.
• Addressing any residual errors, whether under or overcorrection, is part of the post-operative care plan.
• Complications related to surgery, if any, need to be managed appropriately.