Strabismus

What is Strabismus?

Strabismus is a term that originates from Greek, meaning “eyes looking sideways.” It refers to a condition where the eyes are not aligned, often called “squinting eyes,” “crossed eyes,” or “wall eyes.” Normally, both eyes focus equally on an object while the head is straight. In strabismus, one or both eyes might turn inwards or outwards, making them look out of alignment when focused on an object. This issue might occur due to an mistake in how the eye refracts, or bends, light, issues with the eye muscles, or problems with binocular fusion, which is how the brain combines the images from both eyes into one. Recognizing and treating strabismus early usually results in great outcomes. Treatment can include correcting how the eye bends light, eye exercises, covering one eye with a patch, topical medications, or surgery on the eye muscles.

Specific terms are used to describe different eye alignments. Orthophoria refers to perfect eye alignment even when there’s no stimulus present, while orthotropia describes the correct direction of the eyes when viewing an object with both eyes. Both terms describe eyes that are properly aligned.

Heterophoria is when the eyes deviate slightly, but this is controlled by fusion, the brain’s ability to combine images received from both eyes. Heterotropia refers to an eye deviation that can be seen during an eye exam. This deviation might be related to “lazy eye,” or amblyopia.

The terms for different eye deviations are as follows:

• ‘Eso’- means the eyeball deviates towards the nose.
• ‘Exo’- means the eyeball deviates towards the temple.
• ‘Hyper’- means the eyeball deviates upwards.
• ‘Hypo’- means the eyeball deviates downwards.
• ‘Incyclo’- means the upper part of the eyeball rotates towards the nose.
• ‘Excyclo’- means the upper part of the eyeball rotates towards the temple.

If strabismus starts before a child turns 6 months, it is called infantile strabismus. If the eye deviation occurs after 6 months, it is referred to as acquired strabismus, following a period of normal eye alignment.

Additionally, the strabismus is referred to as comitant if the amount of deviation is the same no matter where the person is looking. It is termed incomitant if the deviation changes depending on where the person is looking.

What Causes Strabismus?

Strabismus, also known as squint, is generally categorized as:

* Pseudostrabismus, also known as false squint: This includes pseudo inward squint (pseudoesotropia) and pseudo outward squint (pseudoexotropia).
* Hidden squint (Heterophoria)
* Visible squint (Heterotopia): This can be a squint that doesn’t vary depending on the direction of the gaze (concomitant squint) or a squint that changes with gaze direction (incomitant squint).

Some causes for pseudostrabismus include:

* Pseudoesotropia: This is caused by noticeable inner eyelid folds or a slight deviation of the light reflection in the eye.
* Pseudoexotropia: This may occur due to wide-set eyes or a positive angle kappa, which is another kind of light reflection deviation.

Some causes for Heterophoria include:

* Anatomical causes: This can include uneven eye sockets, abnormal distance between pupils, or a mild weakness of the muscles that move the eyes.

* Physiological causes: Some people may squint inwardly (Esophoria) when they accommodate or adjust for far-sightedness (hyperopes) and outwardly (Exophoria) in nearsighted people (myopes), overly or less convergent use of eyes, or in situations like using a microscope that only limits vision to one eye.

Certain reasons for a visible squint (Concomitant Squint) can include:

* Sensory causes: These could include anything affecting the clarity of an image in one eye like incorrect eyeglasses prescription, difference in power of the two eyes (anisometropia), cloudiness of the lens or cornea (media opacities), blockage of the pupil, macular and optic nerve diseases.

* Motor causes: These can interfere with eye alignment and could include abnormalities in the eye socket, eye-muscle weakness, and issues with focusing.

Incomitant squint can happen due to:

* Neurogenic causes: These may include underdevelopment of certain cranial nerves, tumors, infections, eye trauma, harmful substances like alcohol or lead, migraines, and lesions on certain cranial nerves.

* Myogenic causes: These might consist of in-born defects, muscle damage following trauma, muscle inflammation after a viral infection, and chronic progressive external eye muscle weakness (CPEO). Muscle issues at the junction of nerve to muscle, like Myasthenia gravis, might also cause squint.

Risk Factors

People with certain conditions like Down syndrome, cerebral palsy, Apert-Crouzon syndrome, premature infants with low birth weight, and children who have close family members with strabismus are more likely to develop this condition. It is recommended that all siblings of children with strabismus get screened early for this condition as problems with eye movements are common in families where one person has strabismus.

Risk Factors and Frequency for Strabismus

Strabismus is a condition affecting between 2% to 5% of the general population. In the United States, it impacts 5 to 15 million people. In a health survey, two types of strabismus were observed: exotropia (in 2.1% of the population) and esotropia (in 1.2% of people). Exotropia is more common in people aged 55 to 75 years.

• Half of all childhood cases of esotropia are either fully or partially due to issues with eye focusing.
• Non-focusing related esotropia is seen in 10% of all strabismus cases and is the second most common type of strabismus experienced in childhood.
• Infantile esotropia affects 1 in 100 to 500 people, accounting for 8.1% of all esotropia cases.
• Intermittent esotropia is observed in 1% of the population and is the most common type of exotropia.
• Exotropia is more prevalent in Asian and black populations.
• Women account for 60% to 70% of exotropia cases.

Signs and Symptoms of Strabismus

Strabismus, or crossed eyes, can appear at any age, but it’s commonly detected in children before they turn six years old, with the peak onset at about three years old. Parents often report that their child’s eyes have been crossed since birth. Evidence can often be found in family photographs, which might also reveal which eye the child prefers to use. If the child alternates between eyes, it suggests they do not have vision loss (amblyopia) in one eye due to strabismus. However, if one eye is used predominantly, it could suggest that strabismus has led to vision loss in the other eye. A history of low birth weight could indicate a condition of the retina due to prematurity, which could cause the appearance of strabismus.

When a child suddenly develops strabismus, particularly an older child, it’s important to have a thorough check-up to rule out neurological problems. If the strabismus comes and goes, this suggests that normal two-eye vision is possible some of the time. Photosensitivity, or being sensitive to light, often occurs in intermittent deviation of the eye outward and results from a decreased threshold for light fear. There are other critical factors to consider including allergy history to specific eye drops and certain illnesses that might be critical to prevent serious complications during strabismus surgery.

There are important questions that parents or caretakers should be asked about their child’s eye health, such as when the deviation began, any history of physical illness before the onset of deviation, the nature of deviation- whether constant or intermittent, change in deviation with illness or fatigue, and which eye is predominantly affected.

Below are some commonly seen strabismus conditions along with their clinical features:

• Essential Infantile Esotropia (Congenital Esotropia): Children usually have this condition within their first six months. They have stable, large-angled inward deviation of the eyes, a positive family history, and no neurological problems. Usually there is no refractive issue (need for glasses). Signs of this condition include cross-fixation, an uneven response to moving objects and specific eye movements. Nearly all children with this condition develop a dissociated vertical deviation (DVD), or unequal eye elevation, by the age of three. Treatment involves correcting hyperopia (farsightedness) and treating amblyopia (loss of vision in one eye). Early eye muscle surgery is often recommended.
• Accommodative Esotropia: This commonly begins between ages 2 to 5. It’s often triggered by trauma or illness. There is generally a family history and amblyopia is common. Treatment strategies include prescribing glasses and conducting surgery if the residual esotropia (inward deviation of the eye) is significant.
• Microtropia: This condition is characterized by poor two-eye vision with vision loss in one eye and a very small deviation angle. Treatment involves correcting the difference in vision power between the two eyes and vision loss.
• Intermittent Exotropia: This typically appears around the age of 2 as an outward deviation of the eye that occurs under specific conditions. The suggested treatment strategy includes correcting refractive problems and treating amblyopia. Eye exercises are also often recommended as is eye surgery to adjust the eye muscles.
• Dissociated Vertical Deviation (DVD): This condition typically develops around the age of 2. Signs of DVD include upward movement of the eye that can occur when the other eye is covered or during lack of visual attention. DVD surgery usually involves an adjustment of the upper eye muscle.
• Monocular Elevation Deficiency: This can present as a complete inability to move the eye upward along with a drooping eyelid and a compensatory head position. Surgery is the treatment of choice for this condition.
• Alphabet Patterns: This type of strabismus changes when a person looks up or down.
• Duane Retraction Syndrome: This congenital syndrome typically presents with eye movement limitations with narrowing of the eyelid slit. Surgery is often required with this syndrome.
• Brown Syndrome: This condition, which may be congenital or acquired, presents as limited eye movement upwards and towards the nose.
• Mobius Syndrome: This syndrome presents as an inability to move either eye past the midline. It is due to a congenital absence of the sixth and seventh cranial nerves.
• Oculomotor Palsy: This is presented as a drooping eyelid with defective accommodation, characterized by an eye that is deviated outwards and is rotated externally.
• Lateral Rectus Palsy: This condition is indicated by an inward deviation of the eye and a limitation of abduction.
• Superior Oblique Palsy: This disorder is characterized by a limitation of eye movement downwards and towards the nose and rotating of the top of the eye outwards.

Testing for Strabismus

If a doctor suspects that a baby may have vision problems, they can use a simple technique called the ‘CSM’ method. This involves the use of a torch to look at the reflection on the baby’s cornea as it follows the light moving side by side. If both of the baby’s eyes stay aligned during this exercise, it’s a good sign their vision is functioning correctly.

For toddlers who can’t communicate verbally yet, doctors can use specific picture card tests, known as preferential-looking tests, to assess their vision.

Some children might resist having their eye covered during the tests, especially if it’s the one with better vision. So, doctors have to be patient and use other techniques to achieve the goal.

For older children, several chart tests, such as the Snellen charts and the Sheridan-Gardiner chart, can be used to assess their visual acuity.

Beyond just clarity of vision, doctors also examine an aspect called “stereoacuity,” which is our ability to sense depth or distance. This is done using several tests including the Titmus stereo-fly test, Worth 4-dot test and several other tools.

In cases of strabismus (crossed eyes), people often adopt certain head postures to maintain clear binocular vision. This is called a Compensatory Head Posture (CHP). If a person with strabismus loses their CHP, it could mean they’ve lost their binocular single vision, possibly necessitating surgical intervention.

Doctors also calculate the AC/A ratio while evaluating crossed eyes. This ratio reflects the ability of our eyes to move closer together (converge) as we focus on near objects. It’s typically measured using two techniques, namely the lens gradient method and the heterophoria method.

Several tests, including the Hirschberg test, Krimsky test, cover test, prism cover test, among others, help in measuring the deviation of one eye with respect to the other in strabismus.

To test the movements of our eyes, doctors use different tests that involve either both eyes or just one eye at a time. This examination involves evaluating the quality of certain types of eye movements, called versions, ductions, and vergence.

Surgeons strive to avoid creating double vision (diplopia) after strabismus surgery, so they conduct what’s known as a ‘postoperative diplopia test’ to assess the risk of this complication.

Certain charts like the Hess chart are utilised to assess specific eye conditions that can impair simultaneous vision in both eyes.

Before any surgical procedure related to eye alignment, a ‘forced duction test’ is performed to assess whether the movement of the eyeball is restricted by mechanical causes like excess scarring.

Parks-Bielschowsky 3-step test is commonly used in cases of vertical double vision to identify the weakened eye muscle.

A full assessment of the eye, including an examination of the back of the eye (fundoscopy), needs to be conducted to rule out other eye-related issues.

In adults who suddenly start experiencing strabismus, neuroimaging might be required to rule out more serious conditions such as stroke, myasthenia gravis, thyroid eye disease etc.

The ‘4-Prism Base-Out Test’ and the ‘Prism Adaptation Test’, are very helpful to diagnose small eye deviations, and to evaluate binocular function respectively.

Treatment Options for Strabismus

Strabismus is a condition where your eyes do not line up properly, causing them to point in different directions. The goal of treating strabismus is to realign the eyes, treat weak vision (amblyopia), maintain binocular vision (both eyes working together), and get rid of double vision (diplopia).

There are many ways to treat strabismus:

Observation: In some cases, strabismus can improve itself over time, especially when caused by certain diseases like diabetes or myasthenia gravis. It’s also normal for newborn babies to have inconsistently aligned eyes that get better by their fourth month.

Correcting Vision Problems: Checking for and correcting vision problems is the first step in treating strabismus in children. This involves prescribing appropriate glasses which can often help to relieve the misalignment.

Treating Amblyopia: Amblyopia, or weak vision, can result from strabismus if the eyes are misaligned. Regular use of prescribed glasses can help improve this visual acuity in some cases. Another common treatment method for amblyopia involves covering the stronger eye to force the brain to use the weak eye.

Orthoptics: Orthoptics are eye exercises used to manage intermittent exotropia – a type of strabismus where one or both eyes turn outward. These exercises induce eye muscles to work together, helping to overcome misalignments.

Prismatic Correction: Special glasses called prisms may be prescribed. They help by moving the image closer to the center of vision, thereby enhancing the eyes’ ability to work together as they should. This is generally prescribed for small misalignments.

Medicine and Botulinum Toxin: In certain cases, eye drops and muscle injections that temporarily paralyze an overacting eye muscle can improve strabismus. These treatments often function as a supplement to surgical therapy.

Surgery: Surgical procedures are usually the final resort after nonsurgical methods have failed to correct the condition. This involves adjusting the length and position of the muscles that move the eyes.

When it comes to the specifics of dealing with different types of Strabismus, it becomes slightly more technical. Detailed considerations need to be made for each. For example, early eye alignment is crucial in infancy. Misalignment in newborns is usually treated with glasses and surgery if required. One thing to note is that there are different types of strabismus, and they each require specific forms of treatment. This ranges from surgery and corrective glasses to managing underlying medical conditions.

Remember, maintaining regular communication with medical professionals and adhering to their recommendations is critical for comprehensive, effective treatment.

What else can Strabismus be?

When diagnosing a type of eye defect, doctors will need to consider other possible conditions, categorized based on the kind of defect:

Congenital Esotropia could also be:

• Early-onset accommodative esotropia
• Abducens palsy
• Nystagmus blockage syndrome
• Duane retraction syndrome
• Sensory esotropia
• Strabismus fixus
• Moebius syndrome

Fully Accommodative Esotropia could be confused with:

• Non-accommodative esotropia
• Congenital esotropia
• Cyclic esotropia
• Convergence excess and near esotropia

Intermittent Exotropia might be:

• Infantile exotropia
• Convergence weakness or insufficiency
• Sensory exotropia with poor unilateral vision

By considering these alternative diagnoses, a more accurate conclusion can be made.

What to expect with Strabismus

The outcome for strabismus (or cross-eyed condition) is usually very good if it’s caught and treated early in life. It’s normal for healthy newborn babies to sometimes appear cross-eyed, but this isn’t cause for worry. When babies are around 3 months old, their eyes should start coordinating together properly. After this time, continued occurrences of strabismus should be taken seriously as it could be a sign of an issue.

If strabismus isn’t properly handled before the child reaches 6 to 8 years old, a condition known as amblyopia can develop, which leads to a permanent loss in vision. So, it is essential to know when to consult an eye doctor also known as an ophthalmologist.

Here are some key reasons (conditions) when you should seek an eye doctor’s help:

• Change in the reflex of the pupil or a condition known as leukocoria, an abnormal white reflection from the retina of the eye.
• Constant esotropia – a condition where one or both eyes turn inward.
• Incomitant strabismus – where the level of turned or tilted eyes changes when a person looks in different directions.
• Persistent esotropia appearing at or after 4 months of age.

Possible Complications When Diagnosed with Strabismus

Strabismus, or having crossed eyes, can lead to several issues:

• Blurry vision or ‘amblyopia’
• Reduced depth perception or ‘stereopsis’
• Eyestrain and discomfort, referred to as ‘asthenopia’
• Seeing double images, or ‘diplopia’
• Uncontrollable eye movements, known as ‘nystagmus’
• An unusual posture with the head or neck due to the eye misalignment
• Impaired social and mental growth

If surgery is done to correct strabismus, there might be some side effects:

• Continued double vision, or ‘diplopia’
• Correction not being just right, resulting in eyes still being misaligned
• Scarring, lumps, and long-term inflammation of the conjunctiva – the clear tissue over the white of the eyes
• Thinning of the cornea (the clear front surface of the eye), known as ‘corneal dellen’
• Holes in the white part of the eye, or ‘scleral perforation’
• Loss of an eye muscle
• A severe eye infection, or ‘endophthalmitis’
• Decreased blood supply to the front part of the eye, termed ‘anterior segment ischemia’
• Bleeding behind the eye, or ‘retrobulbar hemorrhage’

Recovery from Strabismus

After a surgery, it’s important to keep in mind a few key things. First, it’s necessary to check if everything is in its proper place and to make sure that nothing has moved too much or too little. If a muscle has slipped or if there are any unforeseen results, it might be necessary to take a closer look right after the operation.

Generally, a small amount of steroids can be used shortly after surgery to keep any swelling under control. As a supplement, oral anti-inflammatory medication can also be taken for the first 2 to 3 days after the procedure to help control inflammation as well.

Preventing Strabismus

It’s important for parents to be informed about what to expect (the prognosis), the benefits, and potential complications when managing strabismus, a condition where the eyes do not properly align with each other. This understanding is crucial as strabismus can affect a child’s physical, social, and emotional development. After discussing with the parents, a detailed treatment plan should be developed.

If a child develops strabismus suddenly, the parents and caregivers should understand that this can potentially be due to a larger health issue, such as a problem with the nervous system. In such cases, it might be necessary to consult with a specialist who can address these underlying causes.

Parents should also understand the importance of continuous treatment. If the recommended procedures are not followed, there is a risk that the child could develop amblyopia (also known as ‘lazy eye’, a condition where vision in one eye does not develop properly) and impaired stereopsis (the ability to perceive depth).