Intermittent Exotropia

What is Intermittent Exotropia?

Intermittent exotropia, the most common type of eye condition known as strabismus, is where one or both eyes turn outward instead of focusing straight ahead. It isn’t always present and usually comes up when a person is looking at something far away, although it can eventually happen even when looking at near objects over time. This condition also goes by other names such as distance exotropia, divergent squint, periodic exotropia, or exotropia of inattention. It’s quite common for newborns to have mild versions of this, seen in 60 to 70% of them, but it typically fades away by the time they reach 4 to 6 months old. This condition often first appears during childhood.

The typical course of this disease begins with exophoria, a condition where the eyes have a tendency to deviate outward but are usually able to focus straight ahead, which could progress into intermittent exotropia and then eventually turn constant. It’s crucial to note, however, that not all cases follow this pattern. Some people might not see their condition progress over time, even without treatment, and a small number might even see an improvement. In a study conducted by Von Noorden, 75% of the 51 untreated patients with intermittent exotropia showed progression, 9% saw no change, and 16% actually improved.

What Causes Intermittent Exotropia?

The exact cause of intermittent exotropia, a condition where one or both eyes turn outward, isn’t entirely clear. However, several different ideas have been suggested by experts to explain why it happens.

1. Innervational factors: This theory suggests that exotropia happens because of an imbalance in the signals sent to the eye muscles, which disrupts the balance between the eyes turning in and turning out.

2. Mechanical factors: This suggestion involves the position of rest of the eyes and physical factors like the positioning, shape, and size of the eye socket and eyeball, the density and volume of the tissue behind the eye, as well as the position, functioning, and elasticity of the eye muscles and supporting structures of the eye sockets.

3. Fusion faculty: This theory says that exotropia occurs when something interferes with binocular vision, the ability of both eyes to work together to form a single image. If the eyes can’t properly merge the images they see, it can cause the eyes to turn inward or outward.

4. AC/A ratio: This is a measure of how much the eyes turn in when focusing. Some researchers suggest that a higher than normal AC/A ratio can be a cause of intermittent exotropia. However, others found that about 60% of patients with certain forms of exotropia had a high AC/A ratio, while 40% had a normal ratio.

5. Refractive errors: This could mean that unresolved vision issues could be a reason for exotropia. For example, people who are myopic (shortsighted) might need less effort to see things close up, and this reduced need for the eyes to turn inward might cause the eyes to turn outward. Similarly, for people who are hyperopic (longsighted), even maximum effort might not lead to a clear vision, affecting their ability to align their eyes, resulting in a low AC/A ratio.

6. Anisomyopia and anisometropia: These conditions involve differences in the power of the two eyes, leading to blurred and uneven images that can interfere with the eyes’ ability to merge images, increasing the likelihood of exotropia.

7. Hemiretinal suppression: This theory suggests that a condition called hemiretinal suppression, where parts of both retinas are inactive, could be a contributing factor to why exophoria (tendency for eyes to deviate outward) might progress to intermittent exotropia.

Risk Factors and Frequency for Intermittent Exotropia

Exodeviation, a type of eye condition, typically begins just after birth. In a study involving 472 patients with intermittent exotropia, a specific type of exodeviation, 204 cases were present from birth. Additionally, some patients started showing symptoms around 6 months old and others between 6 and 12 months. A separate study in China also found 3.24% of people studied had intermittent exotropia.

Research generally shows more females have exotropia than males. A 10-year study in the United States found 205 children with exotropia. Every year, about 64.1 out of 100,000 patients less than 19 years old were identified with exotropia. In these cases, 86% had intermittent exotropia or other underlying conditions like convergence insufficiency or a central nervous system disorder with exotropia.

• Exodeviations typically start just after birth.
• In a study of 472 patients, 204 had intermittent exotropia from birth, 16 showed symptoms around 6 months old, and 72 between 6 and 12 months old.
• About 3.24% of a studied population in China had intermittent exotropia.
• More females than males have exotropia.
• A 10-year study in the United States found 205 children with exotropia.
• Every year, 64.1 out of 100,000 patients less than 19 years old are identified with exotropia.
• Among these, 86% had intermittent exotropia or other underlying conditions.

Signs and Symptoms of Intermittent Exotropia

Intermittent exotropia is a condition where one or both of the patient’s eyes turn outward. Often, patients have no symptoms. Parents are the ones mostly likely to notice signs of the condition, such as their child closing one eye when exposed to bright light, or their eyes wandering especially when the child is daydreaming or tired. This lack of symptoms is down to the patient’s suppression mechanism, which enables them to have normal vision when their eyes are aligned and function normally even when one eye strays.

Here are some symptoms that patients with intermittent exotropia might experience:

• Transient Diplopia: Occasionally, the patient may report momentary binocular horizontal double vision or discomfort related to eye misalignment.
• Asthenopic symptoms: These symptoms occur in the initial stages of the condition when the eyes begin to drift from their normal position. Patients may complain of eye strain, blurring, headaches, or difficulty with reading for long periods.
• Diplophotophobia: This is the condition that causes patients to close one eye in bright sunlight. The bright light can disrupt the balance between the eyes and cause the misalignment to become noticeable.
• Micropsia: This condition might occur when the patient uses accommodative convergence to control the misalignment of the eyes.

The four categories of intermittent exotropia, as classified by Burian, are based on the varying degrees of eye misalignment at different distances:

• Basic type: The difference between the misalignment of the eyes at a distance and up close is less than ten prism diopters. These patients have normal eye functions.
• True divergence excess: After a patch test, the misalignment of the eyes at a distance is more than ten prism diopters greater than up close. These patients may have a high or normal AC/A ratio. Patients with a high AC/A ratio run the risk of overcorrection if surgery is performed based on the distance misalignment.
• Convergence insufficiency: The eyes are more misaligned at near distances than far, with a difference of at least ten prism diopters.
• Pseudo-divergence excess: Initially, the eye misalignment is greater at a distance. However, after a patch test, the difference between the distance and near misalignment drops to less than ten prism dioptres. This suggests that the patient has a greater amount of tonic fusional convergence, which can be disrupted by the patch test.

Kushner further expanded these categories to include factors like tenacious proximal fusion, high and low AC/A ratio, proximal convergence, and pseudo-convergence insufficiency.

Testing for Intermittent Exotropia

Intermittent exotropia, a vision condition where one or both of the eyes turn outward, is identified through an examination of the eye, rather than through specific tests. These examinations usually show that the patient can see well from both eyes. The observations are usually consistent and eye movements are not restricted. A thorough examination of sensory and motor functions should be carried out with measures taken towards near, far, and all directional gazes. These measures help in tracking the changes in the condition over time.

In assessing the severity and control of exodeviation (a condition where the eye turns outwards), both subjective and objective methods are used.

Subjectively, the New Castle Scoring is used to measure how much control a person has over exodeviation. This involves a scoring system ranging from 0 to 9, where 0 represents excellent control, and 9 represents poor control. The score is determined by observing when the deviation is noticed and how often it manifests itself. The total score is figured out by adding the scores from home observations and office observations (both near and far).

Objectively, the control of deviation is assessed by testing the ability to perceive depth, which is known as stereoacuity. This measure is a good indicator of how well one can control the deviation. A normal stereoacuity score suggests good control with little or no suppression of vision.

Moreover, measuring the angle of deviation, a technique that reveals full deviation by breaking the intense proximal fusion, is necessary for patients with intermittent exotropia. It’s also important to measure the eye movement in all directions. This condition can sometimes be associated with other eye movement anomalies. Other tests such as the patch test, lens gradient method and far distance measurement can further help to evaluate and understand the condition.

If there’s a noticeable difference in near and far deviations, the patch test is used to differentiate between real and apparent divergence excess. This test involves patching one eye for 30 minutes and repeating the measurements after removing it.

The lens gradient method is used to identify patients who have both a disparity between near and far deviations and a high AC/A ratio. A diagnosis can be made if there’s an increase of 20 prism diopters or more for near vision after the lens gradient test.

Lastly, the far distance measurement test helps uncover the full deviation by decreasing near convergence. This test involves asking the patient to fixate at a far distance instead of 6 meters. This test has been shown to result in satisfactory outcomes for majority of patients who underwent surgery for maximum angle of deviation.

Treatment Options for Intermittent Exotropia

The management of intermittent exotropia, a condition where one or both eyes occasionally turn outward, can range from simple observation to non-surgical or surgical treatments. These approaches will depend on the patient’s degree of eye deviation, their control over it, and their specific concerns.

Research shows that for children aged 3 to 10 years with intermittent exotropia, there’s a 15% chance of the condition getting worse over three years. For older patients aged 5 to 25 years, without any surgery, studies found that the eye deviation remained the same in 58% of patients, got better in 19%, and got worse in 23%.

Non-Surgical Treatments:

Non-surgical treatments aim to encourage the use of both eyes together, helping the patient recognize double vision when eyes are misaligned, and work on improving control over the eye’s turning (exodeviation). This treatment might be a good fit for patients with small deviations, young patients who can’t have accurate measurements quite yet, or in cases where an overcorrection from surgery could lead to blurry vision (amblyopia) or loss of fixation.

The steps in non-surgical management include:

1. Correcting Refractive Error: If a patient has uncorrected vision problems, it can disrupt the ability to use both eyes together, causing further eye deviation. Therefore, eye tests should be done for all patients, and if needed, glasses should be prescribed. This is especially helpful for patients with myopia (nearsightedness).

2. Orthoptics: These are techniques to help improve control over the eye deviation. Some of these exercises make patients more aware of the eye misalignment and help in controlling the eye’s converging movements.

3. Overcorrecting Minus Lenses and Part-time Occlusion: Specific visual aids may be used to stimulate the eye’s inward movement (convergence) and reduce the outward movement (exodeviation). In some young children, part-time eye patches can be used as a passive method to control the turning of the eyes.

4. Prismotherapy: This method uses prisms to enhance the dual eye image stimulation needed for proper vision. However, large prisms might interfere with vision quality and can be uncomfortable leading to low compliance.

Surgical Treatment:

Surgery can be considered to restore normal binocular function (both eyes working together) and aesthetics. Signs that could indicate the need for surgery include: increased frequency and time of eye turning, struggle with near work (convergence insufficiency), an increase in the basic deviation, the development of suppression (ignoring one eye to avoid double vision), and a decrease in stereo acuity (depth perception).

The surgery can involve either a unilateral (one-eye) procedure, which combines reducing muscle strength on one side (recession) and increasing muscle strength on the other side (resection). Or, it could involve bilateral (both-eye) reduction of muscle strength (recession).

The ideal surgical approach can depend on factors like the severity of deviation, presence of side-related variations (lateral incomitance), or patterns associated with the overaction of specific eye muscles (A and V-patterns).

Another viable option can be the injection of botulinum toxin, a neurotoxic protein, which has shown promising results similar to surgery. In a study of children aged 3 months to 12 years, about 69% showed normal eye alignment just over a year following the injection.

What else can Intermittent Exotropia be?

Intermittent exotropia, a type of eye condition, can be confused with other similar conditions. Determining the difference between these conditions can be made based on the patient’s history and clinical examination. These conditions include:

• Constant exotropia: This condition typically begins within the first six months of a baby’s life. It’s constant and doesn’t disappear on its own.
• Sensory exotropia: This occurs when there’s poor visual function in one eye. It can develop in an older child or an adult, as the eye with the visual issue slowly drifts away.
• Consecutive exotropia: This is when exotropia, an outward eye turn, happens in an eye that used to be inwardly turned (esotropic). It could be due to too much surgical correction or spontaneously happening in an eye with poor vision that’s deviating.

There’s also another condition called Duane’s retraction syndrome. This syndrome is marked by a varying limitation of eye movements, which could appear as either an inward (esotropia) or outward (exotropia) eye turn, or no eye turn at all (orthotropia). Eye movement tests, observing changes in the eye slit, and, if needed, a muscle activity study (electromyography), can help differentiate this condition.

What to expect with Intermittent Exotropia

There really isn’t a universal definition for what counts as a successful surgery in patients with intermittent exotropia, a condition where one or both eyes tend to turn outward sporadically. The variety of treatment methods, different timing of interventions, and scarce long-term patient check-ups make it even more challenging to set up a clear standard of success. However, several studies often deem a surgery successful if it reduces the patient’s misalignment of eyes to 10 units or less, as measured by a tool called a prism diopter.

Research conducted over various time frames showed different rates of success. According to previous studies, successful surgeries often ranged from 50% to 80%. Those studies followed up their patients over periods ranging from half a year to a full 5 years, noticing that the longer the patient was monitored post-surgery, the lower the success rate appeared.

Recent studies also showed fluctuating success rates, typically falling somewhere between 40% and 70%, for all types of intermittent exotropia.

Drawing from his review of surgical outcomes related to different degrees of intermittent exotropia and varying surgical methods, Kushner drew a few conclusions:

* Patients with a high Accommodation Convergence to Accommodation ratio (AC/A) might be prone to developing a condition where the eye turns too far inward, especially at close distances.
* Patients with tenacious proximal fusion, which means their eyes work together well at short distances, have a higher potential of successful surgery.
* The planning of surgery for patients with intermittent exotropia should be based on the most considerable eye deviation consistently recorded for each patient.

Possible Complications When Diagnosed with Intermittent Exotropia

Surgical correction procedures come with a set of potential complications. These can occur due to either anesthesia or the surgical process, which includes moments during and after the operation.

Complications related to anesthesia may include:

• Oculocardiac reflex: a process involving your eyes and heart
• Malignant hyperthermia: a severe reaction to particular drugs used during anesthesia
• Cardiac arrest: a sudden loss of heart function
• Hepatic porphyria: a group of disorders that affect the liver
• Succinylcholine–induced apnoea: temporary cessation of breath due to a certain medication

Surgical complications may occur during the actual operation. These can be:

• Hemorrhage: severe bleeding
• Lost or slipped muscle: muscle getting displaced or lost
• Inadvertent injury to surrounding structures: accidental damage to surrounding tissues
• Globe perforation: damage to your eyeball
• Wrong muscle, or wrong eye surgery: surgical mistakes

After surgery, you may face various complications, which include:

• Diplopia: double vision
• Monofixation syndrome: abnormal vision development
• Loss of stereopsis: loss of perception of depth
• Suture reaction: body’s reaction to stitches
• Conjunctival granuloma: inflammation of the eye surface
• Anterior segment ischemia: lack of blood flow to the front part of the eye
• Retinal detachment: a serious eye condition needing immediate treatment
• Under or overcorrections: not achieving the intended surgical results
• Adhesive syndrome: condition where tissues and organs stick together abnormally

Recovery from Intermittent Exotropia

The healing process after eye alignment surgery largely depends on how the eyes are adjusted post-surgery. Patients might face challenges like double vision, or their eyes may align properly, or might have lingering outward or an emerging inward deviation.

For individuals, who have their eyes perfectly aligned after the surgery:

In young kids, a subtle deviation of eyes inward up to 10 prism diopters (a unit to measure eye deviation) is ideal because eyes usually tend to shift outward after the surgery. These patients should be counseled to undergo post-surgery eye exercises to enhance coordination of eye movements, which will help them adjust to their newfound visual acuity.

For those who have a lingering inward deviation:

A lingering inward deviation of up to 10 prism diopters is ideal. Some patients may even resolve over time with up to 20. Non-surgical treatments should be tried for at least one month due to the higher likelihood of the eyes aligning themselves naturally.

For Children:

There’s a high chance of them developing monofixation syndrome (where only one eye is used for detailed vision) and suppression of lazy eye, hence, the following steps should be taken within 2 weeks after surgery:

1. Refraction test using medications that dilate their pupils (cycloplegics) should be conducted, and any long-sightedness should be fully corrected.
2. Bifocal glasses might be considered if there is more deviation when looking at near objects post-surgery.
3. Eye-patching therapy may be useful, where one eye or the other is patched based on the eye alignment pattern.
4. Prismotherapy, which involves using prisms to fully correct the deviation and maintain two-eyed vision, can also be considered.

If the child remains overcorrected by more than 15 prism diopters, despite these non-surgical measures, another surgery might be necessary.

For Adults:

If their visual system is mature, an overcorrection more than 20 prism diopters might be addressed by non-surgical measures after a waiting period of 6 to 8 weeks. The same non-surgical treatments like corrective glasses, bifocals, or prism therapy, might be considered. Any decision to have another surgery should only be made after 6 months post-surgery.

For those with lingering outward deviation:

Small residual outward deviation (15 to 18 prism diopters): These patients can be managed with non-conservative measures. A full eye correction should be given for any underlying short-sightedness. Certain medications that dilate their pupils can be given twice a day to stimulate eye coordination in long-sighted or normal-sighted patients. Eye exercises should be continued until the eyes are properly aligned. Prism therapy may help to avoid double vision and maintain two-eyed vision.

A large lingering outward deviation (15 to 18 prism diopters): These patients will likely need another surgery. It’s better to wait 8 to 12 weeks before re-operating. Regardless of the type of original surgery, the corrective surgery should be performed with caution, as over corrections are common.

Preventing Intermittent Exotropia

It’s essential to keep patients informed and engage parents or guardians in making decisions about their child’s health. For young patients who have small shifts in eye alignment and can manage them well, simply monitoring the condition might be enough. Sometimes, wearing an eye patch or doing eye exercises may be required. This plan needs to be thoroughly discussed with parents, and they must take an active role to ensure their child follows the treatment correctly.

Parents need to understand that eye exercises could help delay possible surgery. The importance of regular check-ups and parent’s role in monitoring of at-home treatment should be emphasized. All the risks and benefits of different treatment options should be discussed in-depth. Parents also need to be made aware of the potential effects of misaligned eyes (strabismus) on their child’s mental health, as well as its impact on their social interactions and educational performance.