Exodeviations: Etiology, Classification, Epidemiology, Risk Factors, Examination, Presentation and Treatment

Abstract

In this chapter, we intend to present the disease entity, which will include definition, types of exodeviation, symptoms, etiology, risk factors and nonsurgical treatment, as well as surgical management. The types of exodeviations that will be included are pseudoexotropia, exophoria, intermittent exotropia, sensory exotropia, and consecutive exotropia. We will also write about diagnosis of the disease itself, which includes the basics of a strabological examination. There will be special attention paid to the intermittent exotropia considering that intermittent exotropia is the most common type of exodeviations, with a special focus on its treatment. Postoperative treatment options will also be topic to discuss.

1. Introduction

Exodeviations are horizontal deviations of the eye outward from the midline. In the field of strabismus research and treatment, there is no complete agreement regarding the etiology and classification of exodeviations. Exodeviations can vary in severity and can be constant or intermittent. Treatment options may include nonsurgical or surgical procedures depending on the underlying cause and the degree of misalignment. Asymptomatic patients and those with stable fusion mostly do not need treatment.

2. Etiology

The etiology of exodeviations, based on the theories put forth by Von Noorden [1], Duane [2] and Bielschowsky [3], is generally considered a combination of mechanical and innervational factors. Duane primarily proposed that exodeviations stem from an imbalance in the innervational control system that manages the active processes of convergence and divergence. He suggested that an exodeviation that is greater at a distance fixation than at near fixation is caused by hypertonicity of divergence. Conversely, a deviation that is greater at near fixation than at distance fixation is a result of convergence insufficiency. The balance between convergence and divergence plays a significant part, and their interplay may contribute to the exodeviation’s characteristics.

Bielschowsky questioned Duane’s theory, arguing that it did not consider the abnormal rest position associated with exodeviations. He further suggested some of these abnormal deviations could be influenced by anatomical and mechanical factors, such as the orbital shape and axis, interpupillary distance, the globe's size, and the extrabulbar tissues.

Von Noorden [1] and Burian [4] adds an innervational component to basic deviation caused by mechanical and anatomical factors, saying that the interplay between innervation influences that promote or impair ocular alignment is integral to the pathogenesis of exodeviations.

The modern understanding proposes that exodeviations are due to a combination of these theories, with mechanical and anatomical (static) factors contributing to a basic misalignment of the visual axes. This basic exotropia is affected by innervational (dynamic) factors that either maintain ocular alignment via convergence or impair it through divergence. Normal interplay between these forces will provide coarse alignment of the eyes, and any aberration in this is considered as a principal factor in the pathogenesis of exodeviations.

Refractive errors can also modify the innervational pattern that influences the eyes' position, a mechanism that underscores the role of myopia and hypermetropia in the etiology of exodeviations. Factors like anisomyopia and anisoastigmatism, which cause unequal retinal image clarity, could contribute to the pathogenesis of exotropia.

However, it is important to note that these theories and classifications of exodeviations were formulated descriptively, and there still exists a need for further understanding of its etiological intricacies.

3. Classification

Most of the current classifications for exodeviations trace back to Duane who proposed that exodeviations are a result of an innervational misproportion that disrupts the reciprocal relationship between active convergence and divergence mechanisms.

Considering the state of the vergence systems, we can use the concepts proposed by Duane once again (Table 1). These classifications do not bring about any direct etiological implications. But the angle of deviation can provide valuable insights into the nature of exodeviation. For instance, when the exodeviation at distance fixation is larger than that at near fixation, it suggests divergence excess. Conversely, if exodeviation is greater at near fixation than at distance fixation, the condition is indicative of convergence insufficiency. Basic exodeviation implies an angle that is equal at distance fixation and near fixation. Simulated divergence excess is the exodeviation that occurs when the disparity between distance and near deviations is not a fundamental characteristic of the exodeviation, but rather a temporary masking event influenced by persistent convergence innervation. Specific tests, such as the Scobee-Burian occlusion test that involves unilateral occlusion of one eye for a brief period (30–45 minutes), may be necessary to reveal the deviation at near fixation. Following such occlusion, it is often found that the near deviation equals or even exceeds the distance deviation [1].

Due to the state of vergence system	Due to the state of the fusion	Due to the typological terms
Basic exodeviation	Latent exodeviation	Primary exotropia
Divergence excess	Exophoria	Exophoria
Convergence insufficiency	Manifest exodeviation	Intermittent exotropia
Simulated divergence excess	Intermittent exotropia	Constant exotropia
	Constant exotropia	Secondary exotropia
		Sensory
		Consecutive

Table 1.

Classification of exotropia.

The classification based on the state of fusion distinguishes between latent/manifest, intermittent, and constant exodeviation (Table 1) [1]:

• Exophoria: Disorder occurs in states that interrupt fusion, when one eye is closed. This condition is typically detected during testing monocular visual acuity and performing an alternate cover test, when binocular vision is interrupted.

• Intermittent exotropia: It signifies a controlled form of exodeviation where the eyes are generally aligned but in relaxed or uncontrolled states (tiredness, daydreaming); one eye may drift outward. Intermittent exodeviation represents a disrupted state of fusion, occurring in periods when binocular vision is being used.

• Exotropia (constant exodeviation): This is identified when there's a constant loss of fusion and the eyes are constantly misaligned regardless of the distance of the fixation point.

On examining the state of fusion, the capacity of the patient to control exodeviation plays a crucial role. Patients with good control might be able to recover binocular fusion without any cue, but those with poor control might require visual cues such as a blink or refixation to regain binocular fusion.

Speaking in typological terms, exodeviations can be primary or secondary (Table 1).

• Primary exotropia (exophoria, intermittent exotropia, constant exotropia) typically has few or no identifiable secondary causes or associations. It is not linked to an underlying systemic condition or craniofacial anomaly, or is a result of ocular surgery, nor is it a consequence of sensory deficit. Essentially, primary exotropia arises independently, with no discernible etiological factors apart from possible innervational or mechanical elements. Types of primary exotropia often switch among each other [1].

• Secondary exotropia develops as a result of another condition or event. Unlike primary exotropia, this form of exotropia is not innate, but occurs due to underlying circumstances. It presents as a residual or secondary deviation following another condition and can arise either spontaneously in a formerly esotropic patient (one who had an inward turning of the eye) or iatrogenically after surgical overcorrection of an esodeviation. Two distinct forms of secondary exotropia include consecutive exotropia and sensory exotropia [1].

According to the time of appearance: If the deviation is noted at birth or in the first months of life, it is considered congenital or infantile (Table 2). If it arises after that age, it is labeled as acquired.

Due to the time of appearance	Due to the method of fixation
Congenital or infantile	Unilateral
Acquired	Alternating

Table 2.

Classification of exotropia.

With regard to the method of fixation, exodeviations can be either unilateral, with the patient habitually using one eye, or alternating, with the ability to fixate with either eye (Table 2). In alternating exotropia, either eye may deviate outward at different times, unlike constant or intermittent exotropia, which usually affects one eye.

The classification of exodeviations is vital when deciding the most suitable therapeutic approach for managing the condition, whether surgical or nonsurgical interventions are used. Moreover, the monitoring of exodeviations is critical as some exodeviations may progress or decompensate over time, making timely intervention necessary.

4. Epidemiology

Prevalence of exotropia can vary depending on the population sampled. In a meta-analysis study encompassing 56 articles by Hashemi et al. [5], the collected prevalence data suggests that exotropia occurs in approximately 1.23% of the population. This suggests that about 12 out of every 1000 individuals are affected by this condition.

If we look at specific manifestations of exotropia, the prevalence values can differ. For instance, a study conducted in China among preschool children revealed the prevalence of intermittent exotropia to be 3.24% [6]. In this particular population, nearly 33 out of 1000 individuals are impacted. This study did not observe any significant differences with regard to age (p-value = 0.19) or sex (p-value = 0.89). Additionally, among these patients, the most common form of intermittent exotropia was the "basic type", present in 74.7% of cases [6].

On the matter of incidence, Govindan et al. conducted a 10-year retrospective study in Minnesota that revealed an annual age- and gender-adjusted incidence of 64.1 per 100,000 patients aged 19 years or younger. In this study, the majority of cases, about 86% to be precise, were reported to involve intermittent exotropia or convergence insufficiency [7].

It is essential to remember that the prevalence and incidence of exotropia can vary based on geographical locations, populations sampled, and the type of exotropia being considered. As represented in the studies mentioned, while exotropia as a whole might be found at a lower rate in the general population, certain subtypes, such as intermittent exotropia, may present at higher rates within specific demographics.

5. Risk factors

Exotropia has several potential risk factors (Table 3). However, it is crucial to note that individual susceptibility to these factors can vary widely, and their presence does not assure the onset of the condition. Some risk factors for exotropia might include:

• Smoking and substance abuse during pregnancy

• Preterm birth and/or low birth weight

• Genetic factors: Familial history of any form of strabismus is a significant risk. If a parent or sibling has had either exotropia or esotropia, the risk increases.

• Ocular conditions:

  • Uncorrected refractive errors: High degrees of myopia, hyperopia, anisometropia, and amblyopia and impairment of binocular fusion (may be at a higher risk of developing exotropia, either due to the innate need for the eyes to compensate for these vision defects or due to complications from corrective surgeries)

  • Sensory deprivation: Vision loss in one eye (e.g., retinopathy of prematurity, corneal scars, cataracts, glaucoma, severe ptosis, and long-standing vitreous hemorrhage)

• Neuromuscular abnormalities: Abnormalities of the muscles that control the eye, or the nerves that stimulate those muscles, can lead to strabismus, including exotropia. Such problems can stem either from congenital conditions or from damage due to trauma, systemic disease, or degenerative conditions.

• Eye or orbital trauma: Injury to the structures surrounding the eye can impact the muscles that control eye movements and lead to strabismus.

• Systemic conditions: Certain systemic diseases like diabetes, thyroid disorders, cerebral palsy, and other neurologic impairment and craniofacial disorders can be associated with strabismus.

• Age: The onset of exotropia can occur at any age, but it most commonly begins either during early childhood or after the age of 50. Age can thus be seen as an influential factor in its occurrence.

Risk factors

smoking and substance abuse during pregnancy

preterm birth and/or low birth weight

age

genetic factors

ocular conditions (uncorrected refractive errors, sensory deprivation)

neuromuscular abnormalities

eye or orbital trauma

systemic conditions (diabetes, thyroid disorders, cerebral palsy)

Table 3.

Risk factors for exotropia.

These are general risk factors, and how they apply to individual patients will vary [8, 9, 10, 11, 12]. Medical consultation is recommended for the most accurate assessment of risk for any medical condition, including exotropia.

6. Clinical examination of exodeviation

The initial examination involves several steps, starting with a comprehensive history taking, followed by systematic visual examinations.

The very first step in a strabismus evaluation is the collection of a comprehensive and detailed history. Important topics to consider during this history-taking process include the nature of the symptoms the patient is experiencing, when these symptoms were first noticed, how they have changed over time, and any incidents of antecedent illnesses, trauma, surgeries, or blood loss that might be linked to the appearance of strabismus.

Following history-taking, the patient's current ophthalmic treatment regime is evaluated. The use of glasses or contact lenses, their prescription age, duration of use, and their effect on the strabismus are all documented. The use of prisms in their glasses or any history of occlusion therapy is likewise asked about.

The patient is then subjected to certain clinical examinations, divided into categories, which do not differ from other types of strabismus:

• Vision assessment: It incorporates standardized recognition distance and near visual acuity tests and orthoptic assessments, which include various tests such as the Hirschberg’s and Bruckner’s test and so on. These tests help to assess the vision and the binocular status of the patient.

• Assessment of binocular single vision (BSV): BSV should be evaluated through a stereopsis tests (e.g., Titmus test, Lang I, Lang II test), measuring the state of fusion and Bagolini’s test for simultaneous perception. This assessment should be performed at every visit, and the results documented as the condition might worsen over time. Any significant progressive decline in stereopsis can serve as an early indicator for the need for surgical intervention.

• Ocular motility examination: It requires the patient to gaze in nine diagnostic positions (straight ahead, left, right, up, down, upper/left right, lower/left right). The scope of each eye's movements in each direction is observed to identify abnormalities such as limited movements or irregular eye rotations.

• Binocular alignment: Evaluated with a cover/uncover test and alternate cover test. The goal is to identify whether a tropia or deviation in eye alignment exists.

• Angle of deviation: It is measured at near and distance with Prism Cover test and Alternate Prism Cover test.

• Physical examination: The conditions of the eyelids, lacrimal drainage system, the conjunctiva, the cornea, and other ocular structures are carefully examined.

In certain cases, additional techniques and more specialized tests such as Maddox rod testing, Tangent Screen, Bielschowsky’s test, fundus photography, and Hess screen testing may be utilized to evaluate ocular alignment or to measure cyclotropia. Other examinations may include an ocular muscle light reflex test, a red glass test, and an ocular deviation measurement using different devices.

It is worth noting the information obtained from these tests will influence the decision-making process in the management and treatment of the strabismus.

The process of clinical examination specific for exodeviation involves several strategies and tests:

• Fusional amplitudes: Patient's convergence and divergence amplitudes should also be measured. Most patients with intermittent exotropia have adequate convergence amplitudes for near but poor-to-adequate amplitudes for distance. Divergence amplitudes in these patients are usually normal.

• Occlusion test (Scobee-Burian): This test removes the vergence aftereffect, and it is crucial for distinguishing true from simulated divergence excess. Test becomes critical when a patient present with symptoms such as an intermittent exotropia that is present only at a distance, or a deviation at a distance exceeding the near deviation by 15 prism diopters (PD) or more. In the occlusion test, one eye is occluded for a minimum of 45 minutes, and then, the squint measurements are repeated using the alternate prism cover test. Any pronounced increase in the post-occlusion near angle of deviation can be labeled as simulated divergence excess exotropia. Should the near deviation remain the same, this condition is classified as true divergence excess exotropia.

• +3 diopter spherical lens test: This test removes accommodation and accommodative convergence, and it is important in detecting exotropia with high accommodative convergence/accommodation (AC/A) ratio. In this test, the exodeviations for near are measured with and without a +3 diopter spherical lens placed in front of the exotropic eye while using an accommodative target. This method helps estimate the AC/A ratio. The near angle of deviation in exotropia with a low AC/A ratio will increase minimum when +3 diopter spherical lens test is placed, but in exotropia with a high AC/A ratio, it will increase significantly [1].

7. Clinical presentation

Exodeviations encompass a group of ocular conditions wherein a visible deviation of the eye is observed, in an outward direction relative to the straight or forward gaze (Figure 1). The forms of exodeviations include exophoria, intermittent exotropia, (constant) exotropia, and sensory and consecutive exotropia. These ocular deviations can exhibit distinctive symptoms and complications and are an integral part of strabismus disorders. In this chapter, it is also important to mention pseudostrabismus, a false strabismus.

Figure 1.

Alternating exodeviation; exodeviation of right eye (A), and exodeviation of the left eye (B).

7.1 Pseudostrabismus

Pseudostrabismus is a term used to describe the appearance of strabismus even though the eyes are properly aligned. This is often due to certain facial features, such as a wide nasal bridge, prominent epicanthal folds (a fold of skin that comes down across the inner angle of the eye), or close-set eyes, all of which can make it appear as though the eyes are turning outward or inward. In pseudostrabismus, the eye alignment is normal and binocular vision is intact. Over time, as the child’s facial features mature and change, the appearance of eye misalignment often disappears. It is essential to differentiate between pseudostrabismus and true strabismus, since the latter requires treatment to prevent long-term visual impairment.

The differentiation between pseudostrabismus and strabismus can be made using a combination of detailed history taking, visual examination, and diagnostic tests.

To distinguish between pseudostrabismus and true strabismus, a basic examination would involve the observation of corneal light reflexes (Hirschberg test). In true strabismus, the corneal reflex will be off-center in the deviating eye. In pseudostrabismus, despite the appearance, the corneal reflex will be central in both eyes, indicating that the eyes are indeed aligned. There are circumstances when the eyes are aligned but the corneal reflex is displaced from the center for which the kappa angle is responsible. Angle kappa refers to the vergence of the visual axis from the pupillary axis, which is triggered by the anatomical arrangement where the fovea is slightly displaced from the optic axis. A positive angle kappa, which means the corneal light reflex is displaced nasalward, may simulate a pseudoexotropia. This can also make an existing exodeviation look worse than it actually is, or it may even mask or conceal an entire or part of an esodeviation.

The most common cause of pseudoexotropia is retinopathy of prematurity, while other causes include vascular abnormalities, macular scars, choroiditis, and other peripheral retinal disorders that cause traction (e.g., familial exudative vitreoretinopathy, toxocara retinitis, incontinentia pigmenti) [13] and orbital deformities or tumors that cause dystropia but not true strabismus. Pseudoexotropia can also occur in children with temporal dragging of the macula secondary to retinal traction.

Pseudoexotropia can be differentiated from true strabismus with the cover/uncover test and alternate test. In pseudostrabismus, no movement should be visible when either eye is covered.

If there should be any degree of unreliability in the diagnosis of pseudostrabismus, a complete eye examination may be warranted. There should certainly be regular checkups to ensure normal visual development and promptly identify any potential ocular issues.

In summary, pseudostrabismus, while giving the appearance of eye misalignment, does not entail the actual eye misalignment or binocular vision disruption seen in actual strabismus. It is a facet of an individual's physical appearance that typically resolves with growth and maturation and does not typically necessitate treatment.

7.2 Exophoria

Exophoria is a type of latent deviation where the eyes tend to deviate outward, but it is only detectable under particular viewing conditions. Typically, it is observed when binocular vision is interrupted, such as during an alternate cover test or monocular visual acuity testing. This condition is usually asymptomatic. However, patients with exophoria may experience asthenopia, or eye strain, with prolonged near work. Decompensation of an exophoria to an exotropia can occur under certain situations such as when the patient is ill or under the influence of alcohol or sedatives. When exophoria decompensated, diplopia appears. During the regular examination, a rapid change from phoria to tropia can also be observed. This is possible due to fusional amplitude control, relationship between convergence and accommodation, and variable deviation angle at different fixation difference [1]. Treatment is generally recommended when the exophoria becomes symptomatic.

7.3 Exotropia

Exotropia, on the other hand, is a more severe form of exodeviation that is more readily apparent and often manifests even under normal binocular viewing conditions. It is characterized by a manifest and more consistent outward turning of one or both eyes. But from time to time, it is difficult to distinguish between exophoria and exotropia. The exodeviation becomes manifest during times of visual inattention, fatigue, stress, or illness. It is often accompanied by symptoms such as visual disruption and decreased stereopsis (depth perception), and in adults, it can lead to significant psychological stress, anxiety, and depression. Primary exotropia is further divided into intermittent and constant exotropia. Small angle exotropia is generally rare, and the angle is mainly greater than 20 PD.

7.3.1 Intermittent exotropia

The most common type of exotropia and the most common exodeviation of childhood is intermittent exotropia, affecting approximately 1% of the general population [14]. It typically occurs at the age of 2–3 years [15] and often continues into adulthood.

It is an intermittent ocular condition, meaning it appears and disappears over time, often influenced by the focusing attention of the individual. Distance fixation may frequently induce the manifestation of an exotropic shift. On the other hand, near fixation, or focusing on objects close to the eyes, generally results in a normal binocular alignment, signifying that bifoveal fixation can be achieved in such cases. Clinically, symptoms of intermittent exotropia may include squinting or closing one eye when exposed to bright light, eye strain, or complaints of blurred vision. They help reestablish fusion by blinking, which may be one of the presenting complaint [16]. There is also the visual symptom of manifest exotropia, particularly when the individual is inattentive, fatigued, or focusing on distant objects. However, typically, intermittent exotropia does not have diplopia, and the vision remains unaffected, except in cases when the condition progress into a constant form of exotropia. In certain cases, intermittent exotropia may also convert into other types of strabismus. The control of the manifest exodeviation can lessen over time, expanding to both near and distant fixation, in addition to happening more frequently or continuously. Children with intermittent exotropia have less risk of developing amblyopia than those with esotropia.

Intermittent exotropia can be categorized according to the angle of exodeviation and AC/A ratio [17]:

• Basic exotropia is an exodeviation that is approximately the same size at distance and at near fixation

• Convergence insufficiency exotropia is exodeviation that is greater at near compared with at distance fixation. The angle of deviation is at least 10 PD larger at near than at distance. It may be associated with asthenopia or diplopia with reading and near work.

• Divergence excess exotropia is exodeviation that is greater at distance compared with at near fixation. The angle of deviation is at least 10–15 PD larger at distance than at near. As we mentioned earlier, it is important to distinguish true from simulated divergence excess with occlusion test. If there is a difference in the angle of deviation even after occlusion, the true divergence excess is present. The next step is to do a +3 diopter spherical lens test to differentiate low/normal from high AC/A ratio.

• Exotropia with high (AC/A) ratio. The AC/A ratio refers to the relationship between the accommodative convergence (the inward turning of the eyes that occurs when one attempts to see a close object clearly) and the accommodation (the adjustment of the lens of the eye for near vision). A high AC/A ratio indicates that a relatively large amount of convergence is associated with a unit of accommodation. Therefore, exotropia with a high AC/A ratio gives the appearance of a divergent excess and a smaller angle of deviation at near, which will increase and may equal to that at distance after the +3 diopter spherical lens test is performed.

7.3.2 Constant exotropia

Constant exotropia is a condition where the patient is consistently unable to align both eyes on a single object, resulting in a persistent outward deviation of the eye. This is often present in the elderly with long-standing intermittent exotropia that has decompensated or those with sensory, infantile, and consecutive exotropia. The same forms can be observed as in intermittent exotropia. Patients with constant exotropia may have an increased field of peripheral vision and may experience a constriction of their field of vision when their eyes are realigned.

7.4 Sensory exotropia

Sensory exotropia may develop due to visual deprivation in one eye that significantly disrupt or completely destroy the fusion mechanism, resulting in strabismus. This can be the result of a range of pathologic conditions that reduce visual acuity in one eye, including anisometropia, injuries, corneal opacities, congenital or traumatic unilateral cataracts, congenital glaucoma, macular lesions, optic atrophy, and vitreous hemorrhage [1]. The deviation is unilateral, with the amblyopic eye being the one typically affected. In general, when the onset of visual impairment occurs at birth or between birth and 5 years of age, both sensory esotropia and exotropia occur with almost equal frequency. After 5 years of age, sensory exotropia predominates. However, the mechanism leading to exotropia in some patients and esotropia in others with visual impairment remains unclear.

The presence of sensory exotropia suggests an absence of normal binocular function due to the visual disparity between the two eyes. With sensory exotropia, neither normal retinal correspondence (which occurs in normal binocular vision, where similar points on each retina correspond and present a single visual field to the brain) nor anomalous retinal correspondence (a variant of normal binocular vision, often seen in strabismus, where non-corresponding points on each retina provide a single visual field to the brain) is at play. Over time, suppression can become more deeply established, reinforcing the use of only one eye for sight and reducing the chances of establishing or reestablishing BSV.

7.5 Consecutive exotropia

Consecutive exotropia arises as a result of undergoing surgery for the correction of esotropia, which is much more common, or as a spontaneous shift from esotropia to exotropia [1].

There are multiple factors that may contribute to the onset of consecutive exotropia. For postsurgical cases, it may occur due to an overcorrection of the initial esodeviation during the surgical intervention. When consecutive exotropia arises spontaneously, it may be associated with poor vision in the deviating eye, as seen in sensory exotropia. However, not all cases can be explained on this basis alone. High hypermetropia in an esotropic patient may be another contributing factor, as a shift to consecutive exotropia is frequently observed in this group of patients.

8. Treatment

The treatment approach for exodeviations typically depends on the patient's specific condition and can encompass a variety of strategies aiming to improve visual alignment and normal binocular function. The approach may vary from noninvasive measures such as spectacle correction, orthoptics, and occlusion therapy to surgical intervention. The treatment for congenital exotropia begins with amblyopia therapy. It is generally surgical for symptomatic exophoria, intermittent, and constant exotropia [1].

8.1 Nonsurgical treatment

Certain nonsurgical options may be considered to create optimal sensory conditions by stimulating fusion and maintaining active accommodative convergence before surgery or when surgery must be postponed [1]:

• Spectacle (contact lenses) prescription:

  • Correction of the refractive error (astigmatismus, anisometropia, myopia, hypermetropia): It is a first-line treatment. This process can better the retinal image's clarity, which increases the stimulus to fuse and aids control of the exodeviation. Full correction of myopia is required to preserve active accommodative convergence. The correction of any hypermetropic refractive error could decrease the demand on accommodative convergence and thus increase the exodeviation. Therefore, individual patient evaluations need to consider the degree of hypermetropia, patient age, and the AC/A ratio. Typically, in children with exodeviations, a hypermetropia of less than 2.00 Dsph is not routinely corrected. Hypermetropia correction in elderly is mandatory to avoid asthenopia. As a result of a correction, exophoria may worsen and may require treatment [1].

  • Overcorrecting minus lens therapy/over minus correction: This involves the prescription of myopic spectacles, commonly referred to as over-minus lenses. In some instances, overcorrection of myopia may stimulate accommodative convergence, thereby helping manage the exodeviation.

  • Treatment with prisms: The basic principle for prescribing prismatic correction is the elimination of asthenopic symptoms and the restoration of binocular vision. The indication for prescribing prisms is exophoria and manifest strabismus ≤10 PD with the potential of binocularity [18]. Although some strabologists prescribe prisms preoperatively to improve fusional control and bifoveal stimulation [19, 20, 21], Von Noorden [1] do not use prisms preoperatively. Some prescribe prisms postoperatively in patients with a small residual angle of deviation.

• Orthoptics (vision therapy):

  • Passive vision therapy/eye patching for amblyopia treatment is strongly recommended.

  • Active vision therapy/orthoptic exercises (combination of eye exercises and vision-training): The efficiency of orthoptic exercises in the treatment of exodeviations is doubtful. According to critical evaluation of the behavioural vision therapy and systematic review of the efficacy of eye exercises, there was no clear evidence-based studies to support these training and their use remains controversial [22, 23]. Although there is some evidence-based studies that support orthoptic exercises in the treatment of convergence insufficiency according to Arnoldi’s review of convergence insufficiency, convergence exercises are only temporarily effective in improving proximal and tonic convergence, but the effect on fusional and accommodative convergence is little [24]. Sensory training to treat binocular vision anomalies should not be used as a substitute for surgery!

However, recent investigations have suggested that evidence regarding the efficacy of nonsurgical managements of exodeviations is still somewhat inconclusive, with the need for more comprehensive randomized controlled trial studies.

8.2 Surgical treatment

Surgical intervention is usually considered when nonsurgical options do not alleviate the condition. The need for surgery depends on the state of fusional control, the angle size of deviation, and patient age. Surgery in exotropia patients can be challenging, and postoperative care and recovery could require additional supporting measures. A crucial one to note is that strabismus surgery is not cosmetic; it is reconstructive. But in some cases, it can be just cosmetic (e.g., sensory exotropia, constant exotropia).

A key consideration before opting for surgical treatment is the regular monitoring of the patient. Signs indicative of the necessity for surgery include steady deterioration of fusional control, increased frequency of the manifest phase of the strabismus, or development of suppression. Functionally, if the exotropia presents more than 50% of waking hours or induces asthenopic problems, surgery is necessary [1].

The surgical treatment of exodeviations involves several careful steps to ensure a successful outcome and restoration of BSV. When planning the procedure, the surgeon must consider the severity of the condition, the obstruction of vision, the patient's age, and any underlying health conditions.

A step-by-step guide to the surgical treatment of exodeviations:

• Preoperative preparation: Before surgery, it is necessary to provide optimal sensory conditions as described earlier.

• Surgical planning: The surgeon evaluates the type and degree of exodeviation in order to plan the appropriate surgical approach. This planning involves considerations on whether unilateral or bilateral surgery is required.

• Prism adaptation test (PAT): it is used before every surgery, and it serves as a valuable diagnostic tool utilized to determine the "maximum" angle of squint before proceeding with eye muscle surgery. The primary objective of this assessment is to identify the extent of squint that can be effectively corrected while achieving the most optimal binocular outcome. The ultimate goal is to ensure accurate correction and avoid the possibility of undercorrection, thus enhancing the success of the surgical intervention.

• Anesthesia: Patients undergoing exodeviation surgery are typically given a general or local anesthesia, depending on their age and health conditions. The objective is to ensure they remain comfortable and pain-free during the procedure.

• Surgery: In the surgery, the surgeon alters the balance of the muscles by either strengthening the inward-pulling muscles, weakening the outward-pulling muscles, or a combination of both. In most cases, a resection (to strengthen a muscle) or recession (to weaken a muscle) procedure is performed. The eye muscles are carefully detached from their original position on the eye, repositioned, and reattached in a calculated location to adjust the pull of the muscles for better alignment.

• Closure: Once the muscles have been adjusted, the surgeon uses absorbable sutures to close the conjunctiva. Antibiotic eye ointments are applied to prevent infection.

• Immediate postoperative care: After the surgery, patients might occasionally experience double vision in the immediate postoperative period. Surgeons may prescribe short-term topical steroids and antibiotics, along with oral analgetics on a case-by-case basis for pain management and to prevent inflammation and infection.

• Follow-up visits: The postoperative care in patients who undergo surgical intervention should focus on alignment and correction of any underlying refractive errors. If necessary, prisms or occlusion glasses are advised if diplopia persists.

• Ongoing monitoring: Patients should continue to have regular follow-up appointments with their ophthalmologist to monitor the success of the surgery.

Consequently, the treatment administered is multifaceted, taking into account the patient's age, the severity and frequency of the strabismus, the control of the deviation, and the response to initial treatment methods. Thus, the approach is generally individualized to obtain optimal outcomes.

8.3 Exophoria

Asymptomatic patients with exophoria without muscular asthenopia typically do not require treatment. In contrast, symptomatic exophoria is typically approached with a surgical treatment strategy. But there are some situations in which the first attempt is to treat exophoria conservatively.

Nonsurgical treatment

• Spectacle (contact lenses) prescription:

  • Correction of the refractive error as mentioned earlier.

  • Bifocal lenses (the weakest) are prescribed in symptomatic exophoric patient with presbyopia before estimating the increase in exodeviation [1].

  • Treatment with prisms: If treatment with the weakest bifocals fails and patient still has symptoms, it is recommended to prescribe prisms base-in for near vision. Only about half of the angle is corrected with prisms. This is how accommodative convergence is stimulated [1].

Surgical treatment

• A more detailed description is in the chapter Treatment/exotropia/intermittent exotropia.

8.4 Exotropia

The management of exotropia depends on the type of exodeviation and any associated ocular findings. The standard surgical interventions for exophoria or exotropia include the recession of the lateral rectus muscle and resection of the medial rectus muscle. Surgeries can be performed unilaterally or bilaterally, depending on the individual patient's condition and the surgeon's judgment. Surgical treatment for exophoria and exotropia is typically directed at restoring optimal sensory conditions and at normalizing binocular function. The less experienced ophthalmologist might view the treatment of this condition with optimism, particularly given that a substantial number of patients demonstrate potential for reconstructing fusion and improvement in distance stereoacuity following surgery. But basically, the treatment is demanding and frustrating due to temporary improvement and frequent relapses. Postoperative care is crucial for ensuring the effectiveness of the surgical intervention. This includes focusing on eye alignment, managing any blinding complications, and correcting underlying refractive errors.

8.4.1 Intermittent exotropia

The treatment for intermittent exotropia can involve both surgical and nonsurgical options depending on the case's severity and specifics. The management strategies for intermittent exotropia can vary, but the main goals are to correct the eye misalignment, restore binocular vision, and prevent amblyopia. However, treating exotropia, whether intermittent or constant, can be a complex and intricate process.

Nonsurgical treatment

• Observation: In cases where patients are asymptomatic, intermittent exotropia is mild, has good control, and is not increasing; observation may be sufficient, as noted by Hutchinson [14]. The prognosis for normal binocular vision and stereopsis is good if the intermittent exotropia is rarely manifested. This approach entails diligent monitoring for associated issues such as amblyopia, eye strain, diplopia, closing of one eye in sunlight (monocular manifest deviation), increased frequency of deviation, and headaches. Decisions on intervention for children with intermittent exotropia should be made on a case-by-case basis by healthcare professionals and parents.

• Spectacle (contact lenses) prescription:

  • Correction of the refractive error as mentioned earlier.

  • Overcorrecting minus lens therapy/over minus correction: Von Noorden use this treatment only as a temporary measure in patients with a high AC/A ratio [1]. The rationale behind this approach was the anticipation that over-minus lenses could potentially enhance the management of intermittent exotropia by inducing accommodation and accommodative convergence. Despite this initial hypothesis, the findings of a 12-month randomized trial conducted on children aged 3–10 years yielded contrasting results. The trial revealed that while over-minus spectacles did indeed contribute to improved control of distance exotropia while the over-minus spectacles were worn, the beneficial effect did not persist after the treatment regimen was gradually tapered off. Additionally, an association was observed between the use of over-minus lenses and an increased myopic shift in participants [25]. Consequently, based on these outcomes, it is recommended to refrain from prescribing over-minus lenses for the specific purpose of managing intermittent exotropia. This suggestion is informed by the trial's results and the potential for unintended myopic progression associated with their usage.

  • Treatment with prisms: The only indication for prescribing base-in prisms in exotropia is exotropia with small angle (≤10 PD) and with the potential of binocularity, as we mentioned earlier [18]. However, the use of base-in prisms for long-term management is not ideal as they can reduce fusional vergence amplitudes.

• Orthoptics (vision therapy):

  • Passive vision therapy/eye patching involves alternating part-time occlusion of one eye to counteract or diminish suppression. A randomized trial involving 201 young children with intermittent exotropia compared part-time patching to observation, revealing uncommon deterioration over 6 months in both groups (2.2% versus 4.6%, respectively; nonsignificant) [26]. Although exotropia control was slightly better with patching (mean score 2.3 versus 2.8), this distinction may lack clinical significance. A similar study in older children (ages 3 to <11 years), also from PEDIG group, indicated that part-time patching reduced deterioration over 6 months, with infrequent deterioration in both groups (0.6% in patching versus 6.1% in observation) [27]. As with other treatments for intermittent exotropia, therapy discontinuation often results in failure and recurrence [28]. Von Noorden [1] occasionally found it useful using alternating occlusion instead of surgery in patients with small-angle intermittent exotropia.

  • Active vision therapy/orthoptic exercises enhance convergence/divergence amplitudes, fusion strength, and single binocular vision [29]. Typically administered by experienced optometrists, this treatment often involves office-based sessions supplemented by at-home exercises. Alternatively, it can be entirely conducted at home through computer-generated orthoptic exercises. In a randomized controlled trial of 221 children (ages 9–17 years), an intensive program of office-based vision therapy with home reinforcement proved more effective in reducing signs and symptoms compared to various home-based therapies [30]. "Active" vision therapy entailed a comprehensive protocol incorporating diverse exercises targeting improved vergence and accommodation. Conversely, the control intervention consisted of home-based procedures simulating orthoptic exercises without vergence or accommodation training. While this study suggests the superiority of intensive office-based vision therapy over home-based therapy devoid of binocular vision training, further research is warranted to ascertain if office-based therapy surpasses equally intensive home-based therapy [31]. Despite the fact that some countries and some strabologists implement active vision therapy, as mentioned earlier, evidence-based studies are lacking and their effectiveness is questionable [31].

Surgical treatment

Intermittent exotropia may not always require immediate surgery. Taking into account that it does not progress in all patients, patients must be observed for several months. Surgery is typically recommended when certain signs or symptoms are present in the patient, or they develop while the patient is under monitored observation. These may include gradual loss of fusional control demonstrated by increasing frequency of the manifest phase of the strabismus, development of secondary convergence insufficiency, increase in size of the basic deviation, development of suppression evidenced by absence of diplopia during the manifest phase of the strabismus, or a decrease of stereoacuity. In simpler terms, when the exotropia happens more than 50% of the patient's waking hours or causes asthenopic problems, surgery is advised [1]. Despite this, it is important to note that surgical treatment of intermittent exotropia or constant exotropia, especially after a long period of intermittency, is generally aimed at normalization of binocular function.

The ideal age at which surgery should be performed has been a subject of debate. Some experts argue that surgery in visually immature infants should be delayed to avoid consecutive esotropia. On the other hand, others argue in favor of early surgical intervention. According to Baker et al. [32] and Von Noorden [1], surgery for intermittent exotropia should be postponed until the fourth year of life. Earlier surgical intervention is usually recommended if there is a rapid functional deterioration of fusional control despite nonsurgical therapy or if the deviation is constant. Despite the fact that the minimum preoperative angle of deviation should not be less than 15 PD, the angle of deviation greater than 20/25 PD is mostly operated [1]. In conclusion, strabismus surgery stands as the predominant treatment for advanced intermittent exotropia, or when nonsurgical methods prove insufficient [33]. Surgical guidelines advocate intervention for frequent exodeviation manifestation and symptomatic patients. Those conditions in which the patient is asymptomatic with stable fusion at distance and near are considered to be cured [1].

The preferred surgical treatment for intermittent exotropia (and exophoria), varies with the specific attributes of the deviation, including the size and nature of the deviation, the presence or absence of near-distance disparity, and the surgeon's preference. However, some common surgical interventions include:

• Two-muscle unilateral surgery (asymmetrical surgery): In this procedure, the medial rectus muscle of one eye is resected, and the lateral rectus muscle of the same eye is recessed (Figure 2). This aims to adjust the balance of forces acting on the eye and realign the visual axis.

• Bilateral surgery (symmetrical surgery): In this approach, the lateral rectus muscle is recessed in both eyes. Resection of the medial muscles can also be done.

• One-muscle unilateral surgery: In some cases, only one lateral rectus muscle will be recessed. This procedure may be advisable for small-angle deviations or in situations where the risk of overcorrection might be particularly concerning.

Figure 2.

Left eye exotropia before surgery (A) and first postoperative day after two-muscle unilateral surgery (B).

The choice of surgical method depends on several factors. It is crucial to distinguish true from the simulated divergence excess. According to Burian, when there is a true divergence excess, bilateral lateral rectus recession might be the preferred choice. Two-muscle unilateral surgery (recession of the lateral rectus muscle with resection of the ipsilateral medial rectus muscle) on the nondominant eye is the procedure of choice for basic exotropia or the simulated divergence excess [34, 35, 36]. Other authors showed the same effectiveness of bilateral recession of the lateral rectus muscles in case of basic exotropia and in case of simulated divergent excess [37]. Conversely, for near exotropias (convergence insufficiency), both medial rectus resection from 3 to 6 mm might be utilized [1, 38]. However, there are cases where a surgeon chooses to perform a recess-resect procedure for all types of exotropia. When asymmetric surgery is the method of choice, then the nondominant eye is operated on.

A variety of treatments have been proposed for intermittent exotropia; however, there is no universally superior approach for all cases. Studies have shown no statistically significant differences in outcomes between these surgical strategies. Some showed better early postoperative effect with asymmetrical surgery [39]. Long-term follow-up demonstrates a high recurrence rate regardless of the initial therapy chosen [28, 40, 41]. Additionally, the evolving landscape of medical knowledge and practice may provide further insights into refining these treatment strategies. A key objective in this line of treatment is to relieve symptoms, reduce the frequency of manifest deviation, decrease the angle of deviation, and enhance the individual's control over it.

It was also suggested that an initial overcorrection of around 10–20 PD in the immediate postoperative period might be advantageous in terms of long-term outcomes. It is considered as an attempt to extend the effect of the surgical procedure. Less than that causes relapse, and more than 10–20 PD causes consecutive esotropia [1]. Though, it is noted that consecutive esotropia frequently recovers over time, there is no consensus about this, as overcorrection could even provoke diplopia. In general, the response to these different treatment strategies is variable, and more randomized controlled trial studies are needed to conclusively establish the most effective approach.

8.4.2 Constant exotropia

In case of constant exotropia, preoperative nonsurgical treatment is not required. If constant exotropia occurs in early infancy and if there is no data on intermittency, the functional prognosis is poor. Surgery should be performed as soon as the angle of deviation can be measured, which must be at least 15 PD, and when the child alternates freely. Mostly, it is between the first and second year of life. Adults are operated on as soon as the diagnosis is confirmed. Unfortunately, in such cases, the establishment of BSV is not expected [1].

Surgical treatment for constant exotropia is similar to that for intermittent exotropia. However, one notable aspect is that recurrence rates are high, as shown in long-term follow-up studies for the surgical treatment of intermittent exotropia. Therefore, patients may require multiple surgeries to maintain ocular alignment over a long period. Furthermore, it is worth noting that some consider a delay in surgical treatment as a risk factor for the recurrence of strabismus.

8.5 Sensory exotropia

The management goal in sensory exotropia focuses less on restoring binocular visual function and more on alleviating the noticeable eye divergence for aesthetic and comfort purposes. Surgical procedure is the same as that for intermittent exotropia.

8.6 Consecutive exotropia

The specific procedure to correct consecutive exotropia largely depends on individual patient conditions. Consecutive exotropia that arises spontaneously generally requires a surgical intervention to restore normal facial configuration. The goal of the surgery is to realign the eyes to improve cosmetic appearance and potentially visual function. The indications for this surgery are often cosmetic. The most widely used surgical procedures for consecutive exotropia are the same as those for other types of exotropia. However, the exact method may vary based on the specific characteristics of the patient's strabismus. Postoperatively, the patients regularly have follow-up visits to assess their eye alignment, ocular motility, correction of underlying refractive errors, and the possible development of postoperative diplopia.

It is crucial to note that postoperative management and prognosis will vary depending on the specifics of each case, including the extent of the exotropia, the patient's age, and other individual health factors. Therefore, a personalized treatment and postoperative management plan is essential for each case.

8.7 Postsurgical treatment of undercorrections

After exodeviation surgery, in some patients, the residual deviation is visible immediately after the operation and will require additional surgery, while in others, it occurs only a few months or even years after the initially satisfactory result. In such cases, the use of base-in prisms with a power base greater than the residual deviation is recommended to promote convergence and restore fusion amplitudes with the goal of reducing exodeviation [1]. Unfortunately, additional surgery is required in most cases.

8.8 Postsurgical treatment of overcorrections (consecutive exotropia)

According to different authors, the prevalence of surgical overcorrections in patients with exodeviations varies from 6% to 20% [35, 36, 42, 43, 44, 45]. Observation is the therapy of choice for minor degrees of esotropia, which are usually comitant in nature. Postoperative esodeviation of 10–15 PD is desirable and may disappear completely over time, but larger deviations tend to increase. In the first 6 months postoperatively, no other surgery should be performed except when there are significant duction limitations causing an unrelated lateral view.

During observation, several nonsurgical treatment methods can be performed to reduce postoperative deviation or, if this is not possible, to maintain fusion and ensure patient comfort. When diplopia persists for 2 weeks after surgery, miotics or a temporary prescription for hyperopic refractive error can reduce the deviation to the point where the patient will converge. If this therapy is inefficacious, diplopia can be eliminated with alternating occlusion, which will also tend to reduce the angle of successive esotropia. Base outward prisms are a common treatment option, especially when fusion must be maintained under all circumstances, such as in visually immature children or for professional reasons in adults [44, 46]. Many difficulties of this form of therapy that were previously encountered have been eliminated by the use of press-on Fresnel membrane. In the treatment of patients with retained motor fusion, injections of botulinum toxin into the medial rectus muscle have also been shown to be beneficial [47].

The treatment of permanent consecutive esotropia requires considerable patience of the doctor, because the spontaneous reduction of the postoperative angle may take a long time. According to Hardesty and his coworkers [42], consecutive esotropia of less than 15 can be cured with prism therapy alone, whereas surgery usually becomes necessary for larger esodeviations.

Conflict of interest

The authors declare no conflict of interest.