Exodeviation is a horizontal form of strabismus characterized by visual axes that form a divergent angle. The different types of acquired exotropia are intermittent exotropia, sensory exotropia, exotropia with neurologic causes and field defects, and consecutive exotropia.
Certain conditions, such as third nerve palsy, thyroid ophthalmopathy, and iatrogenic trauma following retinal detachment surgery or endoscopic sinus surgery, could cause acquired exotropia; however, these conditions are not discussed in this article.
The deviation usually begins as an exophoria. Exophoria is a condition in which the alignment of the eye is straight when both eyes are open, but either eye drifts outward when covered. During this phase, the patient has bifoveal fixation. This deviation may later progress to intermittent exotropia, during which the deviation may be manifest (exotropia) or latent (exophoria).
When intermittent exotropia develops in a child whose visual system is still immature, bitemporal suppression develops, and the child does not perceive 2 separate images (diplopia). As suppression increases, intermittent exotropia may finally progress to constant exotropia. If acquired exotropia develops in adults, the patient experiences periods of diplopia during the tropic (manifest) phase. See the images below.
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Patient with intermittent exotropia at distance only. Patient is fixing with the left eye. Note the outward deviation of the right eye.
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Patient with intermittent exotropia at distance only. Patient is now fixing with the right eye, showing that he can alternate well.
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Patient with intermittent exotropia at both distance and near. Patient is fixing with the left eye. Note the outward deviation of the right eye.
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Patient with intermittent exotropia at both distance and near. Patient is now fixing with the right eye, showing that she can alternate well.
Esodeviations are more frequent than exodeviations, with a ratio of 3:1.
Acquired exotropia is more common in the Middle East, Africa, and Asia and in those latitudes with higher levels of sunlight. It is less common in the United States and Europe.
Mortality/Morbidity
Diplopia and eyestrain associated with acquired exotropia can affect daily activities, such as driving and reading.
Race
No racial predilection exists.
Sex
Acquired exotropia is more common in females than in males, with a female preponderance of 63-70%.
Age
Contrary to a previous belief, intermittent exotropia can have an early onset, with 25-40% of cases occurring before the second year of life.
Patients may experience asthenopia (eyestrain) during visual tasks, especially after prolonged near work. A common complaint is that a patient loses his or her place on a line while reading and repeatedly restarts on the same line.
Diplopia that is horizontal and crossed (ie, the right eye sees the image on the left, and the left eye sees the image on the right) may develop in some patients.
Some patients are subjectively aware of when the divergence of their eyes occurs, and they are able to volitionally restore binocularity.
Some patients may complain that objects appear smaller and closer because they use accommodative convergence to control the exodeviation.
Children characteristically close one eye in bright light. This action may precede the actual divergence of the eyes, or the parents may notice this phenomenon, which becomes the presenting complaint. Although various theories have been proposed to explain this phenomenon (eg, glare hinders fusion and causes its disruption), it remains incompletely understood.
Some attentive patients may notice an increase in the temporal visual field of the affected eye, called panoramic viewing.
Classification systems
Duane classification
If the deviation is greater at distance than at near, it is called the divergence excess type of exotropia.
If the deviation is greater at near than at distance, it is called the convergence insufficiency type of exotropia.
If little (< 10 prism diopters [PD]) or no difference exists between distance and near deviation, it is called the basic type of exotropia.
Burian classification
The divergence excess type of exotropia occurs when the deviation is greater at distance than at near (same as Duane classification). Burian divided it into 2 types: simulated divergence excess and true divergence excess. The simulated divergence excess type of intermittent exotropia demonstrates an increase in the near deviation after monocular occlusion or with +3.00 diopter (D) lenses placed in front of the habitual (if any) spectacle or contact lens prescription. If no increase occurs in the near deviation with either test, a true divergence excess type of intermittent exotropia is present.
The basic type of exotropia occurs when little or no difference exists between distance and near deviation (same as Duane classification).
The convergence insufficiency type of exotropia occurs if the deviation is greater at near than at distance (same as Duane classification).
Kushner classification
This classification system takes into account the effect of monocular occlusion, the use of either -2.00 D lenses or +3.00 D lenses, and the accommodation convergence-accommodation (AC/A) ratio.
The Kushner classification of intermittent exotropia is depicted in the image below.
A complete ophthalmic examination and an ocular motility examination should be performed in each patient.
Specific parts of the examination are outlined below, followed by physical findings of the different types of acquired exotropia.
Measurement of the deviation
Distance deviation is measured at 6 meters with an accommodative target; target size is 20/70 or smaller.
Far distance deviation is measured when the patient looks out of a window or at any target 50-100 feet away. This method may help obtain the full exotropic angle, which may increase from 10 PD to 30 PD when compared to the distance deviation measured at 6 meters.
Near deviation is measured at 33 centimeters with an accommodative target.
Deviation is measured after monocular occlusion to disrupt fusional convergence. Distance and near measurements are taken after patching either eye for at least 30-45 minutes.
Deviation is measured after using either +3.00 D lenses (near deviation) or –2.00 D lenses (distance deviation) to disrupt the accommodative convergence. The deviation with +3.00 D lenses should always be measured after the monocular occlusion test to avoid an erroneous measurement of a high AC/A ratio.
Assessment of the control of the deviation
This testing is important to obtain a baseline assessment and to monitor deterioration and progression of the intermittent exotropia.
Subjective methods
In home control, parents assess the deviation. The assessment of deviation is categorized as follows: excellent control, where deviation occurs rarely or only at distance when tired, fatigued, or inattentive; good control, where deviation occurs less than 5 times a day and only at distance; fair control, where deviation occurs more than 5 times a day and only at distance; or poor control, where deviation occurs frequently at distance and near.
In office control, ophthalmologists assess the deviation in a clinical setting. The assessment of deviation is categorized as follows: good control, where the patient breaks down only after cover testing and resumes fixation without a blink; fair control, where the patient breaks down after cover testing and blinks to refixate; or poor control, where the patient breaks down without any form of fusion disruption.
Objective methods
Distance stereoacuity provides an objective measure of the control of the deviation and the deterioration of fusion.
Near stereoacuity does not correlate well with the degree of control of the distance deviation.
Amblyopia
Amblyopia does not occur as frequently in patients with intermittent exotropia as in patients with esotropia. This type of amblyopia is usually nonstrabismic and frequently anisometropic.
Assessment of refractive error is an important part of the examination because unequal visual clarity could hinder binocular fusion and lead to the progressive loss of control of the exotropia. Cases of exotropia resolving after the optical correction of a high hyperopic refractive error have been reported.
Sensory changes
Suppression may be noted. Alternate suppression with temporal scotomas (which tend to split fixation) has been demonstrated in 52% of patients with intermittent exotropia.
Retinal correspondence is determined.
Distance and near stereoacuity is assessed.
Lateral incomitances
The size of the deviation differs in the primary position and in the lateral gaze positions.
Horizontal incomitance has been reported in 5-60% of patients with exotropia.
Recognizing lateral incomitance is important since an alteration of the surgical strategy to avoid diplopia in side gaze postoperatively may be required.
Vertical incomitances
The incidence of A- and V-pattern strabismus and oblique muscle dysfunction is lower in exotropia than in other types of strabismus.
The most common pattern associated with exotropia is V-pattern strabismus.
X-pattern exotropia can occur secondary to the overaction of the inferior and superior oblique muscles.
Convergence
When the deviation is greater at near than at distance, convergence insufficiency is possible.
Sensory exotropia
Sensory exotropia is a condition of unilateral divergence as a sequela to loss of vision or long-standing poor vision in one eye.
Sensory exotropia can occur because of visual loss at any age. In younger children, the incidence of esotropia or exotropia occurring in the nonseeing eye is about equal. In adults, the tendency is toward exotropia.
Sensory exotropia accounts for 20-25% of all causes of acquired exotropias.
The deviation angles are characteristically large. An eye with long-standing sensory exotropia often develops any of the several mechanical and innervational abnormalities, especially if the angle is large. These abnormalities include tight lateral rectus muscle syndrome with limited adduction, secondary pseudo–oblique muscle overaction, and shortening and tightening of the Tenon capsule and the conjunctiva over the lateral rectus muscle.
Superior oblique overreactions with A-pattern strabismus are more common than V-pattern strabismus.
On examination, the deviation needs to be measured with the Krimsky test or the light reflex-prism test if the visual acuity in the exotropic eye is poor.
Visual field defects
Exotropia can develop if both eyes have a significant visual field loss. Exotropia may occur with either bilateral homonymous field defects or heteronymous defects (eg, bitemporal field defects). This condition is uncommon.
Exotropia with bilateral homonymous visual field defects
Acquired neurologic disorders may produce bilateral homonymous field defects. Some patients may develop exodeviations, mostly exophorias or small intermittent exotropias.
Characteristically, these patients have normal retinal correspondence and good fusional ability. They do not have significant visual difficulties. Patients rarely complain of diplopia.
Whether the exotropia that develops is a true compensatory phenomenon or a coincidental finding is unclear. The exotropia may be helpful by allowing enlargement of the total visual field.
Exotropia with bitemporal visual field defects
Bitemporal hemianopia, which may occur with lesions (eg, pituitary tumors, aneurysms near the optic chiasma), is rarely associated with exotropia.
Unlike a homonymous field defect, a bitemporal defect always interferes with fusion, and, in cases where strabismus develops, a significant field loss, including central vision, occurs in both eyes.
These patients are usually disturbed by the symptoms of disordered binocular vision. Because of retinal sliding, the patients have a subjective sensation of an elongated target or a duplication of some features of a target. Loss of fixation beyond the fixation target may occur. All of these phenomena make routine visuomotor tasks difficult.
Consecutive exotropia
Consecutive exotropia, also called secondary exotropia, is the type of exodeviation that occurs after surgical overcorrection of an esodeviation.
Hereditary does play a role, but the genetics of this disorder are multifactorial. Exodeviations tend to occur earlier and to be larger in successive generations.
Imaging studies are not routinely required, although they may be beneficial for patients with certain conditions (eg, craniosynostosis, suspected abnormalities of the pulley systems). Repeat surgeries can be guided by the ultrasonic location of muscle insertions.
Nonsurgical treatment is indicated in patients with excellent or good control of the deviation as measured by normal distance stereoacuity and in young children where the risk of surgical overcorrection is undesirable.
Nonsurgical treatment modalities
Correction of refractive error: All kinds of refractive errors, particularly astigmatism and anisometropia, must be corrected. The associated improvement in visual acuity could be associated with increased fusional ability and better control of intermittent exotropia.
Minus lenses: The lenses stimulate convergence through the accommodative convergence synkinesis and help control divergence. The efficacy largely depends on the patient's AC/A ratio. The larger the AC/A ratio, the larger the effect (ie, patients can compensate for larger deviations). Various studies have reported not only an improvement in quality of fusion but also a quantitative decrease in the angle of deviation. Minus lenses range from –2 D to –4 D; they may be most helpful in younger children with exodeviations of 5-15 PD. A study showed that the overcorrecting minus lenses worked well in children aged 2-17 years and that the average reduction in the exodeviation was approximately 10 PD.[1]
Occlusion: Patching the dominant eye or alternate patching of either eye is suggested to interrupt the process of suppression and to reduce the progression of the exotropia.
Prisms: Base-in prisms may aid control and relieve asthenopic symptoms in small comitant exodeviations of up to approximately 20 PD.
Orthoptics: Convergence exercises improve convergence fusional amplitudes and the near point of convergence. Convergence exercises are indicated for patients with symptoms of the convergence insufficiency type of intermittent exotropia. Near point exercises, prism convergence exercises, and red glass convergence exercises are recommended.
Role of botulinum toxin (BOTOX®) injections in the extraocular muscles to treat secondary exotropia: A study showed good results in 383 subjects with exotropia who were treated with BOTOX®.[2] Multiple injections may be required, but they were well tolerated with no permanent adverse effects.
Sensory exotropia
The most important aspect of the management is to find and/or eliminate and/or reverse a treatable cause of the exotropia. Prisms and botulinum toxin injections do not play a significant role in the treatment of sensory deviation.
Exotropia with neurologic causes and field defects
Exotropia with bilateral homonymous visual field defects: Nonsurgical methods of treatment, such as patching, prisms, or botulinum toxin injections, should be tried before surgical realignment.
Exotropia with bitemporal visual field defects: Nonsurgical treatment includes the use of prisms to increase the separation of images and to avoid diplopia without sacrificing the total visual field.
Opinions vary widely with regard to the appropriate timing of surgical intervention for patients with intermittent exotropia. Surgery is indicated in the following:
Poor control of the deviation: When the tropic (manifest) phase is present at least 50% of the time, poor control of the intermittent exotropia is indicated, and surgery should be considered.
Deterioration of control of intermittent exotropia: Serial observations of an increase in the size of the deviation, a progressive deterioration in distance and/or near stereoacuity, a loss of control, and a progressive inability to re-fuse after manifestation of the deviation all indicate deterioration of control, and surgery should be considered.
Bothersome diplopia: Surgery is indicated for patients with this condition.
Severe asthenopia: Orthoptic exercises may be tried. If unsuccessful, then surgery should be considered.
Surgical procedures that can be used are lateral rectus muscle recession, recess-resect procedure (ie, lateral rectus muscle recession and ipsilateral medial rectus muscle resection), and bilateral medial rectus muscle resection. Indications for each surgical procedure are outlined in the image below.
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Management options for various types of intermittent exotropia.
A unilateral recess-resect procedure can also be performed in children with exotropia of the convergence insufficiency type where the exodeviation is more at near and less or absent at distance.[3]
Comparison of treatment options for intermittent exotropia have shown that surgery with preoperative orthoptic/occlusion therapy have the highest success rates compared to treatment with prisms alone or horizontal muscle surgery alone.[4]
Since undercorrection is a frequent sequela to surgery for intermittent exotropia, it is recommended that the largest angle ever measured be taken as the target angle for surgery and that the surgical dose be based on this angle.[5]
Sensory exotropia
If possible, surgery should be confined to the eye with the visual defect. The recess-resect procedure (lateral rectus muscle recession combined with ipsilateral medial rectus muscle resection) is recommended. When indicated, the recess-resect procedure should be combined with recession of the conjunctiva in a long-standing deviation.
Exotropia with bitemporal visual field defects
Surgical correction is difficult because of the varying nature of this alignment. Adjustable sutures may be used, but results tend to be unstable because of the absence of fusion.
Consecutive exotropia
Management depends on the amount of exodeviation and the type of previous surgical procedure.
If the deviation is small, it can be treated with minus lenses, which can be started immediately after surgery. Base-in prisms can be tried for small angle comitant deviations. If the deviation is large, a reoperation is the procedure of choice.
The choice of a reoperation procedure depends on the type of previous surgery, the amount of exodeviation present at distance and near, and any limitation of ocular rotations as a result of the previous surgery.
If the previous procedure was a bilateral medial rectus recession for esotropia, a limitation of adduction may indicate a slipped medial rectus muscle. In such cases, medial rectus advancement must be performed to correct the secondary exotropia.
In situations where adduction is full and the exodeviation is greater at distance than at near, lateral rectus recessions may be considered.
A repeat recess-resect procedure on the same eye or the contralateral eye can be performed when the deviation is the same at distance and near.
If left untreated, intermittent deviations could progress to constant exodeviations, with subsequent development of amblyopia and loss of fusional abilities in young children.
No specific complications are related to surgery for this condition; however, complications of eye muscle surgery in general apply.
Postoperative complications include overcorrection, undercorrection, residual A- or V-pattern strabismus, and diplopia in side gazes due to lateral incomitances.
Some factors that affect the prognosis are as follows:
Some authors believe that surgical alignment before age 4 years yields better results with lower risks for amblyopia. On the other hand, even a slight overcorrection increases the risk of loss of bifoveal fixation at this young age.
Patients with better fusional control preoperatively do better postoperatively.
Correction of refractive error should be continued postoperatively.
Tenacious proximal fusion may predict better postoperative results.
Both motor alignment and sensory functional improvement should be included in the postoperative assessment of response to surgery.
Neepa Thacker, MBBS, MS, DNB, FRCS, Consulting Staff, Department of Pediatric Ophthalmology and Strabismus, Breach Candy Hospital; Head, Department of Pediatric Ophthalmology and Strabismus, Lotus Eye Hospital, India
Disclosure: Nothing to disclose.
Coauthor(s)
Arthur L Rosenbaum, MD,
Disclosure: Nothing to disclose.
Federico G Velez, MD, Assistant Clinical Professor, Department of Ophthalmology, Division of Pediatric Ophthalmology and Strabismus, Jules Stein Eye Institute, University of California, Los Angeles, David Geffen School of Medicine
Disclosure: Nothing to disclose.
Specialty Editors
Francisco Talavera, PharmD, PhD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference
Disclosure: Received salary from Medscape for employment. for: Medscape.
J James Rowsey, MD, Former Director of Corneal Services, St Luke's Cataract and Laser Institute
Disclosure: Nothing to disclose.
Chief Editor
Hampton Roy, Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Disclosure: Nothing to disclose.
Additional Contributors
Michael J Bartiss, OD, MD, Medical Director, Ophthalmology, Family Eye Care of the Carolinas and Surgery Center of Pinehurst
Cooper E. The surgical management of secondary exotropia. Trans Am Acad Ophthalmol Otolaryngol. 1961. 65:595.
Duane A. A new classification of the motor anomalies based upon physiological principles together with their symptoms, diagnosis, and treatment. Am Ophthalmol Otolaryngol. 1897. 6:84.
Parks MM. Concomitant exodeviations. In: Ocular Motility and Strabismus. Hagerstown, Md:. Harper & Row. 1975:113.
Rosenbaum AL. Exodeviations. In: Current Concepts in Pediatric Ophthalmology and Strabismus. Ann Arbor:. University of Michigan. 1993:41.
Rosenbaum AL, Stathacopoulus RA. Subjective and objective criteria for recommending surgery on intermittent exotropia. Am Orthopt J. 1992. 42:46.
Santiago AP, Ing MR, Kushner BJ, Rosenbaum AL. Intermittent exotropia. In: Clinical Strabismus Management: Principles and Surgical Techniques. WB Saunders Co. 1999.
Von Noorden GK. Exodeviations. In: Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 5th ed. St Louis:. Mosby Year Book. 1996:341.
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