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Exodeviations (from Greek εξοτρὀπια, εξο "exo" meaning "to exit" or "move out of”) or divergent squint is primarily a neurologic dysfunction occurring as a result of certain obstacles to development or maintenance of binocular vision and/or defective action of the medial rectus muscles. Small exophorias are found in high frequency in the normal population and 60-70% of normal newborn infants have a transient exodeviation that resolves by 4-6 months of age.1,2,3 Intermittent exotropia is an exodeviation intermittently controlled by fusional mechanisms. Unlike a pure phoria, intermittent exotropia spontaneously breaks down into a manifest exotropia.
Exodeviations are much more common in latent or intermittent form than are esodeviations. Of all exotropia, intermittent exotropia comprises about 75-90% of the cases and is usually preceded by a stage of exophoria.4,5 It usually affects about 1% of the general population. Exodeviations occur more commonly in the Middle East, subequatorial Africa and the Orient than in the United States 5. Jenkins made the interesting observation that the nearer a country is to the equator the higher the prevalence of exodeviations.6
Innervational Factors and Mechanical Factors
Duane championed the view that exodeviations are caused by an innervational imbalance that upsets the reciprocal relationship between active convergence and divergence mechanisms.7 Bielschowsky questioned Duane’s claim that the majority of exodeviations are based on hyperactive tonic divergence. This abnormal position is determined by anatomic and mechanical factors such as orientation, shape and size of the orbits, size and shape of globes, volume and viscosity of reterobulbar tissue, functioning of the eye muscles as determined by their insertion, length, elasticity anatomical and structural arrangement and condition of fascias and ligaments of the orbits.5 Most current theories on the etiology of exodeviations combine the ideas of Duane and Bielchowsky and are of the concept that exodeviations are caused by combination of mechanical and innervational factors.8,9 Neuroanatomic substrates, including the area of the tegmentum of the brainstem in humans and divergence burst cells in the area of the mesencephalic reticular formation have been suggested as possible sites of a divergence center.10 It has been proposed that innervational imbalance in these centers is a trigger to exodeviation. Recent studies have also shown that binocular misalignment of the eyes may be secondary to extraocular muscle pulley position. A muscle pulley consists of a ring or sleeve of collagen, elastin and smooth muscle that encircles the extraocular muscles and is coupled to the orbital wall and other connective tissue structures by similar tissues. Tendons and muscles travel through pulleys by sliding inside thin collagenous sheaths that telescope within the pulley sleeves.11,12,13
Role of defective Fusion
Manifest exodeviation or exotropia are fortunately rare due to good fusional convergence reserves. In 1903, Worth developed a theory that the essential cause of exotropia is a defect of the fusion faculty and indeed is a congenital total absence of the fusion faculty.14 He stated that when the fusion faculty is inadequate the eyes are in a state of unstable equilibrium, ready to deviate either inwards or outwards on slight provocation.
Role of AC/A Ratio
The possibility that a high accommodation convergence to accommodation (AC/A) ratio could have a role in intermittent exotropia has been discussed at length by Cooper and Medow.15 These authors concluded that the AC/A ratio is either normal or just slightly higher than normal in patients who have intermittent exotropia. Kushner in 1988 found that approximately 60% patients with true divergence excess had a high AC/A ratio, and 40% had a normal AC/A ratio.16
Theory of Hemiretinal Suppression
Knapp and Jampolsky have postulated a theory that probably there occurs a progression from exophoria to bilateral bitemporal hemiretinal suppression to intermittent exotropia.17,18 This theory holds that the ability to suppress temporal vision allows the eye to diverge.
Role of Refractive Errors
In addition to interplay between the convergence and divergence mechanisms, refractive errors may further modify the innervational pattern that influences the position of the eyes. In a patient with uncorrected myopia, less than normal accommodative effort is required during near vision thus causing decreased accommodative convergence. According to Donders, this constant under-stimulation of convergence may cause an exodeviation to develop.19 Similarly in patients with a high degree of uncorrected hypermetropia no effort is made to overcome the refractive error by an accommodative effort and clear vision is unattainable.20 This may lead to development of an exodeviation on the basis of an under stimulated and thus underactive convergence mechanism that causes the AC/A ratio to remain low. Thus refractive errors, through their effect on accommodation, are undoubtedly one of the prime causes of misalignment of the eyes. Unequal clarity of retinal images may present an obstacle to fusion, facilitate suppression and therefore contribute to the pathogenesis of exotropia.
Genetics and Risk factors
Although heredity appears to play a role in exodeviations, the etiology of this disorder is probably multifactorial. A positive family history is often noticed.22 Children born with craniofacial anomalies and those with neurologic defects are more likely to exhibit exotropia. Maternal smoking during pregnancy and low birth weight are significant and independent risk factors for the development of horizontal deviations.23
Age of Onset
The onset of the majority of exodeviations is shortly after birth. In a series of 472 patients with intermittent exotropia, the deviation was present at birth in 204 and appeared in 16 at 6 months of age and in 72 between 6-12 months of age.9 In only 24 of the patients did exotropia develop after 5 years of age. With rare exceptions, exodeviatons begin as an exophoria that may deteriorate into intermittent and constant exotropia as suppression develops. Suppression is believed to be the key that unlocks the fusion mechanisms.25
Most studies describe a preponderance of female patients in exotropia.
The natural history of intermittent exotropia remains unclear due to lack of well controlled longitudinal prospective studies of untreated intermittent exotropia. In some cases, an exophoria progresses to an intermittent exotropia that eventually becomes constant. Such deviation usually occurs first at distance and later at near fixation. They may be influenced by decreased tonic convergence with increasing age, the development of suppression, loss of accommodative power and increasing divergence of orbit with advancing age.5 Nevertheless, not all intermittent exotropia are progressive. In some cases, the deviation may remain stable for many years, and in a few cases, it may even improve. Thus the patient should be followed over time to determine whether their exotropia is stable or deteriorating. Von Noorden found that 75% of 51 untreated patients showed progression over an average follow up period of 3.5 years while 9% did not change, and 16% improved.5
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Intermittent exotropia is the most common type of exodeviation and is usually first observed by the parents in early childhood as a spontaneous drifting out of one eye mostly when the child is tired, sick or daydreaming. Adult patients may manifest exodeviation after consuming alcoholic beverages or taking sedatives.
Assessing Control of Intermittent Exotropia
The assessment of control of intermittent exotropia is essential to obtain a baseline evaluation as well as to monitor deterioration and progression of intermittent exotropia.
Home Control: The parents may be told to keep a chart noting the control of deviation at home in terms of the percentage of waking hours the manifest deviation is noticed at home.
Good Control: Patient “breaks” only after cover testing and resumes fusion rapidly without need for a blink or refixation.
Fair Control: Patient blinks or refixates to control the deviation after disruption with cover testing.
Poor Control: Patient who breaks spontaneously without any form of fusion disruption, or who does not spontaneously regain alignment despite blink or refixation.
Distance Stereoacuity: It provides an objective assessment of both control of the deviation and deterioration of fusion. Normal distance stereoacuity usually indicates good control with little or no suppression.
Near Stereoacuity: In a study it was shown that near stereoacuity does not correlate well with the degree of control in intermittent exotropia and that performance in this test is only minimally affected by surgery.26 However, near stereo can be used as a marker for deterioration of the disease; if near stereo is deteriorating, it is likely that the patient’s control is not sufficient to prevent amblyopia from developing.
Measuring the Angle of Deviation
Due to the variable angle of deviation, measurement in a patient with intermittent exotropia can be difficult by routine alternate cover prism testing. A prolonged alternate cover testing should be used in patients with intermittent exotropia to suspend tonic fusional convergence. If after prolonged alternate cover testing, there is significant angle variability or a significant distance/near discrepancy, then a patch test is indicated. Monocular occlusion should be used before +3.00 D lenses to measure near deviation, to avoid misdiagnosing a high AC/A ratio.16 The + 3.00 lenses suspend normal accommodative convergence, whereas monocular occlusion relaxes fusional convergence mechanisms.
• Patch Test - The patch test is used to control the tonic fusional convergence to differentiate pseudo-divergence excess from true divergence excess and to reduce the angle variability. Contrary to the earlier practice of patching one eye for 24 hrs it is now found that patching the eye for 30 minutes is sufficient to suspend the tonic fusional convergence and thus reveal the actual amount of deviation.28
• +3.0 D near add test (lens gradient method) - This test has been devised to diagnose the patients of divergence excess type who have true divergence excess due to high AC/A ratio. This test uses the lens gradient method to measure the AC/A ratio. These patients are the ones who will continue to have a distance-near disparity post-operatively, and may require bifocal spectacles after surgery for a consecutive esotropia at near. This test should be used in patients who have a distance deviation greater than near deviation of 10 prism diopters or more after the patch test. After the patch test while still dissociated, remeasure the deviation at near with a +3.0 add. If the exodeviation at near increases by 20 prism diopters or more the diagnosis true divergence excess with high AC/A ratio is made.
• Far distance measurement - Measuring the deviation by fixating a far object reduces measurement variability and helps uncover the full deviation by reducing near convergence. Combining the patch test and far distance measurement can greatly reduce under-corrections and has improved the overall result. In a prospective randomized trial, 86% of patients who underwent surgery for the largest angle had a satisfactory outcome, compared with 62% who were operated on for the standard 6 meter distance deviation.29 A far distance measurement is considered to be ¼ mile or further.
Patients with intermittent exotropia rarely complain of symptoms. The surprising absence of symptoms is related to a well-developed suppression mechanism. The various symptoms a patient may report in intermittent exotropia are as follows:
• Transient Diplopia: Some patients report occasional transient horizontal diplopia, others will have a vague sense of discomfort or blurred vision when their eyes are deviated.
• Asthenopic symptoms may occur in initial phases when eyes are deviated momentarily. Some patients may notice symptoms like eyestrain, blurring, headache and difficulty with prolonged periods of reading. However, soon the children become asymptomatic due to the development of sensory adaptation.
• Micropsia: Some patients may complain of micropsia that may occur due to the use of accommodative convergence to control the exodeviation.
• Diplophotophobia One symptom that deserves a special comment is closure of one eye in bright sunlight. Bright sunlight dazzles the retina so that fusion is disrupted, causing the deviation to become manifest.26 Thus one eye is closed in order to avoid diplopia and confusion.
Intermittent exotropia has been divided into four groups according to the classification system proposed by Burian.22 This system is based upon the concept of fusional convergence and divergence and relies on measurements of the distance and near deviations.
1. Basic Intermittent exotropia: is present when the deviation in the distance is within 10 prism diopters of the near deviation. Patients with basic deviation have a normal tonic fusional convergence, accommodative convergence (normal AC/A ration) and proximal convergence.
2. Divergence Excess: is present when the distance deviation is 10 prism diopters greater than the near deviation. Kushner found that approximately 60% patients with true divergence excess had a high AC/A ratio, and 40% had a normal AC/A ratio. The group with a high AC/A ratio is prone to postoperative over correction at near if the distance measure is used as a surgical target angle.
3. Convergence Insufficiency: is present when the near deviation is 10 prism diopters greater than the distance deviation.
4. Simulated or Pseudo-divergence Excess: is present when the patient has a larger exotropia for the distance than near but the near deviation increases within 10 prism diopters of the distance deviation after 30-60 min. of monocular occlusion. This occurs because patients with pseudo-divergence excess have increased tonic fusional convergence at near. The prolonged monocular occlusion dissipates tonic fusional convergence thereby disclosing the full latent deviation.
Kushner has attributed disparity between distance and near deviation in intermittent exotropia to proximal vergence after effects and to alterations in AC/A ratio.16 The term “tenacious proximal fusion” has been used to describe the fusional after effects that explain the distance near disparity in patients previously classified as pseudo-divergence excess type. These patients with reduced angle of strabismus at near appear to have a slow to dissipate proximal fusion mechanism that prevents them from manifesting their true near deviation during a brief cover test. Although Kushner’s system is complex it can be used to guide patient evaluation and management.
Similar to Burian’s classification system, distance and near measurements must be obtained. In addition, if a disparity exists between the distance and the near measurement, the AC/A ratio is obtained using the lens gradient method. This is done by using a –2.0 D lens at distance, or by using a +3.0 D lens at near after fusion has been suspended by using 60 minutes of occlusion.
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Medical follow up
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Surgical follow up
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1. Archer SM, Sondhi N, Helveston EM: Strabismus in infancy. Ophthalmology 1989;96:133.
2. Nixon RB et al: Incidence of strabismus in neonates. Am J Ophthalmology 1985;100:798.
3. Archer SM, Helveston EM: Strabismus and Eye Movement Disorders. In Isenberg SJ (ed) The eye in Infancy 1994 Mosby, pg 255.
4. Govindan M, Mohney BG, Diehl NN, Burke JP. Incidence and types of childhood exotropia: a population-based study. Ophthalmology. 2005 Jan;112(1):104-8.
5. Noorden GK von. Exodeviations. In: Binocular Vision and Ocular Motility 5 th ed., 1996 Mosby, pg 343.
6. Jenkins R. Demograhics: geographic variations in the prevalence and management of exotropia. Am. Orthopt. J. 1992,42:82.