Intermittent Exotropia

From EyeWiki

Article summary goes here.

Disease Entity

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.

Disease

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

Etiology

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.

Natural History

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.5Nevertheless, 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 General Pathology

Add text here

Pathophysiology

Add text here

Primary prevention

Add text here

Diagnosis

Add text here

History

Add text here

Physical examination

Add text here

Signs

Add text here

Symptoms

Add text here

Clinical diagnosis

Add text here

Diagnostic procedures

Add text here

Laboratory test

Add text here

Differential diagnosis

Add text here

Management

Add text here

General treatment

Add text here

Medical therapy

Add text here

Medical follow up

Add text here

Surgery

Add text here

Surgical follow up

Add text here

Complications

Add text here

Prognosis

Add text here

Additional Resources

Add text here

References

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.

Assigned editor:
"Error: Invalid time." contains an extrinsic dash or other characters that are invalid for a date interpretation.
Review:
Assigned status Update Pending
.