Courtesy of Brian G. Mohney, MD
Courtesy of Brian G. Mohney, MD
Figure 1B. The same intermittent exotropia patient showing normal alignment.
The evaluation of intermittent exotropia typically includes the parent's observation of frequency combined with an office examination of the deviating angle, level of control and stereoacuity. Although intermittent exotropia is sometimes managed by orthoptics, alternate-day occlusion or over-minus spectacles, most ophthalmologists recommend surgical intervention when there appears to be poor control, evidence of compromised binocular function, such as decreasing stereoacuity, or when pressed by parents for social reasons. However, these indications are poorly defined or have not been rigorously studied, and some outcome measures have recently been shown to vary considerably over the course of one day.3,4 These limitations of assessment, combined with a poor understanding of the natural history, currently prevent a clear understanding of ‘severity' and the indications for intervention.
Intermittent exotropia develops in early childhood and is generally reported to be progressive.5,6 Some authors have observed larger angles of deviation in adults compared with children, thereby implying that there is an increase in angle over time.7 However, other studies have shown that an angle of deviation of less than 25 prism diopters will usually decrease or resolve over time.8-10 None of these reports are prospective and all are subject to some form of selection bias. Moreover, data on stereoacuity and control are incomplete, making it difficult to identify true changes in the condition over time.
While a proper understanding of natural history is critical for the effective management of any disease, the lack of data regarding change over time in intermittent exotropia highlight the uncertain benefits of postponing or proceeding with surgery or other modalities of treatment.
Accurately assessing the severity of intermittent exotropia currently appears elusive to reliable quantification. Previous studies, as well as routine clinical practice, tend to rely on single measures of the deviating angle, level of control and stereoacuity as adequate representations of the condition at any given point in time. By extension, change over time is generally assessed by comparing the differences in the results of these measures from one examination to the next.
However, recently published studies have found considerable variability in the level of control3 and stereoacuity4 during the course of one day, results that question the confidence with which single measures can be interpreted. Improved quantification of these and other measures is essential for assessing change and thereby guiding the clinician as to the optimal timing for any intervention.
Surgery remains the mainstay of treatment for intermittent exotropia and most commonly consists of either a bilateral lateral rectus recession or a unilateral recess resect procedure. Nonsurgical treatments, such as alternate-day occlusion, orthoptics, minus lens therapy and prisms, may be tried as a prelude or adjunct to surgery. However, the long-term efficacy of both surgical and nonsurgical treatments is not well documented. Most studies on the treatment of intermittent exotropia focus on surgical issues, such as the optimum age at which to operate and whether bilateral or unilateral surgery is more effective. Nevertheless, these and other important questions regarding treatment remain largely unanswered and are currently hampered by the lack of robust outcome measures.
Although our knowledge of intermittent exotropia continues to improve, there remain considerable deficits in our understanding. In order to optimize clinical and surgical decision making for the treatment of intermittent exotropia, future work should focus on improving the assessment of the condition's severity and progression, a critical component of which is the acquisition of natural history data.
Randomized controlled trials would be useful for acquiring these data and assessing the effectiveness of various modalities of intervention. Newer avenues of evaluation, such as health-related quality-of-life assessment, may also become important in understanding the impact of the disease and the potential benefits of any interventions. In addition, the use of modern technology, such as eye-position monitors, may enhance our ability to assess how well or poorly individuals can control their deviations over days and weeks.
- Govindan M, Mohney BG, Diehl NN, Burke JP. Incidence and types of childhood exotropia: a population-based study. Ophthalmology. 2005;112(1):104-108.
- Chia A, Roy L, Seenyen L. Comitant horizontal strabismus: an Asian perspective. Br J Ophthalmol. 2007;91(10):1337-1340.
- Hatt SR, Mohney BG, Leske DA, Holmes JM. Variability of control in intermittent exotropia. Ophthalmology. 2008;115(2):371-376.
- Hatt SR, Mohney BG, Leske DA, Holmes JM. Variability of stereoacuity in intermittent exotropia. Am J Ophthalmol. 2008;145(3):556-561.
- von Noorden GK, Campos EC. Exodeviations. In: von Noorden GK, Campos EC, eds. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St. Louis: Mosby, 2002:356-376.
- Nusz KJ, Mohney BG, Diehl NN. The course of intermittent exotropia in a population-based cohort. Ophthalmology. 2006;113(7):1154-1158.
- Kii T, Nakagawa T. Natural history of intermittent exotropia--statistical study of preoperative strabismic angle in different age groups [in Japanese]. Nippon Ganka Gakkai Zasshi. 1992;96(7):904-909.
- Altizer LB. The nonsurgical treatment of exotropia. Am Orthopt J. 1972;22:71-76.
- Newman J, Mazow ML. Intermittent exotropia: is surgery necessary? Ophthalmic Surgery. 1981;12(3):199-202.
- Hiles DA, Davies DT, Costenbader FD. Long-term observations on unoperated intermittent exotropia. Arch Ophthalmol. 1968;80(4):436-442.
The authors state that that they have no financial relationship with the manufacturer of any product discussed in this article or with the manufacturer of any competing product.