Purpose
To describe the long-term refractive error changes in children diagnosed with intermittent exotropia (IXT) in a defined population.
Design
Retrospective, population-based observational study.
Methods
Using the resources of the Rochester Epidemiology Project, the medical records of all children (<19 years) diagnosed with IXT as residents of Olmsted County, Minnesota, from January 1, 1975 through December 31, 1994 were retrospectively reviewed for any change in refractive error over time.
Results
One hundred eighty-four children were diagnosed with IXT during the 20-year study period; 135 (73.4%) had 2 or more refractions separated by a mean of 10 years (range, 1–27 years). The Kaplan-Meier rate of developing myopia in this population was 7.4% by 5 years of age, 46.5% by 10 years, and 91.1% by 20 years. There were 106 patients with 2 or more refractions separated by at least 1 year through 21 years of age, of which 43 underwent surgery and 63 were observed. The annual overall progression was −0.26 diopters (SD ± 0.24) without a statistically significant difference between the observed and surgical groups ( P = .59).
Conclusion
In this population-based study of children with intermittent exotropia, myopia was calculated to occur in more than 90% of patients by 20 years of age. Observation versus surgical correction did not alter the refractive outcome.
Intermittent exotropia, characterized by an acquired, intermittent exodeviation, occurs in approximately 1% of healthy children in the United States and, given its predominance over esodeviations among Asian populations, may be the most prevalent form of strabismus worldwide. Although esotropia has been associated with hyperopia and anisometropia, the refractive error of children with divergent strabismus has not been as rigorously studied. The purpose of this study is to describe the refractive error outcomes in a population-based cohort of children diagnosed with intermittent exotropia over a 20-year period.
Methods
The medical records of all patients younger than 19 years who were residents of Olmsted County, Minnesota, when diagnosed by an ophthalmologist as having intermittent exotropia between January 1, 1975 and December 31, 1994 were retrospectively reviewed. Institutional review board approval was obtained for this study. Potential cases of intermittent exotropia were identified using the resources of the Rochester Epidemiology Project, a medical record linkage system designed to capture data on any patient–physician encounter in Olmsted County, Minnesota. The racial distribution of Olmsted County residents in 1990 was 95.7% Caucasian, 3.0% Asian American, 0.7% African American, and 0.3% each for Native American and other. The population of this county (106 470 in 1990) is relatively isolated from other urban areas, and virtually all medical care is provided to residents by Mayo Clinic, Olmsted Medical Group, and their affiliated hospitals. Patients not residing in Olmsted County at the time of their diagnosis were excluded. Intermittent exotropia was defined in this study as an intermittent distance exodeviation of at least 10 prism diopters (PD) without an underlying or associated neurologic, paralytic, or anatomic disorder.
Data abstracted from the medical records included gender, family history of strabismus, birth weight, gestational age at birth, reported age at onset, and ocular findings. The angle of deviation was primarily determined by the prism and alternate cover technique at both distance and near, although some younger patients were measured by the Hirschberg or modified Krimsky techniques at near. The initial and subsequent refractions were determined in the majority of patients following the topical administration of 1% cyclopentolate in younger patients and by a manifest refraction for older patients. All refractions were converted into their spherical equivalent. Since no patient had greater than 1 diopter of anisometropia, the refractive errors of the right and left eyes were averaged. Myopia was defined in this study as more than or equal to −0.50 diopters. Follow-up was measured from the date of the initial refraction to the last examination at which the refractive error was recorded through August 31, 2007.
Continuous data are presented as a mean with a standard deviation and categorical data are presented as counts and percentages. Progression of refractive error was determined by measuring the difference between the initial and final refraction divided by the total follow-up time per patient through the age of 21 years. Comparisons between groups for continuous variables were completed using Wilcoxon rank sum tests and for categorical variables using Fisher exact tests. All statistical tests were 2-sided, and the threshold of significance was set at P = .05. The rate of developing myopia was estimated using the Kaplan-Meier method.
Results
One hundred eighty-four patients were diagnosed with intermittent exotropia during the 20-year period. One hundred thirty-five of the 184 (73.4%) had 2 or more refractive error measurements separated by at least 1 year, the clinical findings of which are shown in Table 1 . There were 44 (33%) male and 91 (67%) female patients. The mean age at diagnosis for the 135 was 5.6 years (range, 0.9 to 14.9 years). Amblyopia was present in 4 patients (3%). The mean initial angle of deviation was 20 prism diopters (range, 10 to 40 PD) and 14 PD (range, 0 to 45 PD) at distance and near, respectively.
| Characteristics | Findings |
|---|---|
| Number of boys (%)/number of girls (%) | 44 (33%)/91 (67%) |
| Mean age at diagnosis in years (range) | 5.6 (0.9 to 14.9) |
| Number (%) with amblyopia | 4 (3%) |
| Mean initial horizontal deviation at distance in prism diopters (range) | 20 (10 to 40) |
| Mean initial horizontal deviation at near in prism diopters (range) | 14 (0 to 45) |
| Number (%) with inferior oblique dysfunction | 19 (14%) |
| Number (%) with dissociated vertical deviation | 3 (2.2%) |
| Number (%) managed with over-minus correction | 6 (4.4%) |
| Number (%) managed with surgical correction | 54 (40%) |
| Mean follow-up in years (range) | 10.1 (1.0 to 27.1) |
The initial refractive error of the 135 children is shown in Figure 1 , with a mean value of +0.26 (range, −7.75 to +3.13) at a mean age of 5.6 years. Eighty-four patients (62.2%) were initially hyperopic at an average age of 5.0 years; 56 of them (67%) had less than 1 diopter of hyperopia. Thirty-nine of the 135 patients (28.9%) were initially myopic at a mean age at diagnosis of 7.6 years. The remaining 12 patients (8.9%) were plano at an average age of 5.2 years.
The study patients were followed for a mean of 10.1 years (range, 1.0 to 27.1 years). The final refractive error of the 135 children included myopia in 95 (70%), hyperopia in 34 (25%), and plano in 6 (4.4%), at a mean age of 15.9 years. The Kaplan-Meier rate of developing myopia in this population was 7.4% by 5 years of age, 46.5% by 10 years, and 91.1% by 20 years ( Figure 2 ). Of the 135 children, 54 (40%) underwent surgical correction for IXT. The Kaplan-Meier rate of developing myopia in the surgery group versus the observation group is shown in Figure 3 . There was no significant difference in the rate of myopic progression between the two groups ( P = 0.16). Only 6 patients were treated with over-minus correction, but this group was too small for any statistical analyses.
To calculate the annual myopic progression, a subset of patients with 2 or more refractive error measurements separated by at least 1 year and measured before the age of 21 years was reviewed. One hundred and six patients met these criteria with a mean follow-up of 8.2 years (range, 1.0 to 18.8 years). The annual overall progression for the 106 patients was −0.26 diopters (SD ± 0.24). In the 54 patients who underwent surgical correction, the rate of progression was −0.25 diopters (SD ± 0.23) versus −0.27 (SD ± 0.25) in those who were merely observed ( P = .59).
Results
One hundred eighty-four patients were diagnosed with intermittent exotropia during the 20-year period. One hundred thirty-five of the 184 (73.4%) had 2 or more refractive error measurements separated by at least 1 year, the clinical findings of which are shown in Table 1 . There were 44 (33%) male and 91 (67%) female patients. The mean age at diagnosis for the 135 was 5.6 years (range, 0.9 to 14.9 years). Amblyopia was present in 4 patients (3%). The mean initial angle of deviation was 20 prism diopters (range, 10 to 40 PD) and 14 PD (range, 0 to 45 PD) at distance and near, respectively.
| Characteristics | Findings |
|---|---|
| Number of boys (%)/number of girls (%) | 44 (33%)/91 (67%) |
| Mean age at diagnosis in years (range) | 5.6 (0.9 to 14.9) |
| Number (%) with amblyopia | 4 (3%) |
| Mean initial horizontal deviation at distance in prism diopters (range) | 20 (10 to 40) |
| Mean initial horizontal deviation at near in prism diopters (range) | 14 (0 to 45) |
| Number (%) with inferior oblique dysfunction | 19 (14%) |
| Number (%) with dissociated vertical deviation | 3 (2.2%) |
| Number (%) managed with over-minus correction | 6 (4.4%) |
| Number (%) managed with surgical correction | 54 (40%) |
| Mean follow-up in years (range) | 10.1 (1.0 to 27.1) |
The initial refractive error of the 135 children is shown in Figure 1 , with a mean value of +0.26 (range, −7.75 to +3.13) at a mean age of 5.6 years. Eighty-four patients (62.2%) were initially hyperopic at an average age of 5.0 years; 56 of them (67%) had less than 1 diopter of hyperopia. Thirty-nine of the 135 patients (28.9%) were initially myopic at a mean age at diagnosis of 7.6 years. The remaining 12 patients (8.9%) were plano at an average age of 5.2 years.
