INTRODUCTION
Eye care makes up a significant portion of medical services rendered in military medical treatment facilities. For example, during the 6-month period from October 1, 2005 to March 31, 2006, the entire Air Force Medical System coded for 3,385,972 procedures (current procedural terminology codes). Of that number, 107,674 (6.8%) were eye examinations (current procedural terminology codes 92004, 92014, 92002, or 92012). The most common secondary procedure code was for refraction (92015). A refraction, or determination of refractive error, accompanied 71,388 or 66.3% of those eye examinations, accounting for another 4.5% of all coded procedures for the Air Force Medical System.1
With federal service medical treatment facilities spending tremendous resources managing these common conditions, understanding the prevalence and distribution of refractive errors in our beneficiary population may be important for resource planning purposes and understanding the vision demands of our aging dependent population. However, that can be an elusive task. Studies on distributions of refractive error often concentrate on groups of specific races,2-5 age,6-8 occupations,9,10 or geographic residence.5,7 Results of such studies do not represent the racial, sex, or age distribution of our population and also include inherent socioeconomic implications not applicable to military beneficiaries who receive eye care in our clinics at no cost. The purpose of our study is to sample the beneficiary populations randomly presenting at two military bases and to compare the distribution of refractive errors among racial, sex, and age groups and to compare those findings with previous reports.
METHODS
We recorded the spectacle prescription, age, sex, and race of each patient examined in the optometry clinics of Lackland Air Force Base, Texas and Hickam Air Force Base, Hawaii during the 2005 to 2006 calendar years. Recorded prescriptions included sphere power and cylinder power with axis for each eye and bifocal power for both eyes. Age was extracted from the medical record and ethnicity was self-reported. Myopic (nearsighted) prescriptions are represented by negative values and hyperopic (far-sighted) are recorded with positive values. All values are listed in diopters. Statistical analyses and graphical representations were performed using Statistical Package for the Social Sciences (version 10.1; SPSS, Chicago, Illinois).
RESULTS
Distribution of Myopia and Hyperopia
Myopia occurred more frequently than hyperopia. Of all prescriptions for the right eye, 72.8% were for myopia and 20.6% were for hyperopia (Fig. 1 ). The remaining prescriptions were for simple astigmatism. About two-thirds of people are right eye dominant.11 However, despite occasional anecdotal assertions that there may be correlation between "eyedness" and the distribution of refractive errors between right eyes and left eyes, no such correlation occurred in our study (compare Fig. 1 vs. Fig. 2 [p = 0.086, two-tailed test]).
Refractive error distribution trends clearly change with age. Between the ages of 4 and 23 there is a clear trend of increasing myopia. By age 30, this trend reverses with a shift toward hyperopia which continues to approximately age 70. These two trends were clearly present for Caucasians (Fig. 3), African Americans (Fig. 4), Hispanics (Fig. 5), Asians (Fig. 6 ), and Native Americans (Fig. 7). There is another shift toward myopia that occurs about age 70 which we feel certain we can attribute to the onset of cataracts.12 This trend appears to be present for all races at about the same age, but it is most easily observed in the larger sample sizes.
While trends were similar between the races, the degree of myopia and hyperopia was not. Other studies have shown Asian populations tend to be more myopic than other groups.3,13 Our numbers confirmed this. Although Grosvenor13 reports African Americans have a lower incidence of myopia than Caucasians (8% vs. 13%), our results indicate the degree of refractive error in African Americans is significantly higher. In fact, Table I shows Asians and African Americans were significantly more myopic than Caucasians, Hispanics, and Native Americans. Between Asians and African Americans, Asians were more myopic but to an insignificant degree. The least myopic patients were Native Americans. They were significantly less myopic than Asians, African Americans, and Caucasians. They were also less myopic than Hispanics but to an insignificant degree.
Distribution of Astigmatism
Astigmatism can occur in conjunction with both myopia and hyperopia. In our sample population, 68.4% of all cases requiring optical correction required astigmatism (or cylinder) correction. Ninety percent of patients who had astigmatism required <2.00 diopters of correction. Astigmatism correction is always accompanied by an axis value on a O to 180 degree arc for each eye. Figure 8 shows how axis of astigmatism tends to cluster around 180 and 90 degrees, as eye care professionals expect.13 Just over 49% percent of all patients fall within ± 10 degrees of 180 and 17% percent fall within ±10 degrees of 90. The rest fall on oblique axes.
There were some differences in the amount of astigmatism between the races. Table II shows that our Hispanic population was significantly more astigmatic than Caucasians (p = 0.002), African Americans (p = 0.006), and Asians (p = 0.005). They were also more astigmatic than Native Americans but to an insignificant degree (p = 0.94). Native Americans were also more astigmatic than Caucasians (p = 0.257), African Americans (p = 0.258), and Asians (p = 0.142). The amount of astigmatism between the Caucasian, African American, and Asian groups did not differ significantly.
Distribution of Presbyopia
Presbyopia, or problems focusing at near, occurs in all humans around the fifth decade of life and typically requires additional prescription of bifocals for near work. Figure 9 illustrates the distribution of bifocal corrections for the total population. Distribution plots did not differ between the sexes or between races.
The age of onset (mean age requiring +1.25 bifocal) and progression of presbyopia (mean age requiring +2.25) are presented in Table III. Men presented with presbyopic complaints at a slightly higher age than women and required +2.25 bifocals at a slightly later age. Table III shows all races enter the presbyopic years at almost exactly the same age with the exception of the Native Americans. However, we believe this difference is due to the small sample size. Even though there is greater variation within each group for the age at which a +2.25 bifocal is required, both sexes and all races progressed through presbyopia at about the same rate.
Anisometropia
Anisometropia is the condition in which the refractive errors of an individual's eyes differ from each other. This becomes clinically significant when that difference is approximately 2.00 diopters or more when induced prismatic effects and perceived difference in image size between eyes can require contact lenses for best binocular vision.14 Only 3.3% percent of our total population had a difference of 2.00 diopters or more.
DISCUSSION
In addition to shedding light on distribution of refractive errors of our own beneficiaries, we feel this study represents very well a cross-section of the U.S. population by matching the U.S. racial profile and eliminating factors of socioeconomic circumstances and geographic residence. Even though we intended to simply study our beneficiary population, we believe it is worth noting how closely our ethnic demographics matched those of the U.S. population (Table IV). In addition to being a good representation of the U.S. racial distribution, we believe no single group is under-represented due to socioeconomic factors since all have access to eye care at no charge. However, our study population differed significantly from the general U.S. profile in terms of sex as one might expect in a military setting. There is also a disproportionate number of 18- to 21-year-olds in the study sample, principally since Lackland Air Force Base is the only basic training base for the Air Force.
From our results we believe we can make the following general statements regarding refractive error in the military primary eye care setting: (1) myopia is more than three times more common than hyperopia. (2) Asians are the most myopic race, followed by African Americans, Caucasians, Hispanics, and Native Americans. (3) Hispanics are the most astigmatic race, followed by Native Americans, Caucasians, African Americans, and Asians. (4) All race groups become more myopic between ages 4 and 23. (5) All race groups become less myopic (more hyperopic) after age 30. (6) All race groups and both sexes become presbyopic and progress through presbyopia at about the same rate.
Most studies on refractive errors have focused on incidence of ametropia with less discussion in the literature regarding the degree of ametropia, especially with regard to race, sex and age as we find them in our military population.2-8,10-12 Our beneficiary population now consists of around 60% retirees and their family members.15 Recognizing the needs of this aging patient base and understanding the trends noted above may help clinicians and planners manage the resources involved in this significant aspect of the care we provide.
ACKNOWLEDGMENTS
We express our gratitude to the optometry providers and technicians of Wilford Hall Medical Center and Hickam Air Force Base.
© 2008 Association of Military Surgeons of the United States Provided by ProQuest LLC. All Rights Reserved.
Copyright 2008 Military Medicine
