June 15, 2012 — Many children develop myopia (nearsightedness) during childhood, with most cases occurring in children aged 9 to 13 years. For this reason, regular eye exams are critical, according to the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study, a multicenter, longitudinal, observational, volunteer study of nearly 5000 children aged 5 to 16 years. The results were published online on June 11 in the Archives of Ophthalmology.
Robert N. Kleinstein, OD, MPH, PhD, a professor of optometry and public health in the School of Optometry at the University of Alabama in Birmingham, and colleagues conducted the study to determine the incidence of new cases of myopia in children aged 5 to 16 years.
The study was designed to avoid limitations of previous studies, which have largely been conducted outside of the United States, have studied only 1 to 3 racial/ethnic groups, have focused on Asian children, have had no follow-up or short follow-up durations of 1 to 3 years, have used small sample sizes, or have not used cycloplegia to measure refractive error.
Commenting on the study for Medscape Medical News, K. David Epley, MD, explained, "The novelty of [this study] is that it's been done in the United States. All of the previous studies of myopia that have been relatively large or even well-controlled have been outside the United States."
Dr. Epley is a pediatric ophthalmologist with Children's Eye Care, PLLC, in Kirkland, Washington. He is a clinical correspondent for the American Academy of Ophthalmology and vice president of the American Association of Pediatric Ophthalmology and Strabismus.
Long-term Study, Wide Range of Racial Demographics
"It's also much longer-term than most of the other studies looking at myopia in children,” he continued. "The nice thing is that they really looked at kids from 5 years to 16 years of age, and studied them throughout the process of their schooling to see how many of them developed myopia. It's kind of a big effort to be able to do that. They did it across different centers in the US so they could capture as many different racial demographics as possible, so that's unique."
This US study looked at 4927 children from 5 ethnic/racial groups: Asians, African Americans, Hispanics, Native Americans, and whites. Each ethnic group had at least 500 participants. The children were in grades 1 through 8, and were 5 to 16 years old. Each child underwent yearly testing from 1989 to 2009.
Annual tests included measurement of central refractive error after mydriasis and cycloplegia. The researchers defined myopia as -0.75 diopters (D) or more myopia in both principal meridians. They also included myopia defined as -0.50 D or more myopic spherical equivalent refraction to allow the results to be compared with those of other studies.
To show the effect of myopia definition on the number of participants without myopia entering the study and the number of new cases of myopia, 4 other definitions of myopia were used: -0.75 D and -1.00 D spherical equivalent, -0.50 D or more myopia in both principal meridians, and -1.00 D or more myopia in both principal meridians.
At study entry, myopic prevalence ranged from 12.9% for a spherical equivalent of -0.50 D to 8.5% for -1.00 D or more myopia in both principal meridians. Of the 4927 children initially identified, 4556 (92.5%) children were not myopic at study entry.
When using the definition of -0.75 D or more myopia in both principal meridians, 16.4% of the children became myopic during the study period. If researchers used the definition of -0.50 D spherical equivalent or more myopia, 23.4% became myopic. Spherical equivalent-based definitions yielded more cases of myopia than did definitions based on the refractive error in both principal meridians. On the basis of the definition of -1.00 D or more myopia in both meridians, 14% of the children became myopic.
"The incidence of myopia in this study was higher than most of us thought it would be…it's a much larger number," said Dr. Epley.
According to the CLEERE study definition of -0.75 D or more myopia in both meridians, the largest percentage of new cases (18.2%) occurred at 11 years of age. When using the spherical equivalent of -0.50 D or more myopia definition, the largest percentage of new cases (17.4%) occurred at 10 years of age. The number of new cases decreased steadily after age 10 to 11 years.
"A high percentage of children become myopic every year, that is a high percentage become myopic (or become a new case of myopia) at age 8, at age 9, at age 10, at age 11, at age 12, at age 13 and at age 14," said Dr. Kleinstein in an e-mail interview with Medscape Medical News.
More than 75% of new myopia cases were in children aged 9 to 13 years. The age of onset and the development pattern were the same regardless of the myopia definition that was applied.
The largest percentage of new cases occurred in Asians and the smallest percentage occurred in whites. According to the CLEERE study definition of -0.75 D or more myopia in both principal meridians, 27.3% of new cases occurred in Asian children, 21.4% in Hispanic children, 14.5% in Native American children, 13.9% in African American children, and 11% in white children. The ranking was basically the same with use of both CLEERE study definitions, except that African American and Native American rankings were reversed when the -0.50 D spherical equivalent definition was used.
Dr. Kleinstein said that he was surprised by the findings regarding ethnic and racial differences. "It was well-known that 'Asian' children had large amounts of myopia, but now it appears that every ethnic group is affected by myopia," he explained.
The information about ethnic/racial differences is all new, Dr. Epley said.
According to the CLEERE study definition of -0.75 D or more myopia in both meridians, 18.5% of females and 14.5% of males developed myopia.
With the definition of -0.50 D spherical equivalent, 25.5% of females and 21.4% of males developed myopia.
In this study, the difference (1.4%) in new cases in participants with normal birthweight (16.9%) and low birthweight (15.5%) was clinically inconsequential.
"Practicing clinicians need to know that they should be advocates for eye examinations for all children before they start school and periodically thereafter. This needs to be done to ensure that children have the best vision possible for learning as they grow up," said Dr. Kleinstein.
"Practicing clinicians should have myopia as the differential diagnosis for all children that they examine," Dr. Kleinstein said. "They should educate their local school administrators and teachers to be aware that children's classroom and outdoor performance may be affected by their vision," he said.
The take-home message for ophthalmologists and primary care physicians alike, said Dr. Epley, is that "we need to continue screening of children for myopia so we can detect the kids who need glasses, and that screening should continue at least through the age of 12, because then you get to the point where you've captured most of the kids who are going to become myopic."
The authors and Dr. Epley have disclosed no relevant financial relationships.
Arch Ophthalmol. Published online June 11, 2012. Full text
Medscape Medical News © 2012
Cite this: Myopia Incidence 'Much Higher Than Expected' in Large Study - Medscape - Jun 15, 2012.