SAN FRANCISCO, CA--- This month's Ophthalmology, the journal of the American Academy of Ophthalmology, includes reports on the first large-scale visual acuity assessment of preschool children, the Baltimore Pediatric Eye Disease Study. Surprisingly, researchers found that one commonly-used vision test was so inaccurate that its usefulness is questionable. Also of note in this issue are two major, linked studies of infection risks associated with new and standard contact lens types. The conclusions will help eye specialists and lens wearers make better choices and improve use and care practices.
First Major Study of Preschoolers' Visual Acuity Finds Fault with a Standard Eye Test
Visual acuity---the ability to see objects in sharp detail---was evaluated in 1,504 children aged 30 to 71 months as part of the Baltimore Pediatric Eye Disease Study, the first large, population-based study of eye disorders in preschool children. Ophthalmologists agree on the importance of finding and treating vision problems early in a child's life to avoid potentially life-long negative impacts. Prior to the Baltimore study, little data was available on vision disorders in very young children in the United States.
The children were evaluated in a series of vision tests, including the fixation preference test (FPT) and the Amblyopia Treatment Study test (ATS). The FPT measures an aspect of visual acuity known as fixation, the ability to focus images on the macula, the part of the eye that provides detailed vision. In the FPT, each eye is covered and then uncovered to determine how well it maintains fixation. The specific purpose of the ATS test is to measure visual acuity in children 30 months and older. "Decreased" visual acuity in study participants was assessed by comparing the children's results to normal vision standards for their age groups. Tests were repeated for all subjects 60 days later with children wearing corrective eyewear, if needed. Subjects were classified as either non-Hispanic white or black (African-American), and results for the groups were compared. Parents of all subjects completed questionnaires regarding the mother's health during pregnancy, the child's general health and development, and any concerns about the child's ability to perform daily activities.
Overall, the prevalence of decreased visual acuity was low: 1.2 percent in white children and 1.8 percent in black. Prevalence at retesting, with corrective eyewear as needed, was 0.5 percent in whites and 1.1 percent in blacks. Differences between the two ethnic groups were not statistically significant. The most common cause of decreased acuity at initial testing was refractive error: nearsightedness, farsightedness and/or astigmatism. Only one child was found to be legally blind.
Importantly, the researchers found that FPT---considered the "clinical standard" for testing vision in preverbal children and the only test widely available to eye specialists for this age group---did not accurately identify the presence or absence of visual acuity problems in this study population. The inaccuracies became apparent when ATS and FPT results were compared for children able to perform both tests.
"These results call into question the use of FPT for clinical decision making," said lead researcher David S. Friedman, MD, PhD, Wilmer Eye Institute, Johns Hopkins School of Medicine. "Given its poor performance, the accuracy of clinical interventions based on FPT is questionable, as are studies that use FPT as an outcome measure. If the experience of this study can be generalized, the use of FPT should be reconsidered."
Proper Use and Care More Important than New Lens Types for Contact Lens Infection Prevention
Contact lenses are a good alternative to eyeglasses, but are not entirely risk free. Even with today's improved designs, a small but significant number of contact lens wearers develop painful microbial infections of the cornea (microbial keratitis, or MK) that can cause vision loss. The largest combined study of this problem to date was conducted by Australian and United Kingdom researchers, who were especially interested in whether the new silicone hydrogel lenses would reduce infection risk as a result of their improved oxygen permeability. The cornea, the clear front covering of the eye that helps focus light, needs oxygen to remain healthy. The studies looked at use patterns for several types of contact lenses in relation to infection factors, the incidence of MK-induced vision loss, and risk levels associated with specific lens types. Eye specialists and contact lens wearers will be able to make better choices based on these new findings.
Both studies agreed on key points: new lens types, including silicone hydrogel "soft" lenses and daily disposables (DD), did not reduce the risk of MK for wearers. Disease incidence fell within the ranges reported in earlier soft and rigid lens studies: 2.7 to 6.4 cases per 10,000 users per year for daily wear and 9.3 to 20.9 cases per 10,000 users per year for overnight wear. The researchers now think that other factors---such as tear film stagnation, corneal changes beneath the lenses, and slowed turnover of corneal surface cells--may be more important than oxygen permeability in MK development.
Overnight wear of any contact lens, including "extended use" types, was found to be the strongest risk factor for permanent vision loss, which occurred in about 13 percent of MK cases in these studies. DD wearers who developed MK had less severe infections and were least likely to sustain permanent vision loss. DDs do not require lens cleaning or lens cases, where infective agents can grow and thrive.
The Australian study linked use of any contact lens type for less than six months with increased MK risk. Earlier research showed that most new wearers' eyes adapt to lens use over several months: at first the corneal surface thins and sheds cells; later these cells are replaced, the rate of shedding declines, and the ability of bacteria to bind to shed cells also decreases.
"Users of overnight wear contact lenses have an increased risk for developing MK during their early wear experience," says Fiona Stapleton, PhD, University of New South Wales, who led the Australian study. Eye specialists should advise patients accordingly, she adds.
The Australian prospective population survey was conducted from October 2003 to September 2004 with a total of 285 study subjects and 1,798 controls. The U.K. case-controlled study was led by J.K.G. Dart, DM, Moorfields Eye Hospital, and conducted from December 2003 through December 2005 with a total of 367 study subjects and 1,069 controls. The two studies were designed to complement each other and shared key parameters so that results could be directly compared.
Smoking, purchasing lenses via the internet, and higher socio-economic status were additional risk factors in the Australian study. The UK study found risk levels varied significantly depending on the lens brand; a better understanding of how specific brand characteristics contribute to infection is needed so that safer lenses can be developed. Other risk factors in the UK study included improper hand cleaning, younger wearer age, use beyond recommended time spans, and male gender. The differences in risk results between the studies are probably due to prescription and use patterns, the authors say.
Eds: Full texts of the studies are available from the Academy's media relations department.
About the American Academy of Ophthalmology
The American Academy of Ophthalmology is the world's largest association of eye physicians and surgeons--Eye M.D.s--with more than 27,000 members worldwide. Eye health care is provided by the three "O's" - opticians, optometrists and ophthalmologists. It is the ophthalmologist, or Eye M.D., who can treat it all: eye diseases and injuries, and perform eye surgery. To find an Eye M.D. in your area, visit the Academy's Web site at www.aao.org.