1 Department of Public Health and Community Medicine, University of Sydney, Sydney, Australia.
2 Injury Prevention Research Centre, Department of Community Health, University of Auckland, Auckland, New Zealand.
3 Institute for International Health, Faculty of Medicine, University of Sydney, Sydney, Australia.
4 Faculty of Medicine, University of Otago, Dunedin, New Zealand.
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ABSTRACT |
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accidental falls; aged; depth perception; hip fractures; vision; visual acuity; visually impaired persons
Abbreviations: CI, confidence interval; EPIDOS, Epidémiologie de l'Ostéoporose; PAR; population attributable risk
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INTRODUCTION |
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Poor vision is common in the elderly. Common eye diseases like cataract, glaucoma, and macular degeneration are all strongly age-related (1418
), and there is evidence that many elderly people would benefit from a change in eyeglasses (18
, 19
).
Three prospective studies have examined visual impairment in relation to hip fractures (2022
). The Framingham Study found that having poor vision or a difference in visual acuity between eyes was associated with increased risk of fracture (20
). Cummings et al. (21
) found that poor depth perception and reduced contrast sensitivity, although not visual acuity, independently increased the risk of hip fracture. The EPIDOS (Epidémiologie de l'Ostéoporose) Study found that reduced visual acuity was a risk factor for hip fractures (22
). The Beaver Dam Eye Study, a cross-sectional study, found that previous hip fracture was associated with measures of visual function such as visual acuity, contrast sensitivity, and visual sensitivity. In a hospital-based case-control study, Grisso et al. (23
) found that self-reported visual impairment was strongly associated with increased risk of hip fracture.
While these studies have been consistent in reporting an association between visual impairment and risk of hip fracture in older people, there remains some uncertainty about which aspects of impairment are associated with increased risk. Additionally, since none of these studi es provided information about the risks of hip fracture associated with time since last eye examination, information with which to guide recommendations about the most appropriate intervals between eye examinations in older people is lacking. We aimed to examine the associations between visual impairment (both measured and self-reported), time since last eye examination, and risk of hip fracture in a large case-control study of both men and women.
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MATERIALS AND METHODS |
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Demographic information was obtained on each identified individual with a hip fracture. This information was not obtained for people who did not normally live within the Auckland region, those who had sustained a fracture as a result of major trauma such as a traffic accident, and those whose fracture had resulted from a preexisting pathologic condition such as primary or metastatic cancer. A prearranged random sampling procedure was then used to select from the ward registers individuals for whom demographic information had been obtained. These case patients were contacted postoperatively, prior to discharge, and invited to participate in the study.
Age- and sex-matched controls were randomly selected (using computer-generated random numbers) from the practices of a random sample of all general practitioners in the Auckland region. The general practitioners initially contacted the patients, and if consent was given, the study staff invited them to participate.
Next of kin or caregivers (proxy respondents) were invited to participate in the study if a potential respondent was identified as being cognitively impaired, i.e., if 1) next of kin or caregivers indicated that the individual was incapable of understanding and completing the questionnaire or 2) the individual incorrectly answered at least three out of 10 questions on the Mental Status Questionnaire (24). Proxy respondents were also sought for those individuals who died before interview, were too ill, or for some other reason were unable to participate personally.
Study participants completed a face-to-face interviewer-administered questionnaire including a measure of functional impairment (Katz et al.'s Activities of Daily Living scale (25)) and had a number of body measurements taken. Study participants were asked how they would describe their vision: blindness in both eyes, minimal sight, adequate sight, or good sight. They were also asked whether a doctor or optician had told them that they currently had cataracts (excluding those who had already had cataracts removed), glaucoma, or double vision. Participants were asked what type of eyeglasses they wore (if at all) and whether they had been wearing these glasses at the time of the fall (for cases) or at a randomly selected time the previous day (for controls). Study participants were also asked about the length of time since their last eye examination.
Visual acuity was measured using a Snellen visual acuity chart at constant illumination, while the participant was wearing his or her current eyeglasses to correct for distance. Depth perception (stereopsis) was measured using a Randot Stereotest (Stereo Optical Company, Manchester, Connecticut) carried out at 40 cm with constant illumination. The tests were administered by four trained research nurses. However, depth perception tests were only undertaken in a sample of cases and controls, since only one set of testing instruments was available.
Odds ratios, 95 percent confidence intervals, and tests of trend were calculated using logistic regression. Clinically acceptable cutoff points were used when categorizing vision variables, except for minutes of arc, which were categorized into quartiles. In order to compare our results with those of the Framingham Study (20), we also categorized vision into levels that matched those used in that study: good (20/25 or better), moderate (20/30 to 20/80), or poor (20/100 or worse).
Potentially confounding factors were first assessed by examining age- and sex-adjusted associations between confounders and hip fractures. Those confounders that were significantly associated with fracture were put into a model and eliminated one by one if they had no actual confounding effect, starting with the least significant factor in the model (using p values). Confounders were variables that changed the odds ratio of the dichotomous vision variable (visual acuity of 20/40 or better vs. worse than 20/40) by more than 10 percent. Population attributable risks (PARs) were calculated for visual risk factors using multivariate-adjusted odds ratios and proportions of cases exposed (26, 27
).
Since there is a paucity of studies examining risk factors for hip fracture in men, we initially analyzed the data by gender. However, because this resulted in poor statistical power, we combined the two categories.
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RESULTS |
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The age and sex distributions of cases and controls are shown in table 1. There were more women than men in the study, and the average age of cases was 82.2 years as compared with 80.6 years for controls.
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Decreasing levels of self-reported vision were statistically significantly associated with increased risk of fracture (trend p=0.02), while self-reported presence of eye disease was not (table 4). Subjects who reported that they did not usually wear glasses were not significantly more likely to have had a fracture than those who reported usually wearing glasses. Of subjects who normally wore glasses, 516 (65.4 percent) cases had not been wearing their glasses when they fell, compared with 501 (58.8 percent) controls (adjusted odds ratio=1.2; 95 percent CI: 1.0, 1.6). In comparison with having had an eye examination in the past 2 years, increasing time since the last eye examination was statistically significantly associated with risk of fracture (trend p=0.03). In comparison with subjects who had good vision in both eyes, there were significant associations with fracture for persons who had any impaired vision in either eye, moderate vision in both eyes, and poor vision in both eyes (table 5).
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DISCUSSION |
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The Framingham longitudinal study found that having a difference in visual acuity between eyes increased the risk of hip fracture (15), leading the authors to conclude that stereoscopic vision is important in preventing falls and fractures. However, they did not use an actual test of depth perception; they imputed stereopsis from the measurements of visual acuity between eyes. When using similar categories to examine visual acuity, we found that having moderate or poor vision in both eyes was associated with hip fracture. Having good vision in one eye and moderate or poor vision in the other was not associated with fracture, which suggests that instead of the difference between eyes being an important risk factor, binocular visual acuity below a certain point is important.
Cummings et al. (21) found that poor depth perception and contrast sensitivity were associated with an increased risk of hip fracture in White women but impaired visual acuity was not. In contrast, the EPIDOS Study found that a deficit in visual acuity was associated with an increased risk of hip fracture but impaired depth perception and contrast sensitivity were not (22
). The authors suggested that their results differed from those of Cummings et al. because their cohort was older (mean age=80.5 years (22
) vs. 72 years for Cummings et al. (21
)) and that a decline in contrast sensitivity and depth perception may be an early indicator of visual impairment before visual acuity is seriously affected (22
). Our findings do not support this theory; the average age of subjects in our study was similar to that in the EPIDOS Study, yet we found significant associations between stereopsis and visual acuity and hip fracture, with more power to examine these associations.
The Beaver Dam Eye Study, which used retrospective recollection of fractures in a cross-sectional study design, found that both best-corrected, current, and near visual acuity and contrast sensitivity and visual sensitivity were significantly associated with risk of hip fracture after age 40 in subjects aged 60 years or more (2). However, this study examined associations between vision and prevalent fracturesthat is, among people who survived and continued to live in the community.
Grisso et al. (23) found that self-reported visual impairment was significantly associated with increased risk of hip fracture. In their study, visual impairment was defined as present when subjects reported not being able to recognize a friend across the room. Our study confirms these findings. We found that subjects who reported having adequate or minimal vision or being blind in both eyes had a small but significant increase in risk of fracture when compared with those with self-reported good vision.
We found that time since last eye examination was associated with increased risk of fracture. The time period we selected was chosen because the recommended length of time between eye examinations by ophthalmologists and optometrists is generally not less 2 years for an asymptomatic elderly person with no risk factors for eye disease. The Beaver Dam Eye Study found that although vision is relatively stable over a period of approximately 12 months, visual impairment decreases more over a 5-year period among people in their 70s and older (28). We found a statistically significant trend with increasing time since last eye examination over 2 years, which suggests that in order to maintain good vision, elderly people should have their eyes tested at least every 2 years.
There was some potential for bias in this study in that stereopsis was only measured in a subsample of subjects. Case patients who did not undergo the test were more likely to be cognitively impaired, to be older, and to have poorer self-reported vision than cases who did undergo the test. Subjects with cognitive impairment were also less likely to have stereopsis than those without cognitive impairment, which suggests that our results underestimate the strength of the association between lack of stereopsis and hip fracture. This was confirmed by the sensitivity analysis we carried out to assess bias, which found that when missing stereopsis values were assigned imputed values (by age and visual acuity level), the odds ratios rose substantially for the two lowest categories of stereopsis.
We adjusted for proxy respondent status in the multivariate models in an attempt to minimize the amount of bias caused by the differences in percentages of cases and controls who were unable to answer the questionnaire themselves (usually due to cognitive impairment). However, the potential bias caused by the differential participation of cases and controls is difficult to ascertain. No information was collected on reasons for lack of participation in the study, although anecdotal evidence suggests that refusals came from both ends of the spectrum (that is, from the young and healthy as well as the old and frail).
This large case-control study with a well-defined study base had good power to examine associations with vision and hip fracture. Because the most important vision variables (visual acuity and stereopsis) were measured, bias caused by self-report was minimized. However, since vision was measured after the hip fracture had been sustained, it is impossible to be sure that the poor vision was not caused by the incident which caused the hip fracture.
There are a substantial number of elderly people living in the community who have uncorrected refractive error that a change in eyeglasses would improve (18). We found that 40 percent of hip fractures in our study were attributable to poor visual acuity or lack of stereopsis. The attributable risk was 15 percent due to poor visual acuity alonecomparable to the Framingham Study, which found an attributable risk of 18 percent for poor visual acuity in that population. It is likely that correcting refractive error would improve stereopsis, although there have been no studies that have demonstrated the extent to which this would occur.
Our study findings support previous research in that we found associations between impaired visual function, measured or otherwise, and increased risk of hip fracture. We found that both lack of stereopsis and lack of binocular visual acuity were risk factors for hip fracture. This study provides evidence that elderly people should have their eyes tested at least once every 2 years and have any refractive error corrected.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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