Our results indicate mental well-being is a significant mechanism by which VI affects the risk of mortality. Although VI increased the risk of mortality directly after adjusting for mental well-being and other covariates, mental well-being serves as an important indirect pathway from VI to increased mortality risk. In contrast, our results also indicate VI does not have a significant effect on preventive care practice and preventive care practice is not a significant predictor of mortality.
Our study confirms that persons with VI have reduced mental well-being and that mental well-being is a strong predictor of mortality. Using 11 variables to capture emotional well-being from the population-based 2004 SHARE, Monjon-Azzi and associates found a significant relationship between lower vision and well-being.24
In a systematic review of 29 quantitative studies between 2001 and 2008, Nyman and coworkers42
noted working-age adults with VI were significantly more likely to report lower levels of mental health, social functioning, and quality of life. With respect to well-being and mortality, Chida and Steptoe23
performed meta-analysis of 35 studies in initial healthy populations and 35 studies of disease populations and documented positive psychological well-being has a favorable effect on survival in both healthy and diseased populations.
Using latent variables in SEM, we measured the association between VI and reduced mental well-being status. A change of VI status from no impairment to any impairment was associated with more than a half SD decrease in mental well-being. In comparison with other sociodemographic variables, VI had the strongest effect in lowering mental well-being. The effect of VI on mental well-being was stronger than sex, 1-year increase in age, 1-year increase in education, smoking status, or having health insurance.
Similar patterns of association were found when we repeated our analyses using CVD-related mortality as the outcome. VI not only directly increased the risk of CVD-related mortality, it also indirectly increased the risk of mortality through its effect on mental well-being. Furthermore, these associations were slightly stronger in comparisons to results for overall survival; however, the direct and total effect of VI on cancer-related mortality was not significant. These CVD and cancer mortality findings were consistent with previous research. For example, Lee and associates9
found VI increased the risk of CVD-related mortality but not cancer-related mortality in a large population-based NHIS study. In the Beaver Dam Eye Study, Knutson and colleagues12
concluded increased diabetic retinopathy severity was associated with heart disease mortality, and increased nuclear sclerosis severity was associated with stroke mortality; however, there was no significant association between ocular conditions including VI and cancer mortality.
Although the total effects of VI on cancer mortality risk was nonsignificant, there was a significant but relatively small indirect effect on cancer mortality through VI's effect on mental well-being (HR = 1.16 [1.05, 1.27], P
< 0.05). Lower mental well-being is associated with increased risk of cancer mortality, although most studies focused on all-cause mortality and did not report cancer-specific findings.23
A recent Danish cohort study reported that a global quality-of-life question predicted the development of cancer over a 13-year follow-up period (poor versus very good: adjusted HR = 1.90 [1.1, 3.4]).43
One potential mechanism by which VI may indirectly affect cancer mortality is through the influence of low mental well-being on cancer screening behaviors. Higher levels of depressive symptomatology were found to be associated with reduced rates of breast cancer screening behaviors in women.44–47
On the other hand, similar associations were not found between cervical cancer screening and depressive symptoms,45
nor was a direct association found for colorectal cancer in women and men.48
When the analysis was performed by restricting to those participants with “No VI” and “Some VI” by excluding the blind participants, the direct effect of VI on all-cause mortality had the same directionality and was weakened; HR decreased from HR = 1.25 [1.01, 1.55], P < 0.05, to HR = 1.14 [0.91, 1.43], P = 0.25. The indirect and total effect of VI on mortality through mental well-being remained significant; the indirect effect remained essentially unchanged from HR = 1.23 [1.16, 1.30], P < 0.05, to 1.22 [1.15, 1.29], P < 0.05, and the total effect decreased from HR = 1.53 [1.24, 1.90], P < 0.05, to HR = 1.39 [1.11, 1.74], P < 0.05. When the analysis was performed by restricting to those participants with “No VI” and “Some VI” for the CVD mortality analysis, a similar pattern of changes was obtained. These findings indicate a consistent relationship between VI and mortality through mental well-being.
In each of our overall and cause-specific mortality analyses, we found at least some evidence of an indirect effect of VI through its influence on mental well-being. These findings reinforce the notion that the adverse effects of VI on psychological health indicators is a phenomenon that is partially “hidden” from view and should therefore receive greater attention by both clinicians and the research community at large. For example, strategies for the enhancement of well-being and overall quality of life among those living with VI should be further developed. In addition, clinicians treating the visually impaired should be aware that rapid depression screening tools are available to identify subpopulations that may benefit from appropriate follow-up care from mental health professionals.49–51
Our results also indicate VI does not have a significant effect on preventive care practice and preventive care practice is not a significant predictor of mortality. Persons with VI may have more barriers in seeking preventive care.52
Conversely, persons with VI may be more likely to come into contact with the health care system, either because of medical comorbidities or because of ongoing ocular care.53,54
To further study the relationship among preventive care, VI, and mortality, we repeated the analysis replacing preventive care latent variable with just a single preventive care practice indicator: flu shot status. We selected this indicator because the results from a previous report suggested it could lower mortality risk.55
In our multivariable model, however, the direct pathway from flu shot to mortality did not reach statistical significance (HR = 0.97 [0.93, 1.01], P
= 0.15). Therefore, our findings suggest the effect of VI on mortality is not via the pathway of affecting preventive care, although this conclusion is somewhat tempered by the rather limited number of preventive care indicators available in the MEPS dataset.
Although there is no established methodology of assessing mental well-being, the five quantitative mental well-being indicators used in this study are consistent with those of Monjon-Azzi and associates24
and measure overall psychological well-being. Rather than studying each indicator individually, we used a latent variable in the SEM setting to create a more comprehensive composite measure of mental well-being. The advantages of using latent variables within SEM to model mental well-being allow the estimation of mental well-being free of random measurement error. To our knowledge, our group is the first to use such a latent variable to assess the effect of mental well-being on the relationship between VI and mortality. Moreover, SEM allows the assessment of multiple pathways simultaneously.
Limitations of our study include the fact that all variable information was based on participant self-report and was therefore subject to misclassification. For example, self-reported questions like those for VI could be context-specific or influenced by culture, and therefore may be subject to differing interpretations across the diverse sample of adult participants of the NHIS. To our knowledge, the specific VI questions in the MEPS have not been validated, although validations of similar questions have been reported in the literature. For example, concordance of Snellen distance acuity results with the question “Are you able to recognize a face from a distance of 4 meters” was reported to be 79% in one clinic-based study.56
The MEPS distance acuity question was very similar in its structure: “(With glasses or contacts) can you see well enough to recognize familiar people if they are 2 or 3 feet away?” In the Rand Health Insurance Study, the sensitivity and specificity for the question, “Without glasses, can you recognize a friend across the street” were 90% and 78%, respectively, when using 20/100 or worse distance visual acuity in the better seeing eye as the criterion.57
The Rand question, “Without glasses can you read ordinary newsprint?” had a sensitivity of 87% and a specificity of 91% when using 20/100 near visual acuity as the criterion. This question was similar to the one used in the MEPS: “(With glasses or contacts), can you see well enough to read ordinary newspaper print, even if you cannot read.” Sensitivities for both RAND items were lower when cut points were reduced to include mild levels of near and distance impairment (e.g., for 20/40 or worse: distance sensitivity 58%, near sensitivity 59%). The use of visual acuity as a “gold-standard” criterion to evaluate self-reported visual impairment is subject to its own limitations, however, given that other visual system components can affect visual functioning. For example, contrast sensitivity, stereoacuity, and visual acuity are independently associated with self-reported near and far VIs in community-residing populations.58
Information such as history of ocular diseases such as cataract and retinopathy and the causes of visual impairment were not available in MEPS. Furthermore, as indicated previously, the number of assessed preventive care practices was somewhat limited in the MEPS. The preventive care indicators used may not be sensitive enough to reflect the health behavior effect on mortality, and we may have failed to capture with our latent variable other preventive care practices that may be related to both VI and mortality. For example, preventive services targeting the reduction in CVD (e.g., aspirin prophylaxis, pharmaceutical treatment of hypertension, and hyperlipidemia) have been shown to reduce all-cause mortality, although these reductions are somewhat modest (relative risk range, 0.84–0.93).55
In summary, our study shows that VI not only directly increases the risk of mortality but also indirectly increases mortality risk through its adverse impact on mental well-being. Mental well-being represents an important pathway through which VI affects the risk of mortality. Ignoring this indirect effect may lead to an underestimation of the impact of VI on mortality. The persistent increased mortality risk among the visually impaired documented across epidemiologic studies is unlikely to be a result of the impact of VI on preventive care practices. Our study underscores the importance of improving mental well-being of those with VI, which will improve the quality of life and potentially reduce the risk of mortality.