Cataract and mortality
Mortality from vascular and renal disease significantly higher among cataract patients
People with clinically significant cataract have a higher mortality rate, both in general and more especially with regard to vascular and renal diseases, according to the findings of a large study reported by Yifan Chen BM BCh, Medical Sciences Division, University of Oxford, Oxford, UK at the 25th ESCRS Winter Meeting.
“The pathological causes underlying cataract are not fully understood. Therefore, a comprehensive understanding of the relationship between cataract and specific causes of death may provide insights into its pathogenesis or help to provide relevant health screening programmes,” Dr Chen said.
The study used data from the 1999-2008 cycles of the National Health and Nutrition Examination Survey (NHANES), a programme of studies designed to assess health-related and nutritional status of a nationally representative sample of the US civilian population. History of cataract surgery was used as a surrogate for clinically significant cataract because of the increasing rate of cataract surgery and increasingly lower threshold of visual acuity loss required for the procedure, Dr Chen explained.
The study based patient survival on the duration between the NHANES interview and the date of death or 31 December 2015, whichever came first. They confirmed mortality data from the National Death Index and classified the underlying causes of death according to the International Classification of Diseases, Tenth Revision (ICD-10), a globally used diagnostic tool for epidemiology, health management and clinical purposes. In their analysis, they identified deaths from all causes, including vascular disease, cancer, accidents, Alzheimer’s disease/respiratory disease/renal disease and others.
The researchers also considered a wide range of confounding variables including sociodemographic characteristics such as age, gender, ethnicity, education and income, smoking status and alcohol consumption. They also considered comorbidities such as diabetes, hypertension and hypercholesteremia, chronic kidney disease pathology, as well as body mass index and self-reported health status.
In total, the study included data from 14,918 participants aged 40 years and older with a mean age 56.8 years. They had a weighted prevalence of clinically significant cataract of 9.61% (n=2009). After a median follow-up of 10.8 years 3966 participants had died.
The study showed that after multiple adjustments, all-cause mortality remained significantly higher among those who reported a history of clinically significant cataract compared to those without it, with a hazard ratio (HR) of 1.11 (95% CI, p=0.036).
For cause-specific mortality, multivariable Cox models showed that a history of cataract predicted a 34% higher risk of vascular disease-related mortality (p=0.044) and an 85% higher risk of renal disease-related mortality (p=0.028). No significant association was observed between cataract and cancer, respiratory disease, Alzheimer’s disease, accident or other causes for mortality after multiple adjustments.
Many previous studies have investigated the association between cataract and all-cause mortality, but the results are conflicting, Dr Chen said. Moreover, very few studies to date have explored the associations between cataract and cause-specific mortalities and these studies have mainly focused on only a few specific causes such as cancer and vascular diseases.
Dr Chen noted that the finding of an association of clinically significant cataract with higher all-cause mortality compared to those without was consistent with most previous large-scale prospective studies. Previous studies also suggested different strengths of the associations between cataracts and mortality depending on the type of cataract. However, the present study was unable to confirm those findings as the data on cataract type was unavailable.
She added that a few previous studies have also shown an association between cataract and vascular mortality, suggesting the possibility of common pathogenesis pathways. Some hypotheses that have been proposed include a potential role of cumulative oxidative stress and crystallins degeneration, which is a biomarker for ageing and systemic disorders.
Dr Chen also pointed out that no previous studies to her knowledge have reported a significant association between cataract and renal disease-related mortality, whereas in the present study participants with clinically significant cataract had an almost two-fold increase in renal disease-related mortality risk, compared to those without clinically significant cataract.
She noted that hypocalcaemia and urea sequestration secondary to renal insufficiency may both contribute to the higher incidence of cataract. In addition, increased oxidative stress is implicated in the pathogenesis of both chronic kidney disease and cataract.
She added that the strengths of the study included its large sample size, its relatively long duration of follow-up, its detailed enumeration of specific causes of death and the multiple adjustments made for confounding variables. The study’s weaknesses included its potential for recall bias because of its dependence on a self-reported history of cataract surgery. In addition, there may be residual confounding factors and there was no information on the types of cataracts that patients had.
“Further studies are needed to confirm the associations we found between cataract and cause-specific mortality and to investigate the mechanisms behind these associations,” she concluded.