Hypertensive retinopathy doubled in African Americans

Daniel Keller PhD
in Singapore

Tien Yin Wong

AFRICAN Americans have almost twice the rate of hypertensive retinopathy as white Americans, according to a large community-based study.
The new information comes from the
Artherosclerosis Risk in Communities (ARIC) study, an ongoing public health research project. That study followed 15,792 men and women between 45 and 64 years, who were recruited in the late 1980s. They come from four communities around the US.
Researchers of the current study excluded participants with health problems such as diabetes because it could complicate the assessment of retinopathy, leaving 1,860 blacks and 7,874 whites, aged 49 to 73 years, in the present analysis.
The prevalence of retinopathy was 7.7% among blacks and 4.1% among whites. The study is the first to demonstrate such a finding in a group of participants representative of the general public of the US.
It found that blacks had higher normal blood pressures and were more likely to have hypertension and left ventricular hypertrophy.
They were also less educated, had higher HDL-cholesterol levels, higher body mass index and higher blood creatinine levels. But blacks in the study were less likely to have ever smoked or used alcohol.
At the third study visit, when the participants were aged 51 to 72, a 45º retinal photograph was taken of one randomly selected eye for each participant after five minutes of dark adaptation, centred on the disc and macula.
Graders read the photographs according to a standardised protocol and scored them for various lesions: flame-shaped haemorrhages, blot and dot haemorrhages, microaneurysms, cotton wool spots (soft exudates) and disc swelling.
Hypertensive injury to the retina may be evidence of more generalised vascular damage, according to lead author Tien Yin Wong FRCSE, PhD.
“Blood vessel damage in the eye is linked with similar changes in the brain and has been shown to be associated with a higher risk of stroke and death, independent of known risk factors. The higher frequency of this condition in African Americans may explain why they are at higher risk of stroke,” Dr Wong said.
Hypertension-related complications occur more frequently in African Americans than in white Americans, and they include heart problems, kidney disease and stroke, Dr Wong noted.
African Americans, with or without hypertension, were more likely to have retinopathy than comparable whites. Among hypertensives, retinopathy affected 9.1% of blacks and 5.3% of whites. Among normotensive participants, 5.9% of blacks had retinopathy compared to 3.6% of whites.
When researchers adjusted for the severity of hypertension and other cardiovascular risk factors, the prevalence of retinopathy among blacks was reduced by 53%.
“We found that differences in blood pressure explained about half the excess prevalence of retinopathy in African Americans. Thus, controlling hypertension in African Americans is probably one method of reducing the higher prevalence of retinopathy,” Dr Wong said.
The researchers proposed several possible explanations for the remaining excess prevalence of retinopathy in African Americans. First, there are racial differences in socioeconomic status and subclinical macrovascular and microvascular disease.
Macrovascular differences were reflected in measurements of intimal-media thickness of the common carotid artery seen in the study. Higher serum creatinine levels among African Americans indicated microvascular disease.
Second, pathogenic processes involving the endothelium and microvasculature may have contributed to the excess retinopathy observed. And third, there may be differences in susceptibility of the retinal vessels to damage from elevated blood pressure.
“Why they have greater susceptibility, and whether this is related to genetic factors, is not clear,” Dr Wong said.
When diabetics were included in a secondary analysis, researchers found an even higher prevalence of retinopathy among African Americans. The true racial differences in retinopathy may therefore be even higher than those found in the main analysis.
Previous work by Dr Wong has shown that retinopathy is associated with an increased risk of cerebrovascular disease and mortality. That 9.1% of middle-aged hypertensive African Americans have retinopathy has important public health and clinical implications.
The US Joint National Committee on the Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC) and the British Society of Hypertension assert that retinopathy is an indication for initiating anti-hypertensive drug treatment even in patients with only mild hypertension.
Since hypertensive retinopathy is twice as common in African Americans than their white counterparts, Dr Wong suggested that controlling hypertension among African Americans may reduce the disparity. He and colleagues intend to address this issue in future studies.
“We are beginning to recognise that retinopathy occurs in many people, even those without hypertension or diabetes. Thus, many factors, including possibly subclinical hypertension and other factors, like inflammation, may cause retinopathy,” he said.
Dr Wong advises that ophthalmologists be aware that retinopathy is a risk marker of stroke and other cardiovascular diseases. He advises that ophthalmologists refer appropriate patients for a full cardiovascular evaluation and should be aware that African Americans are more likely to have retinopathy than whites.
Previous studies suggesting that blacks of African descent had an excess risk of hypertensive retinopathy have had important limitations. Most consisted of highly selected clinic samples which did not represent the general population. Older studies done before the widespread use of anti-hypertensive medications may no longer reflect the current situation in US communities.
Most previous studies also used clinical ophthalmoscopy, an unreliable method of detecting retinopathy. Dr Wong warned that in daily practice, as in these studies, clinical examination is likely to miss most of these retinopathy lesions, meaning that patients with retinopathy go undetected.
This study by Dr Wong and colleagues used contemporaneous samples of blacks and whites living in the US and overcomes some of the limitations of previous ones. It was population-based, used quantitative and masked evaluations of retinopathy, and standardised assessment of blood pressure and data on other risk factors.
However, in the current study some subjects with normal blood pressure showed signs of retinopathy characteristic of hypertension. Some of these retinal signs are related to aging, according to the authors.
But there is the possibility that some cases of hypertension were missed because isolated blood pressure readings may not have accurately reflected a person's lifetime exposure to hypertension.The study appeared in the journal Hypertension (April 2003 Wong TY, Klein R, Duncan BB et al. Racial Differences in the Prevalence of Hypertensive Retinopathy).

Tien Yin Wong FRCSE, PhD
Singapore National Eye Centre,
National University of Singapore, Singapore
Email: ophwty@nus.edu.sg

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