CHENNAI: Why is every alternate person in rural India above the age of 40 blinded by cataract when only one in five in cities has the disease? This was the question that troubled a group of researchers at Sankara Nethralaya in the city nearly two years ago. Today, they have an answer exposure to ultraviolet radiation.
The team, comprising optometrists and ophthalmologists, found that though the level of ultraviolet radiation was higher in Chennai than in neighbouring Tiruvallur due to pollution, ocular exposure to these rays was higher in the rural areas than in the city, increasing the risk.
“Our study shows that it’s clearly because of the lifestyle changes most city residents have adopted,” said Dr Ronnie George, consultant, Sankara Nethralaya. Screening the profiles of over 800 people equally divided in urban and rural areas the team calculated “the lifetime ocular UV exposure” in rural and urban areas.
“The study proved that no matter where they are or what they do agriculture, poultry or construction labour hats and/or sunglasses are required to block ultraviolet rays from the eye. It has got to do more with common sense than style,” he said.
Arivunidhi K, 47, an agricultural labourer who was operated for cataract in June, understands this very well. “Almost every working member of my family has disease. It progressed faster in my wife and she was operated last year,” he said. While Arivunidhi and his wife Kalaiselvi are among the few lucky ones, many aren’t.
A cataract is a clouding of the lens. India is now home to more than 15 million blind, the bulk of which is due to cataract and therefore reversible. A recent study on cataract conducted by the hospital, in which 7,774 patients were screened, showed that nearly 50% of rural citizens above 40 years were affected. In the urban areas, nearly 20% were affected.
Cataracts are classified according to where they occur. The cataract linked to ultraviolet exposure is the cortical cataract and forms in the outer layer of the lens. People with higher lifetime ocular exposure were more at risk for this type of cataract than those with the lowest exposure.
Lifetime ocular exposure is calculated with formulae the scientists developed based on an individual’s case history, including profession, hours of stay in the sun and place of residence. The hospital sourced the dosage of ultraviolet radiation from Tropospheric Emission Monitoring Systems. According to the international initiative that studies ozone depletion, the UV index in Tiruvallur was 6.9 and in Chennai 7.1.
Since January, the team also measured the extent of ultraviolet radiation in Chennai and the neighbouring districts. “We exposed polysulfone strips, which are capable of absorbing the UV flux from sunlight, and then read the measurement under a collimator, a device that narrows a beam of particles or waves. The gap between the city and the rural areas was evident in this as well,” said Rashima Asokan, an optometrist.
Studies have shown that extensive exposure to sunlight, particularly ultraviolet B radiation, could increase the risk of a major form of cataracts, besides skin cancer. UV-A radiation is more associated with tanning of the skin.
Going by the radiation level, Chennai should have been at a greater risk. “‘We looked at the wherefore, not the why,” Dr George said. The analysis gave the answers. The lifetime ocular exposure for an urbanite was 1.32 against 3.16 for those in the rural areas.
That clearly explained why people in rural areas got the disease at least five years earlier than their counterparts in the city or why the incidence was more than double.
“Where there is enough sun to give sunburns, it’s only wise to block the sun. If glasses are not affordable, we would prescribe at least a hat,” said Rashima.