The untenability of the sunlight hypothesis of cataractogenesis

The excess prevalence of cataract in third world countries led early this century to the hypothesis that sunlight causes cataract. The hypothesis, which ignored differences in diet, culture, poverty and prevalence of other diseases such as diarrhoea, received little support until about thirty years ago when biochemical studies were set up to explore the browning of lens proteins, which is a common feature of cataract on the Indian subcontinent. Initially these studies were encouraging in that exposure to sunlight caused some changes seen in cataractous lenses, but eventually the hypothesis was rejected because the first change in the laboratory was the destruction of tryptophan, but this was not found in brown cataract lenses. A brown nuclear cataract could not be produced artificially in the laboratory using sunlight or UV exposure. Exposure of laboratory animals has produced lens opacities, but in most experiments the doses required have also caused keratitis, conjunctivitis, iritis and inflammation. The cornea seems more sensitive than the lens, which is not surprising, as it gets the first chance to absorb damaging UV. The biochemical rejection of the hypothesis coincided with the re-start of the epidemiological studies. Most of these are simply latitude studies and are no more than a repeat of what was available sixty years ago. They do not help to find a cause. Two studies showed that cataract was less common at higher altitude in the Himalayas, but unfortunately led to opposing conclusions. On the basis of common knowledge that UV exposure was greater at higher altitude, the first altitude study led to the rejection of the sunlight hypothesis.(ABSTRACT TRUNCATED AT 250 WORDS)