There are three types of age-related cataract; nuclear (the most frequent), cortical and posterior subcapsular (least common). The three forms may occur independently but are often mixed with more than one type present when cataract develops, particularly when it is advanced. The Blue Mountains Eye Study (BMES) and other population-based studies have shown that each of these three types of cataract has a different pattern of risk factors, apart from the major influence of increasing age.
Nuclear cataract develops in the nucleus or centre of the lens. As it increases, there is an associated yellow or brown discolouration of the lens.
The BMES demonstrated that smoking, heavy alcohol consumption, sunlight exposure and diabetes increased the risk of nuclear cataract. Nutritional factors appeared to reduce the risk. Higher levels of protein, vitamin A (including beta-carotene) and B-group vitamins (thiamin, niacin and riboflavin) in the diet were associated with around a 40% reduction in the frequency of nuclear cataract.
Cortical cataract develops in the outer shell of the lens as spokes and wedges and commonly causes increasing glare sensitivity.
The BMES identified a number of associations with cortical cataract, including some vascular factors. A history of diabetes or previous heart attack, and a blood factor (fibrinogen) associated with vascular conditions appeared to increase the risk, while regular alcohol consumption and higher polyunsaturates in the diet decreased a person's risk. Women had around 20% more cortical cataract than men at each age, though those who took hormone replacement therapy appeared to have a lower risk.
Posterior subcapsular cataract develops at the back of the lens, often in the visual axis, and so affects vision rapidly and severely. The majority of people needing cataract surgery have some posterior subcapsular cataract present at that time.
The BMES demonstrated many important associations with posterior subcapsular cataract, including that the use of steroid therapy, particularly long-term use of inhaled steroids (Becotide and others) increased the risk of cataract in a moderate, dose-related manner. These findings were published in the New England Journal of Medicine, New Scientist, and elsewhere.
Other associations with posterior subcapsular cataract included long-standing myopia, diabetes, higher salt intake, smoking, blood coagulation factors (fibrinogen) and kidney disease.
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