Visual
impairment
Macular degeneration
[Cataract]
Glaucoma
Diabetic retinopathy
Cataract
What is Cataract?
Cataract is a term describing opacity that develops in the normally
clear lens of the eye. Opacification in the lens causes scattering of
light as it passes through the lens, leading to both reduced vision and
also an increased perception of glare or sensitivity to bright light.
Cataract formation only affects the lens of the eye and does not involve any other eye structures, such as the retina or optic nerve. Early symptoms include glare and sensitivity to bright light or halos appearing around lights, particularly when driving at night. When severity of cataract increases, vision becomes more blurred and colours become duller and darker.
There are no proven medications that can delay or prevent cataracts, although a number of cataract risk factors have been identified, including many that are avoidable.
The development of cataract is part of the normal ageing process. From the age of 40, the proportion of people with cataract doubles with each decade. By their eighties, 80% of people will have some cataract present, including 30% who have had cataract surgery on one or both eyes. Cataract surgery is now the most frequent operation performed in Australia and is highly successful in restoring sight in most cases. Significantly improved vision occurs in around 95% of surgical cases.
Cataract surgery is usually performed as a day case, under local anaesthesia, with light sedation. The newer phacoemulsification operation is now performed in almost all cases. In this procedure, the cataract is fragmented by a high frequency ultrasound and the lens fragments are removed.
Types and Risk Factors
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.
