While we know that hereditary, environmental, and lifestyle factors promote clouding or opacification of the lens of the eye, we still do not know how to prevent it. Therefore, the management of cataract still consists of removing the clouded lens and implanting a synthetic one.
Fortunately, this is now a common outpatient procedure requiring minimal anesthesia. Patients referred for cataract surgery can expect a rapid recovery and substantial improvement in visual acuity. We present here a brief overview of cataract and current techniques to manage it.
THE IMPACT OF CATARACT
Cataract is the most common cause of reversible vision loss worldwide,1 and its prevalence is increasing as our population ages.
In the United States, several large population-based studies have examined the prevalence of visually significant cataract. The Framingham Eye Study2 found that age-related cataract caused degradation of vision to 20/30 or worse in 15.5% of the population as a whole and in 45.9% of people over age 75. The Beaver Dam Eye Study3 used a similar definition of vision loss and found the frequency to be 38.8% in men and 45.9% in women over age 74.
Each year, almost 1.5 million people in the United States have cataract surgery, at an estimated cost of $3.4 billion. However, the cost of not treating cataract is much greater if one considers the effects of cataract-related vision loss on the ability to work and to function independently.
WHAT CAUSES CATARACT?
The lens tends to lose its clarity with age, as well as in association with certain genetic factors, developmental abnormalities, metabolic disorders, medications, and trauma. As yet, we cannot prevent or reverse the clouding.
Data from studies of families and twins provide strong evidence that heredity plays a role in age-related cataract formation. Genetic factors likely account for 50% to 70% of cataract cases and are important in the development of both nuclear and cortical opacities (see discussion of types of cataract below).4,5 Other known risk factors are smoking, sunlight exposure, diabetes mellitus, and the use of corticosteroids.6–9 Alcohol, nutritional supplements, and other drugs are also under study as possible risk factors for cataract.10
Public campaigns to encourage smoking cessation and protection from ultraviolet B light may be useful strategies for delaying the onset of cataract.11 However, the population-attributable risk of these factors is quite low. In interventional trials, antioxidants have not shown clear efficacy in preventing cataract.12 Therefore, in lieu of actually preventing cataract from developing, the focus is on preventing visual disability from cataract by detecting it early and treating it surgically.
TYPES OF CATARACT
There is no universal classification system for cataract. Many types of congenital and developmental cataract exist, and most are recognized in childhood or early adulthood. In contrast, most cases of age-related cataract fall into one of three categories: nuclear, cortical, and posterior subcapsular. Each type has a characteristic set of features and appearance, and two or more types may coexist in the same patient.
With aging, the center or nucleus of the lens hardens and becomes yellowed, owing to the addition of lens fibers. This process, called nuclear sclerosis, is normal. Nuclear sclerotic changes progress slowly over the course of years.
When this type of cataract becomes visually significant, it can result in a myopic (nearsighted) shift in refraction. Therefore, many patients with nuclear cataract have a greater loss of distance vision than of near vision. Many retain the ability to read the newspaper but cannot pass the motor vehicle bureau vision test.
Cortical cataract occurs when discrete opacities form within the outer fibers of the lens (the cortex). These aging changes typically are not visually significant unless they obscure the visual axis. Cortical cataract often causes glare and light scatter during activities such as driving.
Posterior subcapsular cataract
In posterior subcapsular cataract, granular opacities develop within the posterior cortex of the lens. It often occurs in younger patients and causes greater difficulty with near vision than with distance vision. In addition, many patients describe difficulty with glare. This is the type of cataract associated with diabetes mellitus or with corticosteroid use.