The lens in the eye is an unique physical structure that cannot be repaired or replaced, but instead accumulates any damage that may occur to it over time. About 90% of the material in the lens consists of long stringy proteins laid out like cord wood to allow nearly perfect light transmission. Damage to this precise structure can lead to a cataract.
Cataracts are lens opacities that cause 50% of the blindness in underdeveloped countries. In the U.S., cataract surgery is already the most costly surgical expense: some $3.2 billion each year.
Cataracts come in a variety of types. The most common type, called a senile cataract, becomes a vision problem almost exclusively in those over the age of 60, although the underlying damage begins decades earlier. Senile cataracts often start as a discoloration of the lens, with loss of vision occurring as this localized structural damage enlarges to form a distinct opacity.
Diabetes, with its environment of high blood sugars, raises the risk for senile cataract about 40%. Besides the normal stress of aging and an acceleration of damage created in a high blood sugar environment, senile cataracts also appear to be hastened by exposure of the lens to ultraviolet radiation from sunlight, with cataracts being found more commonly near the equator. Cataracts are expected to become even more numerous if loss of the protective ozone layer continues as predicted, because the ozone layer protects us from ultraviolet radiation.
Cataracts are also tightly linked to leakage of calcium and sodium through damaged cell membranes, and to oxidative damage in the lens, and to systemic diseases and poor diets which weaken the antioxidant defenses which the lens needs to protect itself. Good blood sugar control, interestingly, strengthens antioxidant defenses and protects cell membranes from leakage.
Another major risk factor for cataract is another treatment called a vitrectomy which is used to save vision. This procedure involves the surgical removal of the clear vitreous gel in the eye. In one study, 63% of eyes that had had a vitrectomy developed cataracts compared to only 4% in the non-vitrectomised eye. Fortunately, vitrectomy is only used to prevent a more serious form of vision loss, and when treatment is recommended by an ophthalmologist it is clearly necessary.
A different type of cataract, called a sugar cataract, is found only in diabetes. This type occurs at any age but is most flagrant when it occurs in young adults in their twenties who are in very poor control of their Type 1 diabetes. Sugar cataracts can grow rapidly with complete loss of vision in the affected eye in as little as 3 days. Poor blood sugar control activates the aldose reductase pathway in which glucose is converted in one step to sorbitol. Because sorbitol breaks down very slowly in the body, it accumulates in the lens and attracts so much water that the precise structure of the lens needed for vision becomes damaged, and a sugar cataract gets formed. Again blood sugar control is preventive.