Diabetes mellitus can affect lens clarity, as well as the refractive index and accommodative amplitude of the lens. As the blood glucose level increases, so also does the glucose content in the aqueous humor. Because glucose from the aqueous enters the lens by diffusion, glucose content in the lens will likewise be increased. Some of the glucose is converted to sorbitol, the sugar alcohol of glucose, by the enzyme aldose reductase. Sorbitol is metabolized slowly by the lens and accumulates in the lens cell cytoplasm. The resulting increase in osmotic pressure may cause an influx of water, which leads to swelling of the lens fibers. The state of lenticular hydration can affect the refractive power of the lens. Patients with uncontrolled diabetes may show transient refractive changes owing to large changes in their blood glucose level. Acute myopic shifts may indicate undiagnosed or poorly controlled diabetes. People with diabetes have a decreased amplitude of accommodation compared to age-matched controls, and presbyopia may present at a younger age in patients with diabetes than in those without.
Cataract is a common cause of visual impairment in patients with diabetes. Acute diabetic cataract, or snowflake cataract, consists of bilateral, widespread subcapsular lens changes of abrupt onset, typically in young people with uncontrolled diabetes mellitus (Fig 5-16). Multiple gray-white subcapsular opacities that have a snowflake appearance are seen initially in the superficial anterior and posterior lens cortex. Vacuoles and clefts form in the underlying cortex. Intumescence and maturity of the cortical cataract follow shortly thereafter. Researchers believe that the underlying metabolic changes associated with the acute diabetic cataract in humans are closely allied to the sorbitol cataract studied in experimental animals. Although acute diabetic cataracts are rarely encountered in clinical practice today, any rapidly maturing bilateral cortical cataracts in a child or young adult should alert the clinician to the possibility of diabetes mellitus.
Diabetic patients develop age-related lens changes that are indistinguishable from nondiabetic age-related cataracts, except that these lens changes tend to occur at a younger age in patients with diabetes than in those without the disease. The increased risk or earlier onset of age-related cataracts in diabetic patients may be a result of the accumulation of sorbitol within the lens and accompanying changes in hydration, increased nonenzymatic glycosylation (glycation) of lens proteins, or greater oxidative stress from alterations in lens metabolism.
, SmiddyWE, eds. Diabetes and Ocular Disease: Past, Present, and Future Therapies. San Francisco: American Academy of Ophthalmology; 2000:49–53, 266.