A cataract is a change in the clarity of the natural crystalline lens. The development of a cataract is an age-related phenomenon and is more common in elderly individuals. The etiology of the color change is likely a change in water and protein content of the lens and possibly oxidative damage. There are 3 major types of cataracts, and each tends to present with different symptoms. Typically, a cataract will cause slow progressive blurring of vision; however, a nuclear sclerotic cataract (generalized clouding of the lens) can increase the refractive power of the lens inducing myopia, allowing reading vision without the use of glasses. Cortical cataracts (spoke-like opacities in the outer layers of the lens) often cause glare with oncoming light. Posterior sub-capsular cataract (web-like opacities at the posterior pole of the lens) can be highly symptomatic with blurred vision, even when very small. Cataracts can further be classified by severity, from immature (scattered opacities) to mature (totally opaque) to hypermature / Morgagnian (swollen, starting to liquify).

Though cataract formation is a naturally occurring event, several medical conditions can predispose or accelerate formation. Patients with diabetes, hypocalcemia (hypoparathyroid), myotonic dystrophy, high myopia, and inherited metabolic conditions may form cataracts earlier in life. Ocular trauma can also rapidly induce cataract formation.

Related topic: Congenital cataract