A cataract is any opacity of the lens, either localized or generalized. Cataracts are the single largest cause of blindness in the world, blinding an estimated 17 million people.
Types include:; Age-related ("senile") - over 90%; Congenital - 1/250 newborns, 10-38% of childhood blindness; Toxic/nutritional; Systemic disease associated e.g., myotonic dystrophy, atopic dermatitis; Metabolic - diabetes (accelerated sorbitol pathway), hypocalcemia, Wilson's disease; "Complicated" - secondary to associated eye disease, e.g., uveitis (juvenile rheumatoid arthritis, sarcoid, etc.). Also secondary to occult tumor (melanoma, retinoblastoma).; Trauma - heat (infrared), electrical shock, radiation, concussion, perforating eye injuries, intraocular foreign body.
Age-related cataracts are caused by the continual addition of layers of lens fibers throughout life which creates a hard, dehydrated lens nucleus which impairs vision (nuclear cataract). Aging alters biochemical and osmotic balance required for lens clarity, and the outer lens layers hydrate and become opaque, affecting vision.
The cause of congenital cataracts is usually obscure and includes:
Drugs (corticosteroids in first trimester, sulfonamides, etc.);
Metabolic - diabetes in mother, galactosemia in fetus;
Intrauterine infections during the first trimester of pregnancy (rubella, herpes, mumps); and
Other cataract types have in common that a biochemical/osmotic imbalance disrupts lens clarity. Local changes in lens protein distribution lead to light scattering manifest as lens opacity.
Lab tests for cataracts are useful to identify underlying causes, which is often diabetes.
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