In the case of a cataract, the body’s own lens starts to cloud slowly. The natural lens (crystalline lens) consists of proteins, which are densely packed and thus regularly ensure the natural transparency of the lens. Protective proteins (so-called crystallines) ensure that the lens fibrils are arranged regularly. If these protective proteins fail due to oxidative stress then the proteins agglutinate which in turn leads to a progressive loss of transparency – an opacity, clouding of the lens, progresses. This is more or less a natural aging process.
In most cases (about 90%) a cataract is due to aging, the term used for this is senile cataract or age-related cataract. A cataract can also be caused by metabolic diseases such as Diabetes mellitus or lack of calcium (hypocalcemia), hyperparathyroidism or galactosemia. Inflammations within the eye, eye injuries or previous eye surgery, as well as the prolonged use of certain medication (such as cortisone) may lead to the early onset of a cataract.
In rare cases, a cataract may be congenital. Rubella during pregnancy may for example cause cataracts in babies.