Cataracts form when the normally transparent proteins and structural components of the crystalline lens undergo changes that reduce its transparency. The dominant cause is ageing: after approximately age 40, lens proteins progressively undergo chemical and structural alterations, including aggregation and changes in their organisation. The precise molecular sequence is complex and involves oxidative stress, altered protein maintenance, changes in lens hydration and other age-related biochemical processes.
As proteins clump and the lens becomes increasingly opaque, light is scattered rather than passing cleanly through the lens to the retina. The opacity may begin as a small area and enlarge over time. Thus, the fundamental immediate cause of a cataract is loss of lens transparency, while ageing is the major underlying context in most cases. Researchers continue to investigate the detailed mechanisms because not every person develops visually significant cataract at the same age or at the same rate.
Ageing-related cataract is not caused by the lens simply becoming “dirty” or by a removable substance accumulating on the outside of the eye. The lens is a living, metabolically active tissue with a highly specialised architecture designed to remain transparent. Lens fibres contain crystallin proteins arranged in ways that minimise light scattering. With age, proteins can become damaged, aggregate and lose the precise organisation required for transparency.
Oxidative damage is particularly relevant because the lens has limited capacity for replacing mature lens fibre cells. Once these cells have differentiated, they retain their structural components for very long periods. Consequently, cumulative molecular damage can progressively impair transparency. The resulting cloudiness may also cause the lens to become more yellow or brown, which explains changes in colour perception and some aspects of glare.
Several medical conditions increase cataract risk. Diabetes is important because chronically elevated glucose can alter lens metabolism and promote biochemical changes that contribute to opacity. Certain systemic and ocular diseases can also influence cataract formation, although the exact pathways vary. A family history indicates that inherited biological susceptibility can affect the age of onset or pattern of lens changes.
Cataracts may also occur after previous surgery for another eye disorder, including glaucoma, because ocular surgery can alter the environment of the lens. Childhood and congenital cataracts can have genetic or developmental causes and should not simply be regarded as accelerated versions of ordinary age-related cataract. These distinctions are clinically useful because identifying an underlying systemic or ocular cause can affect monitoring and management beyond treating the lens opacity itself.
Physical and environmental exposures can accelerate or precipitate cataract formation. A serious eye injury can directly damage the lens and produce a traumatic cataract, sometimes with an appearance or distribution that differs from age-related opacity. Radiation exposure can also damage lens tissue, which is why previous therapeutic radiation is a recognised risk factor. Prolonged ultraviolet exposure is associated with increased risk, reflecting cumulative environmental stress on ocular tissues.
Smoking is another established modifiable risk factor, with tobacco smoke contributing to oxidative stress and other biochemical effects. Excessive alcohol consumption has also been associated with increased risk. These associations do not mean that every exposed person develops cataracts or that eliminating one factor guarantees prevention; they indicate that environmental and behavioural factors can modify the probability or timing of lens opacification.
Medicines can contribute to cataract formation, with corticosteroids being one of the best-established medication-related associations. The risk depends on factors such as dose, duration, route and individual susceptibility, and the characteristic pattern may differ from that of ordinary nuclear ageing changes. This does not mean people should stop prescribed corticosteroids independently, because the medication may be necessary for controlling serious disease and abrupt withdrawal can itself be dangerous with some treatments.
Instead, known cataract risk is one reason clinicians consider long-term medication exposure when assessing eye health. Other medical treatments can also affect the lens under particular circumstances, especially radiation-based therapies. A medication history is therefore an important part of understanding cataract risk rather than treating cataract as an isolated ocular phenomenon.
The anatomical location and pattern of the opacity reflect different mechanisms and risk factors. Nuclear cataract involves the central lens nucleus and is strongly associated with ageing; cortical cataract involves the lens cortex and can produce spoke-like or wedge-shaped opacities; posterior subcapsular cataract develops near the back surface of the lens and can interfere substantially with the central visual pathway. Posterior subcapsular changes are particularly associated with certain risk factors, including corticosteroid exposure, although associations are not absolute.
Traumatic cataracts may have distinctive shapes depending on the mechanism of injury. Congenital cataracts can affect particular lens regions depending on their genetic or developmental cause. These classifications are more than descriptive labels because the location of opacity helps clinicians understand visual symptoms, progression and potential causation.
Some commonly proposed causes are misconceptions. Ordinary reading, using computers or smartphones, and normal close work do not constitute established primary causes of age-related cataract, although prolonged screen use can cause temporary visual discomfort for other reasons. Cataracts are also not caused by a superficial layer that can be washed away, and ordinary eye drops cannot remove an established lens opacity.
The progression of cataract is influenced by biological ageing and risk factors rather than simply by how much a person “uses” their eyes. Similarly, the presence of cataract does not necessarily mean severe visual impairment is inevitable in the immediate future; many cataracts remain mild before becoming functionally significant. The important distinction is between factors that increase risk and factors that guarantee disease, because cataract formation is multifactorial rather than governed by a single universal trigger.
Prevention is therefore primarily about reducing modifiable risks rather than completely eliminating cataract formation. Avoiding smoking, protecting the eyes from excessive ultraviolet exposure, managing diabetes and other relevant systemic conditions, and using prescribed medicines appropriately can reduce some risks. Sunglasses or other UV-protective eyewear can reduce ocular ultraviolet exposure, particularly in environments with substantial sunlight. Avoiding serious eye trauma through appropriate eye protection is also relevant for occupational and recreational activities.
Nevertheless, age-related biological change remains the dominant driver, so prevention cannot guarantee that cataracts will never occur. Once a cataract becomes visually significant, the definitive established treatment is removal of the cloudy lens and replacement with an intraocular lens rather than attempting to reverse the protein aggregation pharmacologically.
