Cataracts are opacities or loss of optical transparency within the crystalline lens of the eye that interfere with the transmission and focusing of light onto the retina. The lens is normally transparent and changes shape to assist accommodation, but ageing and other influences can alter its proteins, hydration, structure and optical properties, producing areas of clouding.
Cataracts can occur in different anatomical regions, including nuclear, cortical and posterior subcapsular patterns, and different types can produce different symptoms and rates of visual deterioration. Common effects include blurred or hazy vision, reduced contrast, glare, halos around lights, difficulty seeing at night, faded colours and changes in refractive prescription. Cataracts are primarily associated with ageing but can also arise earlier because of trauma, certain diseases, medications such as prolonged corticosteroid exposure, metabolic factors, congenital causes or other ocular conditions. A cataract is not a growth on the surface of the eye and is not simply a film that can be washed away. It develops within the lens itself.
The crystalline lens is composed largely of highly organised proteins and cells arranged to maintain transparency and refractive function. Transparency depends on precise cellular architecture, protein organisation, water balance and the absence of significant light-scattering structures. With ageing, lens proteins undergo chemical modification, aggregation and other changes that can increase light scattering. Oxidative stress and alterations in lens metabolism also contribute.
The lens has limited ability to repair or replace its central fibre cells because mature lens fibres lose their nuclei and much of their cellular machinery. As a result, cumulative molecular changes can persist and progressively affect optical quality. Nuclear Cataracts involve progressive changes in the central lens and can produce gradual yellowing or browning as well as increased density.
Cortical Cataracts involve spoke-like or wedge-shaped opacities in the lens cortex, while posterior subcapsular Cataracts occur near the rear surface of the lens and can disproportionately affect near Vision and glare. The location and density of opacity therefore matter as much as the simple presence of a cataract.
Symptoms of Cataracts usually develop gradually, although some patterns can progress more quickly. Early changes may be subtle, with a person noticing that glasses no longer provide the same clarity, colours seem less vivid or bright lights produce more glare. Night driving can become difficult because headlights create disabling glare or halos and contrast sensitivity falls. Vision may seem blurred even when standard high-contrast acuity remains relatively preserved, because cataracts can degrade image quality through scattering and reduced contrast.
Some nuclear cataracts initially produce a myopic refractive shift, sometimes temporarily improving near reading without glasses in a person who previously required distance correction. This phenomenon does not represent improvement in lens health and may later be followed by substantial visual deterioration. Cataracts can affect one eye more than the other, although age-related cataracts often develop bilaterally over time. Symptoms are not specific to cataracts, so other retinal, corneal, optic-nerve or refractive problems need to be considered when visual function changes.
Risk factors for Cataracts include increasing age, ultraviolet exposure, smoking, diabetes, certain medications, previous eye trauma, intraocular inflammation and some genetic or congenital conditions. Prolonged exposure to ultraviolet radiation may contribute to lens protein and tissue damage, which is one reason appropriate ocular Photoprotection is relevant. Diabetes can alter lens metabolism and is associated with increased cataract risk, while corticosteroids can produce characteristic posterior subcapsular changes.
Trauma can cause lens opacity immediately or years after the event depending on the injury. Cataracts can also be present at birth or develop during infancy and childhood, where their consequences can be particularly serious because visual development depends on receiving clear images. The presence of a risk factor does not guarantee cataract development, and age remains a major determinant for common acquired forms. Prevention cannot eliminate all age-related cataracts, but reducing avoidable risks can contribute to ocular health.
The definitive treatment for visually significant Cataracts is surgical removal of the cloudy natural lens and replacement with an intraocular lens. Cataract surgery involves removing the lens contents through a small incision, commonly using phacoemulsification with ultrasound energy to fragment the lens, followed by implantation of an artificial intraocular lens. Modern intraocular lenses can provide a fixed refractive power and can be designed as monofocal, toric or certain multifocal and extended-depth-of-focus configurations.
The choice depends on ocular anatomy, pre-existing refractive error, retinal and corneal health, visual priorities and tolerance of optical trade-offs. The natural lens cannot simply be made transparent again with medication once a clinically significant opacity has developed. Surgery is generally considered when cataract-related visual impairment interferes meaningfully with activities or quality of life rather than solely because an opacity is visible.
Cataract surgery is highly established but remains a surgical procedure with potential complications, including infection, inflammation, retinal problems, pressure changes, corneal complications and refractive outcomes that differ from the intended result. Modern techniques have greatly improved safety, but risk varies with individual ocular health and surgical circumstances. Posterior capsule opacification can develop after surgery when residual lens epithelial cells proliferate and cloud the capsule behind the implanted lens.
This is sometimes informally called a secondary cataract, but it is not a recurrence of the removed natural lens. It can often be treated with a YAG laser capsulotomy that creates an opening in the cloudy capsule. This distinction is important because a true cataract cannot return after the natural lens has been removed, although postoperative optical problems can have other causes.
Cataracts are often confused with normal ageing, Presbyopia and other causes of visual decline. Presbyopia results primarily from loss of accommodative flexibility and can be corrected by additional near optical power, whereas Cataracts involve loss of lens transparency and can ultimately require surgery. A new spectacle prescription may temporarily improve visual acuity when refractive change is the dominant problem, but it cannot remove significant optical scattering from a dense cataract.
Cataracts also differ from glaucoma and Macular Degeneration because the primary anatomical structure affected is the lens rather than the optic nerve or macula. A person can have more than one of these conditions simultaneously, and cataract surgery may improve lens-related blur without reversing retinal or optic-nerve damage. Therefore, the amount of visual impairment attributable to a cataract must be assessed in the context of the whole visual system.
Cataracts have substantial global significance because ageing populations increase the number of people affected, while access to surgery determines how much preventable visual disability occurs. Cataract remains an important cause of visual impairment worldwide, particularly where surgical services, trained personnel or affordability are limited. The condition illustrates the difference between disease prevalence and avoidable disability: the biological development of age-related lens opacity cannot be completely eliminated, but the resulting visual impairment can often be substantially improved through surgery. Research continues into lens biology, prevention, surgical technology and intraocular-lens design. Appropriate ultraviolet protection, smoking cessation and management of systemic risk factors may reduce avoidable risk, but no proven lifestyle strategy completely prevents ordinary age-related lens ageing.
