What causes the lens to become cloudy in a cataract?

The lens stays clear because its proteins (crystallins) and fibre cells are highly ordered; with ageing and oxidative stress these proteins aggregate and the architecture is disrupted, so the lens scatters light instead of transmitting it.

Why is cataract called a treatable cause of blindness?

Unlike many causes of blindness, cataract can usually be addressed by surgically removing the opaque lens and replacing it, so the visual loss it causes is largely reversible where surgery is available.

Cataract

A cataract is an opacification (clouding) of the crystalline lens of the eye that scatters and absorbs light, progressively reducing vision. Age-related cataract is the most common form and, untreated, is one of the leading causes of blindness worldwide, while remaining highly treatable through surgical replacement of the lens.

Definition

A cataract is a loss of transparency of the crystalline lens, in which the normally clear lens fibres and their proteins become disordered and light-scattering, degrading the retinal image and reducing vision.

Scope

The entry covers what a cataract is, the main morphological types, the biological processes by which the lens loses transparency, and the global burden of cataract-related visual impairment. It treats cataract as a reference clinical topic; it does not provide surgical or perioperative instructions.

Key concepts

Crystalline lens transparency
Lens crystallin proteins
Nuclear, cortical, and posterior subcapsular cataract
Age-related (senile) cataract
Light scatter and reduced contrast
Cataract as a treatable cause of blindness

Mechanisms

Transparency of the crystalline lens depends on the highly ordered, densely packed arrangement of lens fibre cells and their soluble crystallin proteins, together with the absence of light-scattering organelles in the lens core. With ageing and other stresses (oxidative damage, ultraviolet exposure, metabolic factors), crystallins aggregate and the orderly fibre architecture is disrupted, so the lens scatters and absorbs light instead of transmitting it cleanly. Different patterns of this change underlie the principal morphological types: nuclear cataract (hardening and yellowing of the central lens), cortical cataract (wedge-shaped opacities in the outer lens), and posterior subcapsular cataract (opacity at the back of the lens that disproportionately affects near and bright-light vision).

Clinical relevance

Cataract is a leading cause of reversible blindness, and lens-replacement surgery is one of the most commonly performed and effective operations in medicine. This entry explains the condition and its public-health importance as reference material and is not a guide to indications for or conduct of cataract surgery in any individual.

Epidemiology

Cataract is consistently among the top causes of blindness and moderate-to-severe visual impairment in global estimates, with age-related cataract accounting for most cases; the burden is concentrated where access to surgical services is limited, even though the condition is treatable.

Seminal works

liu-2017
bourne-2021

Frequently asked questions

What causes the lens to become cloudy in a cataract?: The lens stays clear because its proteins (crystallins) and fibre cells are highly ordered; with ageing and oxidative stress these proteins aggregate and the architecture is disrupted, so the lens scatters light instead of transmitting it.
Why is cataract called a treatable cause of blindness?: Unlike many causes of blindness, cataract can usually be addressed by surgically removing the opaque lens and replacing it, so the visual loss it causes is largely reversible where surgery is available.