How Cataracts Work: Causes, Symptoms, and Surgical Treatment

This article is for informational purposes only. Consult a qualified healthcare professional for medical advice, diagnosis, or treatment.

What Is a Cataract?

A cataract is an opacity or clouding of the crystalline lens of the eye, the normally transparent structure that focuses light onto the retina. As the lens becomes progressively less transparent, it scatters incoming light, degrading image clarity and reducing vision. Cataracts are the leading cause of reversible blindness worldwide, accounting for approximately 51% of global blindness and affecting an estimated 94 million people. The World Health Organization estimates that 17 million people are blind from cataracts globally. In developed nations where surgery is accessible, cataracts are highly treatable; in low- and middle-income countries, they remain the primary cause of vision impairment due to insufficient access to surgical care. In the United States, more than half of all adults over 80 either have cataracts or have had cataract surgery.

Anatomy of the Crystalline Lens

The human lens is a biconvex, avascular, and transparent structure located behind the iris. It measures approximately 9–10 mm in diameter and 4–5 mm in thickness and is suspended by ciliary zonules. The lens consists of:

• Lens capsule: A thin, elastic basement membrane surrounding the entire lens
• Anterior epithelium: A single layer of cells responsible for lens metabolism
• Lens fibers: Highly elongated, terminally differentiated cells that form the bulk of the lens. Mature lens fibers lose their nuclei and organelles to minimize light scattering.
• Lens proteins (crystallins): Alpha, beta, and gamma crystallins make up ~90% of total protein. Alpha-crystallin acts as a molecular chaperone, preventing the aggregation and precipitation of damaged proteins. This chaperone function maintains transparency.

The lens has no blood supply and relies on the surrounding aqueous humor for nutrients (glucose, oxygen, amino acids) and waste removal. All post-natal lens fiber cells remain throughout life — the lens continuously adds new cells at its equatorial periphery, compacting older fibers toward the center (nucleus).

How Cataracts Form: The Biochemical Basis

Lens transparency depends on the short-range ordered arrangement of crystallin proteins within fiber cells and the precise regulation of their concentrations. Cataract formation occurs when this order is disrupted by protein unfolding, modification, and aggregation into large light-scattering complexes.

Key biochemical events include:

• Oxidative damage: Reactive oxygen species (ROS) generated by ultraviolet radiation, metabolic byproducts, and reduced antioxidant capacity (declining glutathione levels with age) oxidize sulfhydryl groups on crystallin proteins, causing disulfide cross-linking and insoluble aggregates. This is the dominant mechanism in cortical and nuclear cataracts.
• Advanced glycation end-products (AGEs): In diabetes, elevated glucose reacts non-enzymatically with crystallins (Maillard reaction/glycation), creating protein cross-links, browning, and reduced alpha-crystallin chaperone function. This is the basis for diabetes-associated cataracts.
• UV-B radiation: Directly damages lens DNA and generates free radicals within the lens; cumulative UV-B exposure over a lifetime is a major risk factor for posterior subcapsular and nuclear cataracts.
• Water transport disruption: Aquaporin-0 (AQP0) channels maintain lens fiber cell volume and ion balance; mutations or damage to AQP0 allow sodium, calcium, and water accumulation, swelling lens fibers and creating opacities (cortical cataracts).

Types of Cataracts

Risk Factors

• Age: The primary risk factor; nuclear sclerosis is nearly universal by the eighth decade
• Ultraviolet-B (UV-B) exposure: Cumulative, lifelong UV-B exposure significantly increases cortical and nuclear cataract risk; sunglasses and wide-brimmed hats are protective
• Smoking: Doubles cataract risk; oxidants in tobacco smoke directly damage lens proteins
• Diabetes mellitus: 2–5 times higher risk; hyperglycemia-driven glycation and sorbitol pathway activity accelerate opacity formation
• Corticosteroid use: Systemic or topical (eye drops, inhaled) corticosteroids increase posterior subcapsular cataract risk; mechanism relates to steroid-induced inhibition of lens epithelial proliferation
• Myopia: High myopia increases nuclear cataract risk approximately 5-fold

Symptoms and Diagnosis

The progression of cataracts is gradual. Symptoms develop over months to years and depend on type and location:

• Blurred, cloudy, or dimmed vision
• Glare and halos around lights, particularly at night
• Difficulty with bright lights (photophobia)
• Fading or yellowing of colors
• Double vision in the affected eye
• Frequent changes in glasses or contact lens prescriptions

Diagnosis is made by an ophthalmologist using slit-lamp biomicroscopy, which allows direct visualization of lens opacities. Visual acuity, contrast sensitivity, and glare testing quantify functional impact. Biometry (optical or ultrasound A-scan) measures axial eye length and corneal curvature for intraocular lens (IOL) power calculation before surgery.

Surgical Treatment: Phacoemulsification

Surgery is the only effective treatment for clinically significant cataracts; no drops or drugs have been proven to prevent or reverse cataract formation. The decision to operate is based on the patient's visual symptoms and functional impairment, not solely lens opacity grade.

The standard procedure worldwide is phacoemulsification:

1. A small incision (~2–3 mm) is made at the corneal periphery under topical or local anesthesia
2. A circular opening is made in the anterior lens capsule (capsulorhexis)
3. An ultrasonic probe emulsifies and aspirates the lens nucleus and cortex
4. An artificial foldable intraocular lens (IOL) is inserted into the empty capsule bag and unfolds to the correct position

Surgery takes approximately 15–30 minutes and is typically performed as day surgery. Visual recovery is rapid — most patients achieve substantially improved vision within 24–48 hours. Modern IOL options include monofocal (single focus distance), extended depth of focus (EDOF), and multifocal IOLs. Femtosecond laser-assisted cataract surgery (FLACS) automates the capsulorhexis and lens fragmentation steps, improving precision. Complications are rare but include posterior capsule rupture (~1%), cystoid macular edema, and endophthalmitis (<0.1%).