How cataract affects eye lens

0
63
How cataract affects eye lens

Eye cataracts are common causes of loss of vision in the elderly. Learn how a cataract affects the eye lens.

An eye cataract is a very common eye condition that nearly everyone develops after a certain age. While for most people a cataract means an eye operation, it is still a leading cause of blindness in many developing countries. Why does this eye problem occur with advancing age? How exactly does it affect the eye? Read on to find out the story behind eye cataracts.

What is a Cataract

The word is derived from the Latin word ‘cataracta’ which means waterfall, probably for the clouding of vision it causes. Cataracts only affect one part of the eye – the crystalline lens. Each eye is equipped with a biconvex, transparent lens that plays a very important part in focusing light rays entering the eye onto the retina. The lens has the capacity to alter shape so that the eye can have both near and distant vision.

The important thing here is that the eye lens is transparent. It has to be, in order to allow light rays to enter the eye. Cataracts cause the eye lens to become opaque, blocking light entry and leading to loss of vision.

How the Eye Lens Becomes Opaque

There are many factors that help the crystalline lens maintain transparency. These are:

  • The lens has no blood vessels running through it
  • Its cells are tightly packed together
  • It has special arrangement of its proteins and fibres
  • It maintains a relative state of dehydration (requiring a fine water-salt balance)
  • Autooxidation

Conditions that disturb any of the above factors, especially the critical water-salt balance or the lens protein arrangement, could lead to a loss of transparency.

Advancing age brings with it many biochemical changes in the body. In the lens, there is a decrease in the protein levels. There is also a decrease in potassium levels with a corresponding increase in sodium levels, this scenario being the reverse of the normal state. This leads to disturbance of the water-salt balance and cause hydration or water-logging of the lens.

Other age-related changes like nuclear sclerosis, compaction of the nucleus, increase in water-insoluble protein content and pigment deposition may also contribute to the loss of transparency as does age-related oxidative damage.

Eye Changes in Cataract

  • In the initial stages, water-logging causes lens fibers to be separated by fluid. Gradually, small areas of detectable opacities develop with clear areas between them. These opacities occur in different regions of the lens in different types of cataracts.
  • The cataract gradually ‘matures’ with the opaque area increasing in size. The water-logging may cause the lens to sometimes become swollen, a condition called ‘intumescent cataract’. In the final stage the lens becomes completely opaque, the stage of the ‘mature senile cataract’. This stage may sometimes be followed by ‘hypermaturity’ where the lens fibres just liquefy inside the capsule of the lens or the lens becomes shrunken and wrinkled with leakage of water.

As the lens goes through these stages of opacification, the person develops visual disturbances that can go on to blindness. This condition is treated by eye surgery where the opaque lens is removed and replaced with an IOL or intraocular lens.

Cataracts

Cataracts are the most common cause of blindness worldwide. In the United States, cataracts lead to visual impairment more frequently than any other cause, but—since most of them are treatable—cataracts do not typically lead to permanent blindness in this country. (Congdon N, et al., for the Eye Diseases Prevalence Research Group. Causes and prevalence of visual impairment among adults in the United States. Arch Ophthalmol. 2004;122(4):477-85)

What is a Cataract?

  • The lens—the primary focusing apparatus of the eye—is a transparent, elliptical, lamellar structure (similar to a tightly-packed onion) that is enclosed within a thin, clear capsule at the front of the eye.
  • Aging, disease, and environmental factors reduce the circulation, plasticity, and clarity of the lens. Opacities form within the layers of the lens and, over several years, coalesce to form cataracts.
  • Depending on the location of a cataract within the lens, various degrees of visual impairment may result. Cataracts that form beneath the posterior lens capsule, for example, disproportionately impair vision because the opacity is situated at a point where incoming light rays cross before proceeding to the retina.

What are the Signs and Symptoms of a Cataract?

  • Gradual, painless visual blurring
  • Decreased visual acuity (particularly in bright light or when reading)
  • Impaired color perception
  • Decreased contrast sensitivity
  • Glare (especially from bright lights, such as oncoming headlights)

Temporarily enhanced near vision (as a lens opacifies, it causes increased near-sightedness; far-sighted individuals who need eyeglasses to see close objects may experience a period of “second sight”)
Rarely, a cataract will swell and block the drainage of fluid from the anterior chamber of the eye, causing closed-angle glaucoma and pain

What are the Risk Factors for Cataracts?

  • Age (85% are age-related)
  • Gender (women are affected more frequently than men)
  • Corticosteroid use
  • Alcohol
  • Systemic disease (diabetes, autoimmune illness, etc.)
  • Ultraviolet radiation (particularly UV-B)
  • Smoking
  • Eye trauma
  • (From Rosenberg E, Sperazza L. The visually impaired patient. Am Fam Phys. 2008;77(10):1431-36)

How is a Cataract Diagnosed?

Cataracts can be detected during clinical examination in a doctor’s office. Routine examination with a hand-held ophthalmoscope will reveal most cataracts (even those that don’t cause noticeable visual impairment).

Slit-lamp examination—usually performed by an ophthalmologist or optometrist—can provide more details about a cataract’s size and location within the lens.

How are Cataracts Treated?

  • As cataracts develop, patients often require new glasses or frequent changes in old prescriptions.
  • Polarized lenses reduce glare caused by cataracts that don’t yet require surgery.
  • Indirect lighting during reading or other close-up work reduces pupillary constriction and maximizes vision.
  • Cataracts that cause significant visual impairment (best corrected vision is <20/40 or vision is insufficient for accomplishing necessary or desired activities) are candidates for surgery.
  • Modern cataract surgery involves the removal of the entire lens and usually the placement of a silicone or plastic lens to replace the optical power lost by removal of the original lens.
  • Recent developments in intraocular lens technology (e.g., Rezoom or ReSTOR lenses) allow patients to see at all distances, thus reducing dependence on accessory reading glasses.
  • Cataract surgery is typically performed under local anesthesia and IV sedation; patients often remain awake during the procedure, which may last 15 – 20 minutes.
  • Topical herbal preparations, DMSO, and other alternative therapies have not yet shown benefit in the treatment of symptomatic cataracts.

Prevention of Cataracts

  • Cumulative ultraviolet light exposure (i.e., long-term sunlight exposure) is linked to the development of cataracts. People who spend time in the sun should routinely wear UV-blocking sunglasses.
  • Smoking should be avoided, as it has been associated with cataracts (in addition to a multitude of other health problems).
  • Reduce alcohol consumption.
  • Limit the use of corticosteroids whenever possible.
  • Treatment and control of chronic disease, such as diabetes, slows the development of many eye problems, including cataracts.
  • Although the Age-Related Eye Disease Study (AREDS) showed a benefit from antioxidant supplements plus zinc for patients with pre-existing age-related macular degeneration, there is not yet any compelling evidence that such supplements prevent or slow the development of cataracts. (Age-Related Eye Disease Study Research Group. A randomized, placebo-controlled clinical trial of high dose supplementation with vitamins C and E, beta-carotene and zinc for age-related macular degeneration and vision loss. Arch Ophthalmol. 2001;119(10):1417-1436 [corrected version 2008;126(9):1251])

Cataracts are a common, treatable cause of visual impairment in the US. Smoking cessation and avoidance of exposure to ultraviolet light are the major modifiable risk factors.

LEAVE A REPLY

Please enter your comment!
Please enter your name here