Cataract is a clouding of the natural lens within the eye. Usually both eyes are affected, although one may be more severe than the other. Nearly everyone will develop some clouding of the lens by age 60, but the vision may not be affected. It has been found to be the leading cause of blindness (although curable) in people over 40 years, and millions of cataract procedures are needed to be performed annually. However, not all cataracts need to be removed.
Since light must pass through the lens to reach the retina, visual disturbances are the main symptoms of cataract. Cataract within the lens of the eye distorts and blurs visual images. Possible symptoms include:
- A usually gradual blurring of the vision at distance or near which may not be correctable with glasses.
- A shift in the eye's refractive error (or glasses prescription) toward near-sightedness. Some people experience "second sight", or the ability to read without glasses at near due to near-sightedness.
- Glare symptoms, sometimes worse at night (headlights), other times worse during the day. Sensitivity to light, but not pain.
- Halos around lights and double vision (through one eye - double vision can also be caused by misalignment of the eyes, but if one eye is closed the double image disappears).
- Worsening colour vision, although this may be so gradual that it is not appreciated.
- Cataract does not cause pain (except is very advanced cases), redness, scratchy feelings, and cataract is not a "skim growing on the surface of the eye".
Many cataracts may have varying degrees of all of the above types mixed together.
Classification of Cataracts
Cataracts may be described in a number of ways including their location and their cause:
Location in the lens:
Cortical cataract. This is a clouding just inside the lens. These opacities affect the cortex of the lens, and are identified by their unique wedge or spoke appearance. They may cause glare symptoms. This type has been shown to be related to exposure to Ultraviolet radiation.
Nuclear cataract. The nucleus is the centre of the lens. As the lens ages, the nucleus gradually tends to darken, changing from its initial clear appearance to yellow, then sometimes brown. When this darkening begins to affect vision, it is termed a nuclear cataract.
Posterior capsular cataract. This often-dense cataract is located on the posterior of the lens. Due to its location, it often affects near vision more than it affects distance. It tends to affect vision more rapidly, and is common in younger eyes (under 60 especially). This type of cataract can result from long-term use of steroid medications.
Many cataracts may have varying degrees of all of the above types mixed together.
What causes cataract?
It is not fully understood what causes cataract in most cases, why one eye is often worse than the other, and what can be done to slow the worsening of cataract. Ongoing research is being directed in these areas.
Some known causes of cataract include:
- A change in the lens due to normal aging processes. By a certain age, it may be normal to develop some cataract in the lens, although the vision may not always be affected. There may be a family tendency to develop certain types of cataract at certain ages.
- Association with certain medical problems, especially diabetes.
- Association with certain medications, such as long term or high dose steroids.
- There has been established an increase risk for cataract in men who smoke, and an increase risk for cataract extraction in women who smoke.
- Cataract can occur after ocular trauma (sometimes very rapidly in severe trauma), and after intraocular inflammatory problems (iritis).
- Ultraviolet radiation. Studies have shown that there is an increased chance of cataract formation with unprotected exposure to ultraviolet (UV) radiation.
- Alcohol. Several studies have shown increased cataract formation in patients with higher alcohol consumption compared with patients who have lower or no alcohol consumption.
- Nutrition. Although the results are inconclusive, studies have suggested an association between cataract formation and low levels of antioxidants (e.g., vitamin C, vitamin E, carotenoids). Further study may show that antioxidants have a significant effect on decreasing the incidence of cataract.
When does cataract need to be removed?
A cataract does not need to be removed just because it is present; there needs to be a visual impairment present to warrant surgery. Less emphasis now is being placed on a required loss of visual acuity (6/18 vision or worse, in the past), and instead more emphasis is being placed on a loss of functional ability related to visual loss. A cataract does not have to be "ripe" before is can be removed. In other words, one does not have to wait until they have a complete loss of vision before a cataract can be removed.
It is important to have a complete, dilated eye examination, in order to determine if cataract is present, and if it is causing any loss of vision.
The examiner will ask vision related questions regarding daily activities, such as difficulty driving, reading, working, enjoying hobbies, or trouble with glare. These questions help to understand the exact nature of any visual problems that the patient is having.
In many instances, a simple change in glasses prescription can substantially improve the vision, and cataract surgery can be deferred. However, quality of vision problems, such as glare, may remain in spite of new glasses.
After the complete examination, if there is an uncorrectable (with glasses) loss of vision, the examiner should have a good idea as to whether or not cataract is the cause, and whether or not removing the cataract would help to restore the vision. Other eye problems such as corneal disease, glaucoma, retinal problems, or optic nerve problems may limit a full recovery of vision. Other tests may be needed to help to sort this out.
Cataract surgery is considered if a loss of vision is caused by the cataract, and if it seems that improvement would occur in vision with surgery. Generally, if a loss of vision, or other cataract related visual side effects (such as glare) are interfering with a person's lifestyle, cataract surgery is considered.
How are cataracts removed, and what are the risks?
Cataract extraction is a highly refined and successful surgical procedure using state-of-the-art technology. The goal of the surgery is to allow a return of vision as fast as possible and without restricting a person's lifestyle during the recovery period.
Cataract surgery is done as an outpatient operation in an operating room. A person has dilating drops placed in the eye prior to surgery. Anaesthesia is a combination of sedation with local anaesthesia. A person does not have to be "put to sleep" for the operation, but many patients will fall asleep during the operation.
The operation usually takes 30 to 60 minutes to perform.
The cloudy portion of the lens (the cataract) is removed from the eye using a technique known as phacoemulsification. This uses ultrasound to break apart the lens, which is then aspirated using fluid suction. Lasers are not used to remove a cataract at this point. A lens implant is then placed back into the eye where the old lens used to rest.
This implant is a plastic lens of a particular power to help to minimize the need for glasses after the operation. The implant does not need to be exchanged or removed, except in extremely rare circumstances. The opening through which the surgery is done (millimeters in size) may or may not need to be closed with fine dissolvable sutures.
The patient usually returns home within one to two hours after the completion of surgery. Eyedrops are prescribed to help with the healing process. At about a month after the operation, glasses can be prescribed, if needed. If the second eye needs to be operated on, this is usually done 4-6 weeks at the earliest after the first operation.
Generally, cataract surgery is highly successful. However, there are some risks:
- With any surgery, there is risk of infection and bleeding. This is very rare with cataract surgery.
- There is risk that the eye may not be able to see as well as predicted, due to an addition problem with the eye not previously seen.
- There is risk that an unexpected glasses prescription will result, although this is becoming rare with improved equipment used to determine the power of the lens implant.
- There is a chance that the membrane that the lens implant rests on in the eye may become cloudy with time. If this cloudiness begins to affect vision, this membrane can be opened using a laser in a 10 minute procedure done outside of the operating room.
- There is a low (about 1%) risk of retinal detachment after cataract surgery. There is also a slight risk of a usually temporary swelling of the retina after surgery.
- There may be other risks in people with diabetes, glaucoma, macular degeneration, and other eye conditions.
How cataracts affect vision
When vision is clear, the lens of the eye focuses light on the back of the eye, or the retina, which transmits the image through the optic nerve to the brain. When the lens is clouded by a cataract, light scatters so the lens can no longer focus it properly, causing vision problems.
Especially in age-related cataracts, vision disruption can be very gradual. As cataracts worsen, visual symptoms tend to increase in severity. However, some patients may not recognize the change in symptoms because of a cataract’s slow progression.
Cataracts—especially nuclear cataracts—can also affect one’s ability to accurately differentiate colour. As proteins in the eye age, they can develop a yellow or brown tint, causing vision to appear slightly tungsten. While this tinting may not affect the sharpness of one’s vision, it can lead to trouble with reading or determining the difference between darker colours.
The biological processes of cataract formation are becoming more clearly understood, but there is still no clinically established treatment to prevent or slow the progression of cataract. Research on the prevention of cataract has centred on risk factors and the control of diseases, such as diabetes, which are associated with some types of cataracts.
Cataract is a multi-factorial disease; different factors are associated with the development of different types of opacities. Nevertheless, a simple, low-cost and low-risk preventive strategy is to:
- reduce exposure to sunlight eg wear tinted spectacles
- decrease or discontinue smoking
- increase antioxidant vitamin intake through consumption of leafy green vegetables and/or supplements.