Monovision: Correcting Eyesight for Those Above 40?
After the age of 40, we begin to develop presbyopia - commonly known to the Chinese as "lao hua yan" (old age eyes). This condition becomes progressively worse as we age from 40 to 70 years old.
The decreasing flexibility of the lens in a presbyope's eye results in a limited range of focus. Near objects appear out of focus and blur. Majority of presbyopes end up with poor near vision, especially when they wear their distance correction spectacles. However, this may not be the case for myopes who develop presbyopia. Myopes have a "natural" point of focus which is close to their eyes when they do not wear spectacles. The higher the myopia, the closer the object needs to be to the eye in order to be seen clearly. This explains why many myopes take off their spectacles to read up close. LASIK surgeons can also use this effect and intentionally under-correct short-sighted presbyopes to make their point of focus 30 to 35 cm from the eye, which would allow patients to read clearly without spectacles. However, if both eyes are under-corrected, the patient's distance vision will suffer. Fortunately a compromise can be reached to allow patients to maintain both distance and near vision. The LASIK surgeon corrects one of the patient's eyes in order to see distant objects, and under-corrects the fellow eye to enable the patient to see near objects. Often the eye which is corrected for distance vision is the dominant eye (the eye we use to aim at things). Using one eye for distance vision and the fellow eye for near vision is called "monovision". Monovision has allowed many patients over the age of 45 to function without bifocals or reading glasses for many years. Many ladies are happy with this compromise because they no longer need to carry multiple pairs of spectacles, making it extremely convenient to look at price tags during a sale! However, not everyone is suitable for monovision. Some complain of unbalanced vision and loss of stereovision - the ability to see in 3 dimension (3-D). As such, monovision is not advisable for sportsmen or professional drivers as it will cause their judgement of depth and distance to deteriorate, especially in dim light.
The Aging Eye
The eye is like any other part of our body - it continually changes throughout life. At a very young age, most people are hypermetropic - they have "youthful" long-sightedness. Towards the age of 6 years, we usually attain perfect vision, with no myopia or hypermetropia. As we approach school-going age, we have a higher chance of developing myopia or astigmatism. Children should not undergo LASIK (or PRK) as their refraction is likely to still be increasing. Also, a child's healing response is generally more aggressive than an adult's, and this can lead to severe scarring and undercorrection (over-healing). Between the ages of 20 and 40, the refractionof the eye usually becomes very stable. This is the best period of time for one to consider LASIK surgery. Beyond 40 years of age, everyone begins to develop presbyopia. This causes greater difficulty in changing focus and focusing on near objects. As a result, many presbyopes require reading glasses and more light to see clearly. After the age of 45, acute distance vision in dim light is also reduced. Glare from lights is also exaggerated due to an increase in a visual distortion called spherical aberration. Spherical aberration results from changes in the shape and clarity of the human lens as it ages. Cataracts will be the next major development affecting the aging eye. The age at which people develop cataracts varies, but it usually begins to affect people around the age of 60.
Cataracts - clouding of the human lens with age - will block rays of light from entering the eyes. This makes images poor and dim. Cataracts can only be removed with cataract extraction surgery, where an artificial lens is used to replace the original lens of the eye. As a final change, many elderly patients develop astigmatism and an aberration called coma. Coma is a distortion of vision that causes lights to be smeared in one direction. It however, can help compensate for presbyopia and improve near vision somewhat. Cataracts - clouding of the human lens with age - will block rays of light from entering the eyes. This makes images poor and dim. Cataracts can only be removed with cataract extraction surgery, where an artificial lens is used to replace the original lens of the eye. As a final change, many elderly patients develop astigmatism and an aberration called coma. Coma is a distortion of vision that causes lights to be smeared in one direction. It however, can help compensate for presbyopia and improve near vision somewhat.