Presbyopia is a condition where the eye gradually loses the ability to focus at a normal reading distance. It is usually first noticed between the ages of 40 and 50 years when people find that they can no longer focus comfortably on objects closer than at arm's length. Presbyopia is not a disease and it affects everyone.
The eye increases its optical power to focus at near by altering the shape of its lens and this process is known as accommodation. The ability to accommodate decreases with age, to the point that generally the eye is no longer able to focus on close objects at a normal reading distance by about the age of 50. When this occurs, the patient is said to be presbyopic. It is thought that the most likely cause of presbyopia is a loss of elasticity of the crystalline lens inside the eye.
Presbyopia is usually corrected by a spectacle prescription that is designed for close distances. Spectacle options include single vision reading glasses (which will make near objects clear but distant objects blurry) and multifocal glasses (which incorporate progressive lenses that provide clear vision at both distance and near). Less commonly used spectacle forms include 'look-overs' or half-glasses, and bifocal or trifocal glasses.
Presbyopia can also be managed by prescription of contact lenses. The most common form of contact lens correction for presbyopia is monovision in which one eye is corrected for distance and the other eye is corrected for near. There is better than a 80% success rate with this mode of correction with the main disadvantage being a slight reduction in depth perception. Some monovision patients often require 3 contact lenses - 2 distance and 1 near - whereby they can alternate between a distance and a near contact lens in one eye, while always using a distance contact lens in the other eye. This way they can be binocular for sporting and recreational tasks while retaining the use of monovision for work and social occasions.
There are also bifocal and multifocal contact lenses that attempt to correct distance and near vision with the same lens. These contact lenses work by two possible mechanisms - simultaneous vision and alternating vision. Simultaneous vision lenses position the distance and near zones of the lens over the pupil continuously. The major problems with this type of design are patient adaptation and loss of contrast, both as a result of the simultaneous presentation of the distance and near images. Alternating vision lenses (translating bifocals) move to position the distance and near portion of the lens over the pupil during distance and near gaze. The main problem with this design is that translation of the lens on the eye is required for the patient to be able to read as the contact lens 'follows' the eye in all directions of gaze. Bifocal and multifocal contact lenses are available in both rigid and soft lens forms, and the success rate with these lenses (about 50%) is significantly less than that of monovision.
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