Anisometropia: A Common Eye Condition
Lot of people who have to use contact lenses and glasses for vision correction find the name of anisometropia no stranger as the person who has to use contact lenses or glasses has some degree of it. Well, this is not a disease, but an eye condition characterized by the difference in refractive errors between both the eyes. The term is however significantly used when the case of the difference in the refractive error is considerably greater --around 2.00 diopters or more.
There is also a different form of anisometropia which is known as anitmetropia. It is characterised by the difference in power designations for each eye. Example, the right eye may have minus power while the left one, plus power. The refractive error is not so much a problem for the patient, but the real error problems raise their ugly heads when spectacles are used to correct vision.
Corrective lenses have a characteristic of displacing images, and it cannot be sufficiently helped. When a converging or magnifying lens is used for vision correction, the images tend to get moved to the edges of the lens. When a diverging or minifying lens is used, the displacement is towards the centre of the lens. In both the cases, the degree of displacement depends on the power of the lenses of the user. If a patient suffers from anisometropia, the displacement of the images is relatively more between the right and left eyes. While the user moves his eyeball from the centre, the displacement also increases proportionately and it can even cause double vision after a certain extent. People who suffer from presbyopia are the ones affected worst as they need to move their vision from the centre of the lens to the part through which they read or do close works, as in progressive or bifocal lenses. Sometimes, they may have to adjust and tilt their head to have a clear view through the central part of the lens.
This prism imbalance can be corrected in several ways. Bicentric grinding is one of the most used ways. In the process, the eye which has the most plus or minus pair is fitted with the lens having prism ground in half portion of the lens. The wearer can also choose to have two pairs of glasses -- for distant vision, and for close reading. People with anisometropia however prefer to wear contact lenses because of the reduced prismatic effect as compared to the spectacle lenses. The contact lens users can rotate the eyeballs and the lenses stay almost stationary reducing the prismatic effect.
Though the reason of anisometropia is not clearly known, it is thought to be congenital or some outcome of a trauma. The eye with the lesser refractive error becomes dominant and sometimes so much that the non dominant eye loses a lot of its visual capacity (amblyopia). The functionless eye starts to wander and the condition is known as strabismus.