Astigmatism is a condition that usually begins in childhood and lasts throughout a person’s life. It is a condition where the eye has developed a focussing mechanism that is not spherical. The curvature is not uniform in the different meridians of the eye – the best analogy to imagine that the front of the eye is shaped like a lemon instead of an orange, or as a football (soccer ball) instead of a rugby ball.
Astigmatism affects vision at all distances. Depending on the degree of the astigmatism, and whether it is combined with myopia or hypermetropia, people with this condition may be able to see reasonably well or have very poor clarity.
The amount of correction required for astigmats depends on the difference of focussing power between the strongest meridian and the weakest. For example, most people require about 60 units of refractive power (called dioptres) in the total eye for them to be able to see objects in the distance with perfect clarity and comfort. However if a person has 61 units of power instead of the required 60 units along the horizontal meridian and 58 units along the vertical meridian, their vision is astigmatic by three units and their astigmatic refractive error is said to be -3.00D (“minus three dioptres”). This person would probably need correction fairly constantly to see with clarity and comfort at all distances.
A person with 60 units of power along one meridian and 59 units instead of the required 60 along the other meridian has an astigmatic refractive error of -1.00D. Depending on age, this person may be able to function quite normally in the visual world but would probably benefit from a correction for specific tasks requiring very sharp vision.
A person with a difference of 5 units of power between their strongest and weakest meridians has an astigmatic refractive error of -5.00D. This person would usually require their correction all the time, as vision without correction would usually be quite blurred.
There has been less research into what actually causes astigmatism than there has for myopia. Unlike myopia, which can progress in response to the way it is corrected, astigmatism seems to be fairly constant in a person’s life although it can change in response to various factors. Major changes in astigmatism should be investigated as soon as possible to check for the presence of keratoconus.
There are various ways to correct astigmatism. The traditional forms of correction, spectacles and toric contact lenses (especially rigid gas permeables) are still the main ways of correcting this condition although small amounts of astigmatism can be corrected by the newer technologies made possible through orthokeratology and laser refractive surgery. All forms of correction have their advantages and disadvantages, which our optometrist at Vision Excellence at Castle Hill will discuss with you after carefully measuring the refractive error caused by your astigmatism.
An older person who is about to have cataract surgery should discuss post-operation outcomes with their surgeon before having the procedure. Technology now allows toric implants to be possible, meaning it may be possible to reduce the amount of astigmatism during cataract operation and should be discussed with the surgeon prior to operating.