The reason for this is because astigmatism is measured not simply by a number (a ‘scalar’ term mathematically) but it has a specified orientation, direction or axis and is therefore a ‘vector’.
To explain: our spectacle and contact lens prescriptions are defined in terms of the basic units of optical power – dioptres (D) – and we might be told, for example, that we are “minus 3 with minus 1 unit of astigmatism”, which means that we have 3 dioptres of short-sight (myopia) and 1 unit of astigmatism. Astigmatism also has a sign (+ or -) and therefore the prescription would be written as follows:
Right Eye: -3.00DS / -1.00DC axis 100
The capitals DS mean simple ‘dioptre sphere’ (in this case myopia) and the DC stands for ‘dioptre cylinder’. Mathematically a lens (or cornea) with a cylindrical shape would generate the refractive error ‘astigmatism’ and the terms are often used interchangeably. Note that the axis or angle of the astigmatism is 100 – in degrees. In handwritten prescriptions the degrees sign is usually omitted on purpose to avoid possible confusion with a ‘0’.
In the case of the human eye, astigmatism occurs when the front surface – the cornea – is not perfectly uniformly curved, like a football. Instead it is slightly more oval in shape in one direction, like a rugby ball. A football is spherical of course and looks identical from any point of view. A rugby ball however has two curves at right angles to each other. End-on it looks circular but side-on it is oval or elliptical. These two distinct curves, that are mathematically ‘orthogonal’ or at perfect right angles to each other, result optically in two points of focus rather than one. If a person with astigmatism were to look at the spokes of a bicycle wheel for example, some of them would appear in focus but others (at right angles) would be more blurred because lines at a certain angle might be more in focus on the retina at the back of the eye.
There are, in fact, two variants of astigmatism, regular and irregular. Regular astigmatism (as described above) is the more common form where the cornea is more curved in one direction than the other in a very uniform or regular way (like the rugby ball). The two curves (or meridians) are at perfect right angles to each other. This type is easily corrected with glasses, contact lenses or laser eye surgery or lens replacement sugery.
‘Irregular astigmatism’ is less common and occurs where some irregular distortion, perhaps caused by disease or trauma with scarring, has affected the cornea. This type of astigmatism cannot be corrected fully with glasses but contact lenses (usually rigid gas permeable) and/or corneal surgery may be option.
LASIK, LASEK or lens exchange surgery can correct regular astigmatism, whilst also treating your short or long sight. The results of laser astigmatism correction have improved very considerably since the early days of PRK and LASIK, especially with the advent of highly sophisticated ‘tracking systems’ which ensure that the correct angle or axis of astigmatism is treated.
Patients undergoing cataract or refractive lens exchange surgery with significant or high astigmatism can have custom fit Toric (astigmatic) lenses inserted at the time of their surgery to correct their basic refractive error plus their astigmatism. Alternatively, for smaller amounts of corneal astigmatism, limbal (or corneal) relaxing incisions can be created in the cornea by the femto-second laser or by hand for up to 1.5 dioptres.
In the longer term, patients with significant astigmatism following cataract or refractive lens exchange surgery may benefit from laser eye surgery to eliminate or at least minimise their refractive error.