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The Human Eye and Astigmatism

“I've got astigmatism - my last optometrist told me that” a phrase every optometrist hears regularly. Usually it is followed up by the question “What is astigmatism?”

Astigmatism

Astigmatism is not a disease. Just like myopia ('short sightedness') and hyperpia ('long-sightedness'), it is a way of describing how the eye's optical system focuses the light rays. The optical system of the eye has to focus an image of the world around us onto the retina - a sheet of light sensitive cells within the eye. The clear front window of the eye - the cornea, carries out most of the focusing. Just as a camera with a fixed focus can't cope too well with objects very close to it, the eye also needs a mechanism to allow us to bring distance or near objects into sharp focus. A short distance behind the cornea is the flexible lens which has just this function, aided by a 'tyre shaped' muscle able to squeeze it to a more rounded shape to sharpen near vision. Relaxing the 'tyre' brings more distant objects into focus.

Astigmatism - no point!

For some people the front of the cornea is near enough spherical in curvature (like the section of a tennis ball) - so that if you measured the radius of curvature of their cornea, whether measured horizontally or vertically, you would still find the same radius of curvature. Now imagine our 'tennis ball' has been squeezed, top and bottom. It would now have a steeper curve if measured top to bottom and will have flatter curve at right angles to that. Astigmatism is where the optical system of the eye, particularly the front surface of the cornea, has one curvature going across the eye and a different curvature at right angles to this. A 'spherical' cornea will allow a small distant point of light to be focussed on the retina as a point of light. An eye with astigmatism does not have a single position of focus where that small distant light can be focussed as a point. Indeed, the word astigmatism comes from the Greek meaning 'without a point'. If you have been told you have astigmatism don't panic as it is probably as common as myopia and hyperopia. Often small degrees of astigmatism can be tolerated although sometimes the additional strain in attempting to focus can result in sore eyes and headaches, which is when spectacles or contact lenses are prescribed.

Contact Lenses and Astigmatism

Contact lenses do have a number of advantages over spectacles, and every person's eyes are different so the comments here are just to give you some background information to discuss with your eye care professional. If you have only mild astigmatism and this is not combined with other focussing errors, it is unlikely that contact lenses will be advised. However once the astigmatism is over a certain level (probably around the 1.00 D cylinder power) and if combined with a long or short-sighted correction as well, contact lenses really come into their own. Mild astigmatism can often be corrected best using Gas Permeable lenses are discussed elsewhere on the site. For instant comfort Toric Soft Lenses are proving to be very popular.

Toric Soft Lenses

The word toric comes from torus, which describes a surface that has two different curves at right angles to one another - yes, really just another way to describe astigmatic curves. By arranging for the curves to lie on the cornea in a set way these curves can counteract the astigmatism of the eye, allowing sharp vision again.

Soft contact lenses, as the name suggests, are very flexible and float on a thin layer of tears. As you can imagine, a lens to correct astigmatism must never rotate in the eye, but always lie in the direction that will correct the astigmatism. There are a few ingenious toric soft lens designs able to remain stable. Toric soft lenses are stabilised by 'weighting' the base of the lens so that gravity holds it in the right place. Other toric soft lenses use especially thin edges at the top and bottom - the lids rest on these 'chamfers' to hold the lens correctly.

Toric soft lens fitting is a very precise science and, at times, an art too! Considerable skill is needed in measuring the curves of the cornea and calculating the toric curves. Instrumentation is used that can record the corneal curves to the nearest hundredth of a millimetre! However fitting toric soft lenses is more complex that just getting accurate readings of the astigmatism. It is not possible to predict how the tightness of the lids will affect the angle the lens sits in the eye. A correctly calculated toric soft lens may twist a few degrees, throwing the sharpness of the image off. Your eyecare professional will be able to assess the movement of the trial toric soft lens and incorporate an element to bring the correction to sit correctly even with the effect of the lids.

So if you have astigmatism, have a chat with us. At the Terence Mendoza practice we fit a very wide variety of toric soft lens designs, from every major manufacturer. The main manufacturers of toric soft lenses produce a wide range of powers and fittings. If your prescription is outside the standard toric production range we can have lenses custom made.

Visit the Terence Mendoza site for more information on contact lenses and eye care in general.

15 North Street, Rochford, Essex SS4 1BD tel: 01702 544084 / 540838 info@terencemendoza.co.uk