Astigmatism

Astigmatism is not a disease but a feature of the eye's optics: its focusing surfaces are curved unevenly in different directions, so light comes together at two points instead of one and sharpness is lost at every distance. Most people carry a small amount of it and are never bothered. It calls for attention in three situations: when it gets in the way of seeing, when it tires the eyes to the point of a headache, and when it changes quickly.

Why the picture lands in two places at once

Most of the focusing is done by the cornea, the clear front layer of the eye. When it is curved the same way all round, light meets at a single point on the retina. In astigmatism one meridian bends the light more strongly and the one at right angles to it bends it less, so instead of a point there are two focal lines lying at different depths, and the image has no sharp setting at all. Less often the imbalance comes from the lens inside the eye. On a spectacle prescription this is written as two numbers: the strength of the cylinder in dioptres and the axis in degrees.

Astigmatism is either regular or irregular. In the regular kind the surface is smooth and glasses even the imbalance out. In the irregular kind the cornea is curved chaotically, because of a scar or thinning, and glasses do a poor job of it. That distinction decides how the eye is corrected.

How people notice it

Very few describe it simply as blurred vision. The usual account sounds different:

• letters and outlines come with a shadow or a doubled edge;
• squinting and tilting the head become the only way to catch focus;
• heaviness in the eyes, pressure above the brows, a headache after reading or a long stretch of driving;
• street lamps and headlights sprout rays and halos, and evenings are markedly worse than daytime;
• pushing the text further away, the way people do with long-sightedness, changes nothing: there is no sharp distance at all.

Children almost never say that they see badly, because they have nothing to compare it with. What shows instead is behaviour: the child avoids reading, loses the line, holds the book right up to the face, keeps the head at an angle. If astigmatism is strong and sits in one eye only, the brain stops using that eye and amblyopia sets in, a lasting drop in vision that after a certain age can no longer be made up. That is why nothing is left to drift in children.

One distinction is worth remembering: if straight lines look wavy and a patch appears in the middle of the field of view, this is not astigmatism. That is how the retina behaves, and it is looked into separately and without delay.

Where it comes from, and what has nothing to do with it

Most often the shape of the cornea is set before birth, runs in the family and changes little over a lifetime. Acquired astigmatism appears when something deforms the cornea:

• a scar after an injury, an ulcer or severe inflammation of the cornea;
• keratoconus, a gradual thinning and bulging of the cornea;
• conjunctival tissue growing across the cornea, or a lump on the eyelid pressing on the eye from outside;
• previous surgery: cataract removal, corneal transplant, laser vision correction;
• in children, a persistently drooping eyelid that covers part of the pupil.

Reading while lying down, dim light and screens do not reshape the cornea; they cause eye strain, which with astigmatism arrives sooner, but those are two different things. Glasses do not spoil the eyes, and without them a person simply sees worse. Eye exercises train muscles, not the curvature of the cornea, so promises to clear astigmatism away with exercises rest on nothing at all.

Keratoconus, the one thing not to miss

This is the only scenario in astigmatism where time matters. The cornea thins and bulges forward into a cone, the astigmatism grows and the axis rotates. It usually begins before the age of thirty and often unevenly: one eye runs ahead of the other. What should raise concern is not the astigmatism itself but the way it behaves:

• the prescription has to be changed every few months, and the new glasses see worse than the old ones;
• the cylinder grows and the axis shifts from one check to the next;
• glasses give no sharpness anywhere, yet a rigid contact lens suddenly restores it;
• at night a pronounced smear spreads around every light source;
• the person rubs the eyes hard and constantly, or has an allergy or atopic dermatitis.

This is worth knowing for two reasons. Progression is halted by a procedure that strengthens the cornea, but it locks in the shape that has already formed and does not give the old sharpness back: the earlier it is caught, the better the outcome. The second reason is that with keratoconus laser correction is ruled out, because it speeds up the breakdown of an already thinned cornea. That is exactly why a map of the curvature is taken before such surgery, rather than lenses simply being fitted.

When to book an appointment, and when not to wait

It is worth seeing an ophthalmologist if:

• vision is blurred and squinting no longer rescues it;
• glasses or contact lenses have stopped satisfying and have to be changed ever more often;
• a child tilts the head and loses the line, or has never had an eye test at all;
• there has been a blow to the eye, inflammation of the cornea or surgery: the shape of the cornea changes after any of these.

Seek help the same day if vision drops sharply, especially in one eye; if there is pain in the eye, intense redness or light sensitivity; if coloured rings appear around lights together with pain and fog; if vision has changed after an eye injury. Separately, double vision that disappears the moment either eye is covered is no longer a matter of optics but of nerves and muscles, and it needs emergency care straight away.

What happens at the appointment

First an instrument measures the refraction, then your eye doctor tries lenses with a cylinder by hand and checks at which strength and axis you see best; that is what goes into the prescription. Keratometry and topography give a map of the corneal curvature, and those are the tests that tell ordinary astigmatism apart from keratoconus and scarring, while corneal thickness shows whether there is any room for laser correction.

In children the refraction is measured after drops that relax accommodation. Without them the child adjusts the focus involuntarily, the numbers come out too low and the glasses come out wrong.

What it is corrected with

• Glasses with cylindrical lenses. The main option at any age and the only one for small children.
• Soft toric contact lenses. They must sit steadily on the eye, otherwise the axis shifts and the sharpness goes. Sleeping in lenses and rinsing them under the tap lead straight to inflammation of the cornea.
• Rigid gas-permeable and scleral lenses. The way out in irregular astigmatism and keratoconus: the lens builds an even focusing surface in front of the eye.
• Laser correction. It is discussed when the refraction is stable, the cornea thick enough and the topography normal; the surgeon decides after the assessment.
• Toric intraocular lens. It is implanted during cataract surgery and compensates for the astigmatism along the way.

Glasses and contact lenses do not change the shape of the eye, they only compensate for it, so astigmatism itself neither disappears nor grows because of them. New glasses with a cylinder take getting used to: for the first days the floor may look tilted and objects stretched, and this usually settles within a few days. If it has not settled in two weeks, or a headache persists, the glasses should not be worn out of patience but checked again, since the fault is most often in the axis or in the centring of the lenses.

Online consultation

At an online appointment the doctor will work out how long the vision has been changing and how fast, go through previous prescriptions with their cylinder strength and axis, judge whether the picture fits ordinary astigmatism or gives grounds to look for keratoconus, and advise which examinations to arrange. What cannot be done remotely is measuring the refraction and issuing a prescription: instruments are needed for that.

This material is for information only and does not replace medical advice.