Astigmatism

What astigmatism is

Astigmatism is unevenness in the curve of the eye's focusing surfaces. In an eye without it, the cornea — the clear dome covering the iris — curves like a section of a sphere, and light passing through converges on a single point at the back of the eye. In an astigmatic eye the curve is steeper in one direction than the other, closer to the shape of a rugby ball than a sphere, so light is brought to focus in more than one place at once. No single plane holds a sharp image, which is why the result is blur rather than simple short or long sight.

The same unevenness can sit in the lens behind the iris instead of in the cornea. The two are distinguished as corneal and lenticular astigmatism, and a single eye can have both.

It is worth saying early that astigmatism is extremely common and usually unremarkable. Most eyes carry some degree of it, generally from early life. It becomes something to correct only when it is large enough to blur vision, cause strain, or — in a young child — interfere with the development of sight itself.

How it presents

The characteristic sign is blur that does not resolve at any distance. Short sight clears up close, long sight clears further away; astigmatism clears nowhere, which is often what distinguishes it in an eye test.

Alongside that come ghosting or doubling of edges, particularly around lights at night, when a street lamp acquires a smear or a tail. Headaches, aching eyes after reading or screen work, squinting and head-tilting are all common, and in children the strain usually shows up as behaviour — avoiding books, sitting very close to things, losing interest in detailed work — rather than as a reported complaint.

Because astigmatism is usually present from the start and changes slowly, it frequently goes unnoticed. There is no moment at which vision deteriorates, so the blur is simply taken for how the world looks.

What causes it, and what does not

The main cause is inheritance. Corneal shape runs in families, and most astigmatism is something an eye is born with rather than something that develops.

Beyond that, anything that alters the cornea's shape can produce it: an injury, a scar, a corneal infection that heals unevenly, or previous eye surgery. Keratoconus is the condition most associated with it, and is covered below.

Two causes named in older advice do not hold up. Poor posture does not cause astigmatism, and nor does close work. The idea that reading too much, sitting badly or working at near distance can deform the cornea is persistent and has no good evidence behind it. Close work is associated with the development of short sight, which is a different matter entirely and a different part of the eye. Anyone correcting a child's reading posture is doing so for the sake of their neck, not their corneal curvature.

One genuine link is worth knowing for a different reason. Raised blood glucose changes the water content of the lens and can shift the eye's focus temporarily. Vision that fluctuates noticeably from week to week, or blur that arrives alongside thirst and tiredness, is a reason to have blood glucose checked rather than to order new glasses — and a prescription taken during a period of poor glucose control will be wrong once control improves.

Why it matters most in childhood

This is the part the subject's reputation for harmlessness obscures.

The visual system develops by being used, during a window that closes in early childhood. An eye that spends those years delivering a blurred image in one orientation can fail to develop normal sight in that orientation permanently — a form of amblyopia specific to astigmatism, where the problem ends up in the brain's processing rather than in the eye's optics. Glasses then correct the focus but not the vision, because the capacity to use the sharp image never formed.

Caught early enough, correction allows normal development. Caught late, it does not. That asymmetry is the whole argument for having young children's eyes tested rather than waiting for a complaint — and children, who have nothing to compare their vision against, very rarely complain.

In England, vision screening is offered in the reception year, at around four or five. It is a screen rather than a full examination, and it is not a substitute for a sight test where there is any family history or any suspicion.

Having it measured

A routine sight test detects astigmatism and quantifies it. Measurement has two parts: the amount, in dioptres, and the axis, in degrees, describing which direction the steeper curve runs. Both appear on a prescription, which is why an astigmatic prescription has more numbers on it than a simple one.

Sight tests on the NHS are free for a substantial list of people, which is worth knowing because many who qualify do not realise it:

• anyone under 16
• 16-, 17- and 18-year-olds in full-time education, including education at home
• anyone aged 60 or over
• anyone registered as partially sighted or blind
• anyone diagnosed with diabetes or glaucoma
• anyone aged 40 or over with a parent, sibling or child diagnosed with glaucoma
• anyone told by an ophthalmologist that they are at risk of glaucoma
• anyone eligible for an NHS complex lens voucher, or named on a valid HC2 certificate
• recipients of certain benefits, including Pension Credit Guarantee Credit and qualifying Universal Credit awards, and their dependants under 20

The standard entitlement is a test every two years, though an optometrist can recommend more frequent testing where it is clinically justified, and can test sooner if sight changes in the meantime.

Correcting it

Glasses are the usual answer and the simplest. An astigmatic lens is curved unequally to counter the eye's own unequal curve, which is why a new prescription for a first-time astigmatic wearer can briefly make floors look tilted or doorways slightly wrong. It settles within days as the brain adapts.

Contact lenses correct it too, though they have to resist rotating on the eye to keep the correction aligned with the axis. Soft toric lenses are weighted or shaped to stay put. Rigid gas permeable lenses work differently and better for irregular corneas: the lens holds its own shape and the tear film fills the gap beneath it, so an uneven cornea is effectively replaced optically rather than compensated for. That is why rigid lenses remain the mainstay for keratoconus even though soft lenses are more comfortable.

Laser and lens surgery can reshape the cornea permanently. The established options are PRK, which removes the surface layer and reshapes the cornea beneath it; LASIK, which raises a thin hinged flap, reshapes the tissue underneath with an excimer laser and lays the flap back; LASEK, a variant of PRK; and SMILE, which removes a small disc of tissue through a tiny incision without raising a flap. Where astigmatism accompanies a cataract, a toric intraocular lens can correct both at once during the cataract operation.

Suitability depends on corneal thickness, the amount and type of astigmatism, and the stability of the prescription, which is why assessment rather than preference determines what is available to any given eye.

Keratoconus

Keratoconus is a progressive thinning of the cornea, which bulges into a cone shape under the eye's internal pressure and produces astigmatism that is both substantial and irregular. Onset is typically in adolescence or the twenties, with incidence peaking in the twenties, and it can progress over years before stabilising.

Vigorous, habitual eye rubbing is a recognised risk factor, which makes it one of the few things within anyone's control. Whether keratoconus is more common in men or in women is genuinely unsettled — large population studies have found it both ways — so claims in either direction are worth treating with caution.

The important modern development is corneal cross-linking, which uses riboflavin and ultraviolet light to stiffen the corneal tissue and halt progression. It does not restore vision already lost; it stops the deterioration, which makes early diagnosis considerably more valuable than it used to be. Rigid contact lenses handle the optics, and corneal transplant remains the option for advanced disease.

A note on an obsolete operation

Older material on astigmatism lists radial keratotomy among the treatments. It should not be relied on.

Radial keratotomy made spoke-like incisions in the cornea with a diamond blade — not with a laser, as is often stated — to flatten it. It was largely abandoned after excimer laser surgery became available in the mid-1990s, because its results drifted: the ten-year PERK study found 43 per cent of treated eyes had shifted at least a dioptre towards long sight between the second and tenth year after surgery. Anyone who had the procedure decades ago may still be living with that drift, and should mention it before any later eye surgery, since it complicates the lens calculations for cataract operations.

What is reasonable to expect

Most astigmatism is mild, stable and fully handled by a prescription. It is not a disease, it does not generally worsen with use, and nothing about ordinary reading, screens or lighting makes it progress.

The two situations that justify acting rather than waiting are a young child whose eyes have not been tested, and a change in vision that arrives suddenly or alongside other symptoms. Everything else is a matter for the next routine sight test.