What irregular astigmatism is
Astigmatism means the eye focuses light differently in different directions, so light can’t come to a single sharp point.[1] Most astigmatism is regular. The cornea (the clear front window of the eye) is curved more steeply in one direction than the other, but in an even, predictable way. Glasses and toric contact lenses correct it well.
Irregular astigmatism is different. The surface of the cornea is uneven: steeper in one patch, flatter in another, with no consistent pattern. Light is scattered rather than simply bent, which causes:
- blur that doesn’t sharpen fully with glasses
- ghosting or doubled images, often in one eye
- halos, glare, and streaks around lights, especially at night
- a prescription that seems to change, or that never feels quite right
Glasses can’t fully fix it. A glasses lens can correct a smooth, predictable difference in curve, not an uneven surface. In a case report on two thinning conditions of the cornea, the authors note that glasses are limited for exactly this reason.[2] How much you gain by switching from glasses to a rigid lens depends on how poor your vision is in glasses. In a study of 48 eyes with irregular astigmatism, people who already saw 20/20 in glasses gained nothing on average with rigid lenses. Those who saw 20/50 to 20/200 in glasses gained an average of four lines on the eye chart.[3]
What causes it
Irregular astigmatism isn’t one disease. It is what several different conditions do to the cornea. An academic medical center that fitted 262 eyes with irregular corneas grouped them as bulging, thinning conditions (118 eyes), irregularity after surgery (70), scars (57), and corneal dystrophies (17).[5]
- Conditions where the cornea thins and bulges, such as keratoconus, pellucid marginal degeneration, keratoglobus, and Terrien’s marginal degeneration. Keratoconus is the most common cause in scleral lens clinics. In a series of 155 eyes fitted for irregular astigmatism, 67.7% had keratoconus.[6]
- Corneal surgery, including corneal transplants, radial keratotomy, and ectasia after LASIK. Transplants made up 18.1% of eyes in that series.[6] Even after a modern, partial-thickness transplant called DMEK, 23 of 262 eyes (9%) in one series needed contact lenses because of surface irregularity or other optical problems.[8]
- Scars from injury or infection. See corneal scarring.
Irregular astigmatism is also the main reason scleral lenses exist in practice. In an international survey describing 259 scleral lens patients, 87% were wearing them for corneal irregularity.[4]
How a scleral lens works
A scleral lens is a large rigid gas-permeable lens. It rests on the sclera (the white of the eye) and arches over the entire cornea without touching it. Before insertion it is filled with sterile saline, so a layer of fluid sits between the lens and the cornea.
The fluid fills in the dips and peaks of the uneven surface. Light then passes through the smooth front of the lens and an even layer of fluid, so the irregular cornea is largely neutralized optically.[9] Because the lens lands on the white of the eye, it doesn’t have to balance on an uneven cornea the way a small rigid lens does. The lens’s front surface can also carry a prescription, and some designs add optics tailored to the individual eye.[9]
What the research shows
Vision improves, often a lot. A French university hospital fitted 107 eyes with irregular astigmatism from ectasia or a transplant. Vision improved by an average of five lines on the eye chart, and 80% of patients were still wearing their lenses daily after an average of about 22 months.[7] At the academic center with 262 irregular corneas, vision improved by an average of 0.46 on the logMAR scale, where lower is better, and every cause of irregularity they studied improved except one subgroup of just two eyes.[5]
The results hold across causes. In the series of 155 eyes, the best vision with scleral lenses was similar whether the cause was keratoconus, a transplant, or a mix of other conditions.[6]
Irregular corneas tend to do better than regular ones. In a prospective study of 95 scleral lens wearers, 77% of those with irregular corneas were still wearing their lenses at a year, compared with 58% of those with regular corneas.[10]
Breaks may help wearing time. In the 155-eye series, people who took a brief break every 4 to 5 hours to refresh the lens were significantly more likely to wear their lenses successfully.[6]
Other options, compared
Corneal rigid gas-permeable lenses are a long-standing option and can improve vision a great deal over glasses.[3] Hybrid lenses, with a rigid center and a soft outer skirt, are another choice for people who can’t tolerate a corneal rigid lens or can’t keep it centered. In one study of 47 eyes fitted with newer hybrid lenses after rigid lenses failed, most people were still wearing them more than 10 hours a day after more than three months.[11]
| Option | What it does | Typically suited to |
|---|---|---|
| Glasses | Correct regular astigmatism only | Mild irregularity with little effect on vision |
| Soft or specialty soft lenses | Mask mild irregularity | Mild cases |
| Corneal rigid gas-permeable lenses | Give a smooth optical surface on the cornea | Moderate irregularity where the lens centers and is comfortable |
| Hybrid lenses | Rigid center with a soft skirt | Rigid lens intolerance or poor centration |
| Scleral lenses | Vault the whole cornea over a fluid layer | Marked irregularity, or failure of the options above |
| Surgery (cross-linking, transplant, others) | Treats the underlying cause | Depends entirely on the cause; ask a cornea specialist |
Questions to ask a scleral lens fitter
- What is causing my irregular astigmatism, and is it stable?
- Would a smaller rigid lens or a hybrid lens work for me first?
- How much better do you expect my vision to be, and what else in my eye might limit it?
- How many visits does a fit usually take, and what do the fees cover?
- Do you bill my insurance directly, or give me a superbill to submit myself?
- Should I take breaks to refresh the lens during the day?
- What signs mean I should remove the lens and call you the same day?
Common questions
What's the difference between regular and irregular astigmatism?
In regular astigmatism the cornea is shaped a bit like the back of a spoon: curved more steeply one way than the other, but smoothly and predictably. Glasses and toric contact lenses correct it well. In irregular astigmatism the surface is uneven, so there is no single correction that a glasses lens can provide.
How do I know if my astigmatism is irregular?
Your eye doctor can tell from corneal mapping (topography) and from how well glasses correct your vision. Ghosting, doubled images, or vision that stays blurry with an up-to-date prescription are common clues. A trial with a rigid or scleral lens is also a practical test.
Can laser eye surgery fix irregular astigmatism?
Sometimes, in selected eyes, but often not. Many people with irregular astigmatism have a thin or weakened cornea, and some developed it because of earlier laser or other corneal surgery. A cornea specialist can tell you whether any surgical option fits your eye.
Do I need a scleral lens, or would a smaller rigid lens work?
Many people do well with a smaller rigid gas-permeable lens, which can give a large improvement over glasses. Scleral lenses are usually chosen when a smaller lens won't center, isn't comfortable, or the surface is too irregular for it.
Does insurance cover lenses for irregular astigmatism?
Sometimes. Some medical and vision plans cover contact lenses when they are medically necessary, and irregular astigmatism from a corneal condition can qualify. Coverage depends on your plan and whether the practice is in network.
Why do I still see ghosting or shadows with my scleral lenses in?
A scleral lens corrects most of the distortion from an irregular cornea, but some can remain, especially with scarring or a lens that sits off-center. Check each eye on its own to see which one is causing it, then tell your fitter. Fit adjustments, glasses over the lens, or custom optics can sometimes reduce what's left.
Should leftover astigmatism be corrected in the lens or in glasses over it?
Both approaches are used. Building it into the lens needs a lens that stays in the same position on the eye, while glasses over the lens can be simpler to adjust. Your fitter will suggest what suits your eye and how you use your vision.
My fitter says the lens fits well, but my vision still isn't crisp. What now?
A good fit and the right prescription are separate things, so ask your fitter to check your vision through the lens and fine-tune the power. Scarring or other eye conditions can also limit how sharp vision gets. If you still aren't satisfied after several adjustments, a second opinion is reasonable.
Will I still need reading glasses with scleral lenses?
Many wearers do, especially as near vision fades with age. Reading glasses over the lenses are the most common approach, and some people use one eye for distance and the other for near. Ask your fitter which option suits your eyes and daily tasks.