What keratoconus is, and what it means for your eyes

It often starts as glasses that never feel quite right

You may have been back for new glasses more than once, and each time the astigmatism has crept up. Headlights at night smear into streaks. Text on a screen has a faint second copy sitting beside it, even with one eye closed. One eye may be noticeably worse than the other.

These are the kinds of changes people with early keratoconus describe, and they are easy to mistake for ordinary short-sightedness. The difference is that the problem sits in the shape of the cornea, not in the focusing power the glasses are trying to fix. That is why a stronger prescription can disappoint.

Keratoconus is progressive, which means the shape can keep changing for a time. Healthify NZ notes the changes usually start in the early teens and can continue into the 30s, and that the two eyes are often affected to different degrees.

What changes in the cornea

The cornea is the clear dome at the front of the eye. Together with the lens behind it, it bends light so that it lands sharply on the retina. For that to work, the cornea needs to be smooth and evenly curved, like a well-made camera lens.

In keratoconus the cornea becomes thinner and the thinned area bulges forward into a cone. The surface is no longer even, so light entering the eye is scattered rather than focused at one point. Health New Zealand describes it as the cornea becoming thinner and more distorted, with the thinned area bulging forward.

This uneven curve is called irregular astigmatism. Ordinary glasses correct regular astigmatism, where the curve is the same across the whole cornea. They struggle with irregular astigmatism, which is why a contact lens that sits over the cornea and creates a new smooth front surface can work when glasses cannot.

Who gets it, and why eye rubbing matters

Keratoconus usually appears in the late teens or early twenties, although it can start earlier or later. Health New Zealand says it rarely begins after 30. The cause is not fully understood. It runs in some families: the American Academy of Ophthalmology puts it at roughly 1 in 10 people with keratoconus having a parent who also has it.

Eye rubbing is the one factor you can change. Healthify NZ states plainly that keratoconus is known to be associated with eye rubbing, and both Healthify and Health New Zealand advise against rubbing because it can damage the cornea. The itch usually comes from allergy, hay fever or eczema, so treating the itch is part of protecting the cornea.

New Zealand has more keratoconus than many countries. RANZCO, the college for ophthalmologists in Australia and New Zealand, reports that Māori and Pacific peoples are more often affected and tend to have more severe disease, and recommends screening for at-risk young people, including those with Down syndrome. The things that raise the odds are:

• A parent, brother or sister with keratoconus.
• Hay fever, eczema or asthma, especially with itchy eyes and a habit of rubbing them.
• Down syndrome, where RANZCO recommends routine ongoing screening.
• Māori or Pacific heritage, where RANZCO reports higher rates and more severe disease.
• A glasses prescription with astigmatism that keeps rising, or vision that stays blurry through new glasses.

How it is found

A routine eye exam can raise suspicion when the astigmatism is high, uneven between the eyes, or changing faster than expected. The test that settles it is corneal topography, a scan that maps the curve of the cornea point by point. The American Academy of Ophthalmology describes it as working much like a 3D map, and nothing touches the eye during the scan.

The map shows where the cornea is steepening, usually below the centre, often before you notice much yourself. Repeating the map over time is how progression is measured, because day-to-day vision is a poor guide to whether the shape is still changing. Our optometrists also examine the cornea under the slit lamp for the thinning and fine lines that go with keratoconus.

You do not need a referral to have this checked in New Zealand. If you recognise yourself in the signs above, a keratoconus assessment with corneal mapping is the right first appointment. Our post on keratoconus symptoms, diagnosis and treatment goes into the tests in more detail, and signs of keratoconus lists what people notice first.

Two separate jobs: slowing it down and seeing clearly

Keratoconus care has two aims that are easy to confuse. One is to stop the cornea changing shape. The other is to give you clear vision today. No single treatment does both, so many people need two things at once.

Slowing it down is the job of corneal cross-linking. It uses riboflavin drops and ultraviolet light to stiffen the cornea. Health New Zealand describes it as a treatment that may stop keratoconus worsening, while noting you will still need glasses or contact lenses afterwards. In New Zealand it is done by an ophthalmologist, so our optometrists refer you rather than perform it. RANZCO's Choosing Wisely guidance says it is not for every patient with keratoconus: it is indicated when scans show clear progression, and younger patients may be considered earlier because they are more likely to progress.

Seeing clearly is the job of glasses early on and, as the cornea becomes more irregular, specially fitted contact lenses. Corneal rigid lenses and scleral lenses both work by creating a smooth new front surface over the cone. Our guide to scleral versus RGP lenses compares them. These lenses improve vision while they are worn; they do not slow the disease.

Health New Zealand runs a contact lens subsidy for lenses supplied for medical need, claimed through an optometrist or dispensing optician who holds an agreement with Health New Zealand. Who qualifies and what it covers is set by Health New Zealand, not by the practice, so ask us at your assessment. Our insurance and funding page explains how it is handled here.

When this practice is the wrong place

Keratoconus itself is not an emergency, but a few situations are. Call 111 or go to the nearest hospital emergency department, rather than booking an optometry appointment, for sudden loss of vision, a shadow or curtain across your vision, flashes with a sudden shower of floaters, a chemical splash, a red and painful eye with light sensitivity, or any eye injury.

One keratoconus-specific emergency is acute corneal hydrops. The inner layer of a very thin cornea splits, fluid rushes in, and the cornea swells and turns cloudy. It brings a sudden drop in vision, pain and light sensitivity, usually in one eye. Healthify NZ advises seeing a doctor immediately for sudden severely blurred vision or for severe light sensitivity, redness and pain. That means the hospital eye clinic or emergency department the same day, not a routine appointment with us.

If you are unsure how urgent something is, Healthline on 0800 611 116 is free, open 24 hours a day, and can tell you where to go. We can take over the ongoing lens care once the eye has settled.

What to do next

If you have not been diagnosed and the signs sound familiar, book a comprehensive eye exam and mention keratoconus when you book so corneal mapping can be included. If you already have a diagnosis, our guide to the first months after a keratoconus diagnosis sets out what to change, what to monitor and who does what.

Bring your current glasses and any previous prescriptions. The change between them is often the first clue. Our conditions page shows how keratoconus sits alongside the other reasons vision changes.