Corneal topography: what the scan shows and why it is used for keratoconus

The cornea is the clear, dome-shaped window at the front of the eye. It does much of the eye's focusing, so even small irregularities in its shape can change the quality of vision. A corneal topography scan produces a map of the cornea. Moorfields Eye Hospital describes it simply as a quick, painless photo that checks the shape and thickness of your cornea in detail.

The result is a map of the cornea rather than a single number. The Royal Wolverhampton leaflet names Pentacam as one scan used for this, saying the scan gives detail about the corneal thinning, the shape of the cornea and the astigmatism level. Different instruments use different methods, and your clinic will tell you which it uses.

Moorfields explains that in keratoconus the front of the eye is not completely smooth and round, which affects focus and causes short sight and astigmatism, in which the light that passes through the eye forms an unclear image at the back of the eye. A map of the cornea shows that irregularity directly.

Keratoconus is a non-inflammatory condition in which the normally round, dome-shaped cornea gradually thins and develops a cone-like bulge. Moorfields says it normally develops in teenagers or those in their twenties and often gets worse gradually over time, and that the speed of change varies between people. Its leaflet adds that keratoconus is usually diagnosed in young people at puberty, in late teens or early twenties, and affects up to 1 in 450 people depending on ethnicity.

Early keratoconus can be hard to see. Moorfields says that early stages are detectable by a local optometrist and that vision is often unaffected in the very early stages. Moorfields' guidance for optometrists explains why topography is so useful: diagnosis is best confirmed by corneal topography, not least as baseline scans are then available for subsequent comparison. Because most optometry practices do not have topography, patients are usually referred to secondary care on the basis of increasing astigmatism and possible abnormal retinoscopic reflexes.

The same guidance sets out the benefit of early referral. Seeing a person with early-stage keratoconus in a hospital setting gives the earliest feasible baseline corneal topography data, which can be used for subsequent comparison, and it is in the patient's interest to be referred as soon as keratoconus is suspected.

Keratoconus is not treated with drops or medicine. Moorfields says there is no cure for keratoconus and that it cannot be treated with eye drops or medication. Early on, glasses may be enough, and many patients eventually need contact lenses. Corneal collagen cross-linking, which uses ultraviolet light and riboflavin drops to stiffen the cornea, is described by Moorfields as able to stop keratoconus getting worse and effective in over 90% of patients, but only suitable for some people with progressive keratoconus.

That is where repeat topography comes in. The Royal Wolverhampton leaflet says that cross-linking is only suitable when there is evidence of progression, and that during monitoring if any deterioration is detected the team may recommend it. It notes that once you are past your late 30s the cornea starts to stabilise, whereas below this age the cornea is more flexible and progression is more likely.

In short, a single scan tells the team what the cornea looks like today. A series of scans, compared with the baseline, tells them whether it is changing, which is often the deciding question for treatment.

Moorfields describes the scan as a quick, painless photo. It is usually done as part of a wider assessment. Moorfields says that the eye doctor or optometrist will examine your eyes with a microscope called a slit lamp, and that you might also have the topography scan.

Because arrangements differ between clinics, it is sensible to ask in advance whether you need to bring your glasses, whether you should stop wearing contact lenses beforehand and how long you should expect to be at the clinic. Your appointment letter or the eye clinic can tell you.

Your clinician will explain what the map shows. The aim is usually to answer one of three questions: whether keratoconus or another corneal shape change is present, how advanced it is, and whether it has changed since last time. If the result suggests keratoconus, the clinic will discuss monitoring, vision correction and whether cross-linking might be considered.

Moorfields advises that rubbing the eyes may play a role, saying there may be a link between keratoconus and people who rub their eyes frequently, and the Royal Wolverhampton leaflet likewise notes that eye rubbing can make the condition get worse. Moorfields describes a rare complication called hydrops, a sudden, painful loss of vision if fluid from the eye enters a very thin cornea, which it says will settle with time. Any sudden painful change in vision should be assessed promptly by an eye service.

Most people with keratoconus will not need a corneal transplant, according to Moorfields, which says a transplant may be needed in very advanced cases where contact lenses fail to improve vision.