Ophthalmic examination

Corneal topography: what is this examination for?

Corneal topography is a key examination in refractive ophthalmology. In a matter of seconds, without any contact with the eye, it maps the curvature of the cornea with remarkable precision. It is used to screen for keratoconus, to assess eligibility for refractive surgery (LASIK, PRK, SMILE) and to monitor the cornea over time. Here is what this examination measures, how it is performed and how Dr Tourabaly interprets it to guide your care.

Direct answer: a map of the curvature of the cornea

In short. Corneal topography measures the shape and curvature of the corneal surface, point by point. It produces a colour-coded map that reveals irregularities invisible to the naked eye: early keratoconus, irregular astigmatism, corneal scarring. It is a routine examination before any refractive surgery. Performed in a few seconds, without contact or preparation, it is entirely painless. Interpretation by an experienced ophthalmologist is essential to distinguish a cornea eligible for surgery from a fragile cornea that is better left unoperated.

What is corneal topography?

The cornea is the transparent lens at the front of the eye. Its shape — normally regular and spherical — accounts for roughly two thirds of the optical power of the eye. Any irregularity of this surface translates into blurred or distorted vision. Corneal topography uses the reflection of light patterns off the cornea (Placido rings or projected grids depending on the device) to reconstruct, in three dimensions, the curvature map of the entire surface.

Two complementary approaches can be distinguished: topography of the anterior surface (the outer surface of the cornea) and corneal tomography, which analyses the cornea throughout its full thickness, both anterior and posterior surfaces, using diffuse-light systems (Scheimpflug) such as the Pentacam. This second technique provides valuable additional information for detecting subclinical, still asymptomatic keratoconus that would contraindicate refractive surgery.

What is corneal topography used for?

Corneal topography is useful in three main situations:

Keratoconus screening: keratoconus is a progressive deformation of the cornea, which becomes cone-shaped and irregular. Topography detects its early signs — localised thinning, asymmetric bow-tie — well before vision clearly deteriorates. This screening is essential, because unrecognised keratoconus is the main contraindication to refractive surgery.
Eligibility for refractive surgery (LASIK, PRK, SMILE): before any laser procedure, Dr Tourabaly analyses the topographic map to make sure the cornea is regular, thick enough and of a suitable curvature. A cornea that is too thin, too curved or shows a suspicious asymmetry may lead to deferring or contraindicating surgery. This is one of the examinations in the complete preoperative assessment.
Monitoring over time: topography makes it possible to follow the progression of known keratoconus, to check the stability of a cornea after refractive surgery, or to monitor a corneal cross-linking treatment.

It is also useful when fitting rigid contact lenses on irregular corneas, and in the management of certain severe astigmatisms. If you would like to know whether you are eligible for refractive surgery, see our dedicated page: am I eligible for LASIK?

How is the examination performed?

Corneal topography requires no special preparation. It is carried out in the practice, before the clinical examination. Here is how it proceeds:

Positioning: you rest your chin on a chinrest and your forehead against the device’s forehead support. No drops are needed, and no contact with the cornea takes place.
Acquisition: the device projects illuminated rings onto the cornea and captures their reflection in a fraction of a second. The examination itself takes less than 10 seconds per eye.
Analysis: the software immediately generates a colour-coded map. Red and orange areas indicate steeper curvature (a more bulging cornea), and blue areas a flatter curvature.
Interpretation: Dr Tourabaly analyses the map, compares the two eyes, and cross-checks these data with corneal thickness and the other measurements from the assessment to determine whether surgery is feasible and, if so, which procedure (LASIK, PRK, SMILE).

Avoid wearing soft contact lenses in the days before topography: they can temporarily deform the cornea and distort the results. Rigid lenses require a longer break. Dr Tourabaly will tell you how long to observe depending on the type of lenses you wear.

The topographer used in the practice: the Sirius+

In the practice, topography is performed with a Sirius+ (CSO), a device that combines two complementary technologies: Placido-disc topography (projection of illuminated rings analysing the anterior surface of the cornea) and rotating Scheimpflug camera tomography (which reconstructs the cornea in three dimensions, both front and back surfaces). This combination provides a precise analysis of curvature, elevation and corneal thickness across the entire cornea.

The same examination, non-contact and in a few seconds, also allows other useful parameters to be measured before surgery or for follow-up:

Pupillometry: measurement of pupil diameter under low and high light conditions (scotopic, mesopic, photopic). This is an important piece of data before refractive surgery, to anticipate the risk of halos or night-time glare.
NIBUT (non-invasive tear break-up time): a non-invasive analysis of tear-film stability, useful for screening for dry eye before considering laser surgery.

How is the topographic map interpreted?

The topographic map is read like a geographical relief map. A normal cornea shows a symmetrical, regular distribution, with a central zone that is slightly steeper. Several patterns are looked for:

Symmetrical bow-tie: typical of regular astigmatism, entirely compatible with refractive surgery.
Asymmetric or oblique bow-tie: may signal irregular astigmatism or early keratoconus — warranting further investigation.
Inferior cone: localised steepening in the lower part of the cornea, a suggestive sign of keratoconus. Such a pattern leads Dr Tourabaly to investigate further and, if confirmed, to refer to dedicated keratoconus follow-up.

Automated indices (Rabinowitz index, KISA%, Belin-Ambrosio) help characterise the risk, but it is always the ophthalmologist who makes the decision, by cross-checking topography with pachymetry, aberrometry and the clinical examination.

FAQ

Frequently asked questions

Is corneal topography painful?

No. It is an entirely non-contact examination. There is no puff of air, no drops, and no instrument touches the cornea. The device simply projects light onto the eye and analyses its reflection. The examination is over in a few seconds per eye.

Do I need to stop wearing my lenses before topography?

Yes. Soft lenses slightly deform the corneal surface. A break of at least 3 to 7 days is generally recommended before topography for soft lenses, and several weeks for rigid lenses. Your ophthalmologist will specify the duration according to your correction and the type of lenses.

What is the difference between topography and pachymetry?

Topography measures the curvature (the shape) of the corneal surface. Pachymetry measures the thickness of the cornea. The two examinations are complementary and both form part of the preoperative assessment before refractive surgery. Some modern devices (Pentacam, Galilei) perform both measurements in a single acquisition.

Can surgery be performed if the topography is abnormal?

A topographic abnormality is not automatically a definitive contraindication, but it does call for in-depth analysis. Depending on the nature of the abnormality — confirmed keratoconus, mild irregularity or a scar without consequences — Dr Tourabaly will determine whether surgery is feasible, whether prior follow-up is needed, or whether an alternative such as a phakic implant is more appropriate.

Is corneal topography reimbursed?

Topography performed as part of a preoperative assessment for refractive surgery is generally covered within the specialist consultation. Reimbursement arrangements depend on your complementary health insurance and the indication. Dr Tourabaly will inform you during the assessment.

Corneal topography and keratoconus: what is the link?

Keratoconus is characterised by a progressive, irregular bulging of the cornea. Topography is the reference examination for detecting it at an early stage, sometimes before any visual symptom. If keratoconus is suspected, additional examinations (Pentacam, aberrometry) can confirm it and assess its severity. To find out more, see our page on keratoconus.

Scientific sources

Gomes JAP, Tan D, Rapuano CJ, et al. Global Consensus on Keratoconus and Ectatic Diseases. Cornea. 2015;34(4):359-369. PMID 25738235.

Gomes JAP, Rodrigues Frasson TR, Cunha JR, et al. Current indications and outcomes of corneal transplant. Prog Retin Eye Res. 2022;90:101057. PMID 35378256.

Moshirfar M, Hopping GC, Vaidyanathan U, et al. Corneal Topography: A Review of the Current State of the Art. J Ophthalmic Vis Res. 2021;16(4):610-624. PMID 34840665.

Arranging a corneal topography

If you are considering refractive surgery or wish to check the condition of your cornea, book an appointment with Dr Moïse Tourabaly. Corneal topography is part of the preoperative assessment carried out during consultation at the Cachan practice. It is quick, non-contact and an essential step towards a safe medical decision.

This article is intended for information and educational purposes. It does not replace a medical consultation. The indication for and interpretation of a corneal topography are the responsibility of your ophthalmologist. Sources: articles indexed on PubMed; recommendations of the French Society of Ophthalmology (SFO).