Keratoconus

Screening and diagnosing
keratoconus

Keratoconus is not diagnosed by measuring vision, but by mapping the cornea. Modern corneal imaging now makes it possible to recognise the disease well before it becomes visible on clinical examination, and sometimes before it causes any trouble. It is this shift of diagnosis towards ever earlier stages that has changed how the condition is managed.

Why measuring vision is not enough

A sight test measures what you see and the correction that suits you best. It says nothing about the shape or the strength of the cornea. Preserved visual acuity therefore does not rule out keratoconus: at an early stage the distortion remains modest, and spectacle correction can still give a satisfactory result. Conversely, the vast majority of drops in vision have nothing to do with keratoconus.

Slit-lamp examination — the microscope used in the ophthalmology consulting room — has the same limitation. The signs it allows to be recognised directly appear only at an already advanced stage. Waiting for them to become visible means diagnosing late.

Diagnosis therefore rests on imaging that measures the geometry of the cornea itself.

Mapping the cornea: topography and tomography

Two terms recur in reports, and the distinction between them matters.

Topography analyses the front surface of the cornea and draws a curvature map from it: it shows where the cornea is more steeply curved. It is the historical examination, still useful, but it sees only one of the two corneal surfaces.

Topography ring image: a normal cornea and a cornea affected by keratoconus On the left, the image obtained from a normal cornea: regular concentric rings, evenly spaced. On the right, the same image from a cornea affected by keratoconus: the rings are distorted and crowded in the lower part, where the curvature increases. REFLECTED RING IMAGE NORMAL CORNEA Round rings, evenly spaced. KERATOCONUS Rings distorted and crowded towards the bottom. Reflected rings Distorted rings Crowded rings Schematic. Proportions are deliberately exaggerated; this is not an actual scan.

Tomography analyses the full corneal thickness: front surface, back surface, and the distribution of thickness point by point. This complete reading is what makes it possible to detect early forms, because the back surface of the cornea often distorts before the front one.

Several technologies produce these maps — optical scanning imaging, anterior segment optical coherence tomography, concentric disc analysis. They differ in their measuring principle and in their reference values, but share the same objective: to describe the actual shape of your cornea.

What these examinations actually measure

  • Front surface curvature: its maximum value, and above all the location of the steepest point, generally displaced downwards in keratoconus.
  • Posterior elevation: the gap between the actual back surface and a theoretical reference surface. This is often the first detectable abnormality.
  • Corneal thickness, or pachymetry: its value at the thinnest point, the position of that point, and the way thickness increases away from the centre.
  • Overall surface regularity, which accounts for the share of astigmatism that glasses do not correct.

These measurements are never interpreted in isolation. A single abnormal index in a given patient does not establish keratoconus; it is the coherence of the whole, and its comparison with the values expected for a normal eye, that underpins the diagnosis.

Analysing the epithelium

The epithelium is the thin cellular layer covering the surface of the cornea. It has a capacity for remodelling: where the cornea bulges, it thins; where the cornea flattens, it thickens.

This adaptation has an important consequence. By partly smoothing the surface, the epithelium can mask an early distortion and make front surface topography falsely reassuring. Measuring its thickness helps unmask certain early forms, and helps distinguish true ectasia from a surface irregularity of another origin.

Analysing the strength of the cornea

Geometry describes shape; it does not directly report strength. Complementary examinations assess the way the cornea deforms and recovers its position under a calibrated puff of air, which yields information about its mechanical properties.

These measurements provide an additional argument, particularly in ambiguous situations. They do not replace the analysis of shape and thickness.

Corneal response to an air puff: a normal cornea and a cornea affected by keratoconus In each panel, a dotted line marks the resting position of the cornea. Under a calibrated puff of air, a normal cornea moves away from that position and returns to it in a single movement. A cornea affected by keratoconus moves further, overshoots outwards, then returns after several oscillations. The lower graph plots this deformation over time. RESPONSE TO AN AIR PUFF NORMAL CORNEA Rapid response, damped at once. KERATOCONUS Greater deformation, delayed recovery. Resting position DEFORMATION OVER TIME time time A single return movement. Several oscillations before returning. Animated schematic. Amplitudes are deliberately exaggerated; this is not an actual scan.

Early forms and “suspicious” corneas

Corneas do not divide neatly into “normal” and “keratoconus”. Some corneas have parameters that depart from usual values without meeting the criteria for established disease. Depending on the case, these are referred to as forme fruste, subclinical keratoconus, or more simply a suspicious cornea.

Two situations lead to encountering them: the less affected eye of a patient whose other eye carries established keratoconus, and the preoperative assessment for refractive surgery, where the search for these forms is precisely one of the essential points.

A suspicious cornea is not a diagnosis. In these situations, the genuinely decisive information is not a further technical examination, but a repeated comparison over time: it is stability or change over time that settles the question.

When laser surgery is not indicated →

What can look like keratoconus

Several situations produce a distorted cornea or a misleading image:

  • pellucid marginal degeneration, another form of corneal thinning, with a different topography and location;
  • ectasia occurring after corneal refractive surgery, whose mechanism differs even though the appearance is close;
  • marked dry eye or an abnormality of the tear film, which degrade measurement quality;
  • distortion caused by contact lens wear, in particular rigid lenses, which can mimic or mask an irregularity for several days or weeks after removal;
  • a corneal scar or the sequela of inflammation.

Telling them apart relies on the joint analysis of the maps, the clinical examination and the context. This is also why an isolated examination, performed in poor conditions, can lead to a mistaken conclusion in either direction.

Locating severity

Once the diagnosis is made, describing the extent of involvement provides shared reference points from one consultation to the next. Several classifications coexist: the older ones rest on curvature, thickness and the presence of scarring; the more recent ones combine the front surface, the back surface, minimum thickness and corrected vision.

Stage What it usually describes
Early Abnormalities detectable on imaging, corrected vision preserved
Moderate Irregular astigmatism established, spectacle correction increasingly unsatisfactory
Advanced Marked thinning, spectacle correction insufficient, fitting with rigid or scleral lenses usually necessary
Severe Major distortion, very thin cornea, possible opacities

These stages describe a state; they do not predict a course. Early keratoconus may progress and advanced keratoconus may remain stable for years: that is separate information, and it requires repeated examinations.

Progression and monitoring →

How the examination is carried out

Corneal imaging is painless, contact-free and involves no injection. It takes a few minutes and does not require dilating the pupil. You look at a light target; the instrument records.

Two practical conditions govern how reliable the diagnostic result will be:

  • prior removal of contact lenses, several days for soft lenses, often considerably longer for rigid ones — the interval will be specified before the appointment. Patients with known keratoconus who depend on their lenses do not, of course, follow this rule, save exceptions;
  • an ocular surface in good condition: significant dryness or active inflammation must sometimes be treated before the tests can be repeated conclusively.

After the diagnosis

Making the diagnosis does not yet say what should be done. Two questions come next: is your keratoconus changing, and is your vision sufficient for your daily needs. The first is a matter of monitoring, the second of optical correction.

Progression and monitoring →

Treating keratoconus →

A cornea to map, or an examination to compare?

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Understanding the disease before its examinations

Keratoconus →
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