Corneal surgery and cornea disease in Paris and Cachan

ANATOMY AND FUNCTION

What is the cornea? Optical role and structure

The cornea is the transparent, dome-shaped membrane covering the front of the eye, in front of the iris and pupil. It measures roughly 11 to 12 mm in diameter and 520 to 560 microns thick at its centre. Unlike other ocular tissues, it is avascular: it contains no blood vessels, which allows it to stay perfectly transparent and to transmit light without scatter.

Optically, the cornea is responsible for around two-thirds of the eye’s refractive power (about 43 dioptres out of the eye’s total of 60). Its curvature focuses light rays onto the retina. Any change in its shape — distortion (keratoconus), clouding, surface irregularity — directly affects the quality of vision.

The cornea is made up of five distinct layers, from the surface inwards:

• The epithelium (the surface layer, which regenerates within 5 to 7 days), forming the protective barrier against external agents.
• Bowman’s layer (an acellular support membrane that does not regenerate once damaged).
• The stroma (accounting for roughly 90% of the total thickness), made up of collagen lamellae arranged in a perfectly regular pattern — this organisation is the key to transparency.
• Descemet’s membrane (the basement membrane of the endothelium, which thickens with age).
• The endothelium (a layer of non-regenerating cells responsible for the active pumping that keeps the cornea dehydrated and clear).

This lamellar organisation is precisely what makes selective grafting techniques possible: only the failing layer can be replaced, as in DMEK (grafting the endothelium alone), without touching the healthy layers.

CORNEAL CONDITIONS

The main conditions affecting the cornea

Corneal disorders are varied: some affect its shape (keratoconus), others its surface (pterygium, dry eye, keratitis), others still the endothelium (Fuchs’ dystrophy), and some arise after surgery. Each condition has its own dedicated page with the full treatment approach.

REFRACTIVE SURGERY

The cornea at the heart of refractive surgery

Almost all laser refractive procedures act directly on the cornea to alter its curvature and correct visual errors. Understanding its optical role helps to grasp what each technique actually does.

LASIK, PRK and SMILE: reshaping the cornea to correct vision

LASIK, PRK (photorefractive keratectomy) and SMILE reshape the corneal stroma by removing a few microns of tissue. This ablation changes the curvature of the cornea, and therefore its refractive power. For myopia, the centre of the cornea is flattened; for hyperopia, its periphery is modified. The residual thickness after ablation is a key safety parameter: enough residual stroma must be preserved to maintain mechanical strength.

Why topography is done as standard before any laser procedure

Early keratoconus or a thin cornea may look normal on a visual acuity test, but corneal topography reveals abnormalities in curvature or thickness that would rule out a laser procedure. That is why the pre-operative assessment always includes corneal topography (Scheimpflug or Placido) and pachymetry before any surgical decision.

WARNING SIGNS

When should you see an ophthalmologist about the cornea?

Certain symptoms should prompt a corneal ophthalmology consultation, ideally with a full assessment including topography:

• Progressive loss of vision despite a recent correction, or frequent changes of glasses without stable improvement.
• Distorted vision, ghosting or halos around light sources, even with optical correction.
• A foreign-body sensation, burning or chronic watering suggestive of dry eye or a surface disorder.
• The appearance of a spot or a haze on the cornea, or persistent redness in the inner corner of the eye (early pterygium).
• A family history of keratoconus in a young adult aged 15 to 35, even without visual symptoms.
• Plans for refractive surgery (LASIK, PRK, SMILE): corneal topography is done as standard before any laser procedure to detect an underlying corneal abnormality.
• Severe eye pain, sudden watering, photophobia: these signs suggestive of infectious keratitis constitute an ophthalmic emergency.

CORNEAL INVESTIGATIONS

Corneal examinations at the practice

The corneal assessment relies on three complementary examinations, carried out contact-free or with minimal preparation:

Corneal topography

Corneal topography maps the curvature of the front surface (and sometimes the back surface) of the cornea. It produces a colour-coded map of the flat and steep zones. It is the gold-standard examination for detecting keratoconus, identifying a surface irregularity and calculating the parameters for refractive surgery. Analysis of the back surface (elevation topography) is particularly useful for early forms of keratoconus, which are often invisible on the front surface alone.

Corneal OCT (optical coherence tomography)

Corneal OCT produces a high-resolution cross-section of all the layers of the cornea. It makes it possible to visualise the thickness and integrity of each layer (epithelium, stroma, endothelium), to measure the depth of a scar or a deposit, and to monitor the course of a condition during treatment. It is essential for the pre-graft assessment and for follow-up after cross-linking.

Corneal pachymetry

Pachymetry measures the thickness of the cornea at various points. The central value (normally between 520 and 560 microns) is essential for the pre-operative assessment of refractive surgery (LASIK, PRK, SMILE) and for monitoring keratoconus. A value below the surgical safety thresholds may point towards a non-ablative technique such as the phakic implant.

FREQUENTLY ASKED QUESTIONS

Frequently asked questions about the cornea