Corneal diseases — when the clearest tissue in the eye suffers

Structure and function of the cornea

The cornea — in Latin cornea — is the clear, slightly domed disc right at the front of the eye, exactly where a contact lens sits. At its centre it measures around half a millimetre, becoming somewhat thicker towards the edge. You do not see it in the mirror: it is completely transparent. It becomes visible only at the moment it stops being so.

Its job is a double one. It is the protective layer of the eye against dust, wind, fingertips and pathogens. And it is at the same time the strongest lens the eye has: around two thirds of the eye's entire refractive power arises at the cornea — more than in the lens behind it. The light is bent above all at the boundary between air and tear film; that is why the image goes blurred as soon as the tear film breaks up, even though the cornea itself is clear. Every change in its shape acts directly on vision. A dent, a scar or a bulge behaves like a warped spectacle lens that cannot be taken off.

It is built from several layers, three of which govern everyday life. On the outside lies the epithelium, a covering layer that renews itself completely about every one to two weeks — which is why a superficial abrasion usually heals without a trace. Beneath it lies the stroma, by far the thickest storey; it gives the cornea stability and shape, but heals with a scar rather than without a trace. Right on the inside sits the endothelium, a single layer of cells that pumps water out of the tissue without pause and thereby keeps the cornea clear.

This innermost cell layer is the most vulnerable, because the body does not regenerate it. Lost endothelial cells are not replaced; the remaining ones enlarge and cover the area as well. If cell density falls below a critical limit over the course of a life, through a disease or after an operation inside the eye, the pump fails: the cornea takes up water and clouds over. Those affected typically describe it as a fog that is densest in the morning and eases during the day.

One more thing is remarkable: the cornea contains no blood vessels. That is exactly what makes it transparent — vessels would scatter light. It is supplied instead by the tear film from outside and the aqueous humour from inside, and it takes its oxygen directly from the air. Contact lenses sit precisely on this interface, and that is the reason why wearing habits and hygiene with lenses are not a side issue but the heart of the matter.

And it is the tissue with the densest nerve supply in the human body. That is why a defect smaller than a pinhead already hurts so fiercely. The severity of the pain, though, says little about the seriousness of the finding. A harmless abrasion can be almost unbearable, while a deep, dangerous change hurts astonishingly little. That is one of the reasons why, with corneal symptoms, we would rather look once too early.

How you notice that the cornea is involved

• Pain — not just burning or itching, but a sharp, cutting pain
• Marked light sensitivity that makes you screw up your eyes
• A foreign-body sensation that cannot be blinked away
• Veiled vision — seeing as though through frosted glass, often with halos and starbursts around lights at night
• Distorted vision that a new pair of glasses no longer corrects properly
• Redness in a ring directly around the iris, rather than spread evenly across the white of the eye
• Heavy watering and an eye that can barely be opened

This combination separates a harmless conjunctivitis — in which redness and discharge are to the fore and neither pain nor worsening vision belongs to the picture — from a group of urgent findings. The cornea is only one of them. The same pattern of pain, sensitivity to light, a ring of redness and worse vision also arises with an inflammation of the iris (iritis, uveitis) — there typically without purulent discharge — and with a sudden rise in intraocular pressure. From the outside all of it looks the same at first. Which structure is affected — cornea, iris or the pressure in the eye — shows up at the slit lamp, as a rule after staining the surface with a dye that makes every defect visible, and together with a pressure measurement: an examination that takes a few minutes and decides the whole of the treatment that follows.

One exception tolerates no delay. If sudden severe pain, rainbow colours and halos around light sources, an eye that feels stone-hard, nausea or vomiting and badly blurred vision come together — often with a mid-dilated, fixed pupil —, the suspicion is of an acute glaucoma attack. The halos there arise from the same water retention in the cornea as with a corneal finding; the difference is the pressure behind it. That is not something to look at 'later today', but a case for ophthalmic treatment immediately — in this situation sight is lost within hours.

Diagnostics: how we assess the cornea

It always begins with the same thing, and it is the single most important examination there is: the slit lamp. Under high magnification and with a narrow slit of light the cornea can be scanned layer by layer; a dye makes defects of the surface visible that otherwise stay invisible. Only when this finding leaves a question open do the measuring techniques come in. Which ones depends on the question — and some of them we arrange at a specialist centre rather than improvising them.

Topography — the map of the surface

Corneal topography measures the curvature of the cornea point by point and shows it as a colour map: steep areas appear warm, flat ones cool. Nothing touches the eye, and the capture takes seconds. It is the decisive method where we suspect Keratoconus, because it shows an early bulge long before it stands out at the slit lamp. It matters just as much in following the course: whether a bulge is increasing is not a judgement, it is a comparison of two maps. And it explains why glasses on a deformed cornea stop correcting properly at some point.

Pachymetry — measuring the thickness

Pachymetry measures how thick the cornea is at a given point. In keratoconus it shows the thinning at the cone and helps decide which treatments come into question at all. With a weak endothelium, an increase in thickness shows that the cornea is retaining water — often measurable before the clouding gets in the way of everyday life. And it has a role that surprises many: corneal thickness influences the measured intraocular pressure. Without it, a pressure reading taken on suspicion of glaucoma can be judged too high or too low.

Endothelial microscopy — the cell count

This method photographs the innermost cell layer and counts how many cells lie within a defined area and how regularly they are shaped. It is the standard method for putting a number on the reserve of this layer — and because lost endothelial cells do not grow back, that is a reserve in the literal sense. It is indicated where an endothelial dystrophy is suspected, with unexplained clouding, before operations inside the eye and in follow-up after a corneal transplant.

Confocal microscopy — a look at the individual cell

Confocal microscopy shows the cornea in the living eye at a resolution that lets individual cells and nerve fibres be made out. It is not a method for the ordinary case but for a small number of difficult situations: with a corneal inflammation that does not respond to treatment, where fungi or amoebae are suspected, and with unexplained deposits in the tissue. It is not available in every practice; where the findings call for it, we arrange the examination at a specialist centre. The principle stays the same as for every additional investigation here: it is done when its result changes the treatment.

Corneal injuries and foreign bodies

Injuries to the cornea are the commonest reason people come to an eye practice unplanned: a branch in the face, a fingernail while dressing a child, a grain of sand in the wind, a spark while cutting metal. The good news is that the surface repairs itself quickly — a simple abrasion (erosio) usually heals within one to three days without leaving a trace. The bad news is that the depth and the seriousness of the matter cannot be read off the pain — and that in some of those affected the surface stays sensitive afterwards: months later, on opening the eye in the morning, the same pain can suddenly appear, a recurrent corneal erosion. That is treatable; get in touch if it happens.

What you can do straight away:

1. With a chemical in the eye: rinse before you do anything else. Clear water or saline, immediately and for at least 15 minutes — considerably longer with alkalis, lime and cleaning agents. Take contact lenses out as you do, hold the lid open and move the eye in every direction. Wipe visible residues of lime or powder out of the conjunctival sac before rinsing — with a moist cotton bud, having the upper and lower lid turned back for it —, otherwise they carry on burning under the stream of water. Do not neutralise: no acid against alkali or the other way round. Do not stop in order to set off; carry on rinsing on the way in for ophthalmic treatment — along with the information on which substance it was.
2. With loose dust or sand: rinse it out with clean water or saline. If a foreign-body sensation remains afterwards, something is lodged there that has to be seen.
3. Contact lenses out — immediately and for the whole time the symptoms last.
4. Keep the eye still, cover it loosely if need be, and call: +41 44 500 69 00 (Mon–Fri 08:00–12:00 / 13:00–17:30). Outside consulting hours the ophthalmic emergency service applies.

What you should on no account do:

• Do not rub. Rubbing pushes a foreign body deeper and turns a small defect into a large one. It is the commonest mistake and the one with the most consequences.
• Never remove an impaled or firmly lodged foreign body yourself — not with tweezers, a cotton bud, the corner of a tissue or the tip of your tongue. Anything stuck to the cornea is removed under magnification and with anaesthetic drops, not at the kitchen table.
• If a penetrating injury is suspected, do not rinse and do not press. If an object was travelling at speed — hammering on metal, a lawnmower, a cutting disc —, cover the eye loosely, without pressure, and drive straight to the emergency department of an eye clinic. Eat and drink nothing from that moment on — treatment is usually carried out under general anaesthetic, and an empty stomach saves you waiting time.
• No old eye drops from the cupboard, in particular no steroid. With an open cornea or an infection that is starting, it can do considerable harm.
• No anaesthetic drops for self-treatment. They take away the pain — and with it the warning — and delay the healing of the surface.

A category of its own is flash burn: a UV burn of the cornea after welding without protection, after a day on the glacier or on a sunbed. Typically the symptoms only begin hours later, usually in the evening or in the middle of the night, but then with severe pain in both eyes. It heals as a rule within one to two days; what is treated is the pain and the surface. Anyone who welds regularly or works in the mountains spares themselves the repeat with safety goggles and a UV filter — the most effective measure here is the simplest.

Call the same day applies to any foreign body that cannot be rinsed out, to any eye that hurts, is sensitive to light or sees less well after an injury, to any chemical in the eye and to any red eye under contact lenses. Describe to us on the phone what happened — we will tell you whether it can wait until tomorrow. If you are not sure whether your problem belongs here at all, there is the overview of our Eye examinations or the page on Eye inflammation.