The Cornea & External Disease

What the cornea does

Light enters the eye through the cornea first. Because it is curved, it bends that light and provides most of the eye’s focusing power. It must stay perfectly clear and smooth to give sharp vision.

Diseases of the cornea and the eye’s surface (the “external” eye) are treated by cornea specialists.

Close-up of a blue eye; a window-like reflection shows the clear cornea covering the iris and pupil — The cornea is the clear dome over the iris and pupil. It is so clear that you can only see it by the reflection on its surface.

Five thin layers

The cornea is only about half a millimeter thick, yet it has five layers. The outer two layers protect the eye, the middle layer gives it strength and clarity, and the ultra-thin inner two layers hold cells that constantly pump fluid out so the cornea stays clear.

Modern cornea surgeons can treat or transplant only the layers that are sick, without disturbing the rest of your cornea.

Cutaway of the cornea showing its layers, from the surface epithelium through the thick stroma to the single inner layer of endothelial cells — From top to bottom: the protective epithelium, Bowman’s layer, the clear stroma, Descemet’s membrane, and the endothelium. Full transplants (PK) replace every layer; DALK replaces the front layers and most of the stroma; DMEK replaces Descemet’s membrane and the endothelium, and DSAEK adds a thin layer of donor stroma behind them.

Simulated view with corneal disease — Race day with an irregular or swollen cornea, as in keratoconus or Fuchs’ dystrophy: ghosting, haze, and glare. Drag to compare. Simulation for illustration only.

Simulated healthy vision — Race day with an irregular or swollen cornea, as in keratoconus or Fuchs’ dystrophy: ghosting, haze, and glare. Drag to compare. Simulation for illustration only.

Common corneal conditions

Dry eye & ocular surface disease

An unstable tear film or irritated surface can cause burning, grittiness, and fluctuating vision.

Keratoconus

The cornea gradually thins and bulges into a cone shape, distorting vision. It often begins in the teens or twenties.

Keratoconus & crosslinking

Fuchs’ dystrophy

The inner pumping cells slowly fail, so the cornea swells. Vision is often worst in the morning.

Learn about Fuchs’

Scars & infections

Injuries and infections, such as corneal ulcers, can leave the cornea cloudy or irregular.

Corneal ulcers & infections

Growths on the surface

Growths such as a pterygium or Salzmann’s nodules can spread onto the cornea and affect comfort and vision.

Pterygium & surface growths

Healing problems

Some eyes struggle to heal the surface layer, as in neurotrophic keratitis, which leaves the cornea vulnerable.

Neurotrophic keratitis

How it can be treated

1
Medical care
Lubricating and prescription drops, ointments, and treatment of the eyelids and tear film.
2
Specialty contact lenses
Custom lenses can smooth an irregular cornea, such as in keratoconus, and sharpen vision.
3
Corneal crosslinking
Special drops and ultraviolet light strengthen the cornea to help slow or stop keratoconus from progressing.
4
Corneal transplant
Replacing damaged tissue with healthy donor tissue, reserved for cases without less invasive options. Today, often only the affected layers are replaced (DMEK, DSAEK, or DALK) instead of the full thickness (PK).