Corneal Diseases - Information on Diseases of the Cornea

The cornea is the transparent portion of the outer layer of the eye and serves as the “windshield of the eye.” It is formed by keratinocytes, which are the specialized cells of this structure.

Its transparency is due to:

• The parallel arrangement of collagen lamellae
• The absence of blood vessels
• Active fluid removal from the cornea via endothelial pumps

The cornea is innervated by nerves. A clear disc with a diameter of 11.7 millimeters, the cornea is located directly in front of the pupil. It is about 0.6 millimeters thick in the center—roughly half a millimeter—and about 0.8 millimeters thick at the edges. 

The cornea protects the iris and the sensitive interior of the eye @ Vladimir /AdobeStock

Viewed from the outside in under a microscope, six layers can be distinguished in the cornea:

• The anterior corneal epithelium (epithelium corneae) forms the surface of the cornea
• The Bowman’s layer (lamina limitans anterior) adjoins it and, through its structure, helps maintain the outer epithelial layer
• The stroma (corneal stroma) is the middle layer of the cornea and accounts for about 90 percent of its thickness. In addition to connective tissue and nerve fibers, it contains various cell types. These cells perform important functions, such as supplying nutrients, defending against infection, and facilitating repair after injury. The stroma, which consists of regularly layered collagen fibers, is bounded toward the anterior chamber by Descemet’s membrane
• The posterior Descemet’s membrane (lamina limitans posterior) serves as a boundary to the corneal endothelium (endothelium corneae). It is both the thickest and the most resilient membrane in the human body. As a result, it provides effective protection. Its thickness increases over the course of a person’s life.
• The endothelial cell layer is responsible, among other things, for maintaining hydration and thus preserving the transparency of the cornea. The outermost layer of these cells rests on a basement membrane and actively pumps water out of the cornea to keep it clear and prevent it from swelling.

Illustration of the various layers of the cornea

The cornea, located in the outer layer of the eye, acts as a window to the eyeball, enabling sharp vision. 

It has three important functions:

• Protective function: It protects the sensitive interior of the eye from external influences (foreign objects, water). It is therefore comparable to a car’s windshield
• Light transmission: Just like a window, it must allow light to pass through as unobstructed as possible
• Refraction: The regular curvature of the cornea refracts light before it hits the lens of the eye. Small irregularities can lead to visual distortions

The natural refractive power of the cornea is approximately 40 diopters. This means that the cornea accounts for two-thirds of the eye’s total light refraction and thus contributes to image focusing. A key factor in this process is the aqueous humor, which is located behind the cornea.

The cornea must be constantly moistened with fluid to prevent it from drying out. The eyelids perform this task: through blinking, they distribute tear fluid evenly across the cornea. In dry eye syndrome, this lubrication is disrupted, which can impair the transparency of the cornea.

The endothelial layer of the cornea, through its various metabolic processes, regulates dehydration. It ensures internal drainage and maintains transparency.

The density of metabolically active cells in the corneal endothelium decreases with age. Endothelial cells have limited regenerative capacity, which is why they require protection.

The Descemet’s membrane provides protection against infections, injuries, and damage. The epithelium on the surface of the cornea is located in the outer layer of the eye. It can regenerate and ensures that minor wounds heal without scarring.

Among the most common diseases of the outer layer of the eye are defects in the cornea. Depending on their causes, these are classified as inflammatory and non-inflammatory injuries to the cornea.

In ophthalmology, injuries to the outer layer of the eye caused by foreign bodies are common. Profuse tearing flushes the foreign body out of the eye without causing damage to the outer layer of the eye.

Rinsing with an eye wash bottle or under running water helps until the sensation of a foreign body is gone. You should avoid rubbing the eye, as this can lead to superficial damage to the cornea.

A special case is a chemical burn to the cornea, which usually occurs in the context of workplace accidents. This is an acute emergency that requires immediate intervention.

The first step in treating a chemical burn is to rinse the eye thoroughly for 20 to 30 minutes. Next, you should visit an eye clinic for further evaluation to prevent blindness.

A healthy cornea has a uniform curvature. This allows it to optimally refract light entering from all directions before it hits the lens of the eye.

If the cornea is more or less curved in one direction, the refraction of light through the pupil is also altered. This leads to distorted perception, such as round objects appearing as lines or rods.

This condition is called astigmatism. People with astigmatism see objects both near and far as blurry and distorted. They often suffer from headaches. 

In rare cases, a disorder of the lens accompanies the change in the cornea. Astigmatism is often congenital and can be hereditary.

It is usually accompanied by nearsightedness or farsightedness. There are also acquired causes, which include diseases or scarring within the outer layer of the eye.

Most commonly, the abnormalities involve a curvature along the vertical axis (cylindrical astigmatism). A second distinction is made based on whether the cornea is curved in different places (irregular astigmatism) or not (regular astigmatism).

Doctors correct the vision problems associated with the regular form using special glasses or contact lenses. Rigid contact lenses are suitable for irregular astigmatism, but glasses are not. Refractive surgery procedures are suitable for correcting more severe abnormalities, with laser treatment being the most common method.

Keratoconus, which occurs at a rate of one in 2,000, is a non-inflammatory disease of the cornea. It is hereditary, and symptoms typically appear as early as adolescence. In this condition, the cornea thins in the center and bulges forward in a cone-shaped manner. This significantly impairs vision. The prognosis for this condition of the outer layer of the eye varies depending on its severity. The extent of corneal astigmatism can be assessed by projecting rings onto the cornea. Using specialized equipment for precise measurement, keratoconus can be detected even in its early stages—data from pachymetry and information about the curvature of the anterior surface of the cornea provide important clues.

Woman with keratoconus @ Alessandro Grandini /AdobeStock

Corneal opacities result in reduced visual acuity and, in some cases, can also cause pain.

The most common causes include swelling (edema) and scarring of the cornea. Corneal scars on the outer layer of the eye appear as whitish opacities. 

They develop following inflammation, ulcers, or metabolic disorders. A risk factor for inflammation is pre-existing damage to the cornea, such as from improperly fitted contact lenses. Straining the eyes—such as when reading or working on a computer—can further worsen the symptoms.

Inflammation of the cornea (keratitis) can result from a bacterial or viral infection or a fungal infection. If left untreated, this can lead to the formation of a corneal ulcer (ulcus corneae). This can extend into the stroma or as far as the Descemet’s membrane and result in a perforation of the eye.

Extensive scarring of the outer layer of the eye can cause serious damage to the cornea. In some cases, scarring promotes the growth of blood vessels (vascularization). As a result, blood vessels form in the otherwise avascular area of the outer layer of the eye, which further impairs vision.

When cells in the endothelial layer are damaged, fluid enters the cornea. This fluid accumulates, leading to edema and clouding of the cornea.

Fuchs’ endothelial dystrophy (a metabolic disorder) also damages the endothelial cell layer within the cornea, leading to fluid accumulation. In cases of severe visual impairment, only surgical methods—such as penetrating keratoplasty—can help. With endothelial dystrophy, symptoms are often subtle at first and eventually become noticeable through poor vision—especially in the morning.

Today, lamellar keratoplasty is most commonly used. In this procedure, doctors replace only the Descemet’s membrane (Descemet’s membrane endothelial keratoplasty, DMEK). A 3D model of the entire cornea is generated to precisely plan the procedure.

With age or following injuries, degenerative changes can occur in which the cornea loses its homogeneous and clear structure. However, some age-related changes of this kind are not considered pathological.

Corneal Transplant

Opacities in the cornea can very rarely be resolved through conservative, i.e., medical, treatment. Only a corneal transplant can prevent blindness in the advanced stages of corneal damage. In this procedure, the recipient’s cornea is replaced with healthy donor tissue that has previously undergone a thorough examination of its inner pump cells.

1. What is a corneal disease? A corneal disease refers to any inflammatory or non-inflammatory damage to the cornea—the transparent layer of the eye responsible for protecting the eye and refracting light.

2. What causes corneal opacity? Corneal opacity is often caused by scarring following keratitis, ulcers, or metabolic disorders such as endothelial dystrophy, in which fluid penetrates the cornea and clouds the tissue.

3. When should you see an ophthalmologist? Immediate action is required in cases of chemical burns or severe injuries to the cornea. You should also visit a clinic if your vision is progressively worsening or if your prescription changes frequently.

4. How is keratoconus treated? Milder cases can be corrected with rigid gas-permeable contact lenses. In cases of more severe protrusion, corneal surgery or, in advanced stages, a corneal transplant may be considered.