Cornea of the Eye

The cornea of the eye is a transparent, curved structure that is moistened by tear fluid. It is the frontmost part of the eye and serves to protect the eye as well as to refract the light entering the eye. Along with the lens, it is responsible for clear vision.

Function of the Cornea

The cornea is permeated by nerve fibres and is surrounded by conjunctiva. There are no blood vessels in the cornea – it is supplied with essential nutrients by the surrounding aqueous humour, which keeps it transparent.

The transparency of the cornea is crucial for our vision. If it becomes cloudy, so does our sight. The cornea has both an optical and a protective function.

Optical Function of the Cornea

Together with the lens, the cornea is responsible for a large part of the eye's refractive power: light rays passing through the cornea and lens are focused so that they converge at a point on the retina at the back of the eye. The retina then transmits these impulses via the optic nerve to the brain, which interprets them as a sharp image.

Correct refractive power is particularly important because our visual acuity depends on it. A healthy eye has a refractive power of approximately 60 diopters (dpt), with the cornea alone accounting for about 43 dpt.

The cornea has a water content of about 76%. The fluid that surrounds it ensures crystal-clear vision. If the eye lacks sufficient tear fluid, the cornea swells and becomes cloudy.

By the way: Since the cornea plays a vital optical role, refractive errors like astigmatism, farsightedness, or nearsightedness can be corrected by altering the curvature of the cornea. This is exactly what happens in laser eye surgery – the cornea is precisely reshaped using a laser to correct the refractive error, ensuring that the light rays entering the eye are correctly refracted.

Protective Function of the Cornea

Along with the tear film, the cornea has a protective function: it shields the eye from foreign bodies. These cannot penetrate the firm cornea without significant force and are flushed out by the tear film. Blinking distributes the tear fluid over the cornea, keeping it moist, which is essential for unobstructed vision.

Structure of the Cornea

The cornea, along with the sclera (white of the eye), forms the outer part of the eye, with the sclera enveloping the entire eyeball. The cornea only covers the front portion. Behind the cornea are the pupil and the iris (coloured part of the eye). The cornea is nourished by the aqueous humour, which is distributed with each blink. The inside of the eyelids is lined with conjunctiva, which continues onto the cornea.

The cornea is composed of six different layers:

• Epithelium: The surface of the cornea is made up of a flat cell layer, also known as the squamous epithelium or corneal epithelium. This epithelium consists of 5–7 layers of cells stacked on top of each other and renews itself every seven days, which is crucial for the eye's wound healing. Nerve fibres terminate in this layer, making even minor injuries (e.g., from rubbing the eye) very painful. The primary function of this layer is to protect the eye from the invasion of germs and foreign bodies.
• Bowman’s Membrane: Beneath the corneal epithelium is Bowman’s membrane. This is a cell-free layer that provides stability. Due to the absence of cells, it cannot regenerate and heals only by forming scar tissue when injured. It consists of collagen fibrils (cable-like bundles of collagen fibres) and proteoglycans (molecules made of a protein core and polysaccharide chains).
• Stroma: The stroma makes up about 90% of the total corneal thickness and is a connective tissue-like structure. It consists of a large proportion of water and collagen fibres, giving it a leather-like firmness that allows it to withstand pressure from both inside and outside. The collagen fibre bundles (corneal lamellae) are arranged in parallel – if they are damaged, it can severely affect vision. The stroma cannot regenerate itself and heals with scar formation. The high water content also ensures transparency and light refraction.
• Dua’s Layer: This fine layer lies between the stroma and Descemet's membrane. It was only discovered in 2013 by an English ophthalmologist, so it is not mentioned in all literature about the eye. It is a very durable collagen fibre layer that can withstand high pressure.
• Descemet Membrane: Adjacent to Dua’s layer is the Descemet membrane, a second basement membrane composed of a single layer of cells. It thickens with age, making it very resilient. In case of injury or disease, it usually remains intact, preventing the outflow of aqueous humour. Descemet’s membrane is transparent and primarily composed of collagen fibres and laminins (collagen-like glycoproteins). It is the most resilient and elastic layer of the cornea.
• Endothelium: The innermost layer is lined with flat cells. The primary function of the endothelium is to maintain a constant water content. This task is performed by so-called pump cells, which pump out excess aqueous humour, thus regulating water content, which is essential for corneal transparency. The cells do not regenerate but usually last a lifetime. If they die early due to disease, the aqueous humour can no longer be pumped out, and the cornea becomes cloudy.

Problems with the Cornea and Corneal Diseases

Although the cornea is a very resilient organ due to its various layers, it can still be damaged by various diseases, thus affecting the eye. In ophthalmology, many corneal problems and diseases can be diagnosed and treated early with a slit lamp. Every ophthalmologist also offers preventive examinations where the cornea is closely examined.

What Corneal Diseases Exist?

The cornea can be affected by external factors such as foreign bodies, viruses, bacteria, and fungi. Corneal inflammation can have various causes, and injuries can damage the cornea. Additionally, congenital malformations can impair corneal function. No matter the disease affecting the cornea, our vision is usually impaired in most cases.

Astigmatism

Astigmatism is the most common visual impairment affecting the cornea: the curvature of the cornea deviates from its natural shape, altering refractive power and thus impairing visual acuity. Those affected have difficulty focusing on points – they often perceive them as blurred lines, which is why astigmatism is also known as cylindrical vision. It is important to note that astigmatism is not an eye disease but a refractive error.

In most cases, astigmatism is congenital but can also be caused by injuries. Regardless of the cause, it is a refractive error that can be corrected with an appropriate visual aid. A visual aid such as glasses or contact lenses alters light refraction, creating a sharp image on the retina again.

Keratitis (Corneal Inflammation)

Keratitis can have various causes. Bacteria and viruses can enter the eye from outside and trigger an inflammatory process. Keratitis is also often a side effect of other underlying diseases like rheumatism. Wearing poorly fitted contact lenses or poor hygiene when handling lenses increases the risk of keratitis.

The symptoms vary depending on the severity of the inflammation and whether it occurs in one or more layers of the cornea. Red eyes, increased tearing, restricted vision, and eye pain may occur. Keratitis is treated with eye drops, ointments, and possibly tablets, and usually heals without problems. However, corneal scarring may sometimes occur, which can significantly impair vision.

Corneal Opacity

Corneal opacity manifests as a discolouration of the outermost layer of the cornea. The cause is damage to the pump cells, which can no longer drain the aqueous humour, causing it to flow into the cornea, making it swell and cloudy (commonly known as corneal clouding). The transparency of the cornea is no longer guaranteed. The opacity can result from injuries, swelling, or scarring. Metabolic diseases can also be the cause.

Corneal opacity leads to severely impaired vision. In some cases, patients suffer from increased light sensitivity and a foreign body sensation in the eye. Corneal opacity is usually treated with a corneal transplant, as conservative therapy is ineffective. If left untreated, corneal opacity progresses and can lead to complete blindness, depending on the cause.

Dry Eyes

Dry eyes occur when there is insufficient tear fluid. The surface of the eye, including the cornea, is then not adequately moisturised and can be damaged. Wearing contact lenses can cause this problem as they limit the supply of aqueous humour to the cornea. People who blink less frequently, such as during screen work, are at higher risk of developing dry eyes.

Symptoms are varied and can include a foreign body sensation, itching, tearing, and burning of the eyes. The eyes are often red and should be treated promptly to prevent serious damage to the cornea. Dry eyes are often successfully treated with eye drops that mimic tear fluid.

Corneal Injuries

Injuries can cause abrasions on the cornea (corneal erosion). These are often caused by airborne particles, such as those encountered in particularly dusty work environments (grinding, drilling, etc.). Poorly fitted contact lenses can also injure the cornea, as can frequent rubbing of the eyes. Corneal injuries can often lead to inflammation.

A corneal tear can occur if the abrasion penetrates deeper through multiple layers. Depending on the depth and extent of the tear, it can lead to complete blindness. If the cornea is only slightly injured, it will heal on its own within a few days. In any case, an ophthalmologist should be consulted as soon as possible. The ophthalmologist can remove any foreign bodies and treat the injured eye.

Keratoconus

Keratoconus is a typically congenital corneal disease in which the cornea progressively thins and bulges forward into a cone shape. The disease usually begins to develop during adolescence and progresses steadily over several years (10 to 20 years). Between the ages of 30 and 40, it stabilises and no longer worsens.

Depending on the stage, vision can be severely impaired, as the refracted light rays are no longer correctly focused on the retina. For a long time, this can be corrected with suitable visual aids. There are specialised lenses for keratoconus patients that allow most people to live relatively normally despite the disease. In severe cases, when visual aids are no longer effective or cannot be tolerated, a corneal transplant (keratoplasty) may be performed.

Another treatment option for keratoconus is cross-linking, a surgical procedure that strengthens the cornea.

Corneal Ulcer

A corneal ulcer can develop when the epithelial layer is damaged, for example, by a foreign body. Bacteria can then enter the corneal tissue and cause a painful ulcer. The pathogens progressively damage the surrounding tissue, increasing the risk of losing eyesight.

A corneal ulcer is always an emergency and should be treated promptly. Those affected often experience severe pain, red eyes, increased tearing, and heightened light sensitivity. Vision is also significantly impaired. A corneal ulcer is treated with eye drops, ointments, and possibly tablets. In most cases, it heals completely, but scarring may occur.