The cornea is located at the front of the outer skin of the eye and merges into the sclera at the limbus. In contrast to the sclera, it is translucent and constantly requires tear fluid. The cornea is more curved than the sclera and protrudes from the eyeball. Defects in the cornea are among the most common diseases of the outer skin of the eye. They are divided into inflammatory and non-inflammatory injuries according to their causes.
The cornea is the transparent section of the outer skin of the eye and serves as the "windshield of the eye". It is formed by keratinocytes, i.e. the specialized cells of this structure.
The translucency is created by:
The cornea of the eye is permeated with nerves. As a clear disc with a diameter of 11.7 millimeters, the cornea of the eye sits directly in front of the pupil. It has a thickness of around 0.6 millimetres in the middle and around 0.8 millimetres at the edges.
The cornea protects the iris and the sensitive inside of the eye @ Vladimir /AdobeStock
Viewed under the microscope from the outside in, six layers can be distinguished in the cornea of the eye:
Illustration of the different corneal layers
The cornea in the outer skin of the eye acts as a window in the eyeball to ensure sharp vision.
It has three important functions:
The natural refractive power of the cornea is around 40 diopters. This means that the cornea of the eye is responsible for two thirds of the refraction of light and therefore contributes to image focusing. A key factor in this is the aqueous humor, which is located behind the cornea of the eye.
The cornea must be constantly moistened with fluid so that it does not dry out. This task is performed by the eyelids: by blinking, they distribute tear fluid evenly over the cornea.
The endothelial layer of the cornea ensures dehydration with its various metabolic processes . It ensures internal drainage and transparency.
The density of metabolically active cells in the endothelium of the cornea of the eye decreases with increasing age. The cells of the endothelium have difficulty regenerating, which is why they need to be protected.
Protection against infection, injury and destruction is provided by the Descemet's membrane. The epithelium on the surface of the cornea is located in the outer skin of the eye. It can renew itself and ensures that small wounds close again without scarring.
Defects in the cornea are among the most common diseases of the outer skin of the eye. They are divided into inflammatory and non-inflammatory injuries to the cornea of the eye according to their causes.
In ophthalmology, injuries to the outer skin of the eye caused by foreign bodies occur frequently. The strong flow of tears flushes the foreign body out of the eye without causing damage to the outer skin of the eye.
Rinsing with an eye wash bottle or under running water can help. Additional rubbing should be avoided as it leads to superficial damage to the cornea.
A special case is a chemical burn to the outer skin of the eye, which usually occurs as a result of accidents at work. This is an acute emergency that requires immediate intervention.
The first thing to do in the event of a chemical burn is to rinse the eye intensively for 20 to 30 minutes. The next step is to consult an ophthalmologist for further clarification in order to prevent blindness.
The healthy cornea has an even curvature. This allows it to optimally refract the light coming in from all sides 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 an altered perception, e.g. of round objects that appear line-shaped or rod-shaped.
This anomaly is called astigmatism. Those affected see objects near and far out of focus and distorted. They often suffer from headaches.
In rare cases, in addition to the change in the cornea, there is also a disorder in the lens. Astigmatism is often congenital and can be hereditary.
It is usually accompanied by short-sightedness or long-sightedness. There are also acquired causes. These include diseases or scarring within the outer skin of the eye.
The most common deviation is a curvature in the vertical plane (astigmatism). A second distinction is made as to whether the cornea is curved at different points(irregular astigmatism) or not (regularastigmatism ).
Doctors correct the deficits in the regular form using special glasses or contact lenses. Hard contact lenses are suitable for irregular astigmatism, but not glasses. Refractive surgery procedures are suitable for correcting larger deviations, whereby a laser is most frequently used.
With a frequency of one in 2,000, keratoconus is a non-inflammatory disease of the cornea. It is hereditary and the symptoms are already apparent in adolescence. The cornea thins out in the middle and bulges forward in a cone shape. This significantly impairs vision. The prognosis for this disease of the outer skin of the eye varies depending on the severity.
Woman with keratoconus @ Alessandro Grandini /AdobeStock
Clouding of the cornea results in reduced visual acuity and in some cases also leads to pain.
The most common causes include swelling (edema) and scarring of the cornea. Corneal scars in the outer skin of the eye are noticeable as whitish opacities.
They occur following inflammation, ulcers or metabolic disorders. A risk factor for inflammation is previous damage to the cornea, e.g. incorrectly inserted contact lenses.
Inflammation of the cornea (keratitis) can lead to a bacterial or viral infection or fungal infestation. If left untreated, this can lead to the formation of a corneal ulcer (ulcus corneaea). This can extend into the stroma or Descemet's membrane and lead to perforation of the eye.
Extensive scarring on the outer skin of the eye can cause serious injury to the cornea. In some cases, scarring promotes vascularization. This causes blood vessels to form in the otherwise vascular-free area of the outer skin of the eye, which further impairs vision .
Damage to the cells in the endothelial layer causes fluid to enter the cornea. This thickens, leads to edema and clouding.
Fuchs endothelial dystrophy (metabolic disease) also damages the endothelial cell layer inside the cornea. This leads to fluid accumulation. If vision is severely impaired, only surgical methods such as penetrating keratoplasty can help.
Today, lamellar keratoplasty is usually used. Here, doctors only replace the Descemet membrane (Descemet membrane endothelial keratoplasty DMEK).
With age or after injuries, degenerative changes can occur in which the cornea loses its homogeneous and clear structure. However, some age-related effects of this kind have no pathological value.
Transplanted cornea
Clouding in the cornea of the eye can only very rarely be remedied by conservative, i.e. medical, therapy. Only a corneal transplant can prevent blindness in the advanced stages of corneal damage.
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