Although the cornea is the most exposed of all parts of the eye to the influence of external factors, there are several mechanisms of its protection.
The first of them is the role of the eyelids, which, with their reflex closure, prevent the passage of foreign bodies. Blinking also mechanically removes impurities that still reach the eye, and this happens with the great help of tears, which simply wash away foreign substances from the eye. Tears also contain some factors that destroy microorganisms (e.g. lysozyme). After all, the cornea itself is very strong and relatively resistant to injury, and its most sensitive part, the epithelium located on the surface, recovers very quickly in case of injury.
Inflammation of the cornea, keratitis (lat. Keratitis)
- Origin and distribution of keratitis
Similar to conjunctivitis, we distinguish between infectious and non-infectious causes of keratitis.
- Infectious keratitis:
Bacterial keratitis:
Bacteria are by far the most common cause of keratitis, usually Staphylococcus aureus or epidermidis, Streptococcus and Pseudomonas, etc. The entire corneal epithelium (surface layer) is a difficult barrier for most bacteria to penetrate into its deeper parts, so keratitis is much easier to overcome. occurrences in weakened or injured epithelium. An ulcer appears as a result of localized inflammation of the cornea. It is actually a thinned, “bitten” part of the cornea, which is also cloudy due to the accumulation of inflammatory elements. When the deeper parts of the cornea are affected, as a reaction on its inner side, inflammatory elements can also accumulate in the form of small dotted more or less abundant accumulations, until they completely fill the lower part of the anterior chamber. In the most severe cases, there is a rupture (perforation) of the cornea with very serious consequences. This complication can also occur with viral and fungal keratitis.
Viral keratitis:
Herpes simplex and herpes zoster viruses can cause inflammation of the conjunctiva and cornea separately or more often together. Both types of viruses are created in the nerve ganglia, and are activated when the organism becomes mentally and physically weak. Then they travel through the nerve to the eye where they cause disease. Herpes simplex virus causes lesions (injuries) on the surface of the cornea in the form of lines or twigs. Regardless of whether it first affected the epithelium or not, it can penetrate into deeper parts and cause the so-called stromal keratitis, which is characterized by the formation of whitish nodules. At the same time, a reaction from the inside of the cornea can also occur. Acute necrosis (sudden death) of the cornea occurs in patients severely weakened by malignant diseases or AIDS. With herpes zoster, lesions in the form of twigs also appear on the surface, and changes can also occur in the deeper parts of the cornea (nummular and disciform keratitis) as well as on its inner side.
Adenoviruses cause epidemic keratoconjunctivitis described in conjunctival diseases, and on the cornea there are small surface injuries and clouding of deeper parts (subepithelial nodules).
Fungal keratitis:
This disease is relatively rare, and its occurrence is facilitated by the excessive use of antibiotic and corticosteroid eye drops and eye injury by objects contaminated with fungi. The most common causative agents are Aspergillus and Candida fungi. The disease develops over several days, during which a corneal ulcer forms with a reaction on its inner side.
- Non-infectious keratitis
We also call this group keratopathies. The cause is a whole series of factors that weaken the cornea and contribute to its disease. Eyelid and conjunctival inflammations sometimes cause damage and deterioration of the corneal epithelium due to mechanical and metabolic reasons. Foreign bodies, especially those that stay under the upper eyelid, scrape the cornea and thus create microinjuries. A similar type of injury can be caused by unclean and damaged contact lenses. Namely, as they are slightly mobile on the cornea, they damage it by friction.
Smog and other air pollution act mechanically through microparticles dispersed in them and chemically.
Changes in the position of the eyelids cause constant contact of the cornea with the eyelashes. In the opposite case, if there is no complete closure of the eyelids, as sometimes happens with paresis of the facial nerve or thyroid disease, drying of the epithelium is possible, which significantly weakens it. As a result of the influence of all these factors and repeated injury to the cornea, its resistance weakens, so sometimes there is an accumulation of bacteria, viruses or fungi and an infection develops.
- Symptoms
The disease develops over several days. It starts with mild non-specific disturbances in the form of burning and burning in the eyes, increased narrowing and slight redness. As more and more parts of the cornea are affected, there is a strong sensitivity to light and blurred vision. The secretion is abundant, in the case of bacterial inflammations it is more mucous to muco-purulent, and in the case of viral infections it is watery. In case of deeper ulcers, the pain also begins. The eyelids spontaneously close tightly.
- Establishing a diagnosis
The diagnosis is made on the basis of an examination in a specialist clinic. Sometimes the causative agent can be determined even then, and if this is not the case, a swab is taken.
- Treatment
The treatment depends on the cause of the inflammation and the present symptoms. Antibiotic, antiviral or antifungal medicines are used in the form of eye drops or ointments or taken orally. In some cases, anti-inflammatory corticosteroids are used, but you have to be very careful with them because they can aggravate the disease if they are not used correctly. In case of major damage to the surface of the cornea, preparations are also used that accelerate its healing. None of these drugs and preparations are good to use on your own, but you should always seek the advice of a doctor.
When after the inflammation there is cloudiness of the cornea that interferes with the vision more, it is possible to transplant another person’s cornea. This operation is already routinely performed today, and the results are excellent. Clouding and graft rejection occur very rarely.
- Degenerations and dystrophies of the cornea:
Corneal degeneration is a common name for a group of diseases that occurs predominantly in old age. They are characterized by the accumulation of certain substances that are the result of disturbed metabolism in specific areas of the cornea. These accumulations cause its clouding, and if they are located in the area of the pupil, also weakening of vision.
Corneal dystrophies have a similar appearance and form of degeneration as degeneration, but their basis is genetic.
Both groups of diseases are extremely large and fortunately not too common.
- Keratoconus and keratoglobus
These two quite similar diseases are characterized by a disorder of the curvature of the cornea so that it has a protrusion in one part. This condition is unpleasant for several reasons and interferes with vision. First, as we know, a clear and evenly curved cornea is one of the prerequisites for clear vision. On the more curved part of the cornea, light is refracted differently than elsewhere, so it is not possible to fix the image in the macula, which leads to blurred vision. Secondly, in the place of that stronger curvature, i.e. keratoconus, the cornea is weakened, so sometimes there are changes in the water content, which again causes cloudiness. In keratoglobus, the entire cornea is bulging, not just one part of it.
Vision can only be minimally improved with glasses, and the results are better when wearing hard lenses because they press on the eye and thus correct the cornea. In severe cases, a corneal transplant is required.
Literature:
M.Sc. sc. Pavan Dr. Joško, “Eye Diseases”, Zagreb 2003.
