Keratoconus is a disease of the eye, specifically of the cornea. Cornea, which normally has shape of a hemisphere, slowly becomes more thin and protruding, like a cone (conus). The disorder of corneal shape disrupts the regular light input and it’s focus on the cornea so the consequence is impaired vision.
Cornea
Cornea, is a transparent external part of the eye, with a hemispheric shape. It coveres the frontal part of the eye. Like a window into the world, it allows rays of light to enter the eye and vision to be normal. Even though it is transparent and it looks like it lacks some tissue, it is built out of highly organized cell and proteins group. Unlike most of the tissues in our body, cornea does not contain blood vessels which would feed it and protect it from infections. Cornea gets food from tears and eyewash instead, which fills the eye chamber behind it. Cornea needs to be transparent so it can refract the light right. Cornea tissue is organized into five basic layers, and each of them has an important function. These are: corneal epithelium, Bowman’s layer, corneal stroma, Descemet’s membrane and corneal endothelium.
Age
Average age for keratoconus occurence is 16 years, but there is a description for symptom beginning at 6 year olds. Keratoconus rarely occurs after 30.
Gender
Keratoconus occurs equally for both sexes.
Cause
The cause of keratoconus is probably multifactorial, it is a consequence of various factors. It is supposed that keratoconus is caused by biochemical and physical changes of the cornea tissue, even though none of the theories do not explain completely the clinical causes and symptoms.
It is possible that this disease is the final result or the final stage of multiple physical disorders. Keratoconus has a connection with inheritance, hypersensitivity diseases, certain systemic disorders and wearing rough lenses for a long time.
Even though the disease occurs in less cases in the family, one of the main factors is certainly genetic. The proof of the disease being inherited by the descendant’s is the difference for inheritance with identical and fraternal twins, and connection to other genetical disorders like Down and Marfan syndrome. The difference for occurence with different ethnic groups has been spotted.
Categories
Keratoconus can be different according to cone shape, medium keratometric reading or progress speed.
According to the curvature it can be:
- Mild, less than 45 D (diopter) in both eye meridians
- Moderate, 45-52 D
- Advanced, > 52 D
- Severe, > 62 D
According to cone shape:
- Small diameter 5 mm, round shape; it is easily resolved with contact lenses
- Large diameter, > 5mm, oval shape, usually with an offset in the lower part, so it is hard to put the lenses
- The largest diameter, > 6 mm; it infects more than 75% of the cornea and it is very hard to treat it with lenses
Development of the disease
In 90% of the cases the disease infects both eyes. It is most common for it to develop asymmetrically, the diagnosis for the other eye is set around five years after the first eye is diagnosed. The process can be active for five, ten or twenty years and after that it can stabilize, or stop progressing for a long period of time, even for life. During the active phase, progressing of the disease can be very slow or fast, individual variations are very large.
Symptoms
Keratoconus often starts as nearsightedness and astigmatism, so the patients don’t know that they have this condition. The cornea is slowly or quickly getting thinner, it is different for certain patients, and with that process the eyesight is weakened. The sight attenuation can be moderate or very severe, which depends on the quantity of the infected tissue of the cornea. The patients usually have double vision or multiple vision in one eye, so they complain about image distortion. For some patients halos, light circles, photophobia and sensitivity to light appear. Sometimes during the disease, sudden drop of sight can occur because of cornea rupture and the eyewash getting into it. The resultant swelling is called hydropsy. The swelling can last for weeks or months until the crevice heals, and gradually it is replaced with scar tissue. When the sudden swelling occurs, the doctor can prescribe eye drops to relieve the pain, but no doctor can stop the disease from progressing.
Diagnosis
It is relatively easy to diagnose moderate or advanced keratoconus. It’s diagnosis in the early stage, at the beginning of the disease, is much harder and it requires more dana and tests. Patients with keratoconus often change visual aids and diopters in a very short period of time and none of them provides a satisfactory visual correction. Diopters often vary so the tests are different from one examination to another.
The tests that are used to diagnose this condition are:
- Retinoscopy
- Direct ophthalmoscopy
- Keratometry
- Photokeratoscopy
- Examination with a crevical lamp, biomicroscope
Biomikroscope can help noticing a number of classical signs of keratoconus: Fleischer ring, Vogt’s striae, thinning and scarring of the cornea, occurence of different colours in normally transparent cornea, increased visibility of the corneal nerves and hydropsy of the cornea.
Fleischer ring is yellow-brownish or olive pigmentary ring which can, but not necessarily, surround the basis of the cone. It occurs when the hemosiderin pigment precipitates in the epithelial cells. With the progress of the disease, Fleischer ring becomes thinner and more discrete. With a thorough examination this sign can be found with 50% of the patients. Using the cobalt filter can help discovering this sign more easily in the beginning.
Vogt’s striae are small, brush shaped, vertical, but they can also be curved. They are located in the deep parts of the corneal tissue in the are of the largest curvature. They disappear after pressing the apple of the eye through the eyelid. After the pressure is lowered, Vogt’s striae appear again and they are easier to see. Rough lenses sometimes emphasize these stripes.
Thinning and scarring of the cornea are the characteristics of this disease. Scars are usually spotted later when crevices in Bowman’s layer appear. Cornea blurring can occur with patients that never wore contact lenses. Corneal cells die out and they are being replaced with cells from the edge. Blurring can occur for rubbing the eyes or contact lenses that don’t fit very well.
Hydropsy of the cornea is accumulation of the eyewash in the cornea in later stages of the disease when Descement’s membrane ruptures. Sudden occurence of this condition causes sudden loss of sight and a visible spot on the cornea. Hydropsy causes swelling and blurring. When border membrane is recovered, these symptoms can disappear. Sometimes hydropsy fixes the sight for patients that had a cornea that was too rigid. If a scar appears, often the cornea becomes more flat, which facilitates wearing of contact lenses. Hydropsy is more common for patients with Down’s syndrome.
Munson’s sign is noticed without the use of a crevical lamp. It occurs with advanced keratoconus when the cornea is so protruding, that it causes angular incurvation of the lower lid while looking down.
Razzutijev light reflex is a term for dislocation of the light reflex according to the nasal furrow when the light is projected on the side of the temples. It occurs with other eye diseases as well, but it can help with diagnosis when BIOMICROSCOPE is not available.
Reduced eye pressure is often a sign and a result of thinner cornea and / or increased rigidness of the sclera.
Treatment
The loss of eyesight can be cured with appropriate glasses, and later irregular astigmatism requires wearing of firm contact lenses. Contact lenses can improve sight, and they can contribute to disappearing of the scars on the cornea.
Even though with the progress of the disease, the patients can still read and drive, they often think the quality of their lives is reduced.
Frequent checkups with the ophthalmologists are needed, at least once a year, and it is recommended to do it more often so the progress of the disease could be tracked and therapeutic interventions could be undertaken.
When conservative treatment, glasses, lenses and local medication cannot provide satisfactory vision, the alternative is transplantation of the cornea. Of all transplantation procedures, transplantation of the cornea is the most common and the most successful procedure. After the decision that this procedure is needed, the name of the patient is put on a waiting list.
Cornea transplantation is also called penetrating keratoplasty. In this procedure, the medium part of the cornea is removed (“the button”) and a graft is put to that place, corneal tissue of full thickness. Graft is stitched with a thin thread. If that eye also has lens blurring, cataract, it can also be operated simultaneously. Discovering the scar on the cornea and high keratometric values are often prognosis of almost needed surgery.
The procedure usually takes 6-90 minutes. After this procedure, contact lenses are often needed for better visual correction.
Other possibility is partial transplantation of the cornea, lamellar keratoplasty. Only a part of cornea is removed and on that place a graft is installed. This method is technically more demanding and visual sharpness is lower after the procedure, but it takes less time to recover, and possibility of transplant rejection is smaller.
Epikeratophakia is a term for a procedure in which the graft of the cornea is stitched directly above the patient’s cornea. The procedure is low-risk, mostly successful and it is applied with patients that cannot have a different cornea operation.
Excimer laser procedures allow removing of the medial part of the cornea and easier placement of the transplants.