Keratoconus treatment

Keratoconus is the state when the cornea, which has usually spherical shape, is conically convex and irregularly refractive. This results in worse vision. This eye disease is usually bilateral and the degree of affection in most cases varies from one eye to the other. Convexity and attenuation of the cornea usually progress whether slower or faster and lead to development of short-sightedness and astigmatism and gradually also lead to vision degradation.

In early states, keratoconus can be corrected by eyeglasses but with advancing progression lenses lose their ability to adequately correct the visual sharpness, mainly in low light conditions. Wearing of eyeglasses is also made more complicated by quickly changing refraction of the eye and they may not be tolerated as a consequence of anisometropia (huge difference between both eyes in their visual sharpness). Increasing irregular astigmatism also complicates the correction by eyeglasses.

Application of contact lenses is another option for the correction. The pressure of a contact lens on the cornea changes to some degree curvature of the cornea itself which leads to better vision. Hard contact lenses are the most commonly used type of contact lenses. With advancing progression when contact lenses are not able to carry out their function, for example as a result of intolerance, no improvement in the visual sharpness, one must undergo a surgical procedure in order to treat keratoconus.

To stabilize the disease a patient can undergo a procedure called Collagen Corneal Cross Linking. The implantation of the intrastromal corneal ring segments (ICRS) offers also some form of a solution. In a case when none of previous options would lead to satisfactory vision, a keratoplasty procedure is chosen.

Thermokeratoplasty

Thermokeratoplasty is a laser surgical procedure when conus of the cornea is flattened by controlled heating. This leads to changes in a disposition of the stromal collagen. A diode laser transmits infrared radiation which affects collagen fibers in the stroma. At the spot of radiation fibers heat up and this contributes to their concentration. The tissue volume changes in the coagulated spot and this leads to applanation of that particular spot. On the contrary the cornea arches between individual coagulated spots. This is the way the cornea changes its shape. The aim of the therapy is to increase curvature of flat meridians and decrease curvature of steep meridians. Thermokeratoplasty was developed mainly to correct low hypermetropy up to +1,5D. Its advantages have led to attempts to coagulate also the cornea suffering from keratoconus. Nowadays this method is used only marginally. It is used rather experimentally. A lot of complications used to occur after thermokeratoplasty. It was not possible to secure balanced improvement of the visual sharpness and there were also other complications that occurred such as: instability of the corneal topography, damage of the basal corneal membrane, thinner and scarred cornea, and epithelial defects.

Keratoplasty

Keratoplasty (corneal transplantation) is a surgical procedure when damaged corneal tissue is replaced by new corneal graft obtained from donor. Depending on the type of corneal graft we can distinguish perforating keratoplasty when the whole cornea is replaced and lamellar keratoplasty when only a part of the cornea is replaced. The latter can be divided into superficial or deep lamellar keratoplasty.

Perforating keratoplasty

In this type of procedure the cornea is transplanted in its entirety and thus all of its layers are preserved. A graft has usually a diameter of 5-10mm. Mainly corneas of young donors are being preferred because their cornea has more vital endothelial cells which are essential for preserving transparency of the cornea.

Approximately from 1950 perforating keratoplasty has been primary surgical method in keratoconus treatment. Nowadays anterior lamellar keratoplasty is more and more preferred option of keratoconus treatment because it has advantages of lower occurrence of postoperative astigmatism and fewer rejections.

Lamellar keratoplasty

In lamellar keratoplasty approximately 2/3 of corneal layers are transplanted. This process is technically more complex and the visual sharpness is a bit lower than the one obtained after perforating keratoplasty. Therefore lamellar keratoplasty is used to a limited extent in treating high keratoconus and keratoglobus when it is necessary to have structural supports. This technique requires less time for regeneration and poses less risk of rejection of the corneal transplant. It has some disadvantages in form of vascularization and graft straining.

Decision about what type of treatment should be used is done individually by a surgeon after considering all indicative criteria.

Main reasons which influence the decision:

• Correction by contact lenses is not possible for whatever reason.
• Contact lenses do not provide sufficient vision due to corneal scarring.
• Sufficient visual sharpness is necessary with regard to job requirements.
• Zone of ectasis prolongs in the direction towards the corneal limbus.

Some patients are sufficiently satisfied with the visual sharpness 6/15 - 6/18. After couple of years these patients got used to it and created an ability to correctly interpret blurry vision which enabled them to perform well their daily activities.

Surgical procedure involves some risks which a surgeon needs to consider:

• graft rejection, rejection reactions
• infections
• glaucoma
• high postoperative regular or irregular astigmatism
• steep or flat graft

From immunology´s point of view success rate of first keratoplasty in keratoconus is approximately 92%. In some cases second and third surgeries are necessary because of the graft rejection, complications during healing process or rare infection.

Intrastromal corneal ring segments (ICRS)

Implantation of corneal ring segment to the corneal stroma is a surgical alternative to keratoplasty in keratoconus treatment.

This method was originally developed to correct low myopia up to -3 diopters. The development of excimer laser to perform surgeries of eye refractive disorders caused that this method did not become established. Nowadays it is used to reduce corneal astigmatism and to change it from irregular to regular astigmatism in slight forms of keratoconus.

ICRS are small semicircular plastic segments which are inserted into channels in the corneal stroma (approximately into two thirds of the corneal thickness) outside central area and optical axis of the eye. ICRS cause that the central cornea becomes flatter.

The aim of ICRS implantation is to improve the visual sharpness without removing the corneal tissue or touching central part of the cornea. In comparison with keratoplasty ICRS offer simpler and faster surgical procedure, faster healing process, possibility to influence resulting eye refraction (various alternatives of ICRS) and to a certain degree also the possibility of removing ICRS.

Asymmetric radial keratotomy

Opinions on this refractive surgery which corrects keratoconus differ. Even though it is an intervention to the cornea and tends to be labeled as contraindication for keratoconus, some experienced surgeons treat keratoconus in this way.

It is a surgery during which radial incisions are done on the cornea in 70-80% of its depth. Then during healing process the cornea changes its shape and ideally directs rays of light at the retina so the patient sees sharply. Nowadays this method is hardly being used.

In a case when keratoconus is treated in this way it is possible in some specific cases to correct slight or medium advanced states of the disease.

Corneal Cross linking

Corneal Cross linking is a minimally invasive method which improves biomechanical and biochemical stability of the cornea which in the case of keratoconus is disrupted. It is achieved by using eye drops which contain riboflavin (vitamin B2) and applying them to affected cornea (30-45 min.), and subsequent radiation of the cornea by ultraviolet radiation during 30 minutes. This leads to formation of new transversal links between long chains of collagen. These bonds strengthen the integrity of collagen fibers which are primary connective tissue of the cornea.

The aim of using CCL method is to stabilize the cornea and prevent further progression of keratoconus.