The cornea, the transparent front surface of the eye, provides around two thirds of the total refractive power of the eye. This means that it must be in optimal condition for sight to remain clear, as any deviation from normal will lead to large changes in vision.
There are several disorders that can cause the cornea to thin in structure and change in shape, leading to reduced vision that is not correctable with spectacles, standard soft contact lenses, or laser refractive surgeries. The most common of these disorders is keratoconus, and what it is and how it is diagnosed and managed will be explained in this article.
The word keratoconus is composed of the Greek words for cornea and cone, and this describes well what happens in this disease.
It is a corneal ectatic condition, which means it causes the cornea to progressively thin, and this thinning leads to a reduction in strength and rigidity which causes it to bulge forward and downward as the pressure of the fluid within the eye pushes outward.
Keratoconus primarily causes the lower-central area of the cornea to become cone shaped, leading to very high, unstable, and irregular glasses prescriptions along with visual symptoms like ghosting and shadowing.
In cases that are beyond mild, the vision cannot be corrected to normal levels with regular methods like glasses.
Keratoconus is also related to other corneal ectatic conditions like keratoglobus (the entire cornea is thin and bulging) and pellucid marginal degeneration (the entire lower half of the cornea is thin and bulging).
It is related to genetic factors, systemic conditions like connective tissue disorders, and eye allergies or inflammations where someone is frequently and intensely rubbing their eyes.
The condition usually progresses its highest rate in the teens to thirties and stays relatively stable after that point.
Keratoconus might be initially suspected as a glasses prescription that changes frequently in adolescence with associated corneal steepening. Vision might not be very crisp even with the optimal spectacle or soft contact lens prescription.
Using corneal topography or tomography, which capture the curvature and tissue details of the cornea, keratoconus eventually shows up as a pattern of lower-central steepening, bulging, and thinning.
In later stages, it also causes distortion and irregular results for some methods used to get the prescription, along with specific findings in the cornea when viewed through the slit lamp microscope by our eye doctor.
If keratoconus is not discovered before becoming quite severe, it might be found only when the cornea becomes thin and weak enough to let fluid from inside the eye into it, leading to painful blistering and profoundly reduced vision.
In keratoconus that is progressive in a young patient, corneal cross linking will be considered. In this procedure, a surgeon removes the front layer of the cornea and drips a vitamin called riboflavin onto the cornea while illuminating it with ultraviolet light for about a half hour.
The front layer of the cornea regenerates over days to weeks and this procedure helps to strengthen deeper corneal tissues so that keratoconus progression is much reduced. Vision and the prescription continue to change over several months to a year before settling.
Once keratoconus reaches a certain point, even with cross linking completed, special contact lenses (corneal and scleral lenses) are required to deliver optimal vision as only they can compensate for the irregularities in the cornea while glasses and soft contact lenses cannot do this.
Refractive laser surgery cannot be done in someone with keratoconus as it requires removing tissue from an already weakened and thin cornea, which can lead to disastrous thinning and bulging of the cornea in later years.
In some cases, corneal transplantation is still required, where the central keratoconic tissue is replaced with a healthy donor cornea. This is reserved for corneas that have become too compromised to deliver healthy and clear vision even with specialty contact lenses.