In a healthy eye, the cornea has a more or less single curvature over the entire cornea. Objects can therefore be focused in one single focal point to create a sharp image. The pictures on the left show this relation. The cornea is illustrated by the regularly curved line. All light rays converge to a focal point. The second picture shows the related topography of a normal cornea. The homogeneous curvature over the entire corneal surface is colour coded and represented by more or less one color over the entire corneal surface.
In a normal eye of zero dioptres, the focal point is located on the retina (similar to a film inside a camera) and the eye sees precisely without further optical aids. In “normal” eyes suffering from myopia (shortsightedness) and hyperopia (farsightedness), the eye is either "too long" or "too short". In these cases, the focal point is present, but located either in front or behind the retina causing a blurred image without optical aids. Consequently, glasses are necessary to move the focal point on the retina in order to allow for a sharp imaging of the object. This is not the case in keratoconus, where the irregularily shaped cornea does not allow to create a single focal point.
The situation is totally different in eyes suffering from keratoconus. The cornea is irregularly shaped to an extent that the incoming light-rays cannot be focused on one single focal point anymore. Consequently, a sharp imaging of objects is not possible in this eye anymore. As there is no proper focal point, even glasses fail to create a sharp image at the retina. At a certain stage of the disease, the vision is always blurred, with or without glasses.
The illustration on the left shows what a keratoconic cornea with an irregular curvature looks like. The irregular bulging causes a loss of the focal point. A sharp imaging is not possible anymore. The right picture shows a typical topography of keratoconus. In this illustration, the irregular and inhomogeneous curvature is colour coded as well. The steeper areas range from red to purple, whereas the flatter ones are green.
The irregular corneal structure explained above is created by the following process: On an ultrastructural level, the system of collagen fibrils, which comprises the static elements of the cornea, is destroyed (Daxer A. and Fratzl P. Collagen fibril orientation in the human corneal stroma and its implications in keratoconus. Invest Ophthalmol Vis Sci 1997; 38:121-129). The left image shows the orthogonally regular arrangement of the collagen fibrils inside a normal cornea. In keratoconus, collagen fibrils are degraded and dissolved and the regular arrangement of the lamellar structure, which is the basis of the intact corneal biomechanics, is destroyed (illustration on the right). Consequently, a thinning of the cornea takes places. As a result, the elastic tensions within the cornea increase which, in addition, might add to the progression of the degenerative process.
Keratoconus is a mainly genetically caused disease. In Europe and North America, the disease seems to appear sporadic and approximately one out of 1000-2000 inhabitants suffers from keratoconus. However, there are also so-called endemic regions where the disease is diagnosed more frequently. One such endemic region could, for example, be a mountain valley which was more or less isolated from the rest of the region for generations. The same applies to islands and other isolated areas of settlement where, based on a decrease of the gene pool, the probability of genetically caused diseases increases accordingly. Furthermore, there are cultural factors which require consideration. In some regions, such as the Middle East where, traditionally, weddings among relatives are more common than in Europe and North America, keratoconus is diagnosed more frequently. Thus, there are regions in the Middle East where up to 10% of the local population suffer from keratoconus. In addition to a genetic origin, other factors like eye-rubbing are accused to be causative.
In Europe, keratoconus mostly occurs in patients at around 20 years of age (generally between 16 and 25 years of age). However, more aggressive types of the disease, which affect children under the age of 15, exist.
Keratoconus is a progressive disease. This means that the disease is characterized by a continuous decrease of visual acuity. The probability of progression depends on the patient's age. Usually, the following rule applies: the older the patient, the less likely a progression of the disease. However, there are exceptions. There are, for example, patients aged 40 who, even after crosslinking, still suffer from a progression of the disease. A regular medical examination performed by an ophthalmologist is, thus, absolutely necessary.
Given its genetic origin, the disease always affects both eyes. However, it is possible that, clinically, it only shows on one side. The eye which does not show the effects of keratoconus, is referred to as “forme fruste”.
Initially, patients often observe a rather frequent change of the power of their glasses and double contours. Usually, keratoconus is diagnosed by the atypical topographic pattern.