In a healthy eye the cornea is bulged regularly. Thus, objects can be focused in one single focal point and a precise image can be created inside the eye. The image on the left depicts these relations within a normal eye. The cornea is illustrated by the regularly bulged area. As the entire bulging of the cornea is regular, all rays of light emitted by an object are depicted precisely in the focal point. Thus, a precise image is created. On the right there is a topographical image of a normal cornea. The distribution of the bulging upon the cornea is represented by colour coding in this example. Corresponding to the regular bulging the normal cornea shows a rather homogenous colour distribution.
In a normal eye the focal point is located on the retina (similar to a film inside a camera) and the eye sees precisely without further visual aids. In “normal” cases of myopia and far-sightedness this focal point is located either in front or behind the retina; the eye is, thus, either too long or too short. Consequently, glasses are necessary to move the focal point on the retina in order to allow for a precise depiction of the object. Under these circumstances a precise image within the eye can always be created.
The situation when eyes suffer from keratoconus is entirely different. The cornea is irregularly shaped to an extent that the rays of light cannot be focused in one single focal point. Consequently, a precise depiction of objects is not possible. As there is no proper focal point, even glasses fail to move it in order to create a precise image.
The opposite illustration shows what the bulging of an eye suffering from keratoconus looks like. The irregular bulging causes a loss of the focal point. Thus, the emitted rays of light are refracted irregularly on the cornea. Beside the topography there is a typical depiction of keratoconus. In this illustration the local bulging 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 by degeneration (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). Opposite the left image shows the orthogonally regular arrangement of the collagen fibrils inside a normal cornea. Because of hitherto unknown degenerative processes these collagen fibrils are dissolved and the biomechanically advantageous, regular arrangement of the lamellar structure is destroyed (opposite 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 degenerative process. The mechanic stability of the cornea is, thus, decreased which favours a progression of the disease (Daxer A. Biomechanics of the cornea. International Journal of Keratoconus and Ectactic Corneal Diseases 2014; 3: 57-62)
Preliminarily, keratoconus is a mainly genetically caused disease. In Europe, the disease appears 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 isolated from the rest of the region more or less 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, 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 Europe, keratoconus mostly occurs around 20 years of age (between 16 and 25 years of age). However, there are more aggressive kinds of the disease as well which might affect children under the age of 15.
Keratoconus is a progressive disease. This means that the disease is characterized by a continuous decrease of visual acuity. The probability of progression is depending on age. Usually, the following rule applies: the older the patient, the less likely a progression of the disease. However, there are exceptions as well. 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 display the effects of keratoconus, is referred to as “forme fruste”
Initially, patients observe a rather frequent change of the power of their glasses and double silhouettes. Usually, they are diagnosed based on the topography of their eye. Normally, it is characterized by an image resembling the one displayed above which does not leave any doubt as to the nature of the disease (see above). Sometimes, a definite diagnosis is rather difficult in early stages of the disease. In these cases, continuous follow-up assessments are required.