Cornea

The cornea (Latin Cornea) is the crystal-clear covering of the eyeball. It forms the transparent front surface of the eye, through which the iris and pupil are visible.

It has no blood vessels and is particularly sensitive due to the high density of sensory nerves.

It consists of 5 layers from outside to inside:

1. Epithelium: outer protective cell layer capable of regeneration. The tear film adheres to it.
2. Bowman’s membrane: It separates the epithelium from the stroma and serves as the base for the overlying epithelial cell layer. It is not fully regenerable after injury.
3. Stroma: The stroma makes up approximately 90% of the corneal thickness. It contains collagen fibers and is primarily responsible for the shape, strength, and refractive power of the cornea.
4. Descemet’s membrane: It separates the stroma from the underlying endothelium and serves as the foundation of the endothelial cell layer.
5. Endothelium: The single-layered endothelial cell layer has a pump function that regulates the water balance of the stroma to maintain its clear transparency.

The cornea is responsible for approximately 70% of the total refractive power of the eye with an average of 43 diopters. The remaining refractive power is provided by the natural lens.

In a healthy eye, the cornea is regularly curved. This allows an object to be focused to a focal point and a sharp image to be produced on the retina (similar to film in a camera).

In a normal eye, the focal point lies on the retina and the eye sees clearly without further visual aids. In cases of “normal” nearsightedness or farsightedness, the focal point lies in front of or behind the retina – the eye is relatively too long or too short. Then glasses or contact lenses are necessary, or a minimally invasive procedure is possible to shift the focal point onto the retina for clear vision.

Keratoconus

Keratoconus is a non-inflammatory eye disease of the cornea in which it thins and progressively bulges due to insufficient biomechanical stability until it assumes a conical shape. The bulging often appears in the lower area of the eye. Approximately one in 2,000 people is affected, usually with a time delay in both eyes.

Keratoconus usually occurs in the second to third decade of life and typically comes to a standstill after the fourth decade following a usually progressive course. In some patients, the progressive form of keratoconus can not only make wearing special contact lenses impossible but may also require a corneal transplant.

There is a risk that in the early stages, this rare eye disease may be confused with astigmatism and therefore incorrectly treated. Only experienced physicians are able to reliably diagnose keratoconus through examination of the corneal surface, the corneal posterior surface (endothelium), corneal thickness, and if necessary, endothelial cell count determination.

Causes of Keratoconus

Keratoconus is presumably a disruption between the collagen molecules of the connective tissue framework of the cornea, leading to reduced collagen cross-linking and thus reduced biomechanical stability of the cornea.

The causes are multifactorial and not yet fully understood. A genetic predisposition is suggested by the fact that keratoconus often occurs in familial clusters. Immunological factors may also play a role. What is certain, however, is that vigorous and frequent eye rubbing over years, for example in allergy sufferers, represents a high risk factor for the development of keratoconus. There are also associations with other conditions frequently linked to keratoconus, such as atopic dermatitis or trisomy 21. Thyroid dysfunction is also discussed as a possible cause of the disease.

Symptoms of Keratoconus

Possible consequences of keratoconus and the associated irregular astigmatism include unusual fluctuations and decrease in visual acuity, perception of light rings around light sources (so-called “halos”), as well as increased light sensitivity and glare. In many patients, sufficiently good visual acuity cannot be achieved even with glasses correction. Most patients benefit visually from wearing rigid (hard) contact lenses. These improve vision by bridging the irregular corneal curvature, but only while the lenses are worn – they do not halt the progression of the disease.

In advanced stages of the disease, folds (Vogt striae) and permanently persistent corneal scars can form, which can significantly impair vision even with rigid contact lenses.

In extreme cases, corneal hydrops can occur. In very advanced cases, the Descemet’s membrane of the cornea ruptures, allowing fluid to penetrate the cornea from the inside, leading to sudden severe visual impairment.

Course of Keratoconus Disease

Keratoconus usually develops during puberty or early adulthood. It is typically a progressive disease course that can advance until the fourth or fifth decade of life, with severity varying greatly among individuals. Usually both eyes are affected to different degrees. The spectrum ranges from asymptomatic incidental findings to far-advanced disease stages with corneal scarring and the need for corneal transplantation.

If the disease stabilizes at an early stage before the full cone shape has formed, this is referred to as “keratoconus forme fruste.”

Diagnosis of Keratoconus

Early detection of the disease is crucial. Frequently, hints arise from the family history due to the genetic component. It is not uncommon for relatives such as children, parents, or siblings to also be affected and they should undergo ophthalmological screening when a family member is diagnosed. Often, a visual acuity that can no longer be perfectly corrected with glasses first indicates the presence of keratoconus.

Diagnosis is currently made using corneal topography and tomography with modern measurement devices.

At our center, two of the most advanced measurement devices in the world are used:

MS-39 by SCHWIND and Anterion by Heidelberg Engineering

With the help of these devices, the corneal front and back surfaces including curvature radii and corneal thickness are measured, and the probability and severity of the disease are captured in so-called scores.

The examinations are non-contact, hygienic, completely painless, and take only a few seconds.

Essential for keratoconus diagnostics is the longitudinal monitoring of these measured parameters through follow-up examinations at intervals, in order to make statements about the progression behavior of the disease.

Therapy for Keratoconus

The disease is not curable. Nevertheless, there are options to improve visual performance on one hand and to halt possible progression of the disease on the other.

Visual performance can generally be improved by wearing glasses or contact lenses. Keratoconus patients must never undergo refractive laser treatment, so as not to further weaken the already insufficient biomechanical stability of the cornea.

Rigid, i.e., hard contact lenses are the treatment of choice to optimize visual performance. These best compensate for irregular corneal curvatures. If tolerance is poor, additional systems such as mini-scleral lenses or a so-called “piggyback” system (soft + hard contact lens) are available.

In cases of vision-impairing scars or keratoconus stages so advanced that contact lens fitting is impossible, the options are significantly reduced. In these cases, a corneal transplant must be discussed.

The therapy of choice to halt disease progression is crosslinking, described below.

Crosslinking: Collagen Cross-Linking of the Cornea

The gold standard for slowing or completely halting the progression of keratoconus is the performance of so-called crosslinking.

When defined progression of the disease is documented, the costs for this therapy of choice are covered by both private and statutory health insurance.

At our center, the so-called epi-off procedure is performed with one of the most modern and widely used technologies on the market for optimal corneal stabilization. Our department uses the iLink technology from Glaukos with the KXL system.

The first crosslinking was performed according to the so-called Dresden Protocol, which has been further optimized since then. The duration of the procedure at our center is approximately 30 minutes.

During the procedure, the corneal epithelium is first removed either manually or with a laser. Subsequently, the cornea is saturated with a riboflavin (vitamin B2)-containing solution and then irradiated with UVA light. The procedure is performed on an outpatient basis under local (topical) anesthesia.

The procedure is generally considered low-risk and is performed at our center under sterile conditions in the operating room.

The principle of crosslinking is to stimulate collagen cross-linking in the stroma by releasing oxygen radicals in the cornea. This stabilizes the corneal tissue, counteracting the progression of ectatic diseases such as keratoconus and, in most cases, halting them.

In addition to keratoconus, other ectatic diseases such as pellucid marginal degeneration of the cornea or ectasias after laser treatments can also be treated with crosslinking. The decisive factor for indication in all diseases is that the cornea still has sufficient thickness.

What Are the Causes of the Corneal Disease Keratoconus?

Keratoconus is presumably a disruption between the collagen molecules of the connective tissue framework of the cornea, leading to reduced collagen cross-linking and thus reduced biomechanical stability of the cornea.

The causes are multifactorial and not yet fully understood. A genetic predisposition is suggested by the frequent familial clustering. Immunological factors may also play a role. What is certain, however, is that vigorous and frequent eye rubbing over years, for example in allergy sufferers, represents a high risk factor for the development of keratoconus. Thyroid dysfunction could also be a cause.

For this reason, we recommend that our keratoconus patients have their thyroid examined to potentially undergo thyroid hormone therapy. The thyroid hormone thyroxine appears to have a significant influence on corneal biomechanics: it increases the growth and stiffness of corneal tissue and affects the properties of the collagen-protein compound.

Forme Fruste

Forme fruste is indistinguishable from normal astigmatism in its symptoms and is often only discovered incidentally during a thorough ophthalmological examination. It can be corrected with glasses or contact lenses and, if stable, does not need to be treated but should be monitored and regularly checked. Forme fruste is ten times more common than the progressive form.

The Progressive Form of Keratoconus

The progressive form of keratoconus is aggressive and often becomes noticeable as early as the teenage years. The progressive bulging causes corneal curvature, called astigmatism. Due to this irregularity of the cornea, correction with glasses becomes increasingly difficult. Contact lenses are more suitable in this case, as they exert pressure on the cornea and can compensate for the major irregularities. If the disease is not too far advanced, good correction can be achieved with lenses, up to maximum visual acuity.

However, if the corneal bulging continues to increase, even hard contact lenses sit less stably and can fall out. Additionally, with progressive bulging, the cornea below the center becomes increasingly thin and scarred, irreversibly worsening visual acuity.

DIAGNOSTICS

Corneal Topography

The so-called topography, a digital, automated analysis of corneal characteristics, is groundbreaking for the diagnosis of forme fruste or progressive keratoconus.

The keratograph is an instrument for capturing and evaluating corneal topography. A Placido disc (ring system with concentrically alternating black and white rings) is projected onto the anterior corneal surface, the ring-shaped reflection images are captured with a video camera and analyzed by a computer system using Fourier analysis.

A significant advantage compared to ophthalmometer measurement lies in the number of measurement points. In ophthalmometer measurement, only a few measurement points are captured (two central, four peripheral). Keratographs capture 10,000 to 30,000 measurement points depending on the device, resulting in a detailed profile of the cornea. Different display modes can be selected. Corneal topography can be displayed numerically, color-coded, or as a three-dimensional surface map.

Originally, the keratograph was developed for corneal surgery. It has since also found widespread use as a preliminary examination for cataract surgery or LASIK procedures, as well as in contact lens fitting.

Endothelial Microscopy

Our cornea consists of many layers. The innermost layer is the endothelium, whose function is to keep the cornea clear. Dysfunction of this endothelial layer can lead to hazy and blurred vision. With the help of endothelial microscopy, cell density, the number of endothelial cells, and cell morphology can be assessed.

To assess the risk of postoperative corneal irritation, such as after cataract surgery with intraocular lens implantation, endothelial microscopy is recommended. This examination is an elective service.

A disease of the corneal endothelium has significant effects on your vision, for example after or even before cataract surgery.

TREATMENT

A promising treatment for keratoconus is fitting with hard contact lenses. Our team in Munich collaborates with all owner-operated contact lens experts (Söhnges Optik, Docs Optik, Schneider Optik, Wörle Optik, Fassler Optik Germering, and many more) in the interest of stabilizing keratoconus.

Sometimes transplantation of corneal layers (so-called lamellar keratoplasty) is necessary. We collaborate with leading experts in Germany (Prof. Dr. Kruse in Erlangen) and look back on a longstanding, trusted partnership.