Keratoconus: the degenerative and evolutionary disease of the cornea

By Cristiano Antonino On Nov 28, 2022

Keratoconus is a degenerative disease of the cornea that, over time, can worsen and lead to severe vision impairment

What is keratoconus

Keratoconus (from the Greek: Keratos=Cornea and Konos=Cone) is defined as an ocular degenerative, non-inflammatory disease characterised by an abnormal curvature of the cornea, linked to its structural weakness.

It is one of the rare diseases, with a prevalence in the population of no more than one case per 2,000 inhabitants; it is usually bilateral, but asymmetrical, because it affects both eyes with different degrees of development.

Keratoconus has a slow and progressive onset that consists of a wearing away of the corneal tissue

The cornea thins, weakens and begins to sag, deforming until it becomes ‘protruding’ at the apex (corneal ectasia), and assumes the characteristic conical shape.

It characteristically manifests itself in childhood or adolescence and progresses until around the age of 40, although the evolution is highly variable and the first signs can appear in any age group.

Incidence of keratoconus

Keratoconus is classified as a rare disease with a prevalence of about 1 case per 1,500 people.

It occurs more frequently in Western countries and in the Caucasian population.

According to some studies, it affects the female sex more.

What are the causes and risk factors of keratoconus?

The causes of keratoconus are not yet fully understood. There is certainly a genetic component: it is hypothesised that at the root of keratoconus there may be alterations in the genes that control the synthesis, organisation and degradation of collagen molecules, which make up the corneal scaffolding.

Recent studies have identified an increase and abnormal activity of certain enzymes, called proteases, or a reduction in their inhibitors, which are involved in tissue collagen renewal, resulting in thinning and weakening of the corneal structure.

A higher familial incidence of the disease has been found, although in most cases keratoconus presents as an isolated condition with no evidence of genetic transmission; it may also be associated with a predisposition to allergies (atopy) and other ocular or systemic diseases, such as Down syndrome, collagen diseases, Leber congenital amaurosis and some corneal dystrophies.

Repeated ocular trauma over time, e.g. caused by contact lens abuse and especially eye rubbing, and problems with the trigeminal nerve are considered risk factors.

Signs and symptoms of keratoconus

Normally, keratoconus does not cause pain unless there is a sudden perforation of the cornea.

The curvature of the cornea, which is essential for the correct focusing of images on the retina, becomes irregular and modifies the refractive power, producing image distortion and visual impairment: in fact, one of the first symptoms of keratoconus is blurred vision, which in the more advanced stages of the disease becomes poorly amenable to glasses and even contact lenses.

The deformation of the cornea usually results in myopia and irregular astigmatism; more rarely, in some cases where the apex of the cone is peripheral, a hypermetropic defect.

Furthermore, keratoconus is often associated with allergic conjunctivitis, which causes itching and redness; it is sometimes associated with a feeling of discomfort in light (photophobia).

Diagnosis of keratoconus

The diagnosis of keratoconus is made during an ophthalmic examination through the evaluation of the corneal curvature by means of an ophthalmometer or an abnormal shadow image, with scissor movement, during the course of the schiascopy.

In the presence of an irregularity in the images reflected from the corneal surface or projected from the back of the eye, the diagnosis can be clarified by means of

corneal topography (maps of the anterior surface of the cornea)
pachymetry (measurement of corneal thickness);
corneal tomography (maps of the anterior surface, posterior surface and thickness, evaluation of aberrations)
confocal microscopy (detection of abnormalities in the corneal structure); in more advanced cases, characteristic streaks in the corneal tissue or linear brownish deposits of haemosiderin (Fleischer ring) can be observed on simple examination under a slit lamp.

How keratoconus is treated

The treatment of keratoconus varies depending on the stage of the disease and its progression: it ranges from the use of spectacles and contact lenses to surgery.

In the initial stage of the disease, when astigmatism is contained or the keratoconus is not central, spectacles can offer satisfactory correction of the visual defect.

When keratoconus evolves and astigmatism becomes higher and more irregular, correction with traditional lenses is no longer sufficient: in these cases, rigid or semi-rigid (gas-permeable) contact lenses can be used, which allow better correction of the defect, but are unable to halt the progression of the disease.

At a more advanced stage of keratoconus, surgery is the most effective corrective option.

Donor corneal transplantation (perforating, lamellar or mushroom keratoplasty) is a currently widespread and effective surgical procedure

It is performed when the cornea has a central scar or is deformed and thinned to such an extent that it prevents acceptable vision.

The success rate is generally very high (95%) regardless of the severity of the disease and the risk of rejection is low; the lamellar technique (DALK), in which only the altered portion of the cornea is replaced, leaving the posterior layer (endothelium and Descemet’s membrane) in situ, further reduces the risk of rejection and other complications.

Visual recovery after keratoplasty is fairly rapid in the months following the operation, although the final visual result must wait until the suture is removed (one to three years after surgery).

Another surgical option is to insert intrastromal rings in the peripheral portion of the cornea in order to flatten the central area and improve the visual result by reducing the curvature parameters.

Since 2006, a new treatment called corneal cross-linking has become widespread. It is a parachirurgical, minimally invasive treatment that can strengthen the corneal structure in patients with keratoconus, so as to block or slow down its progression; this technique represents a valuable alternative to corneal transplantation if it is applied in the early stages of evolution.

Hence the importance of early diagnosis and periodic specialist check-ups during development, especially in the families of patients with the disease.

The treatment consists of instilling vitamin B2 (riboflavin) in the form of eye drops on the cornea after removal of the epithelium (epi-off technique) or using methods that promote its passage into the stroma through the epithelial barrier (epi-on iontophoretically or with enhancers); after imbibition of the stroma by the vitamin, the cornea is exposed to UV-A radiation.

The aim of the treatment is to increase the cross-linking between the basic collagen fibres in order to strengthen the cornea and prevent, or at least limit, further deformation of its structure; in some cases the treatment results in an improvement of the curvature parameters in the subsequent course.

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