Keratoconus (from Greek: kerato- horn, cornea; and konos cone), is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve.
Keratoconus can cause substantial distortion of vision, with multiple images, streaking and sensitivity to light all often reported by the patient. It is typically diagnosed in the patient's adolescent years and attains its most severe state in the twenties and thirties. If afflicting both eyes, the deterioration in vision can affect the patient's ability to drive a car or read normal print. In most cases, corrective lenses are effective enough to allow the patient to continue to drive legally and likewise function normally. Further progression of the disease may require surgery including intrastromal corneal ring segments, corneal collagen cross-linking, or corneal transplantation. However, despite the disease's unpredictable course, keratoconus can often be successfully managed with little or no impairment to the patient's quality of life.
Keratoconus affects around one person in a thousand. It seems to occur in populations throughout the world, although it occurs more frequently in certain ethnic groups such as South Asians. The exact cause of keratoconus is uncertain, but has been associated with detrimental enzyme activity within the cornea. Environmental and genetic factors are considered possible causes, but the findings are still yet inconclusive. The progression of keratoconus is rapid in patients having Down syndrome.
A simulation of the multiple images seen by a person with keratoconus.
A representation of the multiple images made by extremely high contrast light sources. From left to right: A light source as seen by a healthy eye, uncorrected keratoconus vision, mostly corrected keratoconus vision with contact lenses.
People with early keratoconus typically notice a minor blurring of their vision and come to their clinician seeking corrective lenses for reading or driving. At early stages, the symptoms of keratoconus may be no different from those of any other refractive defect of the eye. As the disease progresses, vision deteriorates, sometimes rapidly. The disease is often bilateral, though asymmetrical in many patients. Some develop photophobia (sensitivity to bright light), eye strain from squinting in order to read.
The classic symptom of keratoconus is the perception of multiple 'ghost' images, known as monocular polyopia. Instead of seeing just one point, a person with keratoconus sees many images of the point, spread out in a chaotic pattern.
The eye examination includes measurement of visual acuity, refractive status and localized curvature of the cornea with a manual keratometer, with detection of irregular astigmatism suggesting a possibility of keratoconus. Severe cases can exceed the instrument's measuring ability.
If keratoconus is suspected, the ophthalmologist will search for other characteristic findings of the disease by means of slit lamp examination of the cornea. Under close examination, a ring of yellow-brown to olive-green pigmentation known as a Fleischer ring can be observed. The Fleischer ring is caused by deposition of the iron oxide hemosiderin within the corneal epithelium. Few subjects exhibit Vogt's striae which are fine stress lines within the cornea. A highly pronounced cone can create a V-shaped indentation in the lower eyelid when the patient's gaze is directed downwards, known as Munson's sign.
A schematic diagram showing change in cornea
Corneal topogram of a keratoconic eye
A more definitive diagnosis can be obtained using corneal topography, in which an automated instrument projects the illuminated pattern onto the cornea and determines its topology from analysis of the digital image. The topographical map indicates any distortions or scarring in the cornea, with keratoconus revealed by a characteristic steepening of curvature which is usually below the centre of the eye. It is of particular value in detecting the disorder in its early stages when other signs have not yet presented.
Despite considerable research, the etiology of keratoconus remains unclear. Several sources suggest that keratoconus likely arises from a number of different factors: genetic, environmental or cellular, any of which may form the trigger for the onset of the disease.
A genetic predisposition to keratoconus has been observed, with the disease running in certain families, and incidences reported of concordance in identical twins. The frequency of occurrence in close family members is not clearly defined, though it is known to be considerably higher than that in the general population, and studies have obtained estimates ranging between 6% and 19%. A responsible gene has not been identified: two studies involving isolated, largely homogenetic communities have contrarily mapped putative gene locations to chromosomes 16q and 20q. However, most genetic studies agree on an autosomal dominant model of inheritance.Keratoconus is also diagnosed more often in people with Down syndrome, though the reasons for this link have not yet been determined. Keratoconus has been associated with atopic diseases, which include asthma, allergies, and eczema, and it is not uncommon for several or all of these diseases to affect one person. A number of studies suggest that vigorous eye rubbing contributes to the progression of keratoconus, and that patients should be discouraged from the practice.
Iatrogenic keratoconus has also been observed following LASIK surgery, caused by removal of excessive stromal bed tissue.
Once initiated, the disease normally develops by progressive dissolution of Bowman's layer,which lies between the corneal epithelium and stroma. As the two come into contact, cellular and structural changes in the cornea adversely affect its integrity and lead to the bulging and scarring that are characteristic of the disorder. Within any individual keratoconic cornea, there may be found regions of degenerative thinning coexisting with regions undergoing wound healing.
The visual distortion experienced by the patient comes from two sources, one being the irregular deformation of the surface of the cornea; the other being scarring that occurs on its exposed highpoints. These factors act to form regions on the cornea that map an image to different locations on the retina and give rise to the symptom of monocular polyopia. The effect can worsen in low light conditions as the dark-adapted pupil dilates to expose more of the irregular surface of the cornea. Scarring appears to be an aspect of the corneal degradation; however, a recent, large, multi-center study suggests that abrasion by contact lenses may increase the likelihood of this finding by a factor of over two. A number of patients complain of chronic eye rubbing and also think it as a possible cause to the disease but it is not so; however, it has been observed that keratoconus progresses faster due to eye-rubbing.
A number of studies have indicated that keratoconic corneas show signs of increased activity by proteases, a class of enzymes that break some of the collagen cross-linkages in the stroma, with a simultaneous reduced expression of protease inhibitors. Other studies have suggested that reduced activity by the enzyme aldehyde dehydrogenase may be responsible for a build-up of free radicals and oxidising species in the cornea. It seems likely that, whatever the pathogenetical process, the damage caused by activity within the cornea results in a reduction in its thickness and biomechanical strength. While keratoconus is considered a non-inflammatory disorder, one study shows that wearing rigid contact lenses by patients leads to overexpression of pro-inflammatory cytokines, such as IL-6, TNF-alpha, ICAM-1, and VCAM-1 in the tear fluid.
Between 10% and 25% of cases of keratoconus will progress to a point where vision correction is no longer possible, thinning of the cornea becomes excessive, or scarring as a result of contact lens wear causes problems of its own, and a corneal transplantation or penetrating keratoplasty becomes required. Keratoconus is the most common grounds for conducting a penetrating keratoplasty, generally accounting for around a quarter of such procedures. The corneal transplant surgeon trephines a lenticule of corneal tissue and then grafts the donor cornea to the existing eye tissue, usually using sutures. The cornea does not have a direct blood supply, and so donor tissue is not required to be blood type matched. Eye banks check the donor corneas for any disease or cellular irregularities.
Spanish-born eye surgeon Ramon Castroviejo successfully performed keratoplasty as early as 1936.
The acute recovery period can take four to six weeks and full post-operative vision stabilization often takes a year or more but most transplants are very stable in the long term. Sutures are removed after the healing process. All cases require a careful follow-up with an eye surgeon (ophthalmologist) for a number of years. Frequently, vision is greatly improved after the surgery, but even if the actual visual acuity does not improve, because the cornea is a more normal shape after the healing is completed, patients can more easily be fitted with corrective lenses. Complications of corneal transplants are mostly related to vascularization of the corneal tissue and rejection of the donor cornea. Vision loss is very rare, though difficult-to-correct vision is possible. When rejection is severe, repeat transplants are often attempted, and are frequently successful. Keratoconus will not normally reoccur in the transplanted cornea; but incidences of this have been observed. The long-term outlook for corneal transplants performed for keratoconus is usually favorable.
A recent surgical alternative to corneal transplant is the insertion of intrastromal corneal ring segments. A small incision is made in the periphery of the cornea and two thin arcs of polymethyl methacrylate are slid between the layers of the stroma on either side of the pupil before the incision is closed. It offers the benefit of being reversible and even potentially exchangeable as it involves no removal of eye tissue.
The principal intrastromal ring available is known by the trade name of Intacs. Internationally, Ferrara Rings are also available. Intacs are the only corneal implants that have gone through the FDA Phase I, II and III clinical trials and were first approved by the Food and Drug Administration (FDA) in the United States.
A pair of Intacs after insertion into the cornea. (In fig)
Clinical studies on the effectiveness of intrastromal rings on keratoconus are in their early stages, and results have so far been generally encouraging, though they have yet to enter into wide acceptance with the refractive surgery community. In common with penetrating keratoplasty, the requirement for some vision correction in the form of spectacles or hydrophilic contact lenses may remain subsequent to the operation. Potential complications of intrastromal rings include accidental penetration through to the anterior chamber when forming the channel, post-operative infection of the cornea, and migration or extrusion of the segments. The rings offer a good chance of vision improvement even in otherwise hard to manage eyes, but results are not guaranteed and in a few cases may worsen.
Early studies on intrastromal corneal rings involved use of two segments to cause global flattening of the cornea. A later study reported that better results could be obtained for those cones located more to the periphery of the cornea by using a single Intacs segment. This leads to preferential flattening of the cone below, but also to steepening the over-flat upper part of the cornea.
A one-time application of riboflavin solution is administered to the eye and is activated by illumination with UV-A light for approximately 30 minutes. The riboflavin causes new bonds to form across adjacent collagen strands in the stromal layer of the cornea, which recovers and preserves some of the cornea's mechanical strength. The corneal epithelial layer is generally removed in order to increase penetration of the riboflavin into the stroma.
One way of reducing the risk of rejection is to use a newer technique called a Deep Anterior Lamellar Keratoplasty, referred to as DALK. In a DALK graft, only the outermost epithelium and the main bulk of the cornea, the stroma, are replaced; the patient's rearmost endothelium layer and the Descemet's membrane are left, giving some additional structural integrity to the post-graft cornea. Because a graft rejection usually begins in the endothelium, the chance of a rejection episode is greatly reduced.
Some surgeons prefer to remove the donor epithelium; others leave the donor's cells in place. Removing it can cause a slight improvement in overall vision, but a corresponding increase in visual recovery time.
A small rupture of Descemet's membrane (magnified view) Patients with keratoconus typically present initially with mild astigmatism and myopia, commonly at the onset of puberty, and are diagnosed by the late teenage years or early 20s. A diagnosis of the disease at an early age may indicate a greater risk of severity in later life. Patients' vision will seem to fluctuate over a period of months, driving them to change lens prescriptions frequently, but as the condition worsens, contact lenses are required in the majority of cases. The course of the disorder can be quite variable, with some patients remaining stable for years or indefinitely, while others progress rapidly or experience occasional exacerbations over a long and otherwise steady course. The course of the disease generally ceases in the third and fourth decades of life.
A small rupture of Descemet's membrane (magnified view)
... leads to corneal hydrops.
In advanced cases, bulging of the cornea can result in a localized rupture of Descemet's membrane, an inner layer of the cornea. Aqueous humor from the eye seeps into the cornea before Descemet's membrane reseals. The patient experiences pain and a sudden severe clouding of vision, with the cornea taking on a translucent milky-white appearance known as a corneal hydrops. The effect is normally temporary and after a period of six to eight weeks the cornea usually returns to its former transparency. The recovery can be aided non-surgically by bandaging with an osmotic saline solution. Hydrops usually causes increased scarring of the cornea. Corneal transplantation is not usually indicated while the hydrops persists.
In 1854 that British physician John Nottingham clearly described keratoconus and distinguished it from other ectasias of the cornea. In 1888 the treatment of keratoconus became one of the first practical applications of the then newly invented contact lens, when the French physician Eugène Kalt manufactured a glass scleral shell which improved vision by compressing the cornea into a more regular shape. Since the start of the twentieth century, research on keratoconus has both improved understanding of the disease and greatly expanded the range of treatment options.
Several other non-inflammatory eye disorders, generally rarer than keratoconus, also cause thinning of the cornea.