Keratoconus
Keratoconus | |
---|---|
Other names | KC, KCN, conical cornea |
Frequency | ~1 in 2,000 people[3] |
Keratoconus (KC) is a disorder of the
While the cause is unknown, it is believed to occur due to a combination of
Initially the condition can typically be corrected with
Keratoconus affects about 1 in 2,000 people.
Signs and symptoms
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/Kc_simulation.jpg/220px-Kc_simulation.jpg)
"... a candle, when looked at, appears like a number of lights, confusedly running into one another" — Nottingham[12]
People with early keratoconus often notice a minor blurring or distortion of their vision, as well as an increased sensitivity to light, and visit their clinician seeking corrective lenses for reading or driving.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d7/Kc_simulation2.png/220px-Kc_simulation2.png)
The classic symptom of keratoconus is the perception of multiple "ghost" images, known as
Genetics
Six genes have been found to be associated with the condition. These genes include
Patients with a parent, sibling, or child who has keratoconus have 15 to 67 times higher risk in developing corneal ectasia compared to patients with no affected relatives.[7][8]
Pathophysiology
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/cb/David_G_Cogan_case_245_keratoconus_thin_stroma_wrinkled_endothelium.png/220px-David_G_Cogan_case_245_keratoconus_thin_stroma_wrinkled_endothelium.png)
Despite considerable research, the cause of keratoconus remains unclear.[18] 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.[19][20][21] Once initiated, the disease normally develops by progressive dissolution of Bowman's layer,[14] 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 characteristic of the disorder. Within any individual keratoconic cornea, regions of degenerative thinning coexisting with regions undergoing wound healing may be found. Scarring appears to be an aspect of the corneal degradation; however, a recent, large, multicenter study suggests abrasion by contact lenses may increase the likelihood of this finding by a factor over two.[22][23]
A number of studies have indicated keratoconic corneas show signs of increased activity by
A genetic predisposition to keratoconus has been observed,
Researches also have shed light on the role of hormones in the pathophysiology of keratoconus. Hormones such as androgen, prolactin, estrogen and progesterone have been shown to influence corneal biomechanics and tissue remodeling, potentially affecting the integrity of the cornea in individuals predisposed to keratoconus.[34] Moreover, fluctuations in hormonal levels during puberty and pregnancy have been associated with the onset or exacerbation of keratoconus in some cases.
Keratoconus has been associated with
Diagnosis
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Keratoconus.svg/220px-Keratoconus.svg.png)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Computerised_Corneal_Topography.jpg/220px-Computerised_Corneal_Topography.jpg)
Prior to any
If keratoconus is suspected, the ophthalmologist or optometrist will search for other characteristic findings of the disease by means of
A handheld
Stages
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b7/Corneal_topography%2C_stage_II_keratoconus_%28Elise_A._Slim_et_al.%29.svg/220px-Corneal_topography%2C_stage_II_keratoconus_%28Elise_A._Slim_et_al.%29.svg.png)
Once keratoconus has been diagnosed, its degree may be classified by several metrics:[47]
- The steepness of greatest curvature from 'mild' (< 45 D), 'advanced' (up to 52 D) or 'severe' (> 52 D);
- The morphology of the cone: 'nipple' (small: 5 mm and near-central), 'oval' (larger, below-center and often sagging), or 'globus' (more than 75% of cornea affected);
- The corneal thickness from mild (> 506 μm) to advanced (< 446 μm).
Increasing use of corneal topography has led to a decline in use of these terms.[47]
Stage | Characteristics[48][49] |
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Stage 1 |
|
Stage 2 |
|
Stage 3 |
|
Stage 4 |
|
Stage is determined if one of the characteristics applies. Corneal thickness is the thinnest measured spot of the cornea. |
Treatment
Lenses
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f3/KC-lens.jpg/220px-KC-lens.jpg)
In early stages of keratoconus,
In people with keratoconus, rigid contact lenses improve vision by means of tear fluid filling the gap between the irregular corneal surface and the smooth regular inner surface of the lens, thereby creating the effect of a smoother cornea. Many specialized types of contact lenses have been developed for keratoconus, and affected people may seek out both doctors specialized in conditions of the cornea, and contact lens fitters who have experience managing people with keratoconus. The irregular cone presents a challenge[19] and the fitter will endeavor to produce a lens with the optimal contact, stability and steepness. Some trial-and-error fitting may prove necessary.[45]
Hybrid lenses
Traditionally, contact lenses for keratoconus have been the 'hard' or RGP variety, although manufacturers have also produced specialized 'soft' or
Scleral lenses
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/7c/Scleral_lens_worn_on_an_eye.jpg/220px-Scleral_lens_worn_on_an_eye.jpg)
Scleral lenses are sometimes prescribed for cases of advanced or very irregular keratoconus; these lenses cover a greater proportion of the surface of the eye and hence can offer improved stability.
Piggybacking
Some people find good vision correction and comfort with a "piggyback" lens combination, in which RGP lenses are worn over soft lenses, both providing a degree of vision correction.[56] One form of piggyback lens makes use of a soft lens with a countersunk central area to accept the rigid lens. Fitting a piggyback lens combination requires experience on the part of the lens fitter, and tolerance on the part of the person with keratoconus.
Surgery
Corneal transplant
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Cornea_transplant.jpg/220px-Cornea_transplant.jpg)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Cornea_Transplant.jpg/220px-Cornea_Transplant.jpg)
Between 11% and 27% of cases of keratoconus
The acute recovery period can take four to six weeks, and full postoperative vision stabilization often takes a year or more, but most transplants are very stable in the long term.
In the US, corneal transplants (also known as corneal grafts) for keratoconus are usually performed under sedation as
One way of reducing the risk of rejection is to use a technique called deep anterior lamellar keratoplasty (DALK). In a DALK graft, only the outermost epithelium and the main bulk of the cornea, the stroma, are replaced; the person's rearmost endothelium layer and the Descemet's membrane are left, giving some additional structural integrity to the postgraft cornea.[62] Furthermore, it is possible to transplant freeze-dried donor tissue. The freeze-drying process ensures this tissue is dead, so there is no chance of rejection.[62] Research from two trials in Iran provide low to moderate evidence that graft rejection is more likely to occur in penetrating keratoplasty than in DALK, though the likelihood for graft failure were similar with both procedures.[63]
Epikeratophakia
Rarely, a nonpenetrating keratoplasty known as an epikeratophakia (or epikeratoplasty) may be performed in cases of keratoconus. The corneal epithelium is removed and a lenticule of donor cornea is grafted on top of it.[19] The procedure requires a greater level of skill on the part of the surgeon, and is less frequently performed than a penetrating keratoplasty, as the outcome is generally less favorable. However, it may be seen as an option in a number of cases, particularly for young people.[64]
Corneal ring implants
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/IntacsAfterInsertion.jpg/220px-IntacsAfterInsertion.jpg)
A possible 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
Corneal intrastromal implantation surgery involving the implantation of a full ring is also available as a treatment option for keratoconus.[67] Evidence supports that the full-ring implant improves vision outcomes for at least a year.[68]
Cross-linking
Radial keratotomy
![](http://upload.wikimedia.org/wikipedia/commons/2/24/Mini_Asymmetric_Radial_Keratotomy_-_MARK.jpg)
Radial keratotomy is a refractive surgery procedure where the surgeon makes a spoke-like pattern of incisions into the cornea to modify its shape. This early surgical option for myopia has been largely superseded by LASIK and other similar procedures. LASIK is absolutely contraindicated in keratoconus and other corneal thinning conditions as removal of corneal stromal tissue will further damage an already thin and weak cornea.[72] For similar reasons, radial keratotomy has also generally not been used for people with keratoconus.[73][74]
Prognosis
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2c/David_G_Cogan_collection_Case_244_photo_2_keratoconus_Descemet_rupture.png/220px-David_G_Cogan_collection_Case_244_photo_2_keratoconus_Descemet_rupture.png)
Patients with keratoconus typically present initially with mild
Corneal hydrops
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Hydrops_%28descemets_membrane_rupture%29_due_to_keratoconus.jpg/220px-Hydrops_%28descemets_membrane_rupture%29_due_to_keratoconus.jpg)
In advanced cases, bulging of the cornea can result in a localized rupture of
Although disconcerting to the patient, 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 nonsurgically by bandaging with an osmotic saline solution. Although a hydrops usually causes increased scarring of the cornea, occasionally it will benefit a patient by creating a flatter cone, aiding the fitting of contact lenses.[76] Corneal transplantation is not usually indicated during corneal hydrops.
Epidemiology
The National Eye Institute reports keratoconus is the most common corneal dystrophy in the United States, affecting about one in 2,000 Americans,[77][78] but some reports place the figure as high as one in 500.[79] In the pediatric populations, ages three to 18, the prevalence of keratoconus was found to be as high as one in 334 children.[80] The inconsistency may be due to variations in diagnostic criteria,[14] with some cases of severe astigmatism interpreted as those of keratoconus, and vice versa.[45] A long-term study found a mean incidence rate of 2.0 new cases per 100,000 population per year.[78] Some studies have suggested a higher prevalence amongst females,[81] or that people of South Asian ethnicity are 4.4 times as likely to develop keratoconus as Caucasians, and are also more likely to be affected with the condition earlier.[82]
Keratoconus is normally bilateral[78] (affecting both eyes) although the distortion is usually asymmetric and is rarely completely identical in both corneas.[14] Unilateral cases tend to be uncommon, and may in fact be very rare if a very mild condition in the better eye is simply below the limit of clinical detection.[45] It is common for keratoconus to be diagnosed first in one eye and not until later in the other. As the condition then progresses in both eyes, the vision in the earlier-diagnosed eye will often remain poorer than that in its fellow.
History
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/29/Conicalcornea.jpg/220px-Conicalcornea.jpg)
The German
By 1869, when the pioneering Swiss ophthalmologist
Society and culture
According to the findings of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK), people who have keratoconus could be expected to pay more than $25,000 over their lifetime post-diagnosis, with a standard deviation of $19,396.
Related disorders
Several other corneal ectatic disorders also cause thinning of the cornea:[45]
- Keratoglobus is a very rare condition that causes corneal thinning primarily at the margins, resulting in a spherical, slightly enlarged eye. It may be genetically related to keratoconus.[19]
- Pellucid marginal degeneration causes thinning of a narrow (1–2 mm) band of the cornea, usually along the inferior corneal margin. It causes irregular astigmatism that, in the early stages of the disease can be corrected by spectacles. Differential diagnosis may be made by slit-lamp examination.[30][93]
- Posterior keratoconus, a distinct disorder despite its similar name, is a rare abnormality, usually congenital, which causes a nonprogressive thinning of the inner surface of the cornea, while the curvature of the anterior surface remains normal. Usually only a single eye is affected.[19]
- Post-LASIK ectasia is a complication of LASIK eye surgery.
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External links
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Keratoconus at Curlie