Keratoconus abbreviated as ‘Kc’ is a non-inflammatory eye condition in which the round dome-shaped cornea progressively thins, causing a cone-like bulge to develop. The reason may be corneal ectasia– the inner layers become weak and change shape– resulting in visual impairment. It is usually Bilateral. However, it may take years after the initial diagnosis in one eye becomes apparent in the second eye. Though it is not a common eye disease, it is by no means rare.
What is the cornea?
The cornea is a highly organized group of cells and proteins. However, it does not consist of any blood vessels. Any tiny particle in the cornea can disrupt the image formation and hence, must remain transparent. Besides, it receives its nourishment from tears and other aqueous content.
The five layers and functions of the cornea
- Epithelium: It blocks the foreign materials from entering the stroma of the cornea. It also has a smooth surface, which helps in absorbing nutrients. The tiny nerve endings are sensitive and may damage with itching or scratching. The foundation is the basement membrane.
- Bowman’s membrane: It is the muscular layer of protein fibers– the collagen. When injured, it leaves behind a scar as it heals. If the scar is significant, it may cause vision loss.
- Stroma: It comprises up to 90% thickness of the cornea. It consists of 78% water and 16% collagen– gives it the strength and elasticity. The shape, arrangement, and spacing are essential for cornea light-conducting.
- Descemet’s membrane: It acts as the barrier against infections and injuries. The collagen fibers of this membrane are different from the stroma.
- Endothelium: It plays a vital role in keeping the cornea free from excess fluids. Without this action, the cornea would swell and become hazy. A healthy eye maintains the perfect balance.
When light falls onto the cornea, it refracts and focuses the light onto the retina via the lens. For perfect vision, the light must pass precisely through the cornea and the lens, which further enters the optic nerve, passing through retina. It finally reaches the brain, which processes the image.
Since the cornea is weak in Keratoconus, it may result in a structural defect– it cannot focus light precisely, leading to visual impairment. Corneal clouding may also accompany the condition.
Morphological Classification
- Nipple cone: a small size (< 5mm) with steep curvatures.
- Oval cone: large (5-6mm) and ellipsoid in shape.
- Globus cone: very large ( > 6 mm) and globe-like.
What are the symptoms of Keratoconus?
The onset of symptoms is generally at puberty and lasts long until the cornea restabilizes its structure. Patients present a defective vision due to progressive myopia and irregular astigmatism (refractive error in which the eye does not focus light due to structural defects). The condition may not improve fully despite the use of glasses.
The cornea is the clear window of the eye and is responsible for refracting most of the light coming into the eye. Therefore, abnormalities of the cornea (i.e., cornea bulging in Keratoconus) severely affect how we see the world, making simple day to day life tasks challenging. In the early stages, it causes slight blurring and distortion of vision and increased sensitivity to light.
A Fleischer’s ring is observed due to the disposition of iron storage molecules (hemosiderin) at the cone’s edges. Fluid build-up in the Descemet’s membrane due to breakage results in acute hydrops.
What are the causes of Keratoconus?
The primary cause of the condition is increase in damaging by-products from the cornea itself. Generally, antioxidants resolve this condition. But low levels of antioxidants (in Keratoconus) can result in weak collagen (primarily involved in keeping the cornea at place). This weakness can result in bulging. The pathological changes are due to progressive ectasia (dilation of tubular structure), which is a consequence of defective synthesis of mucopolysaccharide and collagen tissues.
Ectasia may also be a conseqence of recurrent mechanical stress by rubbing and itching, resulting from hard contact lens wear. Hence, looking for signs of ocular allergy in patients would be the primary step in the diagnosis. The condition may be accompanied by thinning of cornea, leading to abnormal shape. Subsequently, the basement membrane also thickens. There is also evidence of stromal cell apoptosis.
Does genetic inheritance play a role?
Clustering cases within families suggests a genetic risk factor, opening a new route of diagnosis. If any of your parents suffer from the condition, a timely checkup of the cornea is strictly advised. Individuals who may develop Keratoconus have a genetic predisposition for the disorder. An individual genetically predisposed carries a mutated gene/genes but may not express until activated under certain exogenic conditions like mechanical stress– the “two-hit” hypothesis.
Other risk factors
Other diseases and risk factors associated are Down syndrome, Sleep apnea, Asthma, and various non-inflammatory connective tissue disorders.
It is conventionally a non-inflammatory disease because reports suggest no increase in cytokine levels. They observed an increase in IL-6 and IL-7 and a subsequent decrease in TNF-ɑ and IL-13. These changes suggest a complex imbalance between pro-inflammatory and anti-inflammatory agents, rather than a significant increase in pro-inflammatory cytokines.
However, no single theory explains it all. It is a combination of different factors.
Treatment for Keratoconus
The treatment of Keratoconus is based upon the severity and the rate of progression of the disorder. Authentic eyeglasses or lenses can improve vision in mild conditions. However, progressive changes require frequent prescription changes.
Generally, rigid gas permeable contact lenses can help reshape the corneal surface to hide/mask the underlying cone-shaped defect. However, a few may not tolerate these hard lenses and may need piggyback contact lenses (hard are placed over soft lenses), which provide support and comfort. However, these lenses may be unbearable in severe conditions due to progressive bulging and may need surgery.
Besides, the collagen crosslinking medication is an ongoing research, which could stabilize the condition. However, this medication may not fully rule out the need for keratoplasty.
What is keratoplasty– the corneal transplant?
A corneal transplant is a surgical procedure in which the surgeon removes the damaged tissue and replaces it with healthy donor corneal tissue (carrying a risk of graft rejection). They generally reserve this surgery for severe conditions, where the cornea is extremely thin with significant vision compromise. The need for corneal transplant may also arise after an episode of corneal swelling, i.e., corned hydrops that do not respond to other treatment options.
Individuals who have a corneal transplant typically still need to wear cover lenses (hard/soft) because the cornea still does not have a perfect shape.
