This disorder arises when the middle of the cornea thins and gradually bulges outward, forming a rounded cone shape. This abnormal curvature changes the cornea's refractive power, producing moderate to severe distortion (irregular astigmatism) and blurriness of vision. These changes may also disrupt the normal, light-conducting arrangement of corneal protein, causing swelling and sight-impairing scarring of the tissue .
How common is keratoconus?
Studies indicate that keratoconus is an inherited corneal abnormality occurring in about 1 in 2000 people. About 7% of those with the condition have a family history of keratoconus . The majority of keratoconus patients demonstrate a recessive genetic inheritance pattern (where both parents are carriers of the defective genes) however keratoconus may also exhibit sporadic or autosomally dominant heredity patterns.
What is keratoconus associated with?
Keratoconus is found in increased rates in patients with certain other eye conditions such as Retinitis Pigmentosa, Retinopathy of Prematurity, Leber's Congenital Amaurosis, Vernal Keratoconjunctivitis Atopic Keratoconjunctivitis and other corneal dystrophies such as Fuch's Endothelial dystrophy and Lattice Dystrophy.
There are certain systemic diseases which increase the likelihood of a patient developing keratoconus. These include , Ehlers-Danlos Syndrome, Down's Syndrome, Osteogenesis Imperfecta, double jointedness and Addison's Disease. There is a strong relationship between keratoconus and atopic disorders such as asthma, eczema and hayfever.
Keratoconus usually occurs during puberty, or shortly thereafter.
How is keratoconus treated?
Because keratoconus is a genetic condition it cannot be treated with drugs. At first, people may be able to correct their vision with spectacles. As the astigmatism worsens, they must rely on specially fitted contact lenses to neutralise the distortion and improve acuity. Rigid contact lenses most effectively neutralise the distortions created by keratoconus, however there now also exists custom soft contact lenses specifically designed for keratoconus as well as hybrid contact lenses (rigid centre with a soft outer skirt). It is however crucial to have a lens fitted correctly, as a poorly fitting lens can further damage the cornea by causing abrasions and scarring. Contact lenses for keratoconus must be fitted by a contact lens practitioner with experience in keratoconus, preferably in conjunction with computerised corneal topography.
Surgical treatment of keratoconus.
In around 10% of cases keratoconus will progress to the point where the cornea becomes cloudy through scarring and the vision with a contact lens is no longer adequate. If this occurs a person may need to undergo a corneal transplant where the diseased tissue is replaced with a normal donor cornea. This operation is successful in about 90% of cases of advanced keratoconus. Several studies have also reported that about 80% of these patients have 6/12 vision or better with contact lenses after the operation.
There exists numerous forms of corneal transplant including full thickness, partial thickness and single layered corneal transplant. Corneal transplants involving the full thickness of the cornea generally provide the best vision afterwards, however also carry the greatest risk of transplant rejection. Partial thickness and single layer transplants are gaining favour due to their decreased risk of graft rejection.
More recently a process known as corneal collagen crosslinking (CXL) has been developed and has become the mainstay of keratoconus treatment in early stages of the disease. It has been shown in numerous clinical trials to strengthen the cornea through the application of riboflavin (a form of vitamin-B2), followed by treatment with ultraviolet A (UV-A) light. Corneal crosslinking then works to strengthen the cornea by increasing the number of "anchors" that bond collagen fibres together. The goal of this treatment is to identify patients early in the disease process and treat them before they develop significant vision loss from ectasia. Once they have received crosslinking, in greater than 90% of cases their corneal shape will remain stable. The benefits of corneal crosslinking are that patients maintain their good levels of vision for longer with a lower risk of future corneal scarring or the need for corneal transplant.
Corneal crosslinking can also be used in combination with other technologies, in order to improve the vision more rapidly. Tiny plastic inserts known as Intacs can be surgically implanted within the cornea, and have been shown to work well alongside crosslinking. Surface laser vision correction guided by corneal topography has also proven to be a useful technology. Similarly the use of microwave energy to flatten the cornea has been developed however is yet to be practised in mainstream keratoconus management.