Introduction
Keratoconus is a progressive condition, usually presenting bilaterally, that affects the cornea. It leads to the thinning and bulging of the cornea which eventually causes the cornea to become cone shaped. It has a detrimental impact on the patient’s vision. The word keratoconus is derived from Greek and it means ‘cone-shaped’ cornea, with John Nottingham widely acknowledged as the first to define the condition in 1854 (1).
Studies that have investigated its epidemiology have estimated prevalence at between 1.5 and 25 per 100,000 people per year, however there is huge variance due to reasons such as geographical location, ethnicity and the diagnostic criteria used to identify keratoconus (2).
This article aims to provide a short overview into the keratoconus, focussing on aetiology, symptoms, diagnosis and treatment.
Aetiology
The exact cause of keratoconus remains unclear, however it likely has a genetic and environmental component to its disease process. It is associated with genetic conditions such as Down’s syndrome (3) and individuals with a family history of keratoconus are more likely to develop it.
One environmental factor that likely causes keratoconus is persistent eye rubbing. Studies have shown that the odds ratio of developing keratoconus was three times higher in those that rub their eye daily compared to those that do not. Other large risk factors include having a diagnosis of asthma, eczema or allergy (4).
Symptoms
The cornea is a transparent surface that covers the anterior portion of the eye. One of its roles is to act as a structural barrier for the eye but its most important function is to allow light to refract correctly onto the lens which in turn focusses light onto the retina. It contributes around two-thirds of the refractive power of the eye (5). When the shape of the cornea is changed, as seen in keratoconus, it leads to symptoms.
Symptoms are typically first noticed when patients reach around their second or third decades of life and it progresses until their fourth decade (2). Symptoms are usually unnoticed in early disease. Once the disease has progressed, patients typically have symptoms that include blurred vision, worsening of vision and increased sensitivity to light. In addition to this, disease progression is typically associated with a sudden decrease in visual acuity. (6).
Diagnosis
Identification of keratoconus is essential for the management of the disease as the earlier it is diagnosed the better the prognosis. It would reduce the likelihood for a corneal transplant and also reduce the risk of post-refractive surgery ectasia (7). There are a few tests that are used to diagnose keratoconus, mainly to identify corneal thinning.
Optical coherence tomography (OCT) for the anterior segment of the eye can be used to create a map of the cornea and they typically highlight areas of corneal thinning in keratoconus (2). Slit lamp examination is also used for diagnosis as it allows for a detailed review of the cornea. Corneal thinning is seen on examination.
Pachymetry is a device that measures the thickness of the cornea. Ultrasound is used to make contact with the cornea and this also aims to identify areas of thinning associated with keratoconus.
Treatment
The treatment of keratoconus is dependent on disease progression once it is identified. Patients with less severe cases are given glasses to correct their vision. If their vision is unsatisfactory with glasses then contact lenses can be used.
Rigid gas permeable contact lenses are typically used. They help correct astigmatism, improve vision, and protect the eye from ultraviolet radiation which can damage the cornea by inducing oxidative stress. However, the use of them can lead to problems such as increased risk of corneal endothelial changes and some studies have shown it can affect the corneal nerve leading to reduced corneal sensitivity (8).
If conservative management is not effective, then surgical options are considered to treat keratoconus. A procedure called cross-linking is usually done first. Cross-linking involves using ultraviolet light and riboflavin to cause the corneal stroma to strengthen by forming additional cross links. This slows down the progression of keratoconus. It is especially important if the disease is caught early as if there is minimal damage to the cornea at that stage it will help prevent further visual disturbance (9).
If the disease is very advanced, then corneal transplantation is performed as a last resort.
Conclusion
To conclude, keratoconus is a complex condition with its aetiology likely due to a combination of both genetic and environmental factors. Advancements in technology and knowledge has led to better diagnostics and treatment of the condition.