Summary

Keratoconus is a condition where there is progressive thinning and bulging of the cornea, leading to loss of visual acuity. It usually begins between the ages of 10 and 25 and is usually bilateral. The aetiology of keratoconus is unknown.

Background

Pathophysiology

Keratoconus is a progressive, non-inflammatory corneal disorder characterised by the thinning and conical deformation of the cornea. This condition typically begins in adolescence or early adulthood and can lead to significant visual impairment. The exact aetiology of keratoconus is not fully understood, but it is believed to involve a combination of genetic, environmental, and biochemical factors.

At the cellular level, keratoconus is associated with a reduction in the number of keratocytes (corneal fibroblasts) and an imbalance in the extracellular matrix components, particularly collagen. This imbalance leads to a weakening of the corneal structure, resulting in the characteristic protrusion of the cornea. Additionally, there is evidence of oxidative stress and increased levels of proteolytic enzymes, which may contribute to the degradation of corneal tissue.

The disease often progresses through several stages, with early signs including irregular astigmatism and changes in corneal topography. As the condition advances, patients may experience worsening visual acuity, increased sensitivity to light, and distortion of images.

Risk Factors

Several risk factors have been identified in association with keratoconus, including:

• Genetic predisposition: family history plays a significant role in the development of keratoconus, with studies indicating that individuals who have a first-degree relative affected by the condition are at a higher risk of developing keratoconus themselves. However, it is important to note that most patients do not have a family history; only about 10% of offspring of affected individuals go on to develop the condition. This suggests that while keratoconus may exhibit an autosomal dominant inheritance pattern with incomplete penetrance, other factors also contribute to its onset.
• Environmental factors: certain environmental factors may contribute to the onset and progression of keratoconus. These include chronic eye rubbing, which can lead to mechanical stress on the cornea, and exposure to ultraviolet (UV) light, which may exacerbate corneal thinning.
• Associated conditions: keratoconus is often associated with other systemic and ocular conditions, such as atopic dermatitis, Down syndrome, and Marfan syndrome. These associations suggest that underlying connective tissue disorders may play a role in the pathogenesis of keratoconus.
  • Atopic disorder: vernal keratoconjunctivitis
  • Connective tissue disorders: Ehlers-Danlos syndrome, Marfan syndrome, osteogenesis imperfecta
  • Obstructive sleep apnoea
  • Congenital disorders: Leber congenital optic neuropathy, aniridia
• Hormonal changes: some studies suggest that hormonal changes, particularly during puberty, may influence the onset and progression of keratoconus. This is supported by the observation that the condition often manifests during adolescence.

Clinical features

Keratoconus is a progressive disorder where there is thinning of the central or paracentral corneal stroma. It presents with a distorted cone-shaped cornea, which can cause changes in the refractive properties of the cornea. This leads to irregular astigmatism, which cannot be corrected completely with glasses.

The condition may progress, which will require frequent change in glasses. The condition usually presents in the teenage years, with features initially in only one eye. Presenting features include impairment of vision due to progressive myopia and astigmatism. Approximately 50% of normal fellow eyes will progress to keratoconus within 16 years.

Diagnosis

On inspection, there is corneal protrusion in a cone configuration with corneal thinning maximal at the apex of the cornea.

Keratometry is the measurement of the corneal radius of curvature. In keratoconus, the keratometry readings are steep. The highest axis of corneal power on keratometry is used to determine the grading of keratoconus as follows: