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About 2/3 of the focusing power of the eye comes from the cornea. In keratoconus, between puberty and around 35 years of age, the corneal shape becomes steeper and more irregular resulting in poor focus. Keratoconus is thought to be a product of a combination of genetic susceptibility and environmental factors. It is more common in some racial groups (eg South Asian, Arab, Eastern Mediterranian and North African) than others. About 1 in 2000 white Europeans has clinically significant keratoconus, rising to 1 in 450 people with a South Asian background. Mild forms of keratoconus are more common still, with approximately 1 in 50 patients attending a laser eye surger consultation showing signs of keratoconus on corneal shape scans.
Risk factors for progression of the corneal shape abnormality in keratoconus include young age, steep corneal curvature, and asymmetric corneal curvature. In other words, younger patients with a more advanced corneal shape abnormality are at greater risk for further problems if nothing is done to intercept keratoconus progression.
Until recently, there were no treatments available to stop keratoconus getting worse. Patients were given rigid contact lenses once spectacles were no longer effective, and corneal transplantation when contact lenses were no longer tolerated.
Since 2000, Corneal Collagen Cross-linking (CXL) has emerged as a safe and effective method of preventing further corneal shape deterioration in keratoconus, and other treatments for shape correction and visual rehabilitation have been developed. These include intraocorneal ring segment (ICRS) implantation, intraocular collamer lens (ICL) implantation; and custom laser treatments (topography and wavefront guided PRK). The emphasis in keratoconus treatment has shifted towards a much more proactive approach aimed at intercepting disease progression and restoring good unaided vision.
Key aims in keratoconus care
There are two fundamental aims in modern keratoconus care:
- Corneal shape stabilisation
- Restoring good vision
At your initial consultation you will be given advice on each of these aspects of your care appropriate to your age and disease stage. Broadly speaking, patients over 35 years of age have sufficient natural cross-linking to make further disease progression in keratoconus less likely; whereas 90% of teenagers presenting with keratconus will continue to get worse within two years without CXL. CXL is therefore often offered to patients under 35 at an early stage, and we monitor younger patients more frequently for signs of progression. Conversely, we can take a more relaxed approach with older patients, particularly those with mild keratoconus.
3D corneal shape scanning
The pentacam (www.pentacam.com) is the industry standard device for measuring the corneal shape accurately in three dimensions. At your first consultation, pentacam scanning will help us determine the most appropriate initial treatment. Subsequently, pentacam scanning will be used to ensure that your corneal shape is stable by comparing scans between visits. Because keratoconus requires long- term monitoring and travel to London for review may not always be practical, we will give you your pentacam files to keep on a USB stick so that your care can be transferred wherever you end up. Contact lenses, rigid contact lenses in particular, temporarily modify the corneal shape. For accurate scans, you should not use contact lenses for two weeks prior to review visits.
Key points to remember with regard to corneal shape scanning are:
- Bring your USB stick with you
- Leave contact lenses out for two weeks (rigid lenses) or one week (soft lenses)
Pentacam scanning generates a useful classification of keratoconus disease stage according to corneal shape and thickness.
Initial treatment selection is based primarily on this staging and your age.
- Stage I (mild) – monitoring only
- Stage II (moderate) – CXL for progressive disease then monitoring
- Stage III (moderate/advanced) – ICRS followed by CXL for progressive disease then monitoring
- Stage IV (advanced) – corneal transplantation
Whilst broadly correct, this list has important modifiers, altering treatment selection. These will be discussed at your initial consultation. For example, corneal transplantation may be delayed in stage IV disease if specialised contact lens fitting is successful. Also, as above, patients over 35 years of age are not normally offered CXL unless there is evidence of continued shape instability during monitoring.
The initial goal of modern keratoconus treatment is corneal shape stabilisation. We would normally monitor your corneal shape each six months for at least two years according to the following algorithm to make sure your corneal shape is not still changing before any surgery to reduce your dependence on contact lenses and glasses.
Note that Corneal Collagen Cross-linking (CXL) treatment has different forms and can be repeated if necessary.
For monitoring visits, remember to bring your USB stick and leave your contact lenses out. Pentacam scanning is the basis of monitoring in keratoconus and the cornea needs to be allowed to relax into its natural shape prior to the monitoring consultation.
Monitoring visits are relatively brief, requiring scanning and spectacle testing only. Your scans will be reviewed after each consultation and you will be contacted for further review if there are signs of progressive shape change requiring further treatment for shape stabilisation.
Whilst we are waiting for corneal shape to stabilise, you will be offered expert advice on contact lens fitting and spectacle wear. Long-term studies show that corneal shape often improves for up to two years after CXL before stabilizing thereafter. At that stage, fine-tuning of corneal shape with customized laser eye surgery (transepithelial PRK) can be helpful in restoring good spectacle corrected vision. ICL implantation can then re-establish normal uncorrected vision. Our algorithm for reducing dependence on spectacles and contact lenses for patients with stable keratoconus is laid out below.
Each of these procedures (CXL, ICRS, PRK, and ICL implantation) has own risks and benefits. These are discussed in detailed supplementary procedure specific information.