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Corneal Collagen Crosslinking (CXL)

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What is Corneal Collagen Crosslinking?

Corneal Collagen Crosslinking (CXL) is a medical procedure used to treat keratoconous, a condition where the cornea progressively weakens, thins, and bulges into a cone shape. The goal of cross-linking is to strengthen the cornea and halt or slow the progression of keratoconous by reinforcing the collagen fibres within the corneal structure. The procedure involves applying riboflavin (vitamin B2) eye drops to the cornea and activating them with ultraviolet (UV) light, which creates additional bonds, or "cross-links," between the collagen fibres, making the cornea more stable and resistant to further deformation.

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How Corneal Cross-Linking Works:

  1. Riboflavin Application: Riboflavin (vitamin B2) is applied to the surface of the cornea as eye drops. This vitamin acts as a photo-sensitiser, meaning it reacts when exposed to UV light.
  2. Ultraviolet (UV) Light Activation: After the riboflavin has sufficiently penetrated the corneal tissue, the eye is exposed to controlled UV-A light. The interaction between the UV light and the riboflavin generates oxygen radicals, which facilitate the formation of new chemical bonds between collagen fibres in the cornea.
  3. Crosslink Formation: These new cross-links increase the stiffness and strength of the corneal tissue, much like adding reinforcements to a structure, making the cornea more stable and less prone to further thinning or bulging.
  4. Corneal Strengthening: As a result, the progression of keratoconous is slowed or halted, preventing further deterioration in vision.

Types of Corneal Cross-Linking:

(Conventional or Standard) Epithelium Off Crosslinking:

In this approach, the epithelium (the thin outer layer of the cornea) is removed before applying the riboflavin drops. Removing the epithelium allows for better penetration of the riboflavin into the deeper layers of the cornea.

  • Procedure:
    • The epithelium is gently removed.
    • Riboflavin drops are applied for about 20-30 minutes, allowing them to soak into the cornea.
    • UV light is applied for 10-30 minutes, depending on the specific protocol.
    • After the procedure, a soft contact lens is placed on the eye to protect it while the epithelium heals.
  • Benefits: Deeper penetration of riboflavin, which may be more effective for patients with advanced keratoconous.
  • Downside: Longer recovery time, with discomfort and healing needed as the epithelium regenerates.

(Transepithelial) Epithelium On Crosslinking:

In this method, the epithelium is left intact, making the procedure less invasive and reducing recovery time. Special formulations of riboflavin or techniques are used to help it penetrate through the epithelium.

  • Procedure:
    • Riboflavin is applied to the cornea without removing the epithelium.
    • UV light is then applied to activate the riboflavin, creating cross-links in the collagen.
  • Benefits: Faster recovery time and less discomfort since the epithelium is not removed.
  • Downside: Riboflavin penetration may be less effective, particularly in advanced keratoconous, making it potentially less effective for some patients.
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Who is a Candidate for Corneal Collagen Crosslinking?

Corneal cross-linking is most effective for:

  • keratoconous Patients: Especially those with progressive keratoconous (where the cornea is continuing to thin and change shape).
  • Patients with Early-Stage Disease: It is most beneficial when performed in the early stages of the disease, before significant corneal thinning or scarring has occurred.
  • Post-LASIK Ectasia: Cross-linking can also treat corneal ectasia, a rare complication of LASIK surgery where the cornea weakens and bulges.
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Benefits of Corneal Collagen Crosslinking:

  • Infection: Like any surgical procedure, there is a risk of infection, although the use of biological tissue may reduce this risk compared to synthetic implants.
  • Post-Surgical Complications: While the donor tissue is biocompatible, there is still a risk of complications such as migration or displacement of the segments.
  • Continued keratoconous Progression: In some cases, keratoconous may continue to progress even after the procedure, requiring additional treatments such as corneal crosslinking.
  • Need for Additional Vision Correction: Some patients may still require glasses or contact lenses after the procedure, although their vision may be significantly improved.
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Benefits of Corneal Collagen Crosslinking:

  • Halts Progression of keratoconous: The primary benefit of cross-linking is that it stabilises the cornea and prevents the condition from worsening, reducing the need for more invasive treatments such as corneal transplants.
  • Minimal Invasiveness: Cross-linking is a minimally invasive procedure, usually performed on an outpatient basis.
  • Potential for Improved Vision: While the main goal is to stop progression, some patients experience an improvement in vision as the cornea becomes more stable and regular in shape.
  • Short Recovery Time: Recovery time is relatively short, particularly with the epithelium-on version.

Limitations:

  • Does Not Reverse Damage: While it can stop further progression, cross-linking does not reverse any existing corneal thinning or scarring. Patients with advanced keratoconous may not regain lost vision and might need additional treatments, such as contact lenses or implants like INTACS.
  • Effectiveness Depends on Stage: Cross-linking is most effective in the early to mid-stages of keratoconous. In advanced cases where the cornea is very thin or scarred, other treatments might be needed.
  • Potential Side Effects: Temporary side effects may include discomfort, light sensitivity, and blurry vision during recovery. Long-term complications are rare but can include corneal haze or infection.
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Risks and Side Effects:

  • Discomfort and Sensitivity: Especially with the epithelium-off technique, patients may experience pain, sensitivity to light, and irritation for several days after the procedure.
  • Infection: Like any eye procedure, there is a risk of infection, although it is rare.
  • Haze or Scarring: Some patients may develop temporary corneal haze, although this typically clears as the cornea heals.

Recovery Process:

  • Epithelium-Off: Patients may experience discomfort and light sensitivity for several days while the epithelium regenerates. Vision may be blurry initially, and full recovery can take a few weeks. Antibiotic and anti-inflammatory drops are prescribed to prevent infection and promote healing.
  • Epithelium-On: Recovery is generally quicker, with less discomfort, but patients should still expect mild irritation and blurry vision for a short period.
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Success Rate and Long-Term Outlook:

Cross-linking has a high success rate, with studies showing that over 90% of patients experience stabilisation of their keratoconous. It is considered the gold standard for halting keratoconous progression and reducing the need for more invasive procedures like corneal transplants.

However, cross-linking does not cure keratoconous, and some patients may still need to wear glasses or contact lenses to achieve optimal vision. In some cases, combining cross-linking with other treatments, such as INTACS or CAIRS, can provide further improvement in vision.

Conclusion:

Corneal cross-linking is a highly effective, minimally invasive procedure designed to strengthen the cornea and halt the progression of keratoconous. By stabilising the cornea and preventing further damage, it can protect a patient’s vision and reduce the need for more invasive surgeries. While it doesn't reverse the existing damage, its role in preventing further thinning makes it an essential tool in managing keratoconous, especially when diagnosed early.