14.1 Goals

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  Stop or slow the progression of keratoconus in the long term (at least 10 years as per prior studies) by strengthening and stabilizing the collagen fibers in the cornea. ¹

  
  
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  Stabilize or improve both uncorrected and corrected distance visual acuity measurements by postoperative year 1. ^{2 , 3}

  
  
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  Stabilize or decrease steep keratometry values by postoperative year 1. ^{2 , 3}

  
  
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  Visualize a “demarcation line” in the posterior stroma on anterior segment optical coherence tomography (AS-OCT), which is indicative of the depth of the collagen cross-linking treatment into the stroma. ⁴

  
  
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  Ultimately prevent vision loss and/or the need for corneal transplantation from severe progression of keratoconus.

14.2 Advantages

Corneal collagen cross-linking (CXL) stabilizes collagen fibers in the cornea to stop or slow the progression of keratoconus, thereby preventing the development of high degrees of irregular astigmatism that occurs without treatment, which can become progressively difficult to treat with specially fitted contact lenses, and decreasing the risk of serious vision-threatening complications including hydrops, severe scarring, and corneal perforation, which can place the patient at risk of needing corneal transplantation.

14.3 Expectations

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  Stop or slow the progression of keratoconus for at least 10 years following treatment.

  
  
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  Ocular pain in the first postoperative week while the corneal epithelial defect heals.

  
  
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  Uncomplicated healing of the induced corneal epithelial defect without infection or scarring.

  
  
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  Corneal haze and edema in the first postoperative month which then resolves.

  
  
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  Decreased vision for up to 1 month postoperatively which then recovers to the preoperative baseline.

  
  
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  Improvement of uncorrected and/or corrected visual acuity in some patients by postoperative month 6.

  
  
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  As per prior long-term data, 20% of patients, especially those who are under 12 years at the time of their diagnosis, may require repeat CXL within 10 years following initial treatment due to progression, and 4% of patients may require corneal transplantation due to severe progression. ²

14.4 Key Principles

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  CXL utilizes riboflavin, a photosensitizer, with ultraviolet A (UVA) light to strengthen collagen bonds within the corneal stroma, thereby accelerating the normal process of age-related cross-linking that occurs over an individual’s lifetime. ³

  
  
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  When cross-linked, the collagen fibrils of the cornea are stiffened and therefore less likely to stretch and thin the cornea. ⁵

  
  
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  With proper surgical technique, maximum treatment efficacy can be achieved while minimizing long-term risks, such as infectious keratitis, persistent corneal haze, or corneal scarring.

14.5 Indications

As per FDA guidelines, the following are the indications and inclusion criteria for corneal cross-linking. In clinical practice, many surgeons have a more liberal approach (especially for age) in determining candidates for CXL: ³

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  Age 14 years or older.

  
  
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  Corneal topography pattern consistent with keratoconus, namely a pattern of inferior corneal steepening.

  
  
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  Steep keratometry value of 47 diopters or more.

  
  
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  Inferior-to-superior ratio of greater than 1.5 on corneal topography.

  
  
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  Corrected distance visual acuity worse than 20/20 in the treatment eye.

  
  
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  Corneal thickness greater than or equal to 300 microns.

  
  
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  Evidence of keratoconus progression over a 24-month period, including an increase in the steepest keratometry measurement of 1 diopter or more, an increase in cylindrical refractive error by manifest refraction of 1 diopter or more, or an increase in manifest refraction spherical equivalent of 0.5 diopters or more.

14.6 Contraindications

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  Prior history of corneal surgery, excluding prior CXL.

  
  
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  Corneal pathology that could impede epithelial wound healing, such as chemical injury, neurotrophic keratopathy, or limbal stem cell disease.

  
  
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  Prior history of herpetic keratitis, as CXL can cause disease re-activation.

  
  
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  Severe corneal scarring or opacification.

  
  
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  Severe dry eye disease.

14.7 Preoperative Preparation

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  Preoperatively, a careful history is obtained and slit-lamp examination is performed to evaluate for any of the contraindications listed above.

  
  
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  Placido-based corneal topography or, if available, Scheimpflug corneal tomography is obtained to document the presence of progressive keratoconus.

  
  
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  A baseline AS-OCT scan is obtained for comparison with the postoperative scan.

  
  
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  During the preoperative evaluation, the surgeon, the patient, and the patient’s family determine if the procedure can be undertaken under topical anesthesia in a clinic-based setting or if the patient requires general anesthesia in the operating room due to concerns about patient cooperation.

  
  
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  The surgeon should have a detailed preoperative discussion with the patient and the patient’s family outlining the goals of treatment, namely, halting keratoconus progression. Care should be taken to ensure the patient understands that CXL does not reverse the pre-existing changes caused by keratoconus and that it is not a refractive procedure that will eliminate the need for refractive correction.