Photoreactive keratectomy

Photorefractive (laser) surgery:

• used to treat refractive errors such as myopia, astigmatism and hyperopia
• photorefractive (laser) surgery for the correction of refractive errors is safe and efficacious for use in appropriately selected patients (1)
• surgical treatments have been developed to improve refraction by re-shaping the cornea
• corneal re-shaping is achieved using excimer laser ablation. Excimer laser techniques include photorefractive keratectomy (PRK), laser epithelial keratomileusis (LASEK) and laser in situ keratomileusis (LASIK)
  • PRK involves the removal of the corneal epithelium by surgical dissection and excimer laser ablation of a calculated amount of the stromal bed of the cornea
  • LASEK is a modification of PRK; dilute alcohol is used to loosen the corneal epithelium before it is lifted from the treatment zone as a hinged sheet, and then replaced at the end of the procedure
  • in LASIK, a flap is created with a microkeratome; this is lifted before laser ablation and then repositioned

Efficacy of laser surgery:

• a NICE review states that final uncorrected visual acuity achieved was similar for all three techniques (1)
  • PRK
    • data from more than 2000 eyes treated with PRK for myopia showed that a median of 69% of eyes had achieved within 0.5 D of their intended correction, and that 89% had achieved within 1.0 D
  • LASEK
    • data from case series of more than 1800 eyes undergoing LASEK for myopia or astigmatism showed that a median of 75% of eyes were within 0.5 D and a median of 92% of eyes were within 1.0D of their intended correction at 3–6 months follow-up
  • LASIK
    • data from eyes treated with LASIK for myopia or astigmatism showed that 77% were within 0.5 D and 91% were within 1.0 D of their intended correction at 3–12 months

Safety of laser surgery:

• loss of two lines of best-spectacle-corrected acuity in eyes treated for myopia
  • PRK
    • a median of 0.5% (0–20.5%)
  • LASEK
    • 0% (0–8.2%) of eyes treated with LASEK
  • LASIK
    • 0.6% (0–3%) of eyes treated with LASIK
  • patients with high myopia were more likely to lose two or more lines of best spectacle-corrected visual acuity than those with moderate to low myopia
• flap complications
  • may occur during LASIK and LASEK, requiring conversion to PRK or postponement of ablation (with LASIK), and occasionally there may be loss of best spectaclecorrected visual acuity
  • epithelial in-growth has been reported in LASIK ( median of 1.3% (0.0–4.4%)) treated eyes
• ectasia
  • a condition that can result from corneal thinning, is a serious complication related to refractive surgery that can lead to loss of vision
• microbial keratitis
  • has been reported in LASIK studies (occurred in 0–0.16% of eyes) - incidence was similar to, or lower than, that reported for contact lens wearers
• other patient-reported problems included visual difficulty in low light conditions, corneal haze, light halos and problems with glare
  • glare and night vision difficulties were less common after LASIK

Contraindications include (2):

• unstable myopia e.g. occuring in patients aged under 21 years old
• patients on steroids
• keratoconus
• pregnancy - healing is unpredictable

Reference:

1. NICE (March 2006). Photorefractive (laser) surgery for the correction of refractive errors
2. Towler HMA, Lightman S. Ophthalmology. BMJ 1996;312: 889-92