Indications

• recurrent corneal erosion
  
• Reis-Bücklers dystrophy
  
• map-dot fingerprint or stromal corneal dystrophies (e.g., granular, macular, or lattice dystrophies)
  
• keratoconus (specific cases with stable refraction, limited astigmatism, and normal pachymetry)
  
• laser adjustment after keratoplasty, clear lens extraction, or cataract extraction

Inclusion Criteria

• realistic patient expectations
  
• clinically normal eyes
  
• patient age greater than 18 years (required for refractive stability)
  
• postoperative pachymetry not less than 300 μm (ablation depths calculated preoperatively)
  
• postoperative keratometry not less than 36 D (ablation depths calculated preoperatively)
  
• scotopic pupil size less than 8 mm for an excimer laser [provides an optical zone (OZ) <6 mm with no transition zone capability]
  
• good general health (patients with diabetes or HIV treatable if health permits but do not perform simultaneous bilateral surgery; see Contraindications)

Patient Evaluation

Patient Preparation

• Instruct patient to remove soft contact lenses 2 to 3 days before the preoperative examination.
  
• For rigid gas-permeable contact lens wearers, verify the stability of topography before surgery (stability may take 4-6 weeks to occur).

Contraindications

• relative contraindications
  
  
  
  • scotopic pupil size greater than 8 mm (greater risk of having postoperative glare and halos)
    
  • human immunodeficiency virus (HIV) (depends on general health status)
    
  • diabetes (depends on general health status)
    
  • infections that call for surgery to be delayed (e.g., a common cold, sinusitis, and other infectious illnesses)
    
  • ocular pathology (especially autoimmune disease)
    
  • dry-eye syndrome [often associated with systemic diseases such as sarcoidosis, mucin deficiency (e.g., Stevens-Johnson syndrome), avitaminose A, lipid deficiency (blepharitis), and decreased blinking (from contact lens use and post-herpes simplex keratitis)]
  
  
• absolute contraindications
  
  
  
  • unrealistic patient expectations about surgical “guarantees” (e.g., that surgery will not harm vision, the outcome will be piano, or spectacles or contacts will no longer be necessary)
    
  • unstable or progressive myopia or hyperopia
    
  • amblyopia or strabismus (inform patients with very low fusional amplitudes that the procedure will not improve their BCVA)

General Methods

Methods

Advantages

• use for low to high myopia or hyperopia (use laser- and surgeon-specific nomograms to adjust the total amount of myopia and astigmatism to correct by determining the fixed ratio that indicates the coefficient to apply to actual sphere and cylinder to obtain the desired sphere and cylinder corrections)
  
• no need for pretreatment for central island
  
• use of 6-mm OZ with a transition zone of 9 mm (large transitional zones are used to smooth the curve between the actual treatment zone and the remaining untouched corneal stroma)
  
• elimination of the need to perform multi-zones
  
  
  
  • generation of multiple scanning of the corneal surface while removing stroma
    
  • performance of equal passes so that the initial sphere is divided by equal but smaller treatments (sum of small treatments equals target correction)
    
  • introduction of very small rest periods between subtreatments
    
  • decreased haze for high myopes
  
  
• passes (≤6 total) should each last 20 to 30 sec (do not exceed 30 sec per pass)
  
• no need to wet the surface between passes

Disadvantages

• potential for calculation errors (if not automated)
  
• dehydration of corneal stromal (if excessive time between passes causing overcorrection)

Myopia

Equipment Preparation

• Configure the laser to treat all astigmatism (generally, 10-30% is removed from the computed sphere; may vary with different types and brands of lasers).

Nomogram

• patients less than 40 years old
  
  
  
  • removal of 0.25 D of astigmatism to the computed sphere per diopter of cylinder [e.g., −6.00 − 1.00 × 45 degrees = 30% (2 D) and +0.25 D removed to the sphere; because of 1 D of cylinder; the computed ablation = –3.75 – 1.00 x 45 degrees)
  
  
• patients greater than 40 years old
  
  
  
  • removal of an additional 0.25 D to the computed sphere

Results

Results

• hyperopia less than +3 D
  
  
  
  • 92% of eyes within ±1 D of emmetropia
    
  • 8% needing retreatment
    
  • 3% of eyes with mild to moderate haze
    
  • regression of +0.75 to +1 D
    
  • 2% of eyes with loss of more than 1 line of BCVA

Compound Astigmatism

Methods

• Perform a cross-cylinder technique using Vinciguerra’s nomogram for astigmatism greater than 3 D (see Vinciguerra et al., 1999).
  
  
  
  • Dividing the astigmatism in two treatments.
    
  • Perform half of the cylinder in minus cylinder and the other half in plus cylinder (e.g., the treatment of –4.00 – 6.00 X 180 degrees would be piano +3.00 × 90 degrees, piano −3.00 × 180 degrees, and −7.00 D total spherical ablation).
  
  
• removal of 0.25 D of astigmatism to the sphere per diopter of cylinder (see Myopia)

Mixed Astigmatism

Methods

Presbyopia

Patient Preparation

• Try monovision with contact lenses before proceeding with surgery (if the patient feels comfortable with this vision, proceed with the procedure).

Methods

• Although monovision is the only alternative to PRK for improvement of near vision, it is not considered real correction of accommodation.
  
  
  
  • Correct one eye, usually the dominant one, toward emmetropia.
    
  • Undercorrect the other eye with a slight level of myopia (−0.50 to −1.50 D).

Alternative Treatments

LASER IN SITU KERATOMILEUSIS The upper limit for PRK may be between –7 and –12 D. For severe myopia (> –12 D), predictability decreases but the incidence of certain adverse effects (e.g., corneal haze) significantly increases to greater than 10% from less than 1% for low myopia corrections. Inform patients with myopia greater than 10 D and hyperopia greater than 3 D of the greater risks associated with the procedure (60% of patients experience excellent results).

• retreatments
  
• halos
  
• haze
  
• loss of BCVA

Medications

Complications

Identification and classification of patients who require retreatment are subtle because some patients with mild corrections or haze are symptomatic whereas others are completely satisfied with their outcome.

Indications

• corneal haze
  
  
  
  • Early diagnosis is mandatory.
    
  • Retreatments are possible as early as 1 month after surgery before haze formation increases.
    
  • Avoid severe haze by retreating promptly (advanced haze formation can activate mechanisms that create further haze even after retreatment).
    
  • Retreatment of eyes with an aggressive healing pattern that is accompanied by a progressive myopic shift may create the same amount of haze when performed 6 to 12 months after the original treatment.
  
  
• undercorrections, overcorrections, and regression
  
  
  
  • These are correctable within 1 to 4 months after surgery.
    
  • Stable refractive outcomes may result from prompt retreatment.
    
  • Use holmium laser keratoplasty for overcorrections less than +1.50 D.

Results

• up to 95% of eyes within ±1 D
  
• possibility of lost lines of BCVA after retreatment

Phototherapeuthic Keratectomy

Methods

Postoperative Care

• Examine the patient’s eyes 72 hr after surgery or daily until reepithelialization and at 1, 2, 3, 6, 12, and 24 months (for lower myopes, omit 2- and 6-month follow-ups if the patient’s condition does not change).
  
  
  
  • Perform manifest refraction.
  
  
• Evaluate patient’s BCVA and uncorrected visual acuity using a Snellen’s chart.
  
  
  
  • Use corneal topography to check for increased astigmatism or irregular astigmatism.
    
  • Grade haze on a scale of 0 to 3 (clear to completely obscured), as proposed by some authors.
  
  
• After correcting for astigmatism with PRK, perform a vector analysis on preoperative refraction rather than postoperative refraction.
  
• To estimate the quality of astigmatic correction, calculate Alpin’s index of success (defined as the proportion of the remaining astigmatism to treat on the corneal plane divided by the intended correction) using medical computer software.
  
• Analyze refractive outcomes and, if needed, adjust excimer laser nomograms (computer software packages, such as the ASSORT Eye Surgery Analysis Program from the Melbourne Excimer Group in Melbourne, Australia, may help in this task).

Griffith M, Jackson BW, Lafontaine MD, Mintsioulis G, Agapitos P, Hodge W. Evaluation of current techniques of corneal epithelial removal in hyperopic photorefractive keratectomy. J Cataract Refract Surg. 1998;24:1070-1078.

Jackson BW, Casson E, Hodge W, Mintsioulis G, Agapitos PJ. Laser vision correction for low hyperopia. Ophthalmology. 1998;105:1727-1738.

Kim JH, Hahn TW, Young CL. Photorefractive keratectomy in 202 myopic eyes: one year results. Refract Corneal Surg. 1993;9(suppl): S11-S16.

Kitazawa Y, Tokoro T, Ito S, Ishii Y. The efficacy of cooling on excimer laser photorefractive keratectomy in the rabbit eye. Survey Ophthalmol. 1997;42(Suppl 1):S82-S88.

O’Brart DP, Lohmann CP, Fitze FW, Smith SE, Kerr-Muir MG, Marshall J. Night vision after excimer laser photorefractive keratectomy: haze and halos. Eur J Ophthalmol. 1994;4:43-51.

Pop M. Prompt retreatment after photorefractive keratectomy. J Cataract Refract Surg. 1998; 24:320-326.

Pop M, Aras M. Multizone/multipass photorefractive keratectomy: six months results. J Cataract Refract Surg. 1995;21:633-643.

Pop M, Payette Y. Multipass versus single pass photorefractive keratectomy for high myopia using a scanning laser. J Refract Surg. 1999;15: 444-450.

Pop M, Payette Y. Results of bilateral photorefractive keratectomy. Ophthalmology. 2000; 107: 472-479.

Vinciguerra P, Epstein D, Azzolini M, Radice P, Sborgia M. Algorithm to correct hyperopic astigmatism with the Nidek EC-5000 excimer laser. J Refract Surg. 1999;15(suppl):Sl86-S187.