What are the principles of refractory eye surgery?

Principles of Refractive Eye Surgery

Refractive surgery aims to correct refractive errors (myopia, hyperopia, astigmatism, presbyopia) by either reshaping the cornea or implanting an intraocular lens to improve daily function and reduce dependence on glasses or contact lenses. 1

Core Surgical Approaches

There are two fundamental refractive surgical strategies 1:

• Corneal reshaping procedures - Modify the corneal curvature to alter refractive power
• Intraocular lens implantation - Place phakic IOLs in eyes with natural lens, or perform refractive lens exchange

Patient Selection Principles

Eligibility Criteria

Patients must be beyond amblyogenic age with stable refractive error that when corrected results in improvement in visual acuity or function. 1

Key requirements include 1:

• Stable refraction (no significant progression)
• Realistic expectations about outcomes
• Adequate corneal thickness for planned ablation depth (for corneal procedures)
• Absence of visually significant cataract (for phakic IOLs)

Absolute Contraindications

The following conditions preclude refractive surgery 1:

• Unstable refraction
• Corneal abnormalities (keratoconus, ectasias, thinning, edema, neurotrophic keratitis)
• Insufficient corneal thickness for proposed ablation
• Visually significant cataract
• Uncontrolled glaucoma
• Uncontrolled external disease (blepharitis, dry eye, atopy)
• Uncontrolled autoimmune or immune-mediated disease
• Uncontrolled mental illness (anxiety, depression)
• Unrealistic patient expectations

Relative Contraindications for Intraocular Procedures

Additional cautions for phakic IOLs or refractive lens exchange 1:

• Functional monocularity
• Significant eyelid, tear film, or ocular surface abnormalities
• Anterior segment inflammation
• History of uveitis
• Diabetes mellitus
• Shallow anterior chamber
• Pregnancy or lactation

Preoperative Assessment Principles

Comprehensive Refractive Evaluation

Each eye must be evaluated independently with precise measurements, particularly for high refractive errors where vertex distance and astigmatic axis determination are critical. 1

Essential measurements 1:

• Distance visual acuity - Measured at 20 feet in dimly lit room with high-contrast characters
• Subjective refraction - Using phoropter or trial lens set (reproducibility within 0.50 D for spherical equivalent)
• Cycloplegic refraction - Indicated when accommodation cannot be relaxed or when manifest refraction doesn't match symptoms
• Keratometry readings - Must be documented for future cataract surgery planning
• Near vision assessment - Before cycloplegia for presbyopic patients or those with near complaints

Documentation Requirements

Refractive surgeons must maintain records including preoperative keratometry, refraction, and postoperative refraction, providing this data for future eye care including cataract surgery. 1

Informed Consent Principles

The surgeon is responsible for ensuring comprehensive informed consent with documentation, giving patients opportunity to have all questions answered before surgery. 1

Critical Discussion Elements

Patients must be informed about 1:

Refractive outcomes:

• Range of expected outcomes and possible residual refractive error
• Procedures for reduction of residual error (enhancement procedures)
• Loss of accommodation following refractive lens exchange
• Need for reading/distance correction postoperatively

Vision quality changes:

• Changes in visual function not measured by acuity testing (glare, low-light function)
• Night-vision symptoms (glare, haloes) developing or worsening - particularly important for high ametropia or those requiring high-level visual function in low-light
• Effect on ocular alignment
• Monovision advantages and disadvantages for presbyopic patients

Complications:

• Loss of best-corrected visual acuity
• Corneal endothelial damage leading to edema
• Irregular pupil, microbial keratitis, endophthalmitis
• Intraocular inflammation, cystoid macular edema
• Retinal detachment (especially with myopic refractive lens exchange: 2-8% cumulative risk) 1
• Development/progression of cataract
• Development or exacerbation of dry eye symptoms, neuralgia 1
• Recurrent erosion syndrome
• Influence on IOL calculation accuracy for subsequent cataract surgery

Postoperative care plans - Setting and providers of care 1

Surgical Technique Principles

Corneal Refractive Surgery (PRK Example)

All instrumentation must be checked and calibrated before the procedure, with surgeon confirmation of patient identity, operative eye, and correct treatment parameters entered into the laser computer. 1

Key technical principles 1:

Alignment and positioning:

• In significant astigmatism or wavefront-guided treatment, ensure torsional alignment
• Axis alignment is crucial - large reduction in effect occurs if astigmatic ablation misaligned with true axis
• Place reference marks on operative eye while patient seated upright to compensate for ocular cyclotorsion when supine
• Use tracker or iris registration to maximize astigmatic ablation accuracy
• Maintain proper head position so facial/corneal planes parallel to floor and orthogonal to laser beam

Epithelial management:

• Remove epithelium mechanically (brush, blade, epikeratome), chemically (~20% ethanol), or by laser
• Expeditious removal minimizes nonuniform/excessive stromal drying, reducing unanticipated outcomes
• Remove enough epithelium for full planned laser optical zone diameter plus peripheral transition zones

Mitomycin-C use:

• Often used off-label to reduce corneal subepithelial haze risk, particularly with high corrections or prior corneal surgery
• Concentration 0.02% (0.2 mg/ml) for brief period (15 seconds) shows no significant endothelial cell count reduction 1

Safety Thresholds for LASIK

For LASIK procedures, minimum 250 μm residual stromal bed thickness is suggested as safe, though no absolute value guarantees ectasia will not occur. 1

Additional ectasia risk factors 1:

• Abnormal topography
• Percentage of tissue altered (PTA) ≥40%

Intraocular Refractive Surgery Principles

Phakic IOLs allow correction up to 20 D of myopia and have optical/structural advantages over keratorefractive surgery at high refractive levels. 1

Considerations for procedure selection 1:

• Patients with thin corneas or atypical topography at increased risk of corneal complications with keratorefractive surgery
• Intraocular refractive surgery may be alternative in these situations
• Retinal detachment following refractive lens exchange in high myopia: 2-8% cumulative risk over time
• Phakic IOLs not associated with increased retinal detachment risk compared to other intraocular interventions in highly myopic patients
• In highly myopic eyes ages 30-50 years, relative risk of BCVA loss less for phakic IOLs than refractive lens exchange
• 10-year refractive stability of phakic IOLs superior to LASIK, at expense of reduced endothelial cell count

Anesthesia Principles

Intraocular refractive surgery may be performed using general or local anesthesia (retrobulbar, peribulbar, sub-Tenon's injection, topical, intracameral), with planned mode discussed with patient regarding pain, discomfort, consciousness level, visual experiences, and complications. 1

For intraocular procedures 1:

• Topical or local anesthesia with sedation generally used depending on implant type
• Intravenous access generally recommended to treat potential adverse events when sedation/analgesic agents administered
• Given lack of evidence for optimal anesthesia strategy, type should be determined by patient needs and preferences of patient and surgeon

Postoperative Management Principles

Medication Regimens

Postoperative regimens of topical antibiotics, corticosteroids, NSAIDs, and oral analgesics vary among practitioners - the operating surgeon decides use of any or all products singly or in combination. 1

For PRK 1:

• Bandage contact lens usually applied
• Judicious short-term use of dilute topical anesthetics can help control postoperative pain

Complication Monitoring

Persistent diffuse lamellar keratitis (DLK) unresponsive to corticosteroids should prompt consideration of microbial keratitis or interlamellar fluid due to increased IOP measured peripheral to LASIK flap, intraocular inflammation, or endothelial decompensation. 1

For extensive DLK, interface should be irrigated to minimize stromal loss 1

Special Considerations

Dry Eye Management

Pre-existing untreated dry eye disease has been identified as risk factor for postsurgical dry eye symptoms, and as part of informed consent process, common adverse effects such as dry eye and eventual presbyopia must be emphasized with patients considering corneal refractive surgery. 1, 2

Pupil Size Assessment

Published studies have failed to demonstrate relationship between pupil size and quality of postoperative vision, minimizing importance of pupillometry in preoperative workup. 1

Refractive Stability Monitoring

The refractive stability of patients undergoing corneal refractive procedures and treated with diclofenac sodium ophthalmic solution 0.1% has not been established - patients should be monitored for one year following use in this setting. 3

Bleeding Risk

With some NSAIDs, there exists potential for increased bleeding time due to interference with thrombocyte aggregation, and ocularly applied NSAIDs may cause increased bleeding of ocular tissues (including hyphemas) in conjunction with ocular surgery. 3