Am I a Candidate for LASIK? What the Assessment Involves

Laser vision correction has an excellent safety record in appropriately selected patients and is one of the most commonly performed elective procedures worldwide. The critical phrase is appropriately selected. A thorough pre-operative assessment is not a formality — it identifies the patients for whom LASIK is genuinely safe, and those for whom it is not. Occasionally the assessment results in a recommendation to proceed with PRK instead, or to avoid laser surgery altogether.

Why patient selection matters

The complications most feared in laser vision correction, including post-LASIK ectasia where the cornea progressively weakens and bulges after treatment, are almost always preventable with rigorous pre-operative screening. Modern corneal imaging detects subtle shape abnormalities that would have been invisible a decade ago. A well-selected patient has a high probability of an excellent, stable, long-term outcome.

Skipping or shortening the assessment is a false economy. The 60 to 90 minutes invested in a comprehensive evaluation determines whether a patient's anatomy and health genuinely support the planned procedure. Where the assessment reveals a concern, the recommendation to modify or decline surgery is in the patient's best interest, even if disappointing.

Prescription range and stability

The prescription must be stable for at least 12 consecutive months before LASIK is performed. A prescription that is still changing will lead to regression and dissatisfaction after surgery. Most surgeons prefer to treat patients aged 21 or older, as myopia often stabilises in the early-to-mid twenties. The table below summarises the treatable range for each refractive error type.

For patients over approximately 45, the natural loss of accommodation (presbyopia) means that achieving excellent uncorrected distance vision will come at the cost of near vision without reading glasses. This trade-off must be explicitly discussed at the consultation, and some patients in this age group are better suited to refractive lens exchange.

Corneal topography and tomography

Corneal topography maps the curvature of the front corneal surface; tomography adds measurement of the posterior surface and corneal thickness at every point. This is the most critical part of the LASIK assessment. An irregular topography pattern, even without symptoms, may indicate a subclinical ectatic tendency that contraindicates LASIK and cannot be detected by standard slit-lamp examination alone.

The assessment looks for a normal, regular corneal shape; absence of keratoconus or forme fruste (subclinical) keratoconus; adequate and even corneal thickness; and normal posterior corneal elevation. Forme fruste keratoconus is a finding in which the corneal shape shows early ectatic features without frank clinical disease. Operating on such a cornea with LASIK carries a meaningful risk of accelerating ectasia.

Corneal thickness

Average corneal thickness is approximately 540 microns. After flap creation and laser ablation, the minimum safe residual stromal bed thickness is generally 250 to 300 microns. Each dioptre of myopia correction removes approximately 12 to 15 microns of tissue. Where the arithmetic does not leave adequate residual tissue, PRK is the safer alternative as it avoids flap creation and preserves more stromal depth.

An example: a cornea measuring 500 microns with a standard 110-micron flap leaves 390 microns of stromal bed. Treating minus 4.00 dioptres removes approximately 55 to 65 microns, leaving a residual bed of 325 to 335 microns, safely above the threshold. For a cornea measuring 460 microns with the same correction, the residual bed falls to approximately 285 to 295 microns, and the risk profile warrants careful review. The calculation is performed for every patient as part of the pre-operative work-up.

Dry eye disease

LASIK transects corneal nerves in the flap creation zone, temporarily impairing the sensation that drives reflex tearing. Pre-existing dry eye disease is significantly worsened after LASIK and can produce persistent discomfort and fluctuating vision. Dry eye is assessed with symptom questionnaires, slit-lamp examination, and tear film quality measurements before any surgical recommendation is made.

Patients with significant dry eye may be advised to complete a course of dry eye treatment before reassessment, or may be better served by PRK, which causes less severe disruption to corneal nerves. For a detailed explanation of how dry eye intersects with laser surgery, see the guide to what to expect from LASIK surgery.

Pupil size in dim light

In mesopic (dim light) conditions the pupil may dilate beyond the laser treatment zone, producing halos and starbursts around point-source lights at night. Modern excimer lasers with large optical zones have substantially reduced this risk, but large scotopic pupils remain a relevant factor for patients with demanding night vision requirements such as pilots, emergency workers, and frequent night drivers.

Pupil diameter is measured in dim illumination as part of the standard assessment. Where a large pupil is identified in a patient with demanding night vision requirements, this is discussed at length before any recommendation is made.

LASIK versus PRK: when the flap is not the right choice

PRK (photorefractive keratectomy) uses the same excimer laser as LASIK but without creating a corneal flap, preserving more residual stromal tissue and eliminating flap-related complications. PRK is preferred for thin corneas, borderline topography, dry eye disease, and contact sport participation. Visual outcomes at 12 months are equivalent to LASIK; the trade-off is a longer recovery period.

For a detailed account of what the PRK procedure and recovery involve, see the guide to PRK: what to expect before, during, and after surgery.

What the consultation involves

A comprehensive LASIK candidacy assessment takes 60 to 90 minutes and includes visual acuity, manifest and cycloplegic refraction, corneal topography and tomography, pupil size measurement in dim light, dry eye evaluation, and a dilated retinal examination. Results are reviewed in full before any recommendation is made. Borderline cases are re-examined before a final decision.

At the conclusion of the assessment, one of four recommendations is made: proceed with LASIK; proceed with PRK as the safer alternative; defer to allow prescription stability or dry eye treatment; or decline laser refractive surgery and discuss alternatives such as refractive lens exchange. A recommendation not to proceed with LASIK is not a failure of the process — it is the process working correctly. For a full comparison of laser refractive outcomes by procedure type and prescription range, see the guide to laser refractive surgery and refractive lens exchange outcomes. For a summary of all refractive surgery options at the practice, see the refractive surgery overview.

References

1. Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115(1):37–50.
2. American Academy of Ophthalmology. Preferred Practice Pattern: Refractive Errors and Refractive Surgery. San Francisco: American Academy of Ophthalmology; 2023.
3. Shortt AJ, Bunce C, Allan BD. Evidence for superior efficacy and safety of LASIK over photorefractive keratectomy for correction of myopia. Ophthalmology. 2006;113(11):1997–2008.