How Your Eyes Are Measured: A Step-by-Step LASIK Consultation Walkthrough
Your LASIK consultation involves more testing than any glasses prescription you've ever had. Each measurement serves a specific purpose — and together, they determine whether you're a candidate, which procedure fits your eyes, and exactly how the laser should be programmed. Here's what happens, step by step.
A thorough LASIK consultation includes 8–12 distinct measurements taking 90–120 minutes. This is one of the most comprehensive eye exams you'll ever receive. Every test either confirms candidacy or programs the laser — nothing is optional.
The Complete Test Sequence
1. Visual Acuity (5 minutes)
The familiar letter chart — reading the smallest line possible with and without glasses/contacts. This establishes your baseline and the goal the laser is targeting.
2. Manifest Refraction (10 minutes)
The "which is better, one or two?" test using the phoropter (the mask-like device with rotating lenses). This determines your subjective prescription — the one that gives you the best-corrected vision.
3. Autorefraction/Aberrometry (5 minutes)
A machine objectively measures your refractive error by analyzing how light bounces off your retina. The iDesign and WaveScan systems also map higher-order aberrations — irregularities that glasses can't correct but wavefront-guided LASIK can.
4. Corneal Topography (5 minutes)
A color-coded map of your cornea's curvature. Think of it as a contour map of a mountain — it shows steep areas, flat areas, and any irregularities. Two technologies are commonly used:
- Placido disc topography: Projects concentric rings onto the cornea and measures their reflection. Maps the front surface only.
- Scheimpflug imaging (Pentacam): Rotates a camera around the eye to map both front and back corneal surfaces, plus thickness at every point.
Topography is the primary screening tool for keratoconus — a contraindication for LASIK. Even subclinical keratoconus (too mild to cause symptoms) can be detected on topography by the characteristic inferior steepening pattern. This is why topography is non-negotiable.
5. Pachymetry — Corneal Thickness (5 minutes)
Measures the thickness of your cornea at its thinnest point, typically using ultrasonic or optical methods. The average cornea is 540 microns. LASIK requires enough tissue to create the flap (about 110 microns) plus the ablation (varies with prescription) while leaving at least 250–300 microns of residual stromal bed for structural integrity.
6. Pupillometry (2 minutes)
Measures pupil diameter in different lighting conditions. Large pupils in dim light (over 7mm) were historically a concern for halos and glare, though modern large-zone treatments have largely addressed this. The measurement still helps the surgeon optimize the treatment zone diameter.
7. Slit-Lamp Biomicroscopy (10 minutes)
The surgeon examines your eye under magnification, checking the cornea layer by layer, the iris, lens, and anterior chamber. Looking for: corneal scars, dystrophies, dry eye signs (conjunctival staining, meibomian gland quality), cataracts, and any structural abnormalities.
8. Intraocular Pressure (2 minutes)
IOP is measured to screen for glaucoma and establish a baseline. Air puff tonometry (the device that blows air at your eye) or Goldmann applanation tonometry.
9. Dilated Fundus Exam (20 minutes + dilation time)
Drops dilate your pupils so the surgeon can examine the retina, optic nerve, and vitreous. Looking for: retinal tears, lattice degeneration (thin retinal areas), macular pathology, and optic nerve head appearance. Dilation takes 20–30 minutes to reach full effect and blurs near vision for 4–6 hours afterward.
10. Tear Film Assessment (5 minutes)
Schirmer's test (paper strip in lower eyelid measures tear volume) and tear break-up time (TBUT — fluorescein dye to see how quickly tears evaporate). Identifies dry eye before surgery so it can be treated pre-operatively.
11. Cycloplegic Refraction (10 minutes)
After dilation, refraction is repeated. The cycloplegic drops paralyze the focusing muscle, revealing your "true" prescription without any accommodative effort. This is especially important for younger patients who may be unconsciously compensating.
What You Should Bring to Your Consultation
- Current glasses and/or contact lens prescription
- Previous eye exam records (if available)
- List of medications (including eye drops)
- Medical history, especially autoimmune conditions, diabetes, or prior eye surgery
- Stop wearing soft contacts 3 days before; rigid/hard contacts 2 weeks before (contacts alter corneal shape)
At JCI-accredited clinics in Colombia, this entire workup is performed in a single visit — typically the day before or morning of surgery. In the US, the consultation and surgery are often scheduled weeks apart. Colombia's same-week model means you spend less total time away from work and daily life.
Frequently Asked Questions
Contact lenses temporarily reshape the cornea. Soft lenses alter the surface for 1–3 days; rigid lenses for 2–4 weeks. Your measurements need to reflect your natural corneal shape, not the shape your contacts imposed.
Your near vision will be blurry for 4–6 hours and you'll be light-sensitive. Bring sunglasses and plan not to drive. In Medellín, use Uber or a taxi back to your hotel.
Good surgeons will discuss borderline findings openly. Common scenarios: slightly thin cornea (may recommend PRK instead of LASIK), early dry eye (treat first, then reassess), or mildly suspicious topography (may recommend additional testing). Borderline doesn't mean no — it means the surgeon is being thorough.
Nothing is painful. The dilation drops may sting for 2–3 seconds. The air puff tonometer is startling but not painful. The rest involves looking at lights and lenses.