Eye Exam: What to Expect and Why It Matters Before LASIK

Standard Eye Exam vs LASIK Pre-Operative Evaluation

A standard comprehensive eye exam performed by an optometrist or ophthalmologist covers: distance and near visual acuity, manifest refraction (glasses prescription), cover test (eye alignment), slit lamp exam (anterior segment), dilated fundus exam (retina and optic nerve), and intraocular pressure measurement. This exam confirms your prescription and screens for eye disease but is not sufficient for LASIK candidacy evaluation.

A LASIK pre-operative evaluation adds multiple advanced diagnostic tests specifically to determine whether laser surgery is safe and appropriate for your individual eyes. This evaluation takes 60 to 90 minutes and should be performed by or under the direct supervision of an ophthalmologist, not by an optician or technician alone.

Corneal Topography: Mapping Your Corneal Surface

Corneal topography maps the curvature of the anterior corneal surface using a Placido disk system (which projects concentric rings onto the cornea and analyzes their reflection) or an elevation-based system. The topography map identifies: regular astigmatism, irregular astigmatism, early keratoconus patterns (inferior steepening, skewed axes), post-surgical changes, and corneal surface irregularities. Keratoconus or suspicious topography patterns are among the most common reasons for declining LASIK candidacy. Corneal topography is non-contact, takes less than 1 minute, and is painless. See keratoconus diagnosis and corneal structure.

Pachymetry: Measuring Corneal Thickness

Pachymetry measures corneal thickness across the entire corneal surface. This determines whether sufficient tissue exists to safely create a flap and perform the planned ablation while leaving an adequate residual stromal bed (minimum 250 microns). Ultrasound pachymetry (brief probe contact) or optical coherence tomography-based pachymetry (non-contact) are both routinely used. A pachymetry map at minimum 500 microns centrally is typically required for LASIK candidacy, though the actual threshold depends on the planned ablation depth. See corneal thickness requirements.

Wavefront Analysis: Measuring Higher-Order Aberrations

Wavefront analysis uses a Hartmann-Shack sensor to measure how light passes through the entire optical system of the eye. It captures not only the standard prescription (sphere, cylinder, axis) but also higher-order aberrations (coma, spherical aberration, trefoil) that contribute to visual quality issues beyond simple blur. The wavefront data is used to program a wavefront-guided LASIK treatment and is also useful for screening patients who have significant pre-existing HOAs that might make standard treatment suboptimal. See wavefront technology and wavefront-guided LASIK.

Dry Eye Testing Battery

Pre-LASIK dry eye assessment is critical because LASIK worsens dry eye temporarily. A thorough evaluation includes: Schirmer test (tear production measurement), TBUT (tear film breakup time), corneal and conjunctival fluorescein staining, tear osmolarity (TearLab), MMP-9 inflammatory marker testing (InflammaDry), slit lamp evaluation of lid margins for blepharitis and MGD, and meibomian gland expressibility. Patients with dry eye should be identified and treated before surgery. See the full discussion at LASIK candidacy with dry eyes.

Dilated Fundus Examination

A dilated fundus exam (pupils dilated with eye drops) allows visualization of the retina, macula, and optic nerve. This is essential before LASIK to rule out: retinal holes, tears, or lattice degeneration (areas of retinal thinning at risk of detachment); diabetic retinopathy; macular disease that would limit the achievable post-LASIK visual acuity; and optic nerve abnormalities suggesting glaucoma. The dilation causes blurry near vision and light sensitivity for 4–6 hours, which is why patients should bring a driver to the pre-operative evaluation if possible. Note: dilation also temporarily changes refraction, so the prescription measurement is taken before dilation.

Timeline and Cost of the Pre-Operative Evaluation

The pre-operative evaluation typically takes 60 to 90 minutes. Contact lens wearers must discontinue lenses before the evaluation: soft lenses 2 weeks, toric soft lenses 3 weeks, rigid gas permeable lenses 4–8 weeks. At many LASIK centers, the pre-operative evaluation is offered free as part of the consultation for patients who are considering surgery. At stand-alone ophthalmology practices, the evaluation may cost $100–$300 if performed separately from the surgery fee. Always confirm what the pre-operative evaluation includes when scheduling your consultation. See what to expect at your free LASIK consultation.