The pre-operative assessment for refractive surgery

Laser refractive surgery is tailored to each eye. Every cornea has its own shape, thickness and curvature. Every patient comes with their own lifestyle and expectations. The pre-operative assessment makes it possible to tailor the procedure to both of these dimensions.

This assessment has four objectives.

Understanding your plans and your lifestyle

Before the measurements, I listen. Why you are coming today, what you expect from surgery, how you live with your glasses or contact lenses, what you are afraid of. The discussion guides the choice of technique.

Checking that your eyes are suitable for surgery

Laser surgery is possible for most patients. But there are situations where it is not: a cornea that is too thin, signs of weakness, a correction beyond the limits of the laser. My role is to detect these beforehand.

When laser surgery is not the right option, I say so clearly. I would rather not operate than take the slightest risk with your eyes.

Choosing the most suitable technique

LASIK or TransPRK for most cases, SMILE or presbyopia strategies depending on the indication. No technique is better than the others in absolute terms; there is the one that matches your eye and your correction. The assessment tests make it possible to decide on objective criteria. Your lifestyle and your professional or sports constraints also guide the choice.

Everything about LASIK

Everything about TransPRK

Everything about SMILE

Anticipating the result and screening for anything that could be a problem

The assessment also lets me give you a precise idea of the expected result. This is when I screen for the most subtle signs of weakness, early keratoconus being the most important example. My hospital experience in corneal surgery, with more than 350 corneal transplants, taught me to spot them.

The assessment starts with time to talk. Understanding who you are, why you are coming and what you expect from this surgery is as necessary to me as measuring your cornea. The chosen technique and the correction strategy are largely built around this conversation. So is the pace.

I often say that I don’t operate on eyes, I operate on patients.

Why now: your motivation and your expectations

I start with the obvious questions: what brings you here, and why now? Being tired of glasses, contact lens intolerance, a life plan, a sport, a demanding job, a cosmetic reason. All of these reasons are valid. I try to understand what you really expect from surgery. Complete independence from glasses? Sharp distance vision? Being able to read with nothing on your nose?

I weigh this goal against what surgery can actually achieve for your eyes. Refractive surgery has its limits. My role is to match, as far as possible, your plans with what the technique allows, and to tell you clearly when the gap is too wide.

Your daily life: visual habits, work, driving

How many hours do you spend in front of a screen? Do you read a lot? Do you drive a lot, especially at night? These answers matter. Prolonged screen work makes post-operative eye strain more likely in the first weeks, and sometimes leads to prescribing rest glasses after surgery. Intensive night driving calls for caution regarding the management of halos and the correction strategy, particularly in presbyopic patients.

As for contact lenses, I look at your real comfort, beyond mere tolerance. How long do you wear them each day? Do you suffer from discomfort at the end of the day, dryness, red eyes? These signals weigh in the decision, particularly when there is a tendency to dry eye.

Sports and profession, when lifestyle drives the technique

This factor directly influences the choice of technique.

For combat sports (boxing, MMA, krav maga), intense contact sports, law enforcement, the military, firefighters, in short any situation where your eye is exposed to a risk of trauma, I often favor TransPRK over LASIK. TransPRK does not create a corneal flap and offers additional mechanical safety in case of a blow to the eye. In boxing, with gloves, a LASIK flap is not a problem: it is mostly a question of peace of mind.

LASIK remains excellent for the vast majority of patients, athletes included. The risk of the flap shifting in everyday life is very low. But for profiles exposed to repeated trauma, I sometimes prefer to remove this variable.

For visually demanding professions (pilots, surgeons, precision work, night work, artistic professions), expectations in terms of quality of vision are often higher than average. We discuss this in detail, and the surgical strategy is adjusted.

Your anxieties or fears, a topic in their own right

Refractive surgery is elective surgery. No one has to have it. It is a choice, and that choice brings a lot of apprehension. Will it work? Does it hurt? What if something goes wrong? I hear these questions every day, and I take the time to answer them one by one.

I prefer to explore these fears with you. Some patients are afraid of the laser itself, others of the operating room, others of the result, and others still of a specific event they have heard about. Understanding exactly what worries you makes it possible to address the right point. When a patient needs it, I offer two consultations rather than one, or a prior contact lens trial to test the correction strategy (particularly for presbyopic patients considering monovision). There is no rush.

Beyond the consultation, you can write to me at any time, before as well as after the operation. I answer my patients personally.

Suitability for surgery is not just about corneal thickness, contrary to a widely held idea. It depends on several parameters, and it is their combination that determines the decision.

There are in fact dozens of statistical formulas assessing the risk of post-operative ectasia (the feared adverse effect of LASIK), each giving more or less weight to one parameter or another. My role as a surgeon is to review all of this data comprehensively, and to make an informed diagnosis of suitability.

Total corneal thickness

The standard threshold for LASIK is 500 microns pre-operatively. Below that, we tend to opt for TransPRK, which does not require a flap and better preserves residual thickness. Thickness alone does not settle the matter, however: a thin but regular cornea may be suitable, whereas an average but irregular cornea will not be.

Corneal regularity, screening for keratoconus

Keratoconus is the main contraindication to refractive surgery. This progressive deformation of the cornea, which stretches and bulges into a cone, can be triggered or worsened by LASIK in a predisposed patient. It must therefore be screened for before any procedure.

Advanced forms are easy to recognize; early, subclinical forms are less so. Subtle asymmetry, localized thinning, epithelial irregularity: all subtle signs that require a trained eye. My hospital background in corneal surgery (Hôtel-Dieu with Prof. Bourges, Fondation Rothschild with Prof. Gabison and Prof. Gatinel, then former Chief Resident (Chef de Clinique) at Cochin Hospital) trained me to detect them, and to tell truly suggestive signs from false alarms.

The residual posterior stromal bed

This is the thickness of intact cornea left behind the laser treatment. In LASIK, about 110 microns are taken up by the flap, then an additional 15 microns per diopter of myopia corrected. What remains is the residual stromal bed. It is what guarantees the long-term stability of your cornea.

Animated simulator of the residual stromal bed. In LASIK, the femtosecond laser cuts a 110 µm flap attached by a hinge, the excimer sculpts only the central optical zone (≈ 75 µm), then the flap is laid back flat. The residual bed is about 365 µm. In TransPRK, the laser removes the central epithelium (≈ 50 µm, which regenerates) then sculpts the stroma without creating a flap: the residual bed remains higher, about 425 µm. For the same correction, TransPRK thus preserves around sixty microns more, because it does not sacrifice a layer of stroma in a flap. © Dr Nahaisi.

Standard recommendations set the minimum threshold at 280 microns. Some studies go as low as 250 microns, but this is a reduced margin that assumes all the other parameters are perfectly reassuring. For my part, I take a more cautious approach, at 300 microns, to ensure an additional safety margin.

I do this calculation for each eye when preparing the treatment. The laser software (below) simulates the ablation and displays the expected residual stromal bed (“RST”, residual stromal thickness), which I systematically check before approving surgery.

Treatment plan for a nearsighted, astigmatic eye (anonymized example). On the left, the ablation profile: the laser removes tissue in the center, in red, and barely touches the periphery. On the right, the “RST manager” calculates the residual stromal bed from the corneal thickness (552 µm), the flap thickness (110 µm) and the ablation depth (134 µm). 308 µm remain, above the safety threshold. I do this check for every patient before approving surgery.

When the expected residual bed would be insufficient with LASIK, because the cornea is thin or because the correction is high, TransPRK becomes the technique of choice. For very high corrections beyond -10 diopters, the phakic implant (ICL) is also an option to consider, with referral to a specialized colleague.

Corneal curvature, anterior and posterior keratometry

We measure the curvature of your cornea, both on its front surface (anterior keratometry, as mean and maximum values) and on its back surface (posterior keratometry). A marked asymmetry between the two surfaces can be a sign of weakness.

The starting curvature also determines the possible range of correction. A cornea that is too flat in a nearsighted patient, or too steep in a farsighted patient, limits the laser’s room for maneuver: in the first case, because an already flat cornea would have to be flattened further; in the second, because it would exceed reasonable limits of post-treatment steepening. This point is detailed on the dedicated pages.

See the Myopia page

See the Hyperopia page

Eye rubbing

Chronic rubbing is one of the main causes of corneal deformation and acquired keratoconus. When the topography shows a sign of weakness, I systematically try to find out whether there is a risk behavior: rubbing in the morning on waking, allergic itching, sleeping on your stomach with your head in the pillow, a hand or elbow pressed under the eye during the night.

Identifying and changing these behaviors is sometimes enough to stabilize a fragile cornea and consider surgery after a few months of monitoring.

Summary: the parameters analyzed and what they determine

At the end of the consultation, I give you a clear answer: suitable or not, which technique, what expected result. If surgery is approved and you want to go ahead, you also leave with the complete surgical file.

The surgical file

It contains the appointment notice for surgery day, the prescription for the treatments to take before and after the operation, the informed consent form to sign, and the quote to send to your complementary health insurance to find out how much they cover.

See fees, quotes and complementary health insurance reimbursement

The date of the procedure

You can schedule your operation date at the end of the consultation, or later. No obligation. Some patients schedule their surgery in the following weeks. Others come back to me several months or several years later. Others never follow up, and that is perfectly legitimate. What matters is making your decision calmly and being ready.

In any case, I remain available. To get back to me, you can use Doctolib messaging, email or phone. I answer my patients personally, before and after surgery.

See the complete patient journey, step by step