The least spectacular step of the journey — no procedure, no laser, only measurements — is also the one that determines everything else.
Before any decision, there is an assessment. Each test has a specific role: together, they map your eye and show which correction is possible, which is advisable, and which is not. Here are those tests, one after another, and what each of them tells us.
A preliminary point: if you wear contact lenses, you will need to stop wearing them for a while before the assessment. They temporarily alter the shape of the cornea and distort the refraction, topography and biometry. As a general rule, allow one to two weeks without soft lenses, and at least three to four weeks for rigid lenses; you will be told the exact interval when you book your appointment.
The orthoptic evaluation
I always begin with an orthoptic evaluation. Orthoptics studies how the two eyes work together and how the brain fuses their two images into one. This test looks for a weakness in that fusion or for a latent squint — a tendency of the eyes to drift, usually compensated and therefore invisible in daily life, also known as a phoria. Why place it first? Because a refractive correction changes the visual balance between the two eyes: when that balance relied on an already fragile effort of compensation, the procedure may, in rare cases, bring out double vision — diplopia. Detecting this risk before any decision means it can be taken into account in the indication and, where necessary, the procedure can be adapted or set aside. This test also identifies the dominant eye, the one the brain favours. This matters when monovision is being considered, which sets one eye for distance and the other for near vision in order to compensate for presbyopia: the dominant eye is, in principle, the one set for distance.
Refraction
Refraction measures your optical error — myopia, hyperopia, astigmatism — in other words, the correction your eye needs to see clearly. I measure it in two ways. Manifest refraction is determined from your responses to trial lenses: it is the correction with which you see best. Cycloplegic refraction is obtained after instilling drops that temporarily relax accommodation, the mechanism by which the eye adjusts its focus. In younger patients, and particularly in those with hyperopia, accommodation can mask part of the error; cycloplegia reveals this hidden error. It is by comparing the two measurements that the correction to be treated is set.
Assessing the ocular surface and tear film
The tear film and the surface of the eye are examined specifically. Laser treatment temporarily increases dry eye in the months following the procedure; an untreated dry eye is a common cause of discomfort and dissatisfaction. An irregular surface also distorts topography and biometry measurements. I therefore assess it beforehand and, if necessary, treat it before finalising the measurements.
Corneal topography and tomography
Topography maps the surface of the cornea, the eye’s first lens. It reveals astigmatism, but above all irregularities in shape. Current devices go further: they also analyse the back surface of the cornea and measure its thickness across its whole extent. This is known as tomography. It is the most important test for the safety of a laser treatment, because it can detect keratoconus — a progressive condition that thins the cornea and makes it bulge — sometimes at a very early stage. A cornea showing these signs is a contraindication to laser treatment: removing tissue would further weaken a structure that is already weakened. Other solutions, such as an implant, sometimes remain possible.
Corneal thickness (pachymetry)
Measured during tomography, corneal thickness deserves particular attention, because it determines how much tissue can be removed while preserving a sufficient reserve. A thin cornea reduces the available margin and may limit, or even rule out, laser treatment. This measurement weighs directly in the choice between a corneal correction and a correction with an implant.
Corneal biomechanics
The cornea is not just a shape: it is a tissue with its own mechanical properties, able to deform under stress and then return to its place. To assess them, a device sends a brief puff of air that deforms the cornea for an instant; the way it gives way and then recovers indicates how strong it is. Where topography describes the shape of the cornea, biomechanics assesses its resistance. None of these measurements is sufficient on its own, but together they refine the detection of corneas at risk of ectasia, a progressive deformation comparable to keratoconus, but triggered by the procedure.
Optical biometry
When the discussion concerns the crystalline lens — cataract surgery or refractive lens exchange (RLE) — or a phakic implant, I measure the length of the eye, its internal dimensions and the curvature of the cornea. These measurements determine the power of the intraocular lens (IOL) to be chosen and, for a phakic implant, its size, so as to come as close as possible to the intended result. Precision is decisive here: the implant is calculated for your eye, and for your eye alone.
Endothelial cell count
The inner surface of the cornea is lined with a single layer of cells, the endothelium, which keeps the cornea transparent by regulating its hydration. These cells do not renew themselves: their number decreases slowly with age, and any surgery inside the eye can cause some of them to be lost. A specular microscope photographs and counts them in a few seconds, without touching the eye. Before a phakic implant, this measurement is essential: a count that is too low for your age is a contraindication to the implant, and the initial value then serves as a reference for regular monitoring. Before lens surgery, it helps identify a fragile cornea, for example Fuchs’ dystrophy, which may change the surgical strategy.
The general eye examination
The assessment is not limited to measuring the optical error. Slit-lamp examination explores the front of the eye and the transparency of the crystalline lens; measuring intraocular pressure contributes to screening for glaucoma; examination of the fundus, with the pupil dilated, checks the condition of the retina, particularly its periphery in patients with myopia. Before considering a correction, we need to make sure the eye is healthy: refractive surgery is intended for an otherwise healthy eye, and must never push another condition into the background.
What the assessment allows us to decide
This assessment is not an administrative formality: it is the step at which the indication is built. Its results may confirm a technique, point towards another, or conclude that no procedure is appropriate at present.
To understand how these measurements translate into a decision, you can read the article: “The indication comes before the technique”.
This page presents general guidance drawn from my practice and current scientific literature. Each situation is assessed individually in consultation, on the basis of a complete preoperative assessment. It is not a substitute for personalised medical advice. — Dr Alexandre Balon, ophthalmic surgeon, Clinique Saint-Pierre Ottignies.