The Eye Examination Before Vision Correction: What Each Test Measures

Why the examination matters more than the procedure

The procedure itself takes minutes and is largely automated. The examination takes considerably longer and is where every decision is actually made: whether any procedure is suitable, which one, how much tissue can be removed, and what the eye is likely to do afterwards. If you take one thing from this page, take that.

The reason is structural. A refraction tells you what correction would bring your vision to normal. It says nothing about whether your cornea has enough tissue to spare, whether its shape is regular enough to be reshaped predictably, whether your tear film will tolerate it, or whether there is something at the back of the eye that needs attention first. Two people with the same prescription can be quite different candidates, and only the measurements tell you which is which.

Refraction: what your prescription says

Refraction is the measurement of the lens that brings your focus onto the retina, written as a prescription. Three numbers matter for surgery. Sphere describes the overall short or long sightedness, in dioptres: negative for short sightedness, positive for long sightedness. Cylinder describes astigmatism, where the cornea is curved more in one direction than another. Axis is the direction of that astigmatism, in degrees of a circle.

The assessment measures refraction twice: once as it is, and once after dilating drops relax the focusing muscle, which is the difference between the prescription you wear and the prescription your eye really has. That second measurement is the one a surgical plan is built on, because it is not influenced by how hard your eye has been working to focus.

The cornea: topography, tomography and pachymetry

Three measurements of the cornea decide the arithmetic of any laser procedure. They are taken with different instruments and answer different questions.

Topography

A corneal topographer maps the surface curvature of the cornea, producing the familiar colour map. It is used to detect irregular astigmatism and keratoconus, including the early form that cannot be seen at a slit lamp, and to check that the cornea is a shape a laser can predictably reshape. An irregular map is one of the reasons a clinic may decline to treat an eye entirely.

Tomography

Tomography goes further by imaging the cornea in three dimensions, including the thickness of its layers at each point, and by measuring elevation rather than just curvature. In practice, a scanning instrument gives the clinician a cross-section of the cornea rather than a picture of its surface, which is what makes subtle ectatic change visible.

Pachymetry

Pachymetry is the measurement of corneal thickness, in micrometres, and it is the number that most often decides what can be done. A laser procedure removes tissue; a flap, where one is used, consumes tissue of its own; and what remains underneath has to be thick enough to keep the cornea structurally sound. Corneal thickness varies between people, and it is why two identical prescriptions can get two different answers.

Pressure and the back of the eye

Your intraocular pressure is measured with a tonometer, which is a minute-scale reading that matters for glaucoma screening. It also carries a complication specific to refractive surgery: because the procedure changes the thickness and stiffness of the cornea, pressure readings afterwards are not read the same way as they are in an untreated eye. That is one reason to keep the preoperative record.

The retina and optic nerve are examined too, usually with dilating drops. This is partly routine eye health, because a dilated examination is how retinal tears, diabetic changes and early glaucoma are found, and partly because a retinal problem changes the order of priorities: it is treated first, and the refractive plan comes afterwards.

The tear film and the surface

Dry eye is the finding most likely to change a treatment plan, and it is assessed rather than assumed: the stability of the tear film, the health of the surface and the pattern of any staining, together with what you say about symptoms, comfort with contact lenses and how much of your day is spent on screens.

It matters because every refractive procedure disturbs the corneal nerves that keep the eye moist, and an eye that is already struggling may be made uncomfortable for months afterwards. The useful outcome of this part of the assessment is not a pass or fail: it is knowing whether the surface needs treating first, whether a technique with a smaller incision is preferable, or whether the honest answer is that this is not the right time.

Getting your numbers

Ask for a copy of your measurements, and expect to be given them: refraction, corneal thickness at the thinnest point, the shape map, the pressure reading and the fundus findings. It is a short document, and it is the most useful thing you will own afterwards.

It is also the honest way to compare clinics. Two clinics can quote differently for the same eye, and the reason is often visible in the arithmetic: how much tissue the plan leaves behind, which technique it uses, and what is included in the price. A clinic that will not share your numbers is telling you something about how the rest of the relationship will go.

Examination questions, answered

Each question below opens the page that answers it in full, where the long version of the answer is. The direct answer is printed here so you can tell whether it is the question you are asking.

• What happens during a comprehensive eye examination?

  A comprehensive eye examination covers vision and refraction, eye pressure, the external eye and eyelids, the cornea, and the retina and optic nerve, usually with dilating drops so the back of the eye can be seen properly.

• How long does a comprehensive eye examination take?

  A full examination with dilation usually takes substantially longer than a routine sight test, partly because some measurements need repeat readings and partly because the drops need time to work.

• Why is a full eye examination needed, not just a refraction?

  A refraction only establishes what prescription corrects your vision. It says nothing about corneal thickness, corneal shape, tear film or the health of the retina, and those are the findings that decide whether surgery is safe and which technique is appropriate.

• Why does the doctor dilate my eyes?

  Dilating drops widen the pupil so the retina and optic nerve at the back of the eye can be examined properly. Without dilation a large part of the retina simply cannot be seen.

• What is corneal topography?

  Corneal topography maps the surface shape of the cornea, producing a colour-coded elevation and curvature map that shows how regular the surface is and where the steep and flat areas lie.

• What is corneal tomography?

  Corneal tomography measures the shape of the cornea in three dimensions, including the thickness profile across it, rather than describing the front surface alone.

• What is pachymetry?

  Pachymetry is the measurement of corneal thickness. It is one of the central measurements in refractive planning, because it determines how much tissue is available to be removed.

• What is an eye pressure test?

  An eye pressure test measures the fluid pressure inside the eye, which is one of the main risk factors for glaucoma. It is quick and is done with a small instrument that touches the front of the eye after numbing drops.

• What is a retinal scan or photograph?

  A retinal photograph records the appearance of the retina, and it is used as a baseline so that any future change can be compared against something. An OCT scan goes further and images the retinal layers in cross-section.

• What is an OCT eye scan?

  An OCT scan uses light to build a cross-sectional image of the retina, which shows its layers and any fluid or thinning that a photograph would not reveal. It is quick and does not touch the eye.

• Why should the retina be examined before vision correction?

  The retina should be examined because a refractive assessment is also an opportunity to find conditions that have nothing to do with the cornea and that would change how your eyes are managed, particularly in higher short sightedness.

• How often should adults have an eye examination?

  Adults without symptoms or risk factors are generally advised to have an examination every couple of years, and more often where there is a relevant condition such as diabetes, a family history of glaucoma, or a higher prescription.

What an examination can find that you did not know about

Consultations regularly find things the patient had no reason to suspect. Keratoconus that has never been symptomatic, a retinal tear that would have become a detachment, glaucoma in its early stage, diabetic changes in someone who did not know their blood sugar was a problem, and a tear film that explains years of intermittent discomfort.

Some of those findings stop surgery; some change which technique is used; some are unrelated to refractive correction and are simply better found early. All of them are reasons why the examination comes before the decision, and never after it.