What an OCT Scan Shows and Why We Do One

If you have been offered an OCT scan at an eye examination, it is reasonable to ask what it adds beyond the usual tests.

The short answer is that a standard examination looks at the surface of the retina. An OCT scan looks underneath it.

What it actually does

OCT stands for optical coherence tomography. It uses light waves to build a cross sectional image of the retina, in much the way an ultrasound uses sound waves, but at far higher resolution.

The result is a view through the layers of the retina at a scale measured in microns. Ten distinct layers become individually visible, along with anything sitting between or within them.

Nothing touches your eye. There is no injection, no discomfort, and the scan takes seconds.

What it detects that an examination cannot

Several conditions begin beneath the surface, and by the time they are visible from above, they are no longer early.

For glaucoma, OCT measures the thickness of the nerve fibre layer around the optic nerve. Thinning is measurable before any loss appears on a visual field test, which matters enormously in a condition where lost nerve fibres do not regenerate.

For macular degeneration, it reveals fluid and structural change within the macula, and is the clearest way to distinguish stable dry disease from a wet conversion that needs urgent referral.

For diabetic eye disease, it identifies swelling at the macula, which is the most common reason vision is lost in diabetes and is treatable when caught.

It also assists in assessing epiretinal membranes, macular holes, and the vitreous as it separates from the retina with age.

Why the baseline matters as much as the scan

A single scan tells you what your retina looks like today. Its greater value is as a reference point.

Most of these conditions are diagnosed by change over time rather than by any single measurement. A nerve fibre layer that is thin might always have been thin, or it might have thinned by fifteen microns in three years. Only the second is glaucoma, and only a prior scan can tell you which you are looking at.

This is why we recommend a baseline scan even when everything appears normal. The value accrues later.

Measuring axial length in children

The same technology measures the length of the eye from front to back, and this is central to how myopia is monitored.

Prescription is a crude measure of progression, since it moves in steps and reflects several variables. Axial length measures the underlying change directly, in millimetres, and detects it earlier. It is the number that tells you whether a myopia management plan is actually working. We use it at every review, as set out in our guide to myopia in children.

What to expect

You sit at the instrument, rest your chin, and look at a target while the scan runs. It usually takes a few seconds per eye.

Dilating drops are not always required, though they improve the view in some cases and would be discussed beforehand. If they are used, vision will be blurry at near and light sensitive for a few hours, so you should arrange not to drive.

The images are available immediately, and we will go through them with you rather than simply filing them.

Where to from here

OCT has moved from specialist equipment to a routine part of a thorough eye examination, largely because the conditions it detects are the ones where early intervention makes the greatest difference.

At Noranda Optometrist, OCT imaging is part of how we assess adults and how we monitor children in myopia management. If you have never had a baseline scan, it is worth having one while everything is normal.