What is an OCT scan and why might your optician recommend one?

If you have had an eye test recently, your optician may have mentioned an OCT scan, or offered one as part of your appointment. You might have said yes without quite knowing what it was, or declined because you were not sure whether you needed it.

It is one of the most useful pieces of technology available in modern optometry, and most people know very little about it. This guide explains what it does, what it can detect, and why it is becoming a routine part of eye care for many patients.

What is an OCT scan?

OCT stands for optical coherence tomography. The name sounds complex but the principle is straightforward. The scanner uses light waves to produce a highly detailed, cross-sectional image of the retina, the light-sensitive layer at the back of your eye.

Think of it like an ultrasound, but using light instead of sound, and producing images of extraordinary detail. Where a standard eye examination lets an optician see the surface of the retina, an OCT scan lets them see through it, examining each individual layer in cross-section.

The scan itself is quick and entirely painless. You rest your chin on a support, look at a small light inside the machine, and the scan is done in seconds. There is no contact with the eye and no discomfort involved.

What can an OCT scan detect?

The real value of OCT scanning is its ability to identify changes in the structure of the retina before they cause any symptoms. Many of the most serious eye conditions begin with subtle structural changes that are invisible to the naked eye and do not affect vision in the early stages. By the time a person notices something is wrong, the condition may already be significantly advanced.

OCT scanning is particularly useful for detecting and monitoring:

Age-related macular degeneration. OCT can identify the earliest structural changes associated with AMD, including the build-up of deposits beneath the retina and the presence of fluid that indicates wet AMD is developing. Catching these changes early is critical, particularly with wet AMD where prompt treatment significantly affects the outcome. You can read more about AMD and why early detection matters in our dedicated guide to macular degeneration.

Glaucoma. Glaucoma causes gradual damage to the optic nerve, often without any noticeable symptoms until significant vision loss has already occurred. OCT can measure the thickness of the nerve fibre layer around the optic nerve, detecting thinning that indicates damage before it would be apparent in a standard visual field test. For more on what glaucoma looks like in its early stages, see our post on the early signs of glaucoma.

Diabetic eye disease. Diabetes can cause changes to the blood vessels in the retina, leading to swelling, leakage, and eventually significant vision loss if unmanaged. OCT can detect retinal swelling caused by diabetic macular oedema at a very early stage, allowing intervention before damage becomes permanent.

Macular holes and epiretinal membranes. These are structural problems affecting the central part of the retina. They can cause distortion and central vision loss and are readily visible on an OCT scan, often at a stage where treatment is still straightforward.

Retinal detachment and vitreous changes. OCT can show the relationship between the vitreous gel and the retina, helping to assess whether floaters or flashes are associated with any traction on the retinal surface. If you have been experiencing floaters or flashes, an OCT scan can provide useful additional information about what is happening at the back of the eye.

How is it different from a standard eye test?

A routine eye test checks your vision, assesses your prescription, measures eye pressure, and examines the visible structures of the eye including the retinal surface, optic disc, and blood vessels. It is a thorough and important examination.

What it cannot do is see beneath the surface of the retina. An OCT scan adds that dimension. It does not replace a standard eye test but it substantially extends what can be assessed during an appointment.

A useful way to think about the difference: a standard eye examination is like looking at the surface of a road. An OCT scan is like seeing the layers beneath it, including any subsidence or damage developing underground before it breaks through to the surface.

Who should consider having one?

OCT scanning is useful for almost anyone, but it is particularly worthwhile for people with one or more of the following:

• A family history of glaucoma, macular degeneration, or diabetic eye disease
• A personal diagnosis of diabetes or high blood pressure
• Age over 50, when the risk of AMD and glaucoma increases significantly
• High myopia (short-sightedness), which is associated with higher risk of certain retinal conditions
• Any recent change in vision, such as new floaters, flashes, distortion, or a change in central clarity
• A previous finding on a standard eye test that warrants closer monitoring

That said, many people without any specific risk factors choose to have an OCT scan simply for a thorough baseline assessment. Having a detailed image of your retinal layers on record means that any future changes can be compared against a known starting point, which significantly improves the ability to detect problems early.

Is it available on the NHS?

OCT scanning in the community, meaning at your local optician rather than a hospital, is not routinely funded by the NHS. It is usually offered as an additional private service alongside a standard eye test, at a modest additional cost.

If an OCT scan identifies something that needs further investigation or treatment, you would typically be referred into the NHS hospital eye service at that point, where more detailed scanning and any necessary treatment would be provided.

The value of having it done at your optician is the ability to detect problems early, in a familiar setting, without waiting for a hospital referral. Many conditions caught at the community OCT stage can be monitored and managed with far better outcomes than those picked up later.

What happens after the scan?

Once the scan is complete, your optician reviews the images with you. Most findings are entirely normal, and being shown a healthy retinal scan is reassuring in its own right. If anything requires attention, your optician will explain what they have found, what it means, and what the appropriate next step is.

In many cases that next step is simply to monitor at the next appointment. In others, a referral may be appropriate. Either way, you will leave with a clear picture of what is happening and what, if anything, needs to be done.

The images are stored and form part of your records. At future appointments, they provide a baseline for comparison, which is one of the most clinically valuable aspects of having a scan done regularly rather than just once.

How often should you have one?

For most people without specific risk factors, having an OCT scan every two years alongside a routine eye test is a sensible approach. For those with conditions being actively monitored, or with higher risk profiles, your optician will advise on a more frequent schedule.

If you are not sure how often you should be having your eyes tested in general, our guide on how often to get an eye test covers the recommended intervals and what affects them.