Optical coherence tomography, or OCT, uses a beam of harmless light to build a cross-section image of the eye. It shows detail that once could be seen only under a laboratory microscope, and measures the layers of the retina and optic nerve to a thousandth of a millimetre.
What the test is like
You sit at the machine with your chin on a rest and look at a target light. Each scan takes a few seconds. Nothing touches your eye, and it does not hurt. Your pupils may be widened with drops first, which blurs your vision for a few hours.
What it shows
The retina and macula
The macula is the small central part of the retina you use for reading and recognising faces. OCT can show:
- fluid or swelling in the retina (macular oedema), for example from diabetes
- a small break in the macula (macular hole)
- a wrinkled film on the surface of the retina (epiretinal membrane)
- thinning or loss of layers (atrophy)
- deposits and bleeding, as in macular degeneration
The optic nerve and glaucoma
OCT measures the thickness of the nerve fibres around the optic nerve. In glaucoma these fibres are slowly lost. The results are compared with a database of healthy eyes of the same age, and colour-coded charts show how far each measurement is from the expected range. OCT can show damage before you notice any loss of vision.
The front of the eye
OCT can also image the cornea and iris, and measure the drainage angle, which helps assess the risk of angle-closure glaucoma.
Why repeat scans matter
One scan is a snapshot. Scans repeated over months and years show whether something is changing, which matters most in glaucoma and retinal disease. Results from different OCT machines cannot be compared reliably, so it helps to have your follow-up scans on the same machine, where the software can compare them and highlight real change.
What OCT cannot do
OCT is very sensitive, but it does not make a diagnosis on its own. Your ophthalmologist reads the scan alongside your examination, your history and your other tests.
This guide explains; it does not diagnose. Your doctor’s advice about your own eyes comes first.