What is an OCT scan and what can it tell you?

How does it work?

OCT stands for optical coherence tomography. The device sends a harmless beam of light into the eye and measures how the layers of the retina reflect it — comparable to ultrasound, but with light instead of sound. The result is a razor-sharp cross-section showing the individual layers of the retina, down to thousandths of a millimetre.

For you the test is simple: chin on the rest, look at a light, hold still for a few seconds. No contact, no radiation, usually no drops.

What does an OCT show well?

The scan excels in two places. The first is the macula, the centre of sharp vision. Fluid, swelling, a small hole, pulling scar tissue or the hallmarks of macular degeneration: an OCT often shows them earlier and more clearly than any other test. The second is the optic nerve and the nerve fibre layer around it. That layer's thickness is compared against a normative database — thinning can be an early sign of glaucoma, before anything is noticeable in the visual field.

At least as valuable as a single scan is the repeat scan: comparing measurements over months and years reveals whether something is stable or changing. For monitoring glaucoma and macular degeneration, that trend often matters more than the single snapshot.

What does an OCT not show?

An OCT is not a complete eye examination. The far edges of the retina fall outside the scanning field — a peripheral tear can remain invisible. It does not measure your prescription, nor your eye pressure, and it says little about cataract. A 'green' result within the normative range is reassuring but not a guarantee: some abnormalities require other tests.

The reverse also holds: a 'red' box in the report is not yet a diagnosis. The comparison against the database can be off because of a large or small optic nerve, a high prescription, or plain anatomical variation. Interpretation remains human work.

When is an OCT worthwhile?

With complaints about central vision (distortion, a spot in the middle), with elevated eye pressure or glaucoma in the family, for monitoring known macular or nerve conditions, and as a baseline against which later scans can be compared. Many opticians offer an OCT as part of an extended eye exam — a fine low-threshold first sieve, as long as it is clear that an abnormal scan calls for professional assessment.

What to do with a result that raises questions?

Ask for the image and the measurement report — you are entitled to them — and have someone explain what is abnormal and why it is or is not urgent. A second look at an existing scan is often more useful than immediately making a new one. And keep your reports: with OCT especially, the information lives in the trend.