What an OCT scan shows that a sight test cannot

A sight test measures how well you see. It does not always tell you why. An OCT scan is the part of your appointment that looks behind the answer.

Light, not X-rays

OCT stands for optical coherence tomography. It works a little like ultrasound, but with light: a beam scans across the back of the eye and the machine builds a cross-section from the reflections. The result is an image of the retina in layers, each a fraction of a millimetre thick.

It takes about four seconds an eye. You rest your chin, look at a small light, and it is done.

What it finds

The retina can change a long way before your vision does. OCT picks up three things early:

• Swelling or fluid under the macula, the first sign of wet macular degeneration and of diabetic eye disease.
• Thinning of the nerve fibre layer, which is how glaucoma shows itself years before it affects your side vision.
• Tiny holes, puckers and membranes on the surface of the retina that a photograph cannot see.

Why the second scan matters most

One scan tells us what your retina looks like today. A second scan a year later tells us whether it is changing, and change is what matters. A nerve that is naturally thin is normal. A nerve that is thinner than it was last year is not.

That is why we keep every scan, and why the OCT is part of every private test rather than something we offer on the day. The value is in the comparison.

What happens if we find something

Most of the time, nothing. When we do see a change, we show it to you on the screen, explain what it means and what happens next. Some changes we monitor here with a repeat scan in three or six months. Some need a specialist, and we write that referral the same day.