Optical Coherence Tomography (OCT) of the Macula

An OCT scan, or optical coherence tomography, is an extremely useful eye test that takes cross-sectional images of different parts of the eye, such as the retina, the macula, the optic nerve and the cornea

Page contents

1. How OCT works
2. Which macular conditions is this test recommended for?
3. How long does the test take?
4. Is the test painful?
5. Can the test ever not be carried out?
6. Is any preparation needed?
7. What exactly OCT sees inside the eye
8. Macular OCT: what it shows in each condition
9. OCT in glaucoma
10. OCT angiography: imaging blood vessels without an injection
11. OCT before and after cataract surgery
12. How the result is read
13. OCT or fluorescein angiography?
14. OCT of the front of the eye
15. How often OCT is repeated
16. The test step by step
17. Who should have an OCT scan even without symptoms
18. Symptoms that need an OCT scan without delay
19. Questions worth asking about your result
20. OCT in everyday practice: an example
21. Preparation and practical information
22. Myths about OCT
23. Why it matters where you have the test
24. Frequently Asked Questions about OCT Scans

How OCT works

OCT uses a beam of light (low-power infrared light): based on the reflection it receives from the tissue being examined, and with the help of specialised software, it forms an image. The detail of the structures we examine is enormous and the information we obtain is extremely important.

This test complements other eye examinations, such as fundoscopy, by providing information not only about the surface of the macula but also about what is happening within it and behind it.

Which macular conditions is this test recommended for?

The main conditions for which the test is useful are

–Age-related macular degeneration (AMD)

-Diabetic macular oedema (DME)

Retinal vein occlusion (CRVO-BRVO)

Central serous chorioretinopathy (CSC)

Of course, for most macular conditions we can obtain useful information from this test, information that helps us greatly in making a diagnosis.

How long does the test take?

In recent years the test has become shorter and shorter. With the modern equipment we have, a few seconds are enough to image the macular area with great accuracy.

Is the test painful?

Not at all. The test is carried out from a distance, nothing touches the patient’s eye and all that is needed is to focus on a target for a few seconds.

Can the test ever not be carried out?

In very rare cases where the optical media of the eye are very cloudy, mainly when a cataract is very advanced or the cornea is very cloudy, the quality of the image we obtain is limited. Even in these cases, however, we can usually obtain useful information.

Is any preparation needed?

With our equipment, no preparation is needed, not even eye drops to dilate the pupil (mydriasis). In addition, very little cooperation is required from the patient.

What exactly OCT sees inside the eye

Optical coherence tomography works with light, in a similar way to ultrasound, which uses sound. A beam of light scans the retina and the device records how it is reflected from each layer. The result is a cross-sectional image, as if we had cut the retina into thin slices and examined it under a microscope, without any incision and without touching the eye.

The resolution reaches a few micrometres, enough to distinguish the roughly ten layers of the retina. This shows precisely whether there is fluid, where it is, whether a layer has thinned or whether something is “pulling” the retina from the surface. The same technology is used to examine the macula, the optic nerve and, with a special lens, the front of the eye.

Macular OCT: what it shows in each condition

Macular degeneration

In the dry form, OCT shows drusen, the deposits under the retina, and areas of atrophy. In the wet form, it detects fluid within or under the retina and the membrane of abnormal new blood vessels. It is the test that guides every decision about injections: if there is fluid, treatment continues; if it has dried up, the intervals can be extended. Read more about macular degeneration.

Diabetic macular oedema

In people with diabetes, OCT measures the thickness of the macula accurately and reveals oedema even before vision is affected. It is used for diagnosis and for monitoring the response to treatment. See our page on diabetic retinopathy.

Epiretinal membrane

A thin, transparent membrane forms on the surface of the macula, contracts and “wrinkles” it. The patient sees wavy lines or a blurred image. OCT shows the thickness of the membrane and the distortion it causes, and helps decide whether and when vitrectomy is needed.

Macular hole

A small opening in the centre of the macula causes a “gap” in the centre of vision. OCT confirms the diagnosis, measures the size of the hole and distinguishes a true hole from other conditions that look similar. The size affects the chance of successful surgery.

Vitreomacular traction

When the vitreous does not separate completely from the retina, it can pull on it at the macula. OCT is the only way to see this traction clearly and to monitor whether it resolves on its own.

Central serous retinopathy

Fluid collects under the retina, usually in young men and at times of stress or after steroid use. OCT shows the fluid and how it resolves. Find out more on our page about central serous retinopathy.

Vascular occlusions

In retinal vein occlusion, OCT records macular oedema and guides injections, just as in macular degeneration.

OCT in glaucoma

In glaucoma, OCT measures the thickness of the nerve fibre layer around the optic nerve and of the ganglion cells in the macula. These are the cells the disease destroys. Their thinning may appear before any loss becomes visible on visual field testing, which is why OCT is so valuable in diagnosing early glaucoma. With repeated measurements, the doctor can see whether thinning continues despite treatment.

Care is needed in interpretation, however. In people with high myopia, the shape of the optic nerve is different and the device may “colour” a normal result red, because its database does not include such eyes. On the other hand, in advanced glaucoma the values reach a floor and no longer change, so monitoring relies more on visual fields. OCT does not replace clinical judgement; it complements it.

OCT angiography: imaging blood vessels without an injection

A development of OCT, OCT angiography, images the blood vessels of the retina and choroid by detecting the movement of red blood cells. No dye injection is needed, unlike conventional fluorescein angiography, and it takes a few minutes. It is useful for detecting new vessels in wet macular degeneration, areas without blood supply in diabetes and vascular occlusions, and for monitoring without repeated dye injections. It does not replace fluorescein angiography in every case, because it does not show fluid leakage, but it significantly reduces the need for it.

OCT before and after cataract surgery

Before cataract surgery, macular OCT reveals problems that are not always visible behind the cloudy lens, such as an epiretinal membrane or early degeneration. This helps give the patient a realistic idea of the vision they will have afterwards, and helps choose the right intraocular lens. For example, multifocal intraocular lenses are not a good choice when the macula is not healthy.

After surgery, if vision does not improve as much as expected, OCT detects post-operative macular oedema, a complication that is treated effectively with drops when diagnosed early.

How the result is read

An OCT result includes cross-sectional images and colour maps. On the thickness map, warm colours, red and yellow, show greater thickness and cool colours, blue and green, show less. In optic nerve analyses, the values are compared with a database of healthy people of the same age: green means normal, yellow borderline and red outside normal limits.

The colours can be misleading, however, when read without clinical knowledge. A poor-quality image, incorrect detection of the layer boundaries by the software or an eye with unusual anatomy can give falsely abnormal results. This is why the ophthalmologist always examines the cross-sectional images themselves and not just the numbers.

OCT or fluorescein angiography?

The two tests answer different questions. OCT shows structure: thickness, layers and fluid. Fluorescein angiography shows how the blood vessels function: where they leak, where there is no blood supply and where there are new vessels. OCT is quick, painless and can be repeated as often as needed, which is why it is the main monitoring test. Fluorescein angiography, which requires an intravenous dye injection, is done less often, when needed for diagnosis or for planning laser treatment.

OCT of the front of the eye

With a special attachment or a dedicated device, OCT can also image the anterior segment of the eye. It measures the thickness of the cornea, shows the width of the angle through which fluid leaves the eye, which is crucial in angle-closure glaucoma, and helps assess intraocular lenses and choose the size before ICL lenses are implanted. In keratoconus, it complements topography with measurements of the corneal epithelium.

How often OCT is repeated

OCT involves no radiation and has no side effects, so it can be repeated as often as is clinically needed:

• Injection treatment for wet macular degeneration, diabetic macular oedema or vein occlusion: At every visit, usually every 4 to 12 weeks.
• Dry macular degeneration: Usually once a year, or sooner if vision changes.
• Glaucoma: Once or twice a year, together with visual fields.
• Hydroxychloroquine: Every year after the first years of treatment.
• Epiretinal membrane or traction: Every few months, to see whether it is progressing.

The test step by step

1. You sit in front of the device and rest your chin and forehead on the support.
2. You look at a small light inside the device. You will see a red or blue line moving; you do not need to follow it.
3. Each scan takes a few seconds. Try not to blink during the scan, and blink freely between scans.
4. In some cases, pupil dilation is needed for a better image, especially when the pupil is very small or there is a cataract.
5. The images are available immediately and the doctor reviews them with you.

Who should have an OCT scan even without symptoms

OCT is not a routine test for everyone, but in certain groups it is very valuable even when vision is good:

• People over 50 with a family history of macular degeneration
• People with diabetes, especially long-standing diabetes or findings on fundoscopy
• People with high eye pressure, a suspicious optic nerve or a family history of glaucoma
• People taking hydroxychloroquine or other medicines with possible retinal toxicity
• People with high myopia, who have an increased risk of macular conditions
• People planning cataract surgery, especially with a multifocal intraocular lens

Symptoms that need an OCT scan without delay

If you notice that straight lines look wavy, that a blurred or dark spot has appeared in the centre of your vision, that objects look smaller or larger with one eye, or that colours look more faded, book an appointment soon. These symptoms usually point to a problem in the macula, and OCT is the test that will show the cause. Check each eye separately by covering the other, because with both eyes open a change in one eye often goes unnoticed.

Questions worth asking about your result

• Is there fluid or swelling in the macula?
• Is my nerve fibre thickness normal for my age?
• Has anything changed since the previous test?
• Do I need treatment or just monitoring?
• When should I repeat the test?

OCT in everyday practice: an example

Take a 72-year-old woman with dry macular degeneration who is monitored every year. One morning she notices that the lines between the bathroom tiles look crooked with her right eye. Her vision on the eye chart is almost the same as before, and on fundoscopy the change is minimal. On OCT, however, a small amount of fluid under the retina is clearly visible: the degeneration has become wet. Injections start the same week and her vision is preserved. Without OCT, the diagnosis would have been delayed and the damage would have been greater.

This is the most important advantage of the test: it detects changes at the earliest stage, when treatment has the best results.

Preparation and practical information

No fasting or special preparation is needed. If you wear contact lenses, you can keep them in, unless another test that requires their removal is also carried out. If your pupils are dilated, you will be given instructions and it is best not to drive afterwards. Bring any previous OCT scans, even if they were done on a different device; the doctor will compare them with the images, even if the numbers are not fully comparable between different devices.

Myths about OCT

• “It is an X-ray and involves radiation.” No. OCT uses near-infrared light, without ionising radiation. It is completely safe, even in pregnancy and in children.
• “The light damages the eye.” The intensity is very low and causes no damage, however many times the test is repeated.
• “If everything is green, I am definitely fine.” The colours are an aid. The final assessment is made by the doctor, based on the images and the clinical examination.
• “OCT replaces fundoscopy.” OCT examines the centre and the optic nerve in great detail, while fundoscopy checks the whole retina, including the periphery.

Why it matters where you have the test

The value of OCT depends on two things: the quality of the scan and the interpretation. A good scan needs proper centring, adequate signal quality and the right scanning protocols for the question we want to answer. Interpretation requires knowledge of the clinical picture and the patient’s history. When the test is carried out and assessed by the same ophthalmologist who monitors you, comparisons from visit to visit are more reliable, because the same device is used, along with the follow-up function that scans exactly the same spot every time.

If another doctor has asked you to have an OCT scan, mention it when booking your appointment, so that the right scanning protocol is used and you receive the results with a brief report. Keep the results in a folder with your other tests, so that they are available for your doctor and for future comparisons whenever needed. A series of tests over time always tells much more than a single measurement, however carefully it has been done. To book an OCT scan, call 210 677 0007.

Frequently Asked Questions about OCT Scans

Medically reviewed by: Dr. Vasileios Skouteris MD, FEBO