3D OCT – What is it and is it necessary? By Optometrist Amy Ogden
Having just finished my first full testing month (and what a busy month it’s been). I’ve decided to write a little introduction to my favourite bit of testing equipment, the 3D OCT machine. Often when you call to make an appointment to see one of the Optometrists, the 3D OCT scan will be mentioned and offered to you. Many people are unsure of what a 3D OCT Scan is. This is often overlooked, and their opportunity to utilise this wonderful medical technology is missed. I thought it would be a good idea to write a little introduction into why this equipment is so wonderful. What I use it for and the benefits of using it, as opposed to the fundus camera (I could go on all day, this is just the basics).
OCT stands for Optical Coherence Tomography. Essentially it works by sending light rays to the back of the eye. Due to the layers of the retina having different reflective properties, the image formed is a greyscale image of the hypo and hyper reflective layers (near infrared light is used, so there is no likely damage to the eye). The principle is rather like an ultrasound scan as it is an echo technique. Only using light waves instead of sound waves. I can then see any abnormalities in each layer due to the differences in the reflective properties of tissues. For example blood usually casts a shadow, whereas exudates are hyper-reflective.
Why is this important?
Our retina has 10 layers;
Inner limiting membrane
Retinal nerve fibre layer (RNFL)
Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
External limiting membrane
Retinal pigment epithelium (RPE)
The Volk lens
When I’m using my Volk lens (when you sit at the microscope), or my ophthalmoscope (when I use the hand-held microscope and get quite close), I can’t see all of these layers; I can’t see them all when I take a picture of the back of the eye with the fundus camera either.
I can understand at this point if you’re wondering why this matters, what does it matter if I can’t see the photoreceptor layer? Or the retinal pigment epithelium? It is always useful to see these layers, even if they are healthy, to be sure they are and to have a baseline to refer to further down the line.
Sometimes, you may be unaware of a problem or having unspecific symptoms, and seeing the OCT scan will bring a very subtle change to my attention, something that may only have manifested itself on a fundus picture or to me on Volk once further disease progression has occurred.
For example in the photoreceptor layer, which I mentioned above, there could be changes that may be indicative of a condition called Retinitis Pigmentosa. In the RPE, this is where we can monitor for some of the signs of Age Related Macular Degeneration (AMD) and subtle changes in the pathology here can help us in determining if the Dry AMD changing to Wet AMD, this is important as Wet AMD is something we need to treat urgently, and fast diagnosis is key.
What can OCT test for?
3D OCT is also useful test for those at risk of glaucoma, combined with the visual field screener, IOPs (pressure test) and the ocular examination, using the OCT is another tool in our belt to help as a diagnostic aid. This is because it provides a scan of the optic nerve head, and the macula and helps us to pinpoint any suspicious findings; this along with the tests mentioned above helps us keep on top of early detection and precise monitoring. OCT can also measure the drainage angle in the eye, this is important for a condition called angle closure glaucoma.
(I’ve only used a handful of pathology examples here)
How is it performed?
I know I mentioned an ultrasound above, and some of you will be thinking about the “jelly” used in those types of scans. Not for OCT. The OCT is very similar to the fundus picture without the “bright flash”.
You sit at the OCT with your chin on the rest, and I will line you up with the lens. You will be asked to “look at the green cross” and will then be instructed to blink “freely and then keep your eyes open”, only for a short time, whilst the scan is completed. I will repeat this for the different types of scans you require, for both eyes.
We will then go through all scans together and I will explain the findings. Should any further action need to be taken, we will make an action plan together.
The best bit?
I can send a copy of these scans directly to the hospital should it be required. Which makes triage a lot more efficient.
Are OCTs compulsory?
Absolutely not. They are not included as part of the NHS sight test and therefore are an optional extra. I think they are a brilliant bit of extra information to have, and a fabulous monitoring tool, but they are not compulsory for me to carry out a thorough eye examination, however if I feel that you would benefit from the OCT I will let you know, but the choice will always be yours!