Posted 14/06/2018 16:55:40
This blog explains my some of my experiences over two year with an optical coherence tomography machine in clinical practice. It touches on its main uses, hopefully in an understandable manner, it is a very complicated machine so that explaining its value is by its own nature a little complicated.
Optical Coherence Tomography (O.C.T.) -An Optometrists Perspective
Two years ago I bought a Heidelburg O.C.T., I say bought, but at fifty five thousand pounds including add-ons I will still be paying off the loan for the fore see-able future.
This blog discusses what it does and was it worth it.
What is an O.C.T. Machine ?
Many practices now have fundus cameras, I've had mine for about fifteen years; it takes a digital picture of the surface of the retina at the back of the eye giving a base line picture of the optic nerve – the “wire” that connects the eye to the brain and the macula, an area the size of a pin prick that we use to read, as well as the rest of the retina that we use to take in the whole picture.
An O.C.T. Machine also takes a digital photograph of the surface of the retina, usually in black and white. What makes it different from a camera is that it uses a low power lazer ( harmless to the eye) which scans the retina, point by point, giving a layer by layer three dimensional model of the retina in microscopic detail.
How does this help ?
The retina works like the chip in a camera converting visible light into electrical energy signals which get converted into a picture by the computer, in our case the visual cortex in the posterior portion of our brains. Each pixel on a chip represents a light receptor cell in the eye.
The analogy ends here, unlike a camera chip, the wiring lies in front of the receptor cells and is composed of several layers which cross link so there is not a direct one to one relationship for each cell and nerve fibre. My view is that this is to create an averaging effect and allow for any occasional cell death.
Macular degeneration shows up first as a sub-retinal change , visible only by O.C.T. Progressing to intra-retinal changes possibly still visible only by O.C.T.
When these changes are accompanied by bleeding, obvious fluid accumulations or scarring they then become directly visible with standard techniques.
Certainly my policy before I had this tool was to consider any sudden change of vision or distortion as wet macular degeneration and rapidly refer for possible treatment, since the longer the delay in treatment the worse the outcome.
I will consider the disease process and treatment in a future blog.
I vividly remember one youngish patient (anybody of my age is youngish) I saw some years ago with only one functioning eye and no other symptoms in his good eye. It was only because I compared his recent photograph with a previous one that I spotted a faint bleed, had I missed it he would not be able to drive today- I still have nightmares about that one. This machine takes the luck out of diagnosis.
O.C.T. S are also useful for glaucoma detection as they record changes in thickness of all the retinal layers, specifically the nerve fibre layer and ganglion cell layer which thins progressively in glaucoma if untreated. I will consider glaucoma in a future blog.
So two years of machine use, what's the benefit ? I am relieved to say it hasn't picked up any glaucoma's that I didn't already know about, although subtle changes in time may reveal some cases that have been hiding.
I have also been able to explain several cases of poor vision ( not treatable ) and and a good many fine retinal changes that just need monitoring, but that I and the patient were previously blissfully unaware of.
Where this machine really has made a huge difference is in the early detection of wet macular degeneration, this condition can blind if untreated and quickly. Ideally wet macula's should be treated with an injection in the eye within two weeks of diagnosis and if it were my eye, on the same day.
To date I am picking up about one wet macular a month with this machine that I would not spot otherwise as they are in patients with perfect vision, no symptoms and no visible signs on standard direct observation. These patients attended for a routine eye test- it can be quite a shock to be then be told that you need relatively urgent treatment to save you sight and while I hate worrying patients, that is the case.
In addition to this there have been a slightly lesser number of symptomatic patients who I would have referred any way but the machine confirms the diagnosis.
It is my view that wet macula degeneration can remain at a pre-clinical stage for some months before the leak ( its called wet because it leaks) causes significant distortion and or sight loss. This would explain why I'm picking up such a high level of asymptomatic patients, I cannot believe that at least 24 patients over the last two years just happened to get a wet macula in the month they were due for an eye test.
The aim of treatment is to maintain vision and in some cases improve it from the point of diagnosis so that, clearly early diagnosis is of great benefit.
So, to answer my original question, was it worth the investment ?
The machine was out of action last month- I was bereft without it and just incidently, we tested it on the engineer who came to fix it and found a problem in his eyes.