When deciding whether or not to have an OCT scan, people sometimes say to me "See if you find anything during the eye examination that requires a scan."
But if I've found a problem just from looking in the eye, I usually don't need a scan to confirm it. The point about the OCT is that it enables us to see beneath the surface, to give information that I don't already know; to see what I haven't already seen.
This is particularly useful for glaucoma and macular degeneration, which I talk about separately. But it also shows up diabetic changes in the eye; it helps to distinguish between benign and malignant pigment spots; and it's even been used to diagnose a child at risk of getting colon cancer.
Using the Topcon OCT, we also get a photograph of the retina, which is really useful for seeing the changes to blood vessels that occur with raised blood pressure or high cholesterol levels. When you can combine the surface view and the view underneath, you get the complete picture. I recently saw a lady who had quite poor vision, especially in one eye, and cataracts. The OCT revealed a hole in the macula in one eye. But it also showed that a cataract operation in the other eye would restore her vision to normal.
In the 18 months since we've had the OCT, we've done thousands of scans. Most of them have shown normal, healthy eyes. But some have shown problems at a much earlier stage than we would otherwise have detected them. This has meant earlier and therefore more effective treatment, and much sight has been saved. I'm often thankful that we now have this technology.
David Donner
Showing posts with label Ocular Coherence Tomography. Show all posts
Showing posts with label Ocular Coherence Tomography. Show all posts
Wednesday, 10 November 2010
Monday, 1 November 2010
Glaucoma and OCT
What is glaucoma?
That’s not as easy question to answer as you might think. One study, looking at a group of patients aged over 80, found that the prevalence of glaucoma amongst them varied between 0.5% and 6% depending on which definition of glaucoma they used.
Because glaucoma is generally a slowly progressive disease, it’s often difficult to tell whether it’s started or not unless you have accurate measurements from before.
Glaucoma is not the same as high pressures.
There’s a circulating fluid within the eye that has its own pressure, known as the intraocular pressure. Many people will be familiar with the measurement of this pressure, which is often by an instrument that blows air into the eye. Frequently, when I do this test, I hear the comment “this is the glaucoma test, isn’t it?”
Well, actually it isn’t. Raised intraocular pressure is just a risk factor for glaucoma. So glaucoma is now described as a group of diseases that cause progressive damage to the nerves at the back of the eye, with or without raised pressures. The most common of these is Primary Open Angle Glaucoma (POAG).
Half of all cases of glaucoma are undiagnosed
This refers to cases of POAG, according to surveys across several industrialised countries. In one study in East Anglia, 90% of those with undiagnosed glaucoma were found to have normal pressures.
If that isn’t bad enough, the figures in other parts of the world are far worse. The percentage of glaucomas that are undiagnosed was found to be 75% in Bangkok, 93% in India, and 100% in Mongolia (i.e. every case they found was undiagnosed). No wonder glaucoma is the leading cause of irreversible blindness in the world.
So how can you tell if someone has glaucoma or not?
Visual fields
As nerve fibres get damaged, there comes a point when vision starts getting lost. But it won’t appear as a big splodge in the centre of your vision; it’s the more peripheral areas that get affected first. The reason people don’t notice this is because the brain fills in the gaps from neighbouring images. By the time it is noticed (for instance when the car wing mirror keeps getting knocked) quite a lot of vision has already been lost.
We routinely test the field of vision, but analysis isn’t always straightforward. Factors affecting the visual field include pupil size, high prescriptions, other eye conditions, and normal aging. The best field tests are able to take these factors into account by examining in more detail those areas most likely to be affected by glaucoma, but these can be quite long and demanding. And, by the time some field is lost, quite a bit of nerve damage has already taken place.
The optic nerve
When we look into the eye, we can see where the optic nerve comes into the retina. As nerve damage occurs, it’s possible to see changes occurring to this part of the nerve. There’s a paler part in the centre, known as the cup, and this gets larger when the nerve gets damaged. The overall size of the nerve doesn’t change, so if the cup gets abnormally large in comparison, glaucoma is indicated. The difficulty is in deciding what’s normal and what’s abnormal, because some people (especially if they are short-sighted) have larger optic nerves than others, and this also affects the size of the cup.
Imagine you have two sets of equal numbers of cut flowers. One set goes in a narrow vase, and the other goes in a wide vase. In the narrow vase, the flowers will be bunched up with no space in the middle. But in the wide vase, the flowers will be arranged towards the outside, and the space in the middle will be the equivalent of the cup in the optic nerve. As the flowers die, the “cup” increases in size.
In the optic nerve, you can’t be sure that nerves have died (at least not in the early stages) unless you knew how many were there before. And this is where the OCT comes in.
OCT and Glaucoma
The OCT can measure the thickness of the nerve fibre layer, which indicates how many nerves are coming into the main optic nerve from all over the retina. This helps in the diagnosis of glaucoma in two ways.
Firstly, it compares the measurement against an average of people of the same age, sex and ethnicity, so it shows the likelihood of glaucoma by virtue of being out of step with average. The, secondly, it stores those measurements. Then, if they are repeated some time later, any deterioration can be picked up.
In one study (Paul 2010) OCT technology was shown to predict field defects that developed on average four years later.
Treatment
People often ask me if anything can be done about glaucoma once it’s been detected. The answer is “Yes, very much so”. The usual treatment is drops, to bring the pressures down, even if they aren’t especially high. But it needs to be detected early, because any vision that’s been lost can’t be regained. Also, once some nerve fibres died, the remaining ones seem to be especially sensitive to even normal pressure.
Closed angle glaucoma
In about 10% of all cases of glaucoma, the iris gets in the way of the drainage system. This is known as closed angle glaucoma because the angle between the drainage system and the iris gets very narrow and can even close completely. When this happens, the pressure builds up very rapidly in an acute attack. The eye becomes red and painful, and vision loss can occur rapidly without treatment. Unfortunately, two-thirds of those with closed angle glaucoma develop it slowly without any symptoms prior to an attack.
Another use of the OCT is that we can use it to see whether the angle is open or close, so we can detect those at risk of an acute glaucoma attack. Often, all that’s required is a little laser treatment to allow fluid to get through and prevent an attack.
Conclusion
OCT is now playing a vital part not only in the early detection of glaucoma, but also in monitoring the effectiveness of glaucoma treatment.
Contact me for more information on Glaucoma and OCT
David Donner
That’s not as easy question to answer as you might think. One study, looking at a group of patients aged over 80, found that the prevalence of glaucoma amongst them varied between 0.5% and 6% depending on which definition of glaucoma they used.
Because glaucoma is generally a slowly progressive disease, it’s often difficult to tell whether it’s started or not unless you have accurate measurements from before.
Glaucoma is not the same as high pressures.
There’s a circulating fluid within the eye that has its own pressure, known as the intraocular pressure. Many people will be familiar with the measurement of this pressure, which is often by an instrument that blows air into the eye. Frequently, when I do this test, I hear the comment “this is the glaucoma test, isn’t it?”
Well, actually it isn’t. Raised intraocular pressure is just a risk factor for glaucoma. So glaucoma is now described as a group of diseases that cause progressive damage to the nerves at the back of the eye, with or without raised pressures. The most common of these is Primary Open Angle Glaucoma (POAG).
Half of all cases of glaucoma are undiagnosed
This refers to cases of POAG, according to surveys across several industrialised countries. In one study in East Anglia, 90% of those with undiagnosed glaucoma were found to have normal pressures.
If that isn’t bad enough, the figures in other parts of the world are far worse. The percentage of glaucomas that are undiagnosed was found to be 75% in Bangkok, 93% in India, and 100% in Mongolia (i.e. every case they found was undiagnosed). No wonder glaucoma is the leading cause of irreversible blindness in the world.
So how can you tell if someone has glaucoma or not?
Visual fields
As nerve fibres get damaged, there comes a point when vision starts getting lost. But it won’t appear as a big splodge in the centre of your vision; it’s the more peripheral areas that get affected first. The reason people don’t notice this is because the brain fills in the gaps from neighbouring images. By the time it is noticed (for instance when the car wing mirror keeps getting knocked) quite a lot of vision has already been lost.
We routinely test the field of vision, but analysis isn’t always straightforward. Factors affecting the visual field include pupil size, high prescriptions, other eye conditions, and normal aging. The best field tests are able to take these factors into account by examining in more detail those areas most likely to be affected by glaucoma, but these can be quite long and demanding. And, by the time some field is lost, quite a bit of nerve damage has already taken place.
The optic nerve
When we look into the eye, we can see where the optic nerve comes into the retina. As nerve damage occurs, it’s possible to see changes occurring to this part of the nerve. There’s a paler part in the centre, known as the cup, and this gets larger when the nerve gets damaged. The overall size of the nerve doesn’t change, so if the cup gets abnormally large in comparison, glaucoma is indicated. The difficulty is in deciding what’s normal and what’s abnormal, because some people (especially if they are short-sighted) have larger optic nerves than others, and this also affects the size of the cup.
Imagine you have two sets of equal numbers of cut flowers. One set goes in a narrow vase, and the other goes in a wide vase. In the narrow vase, the flowers will be bunched up with no space in the middle. But in the wide vase, the flowers will be arranged towards the outside, and the space in the middle will be the equivalent of the cup in the optic nerve. As the flowers die, the “cup” increases in size.
In the optic nerve, you can’t be sure that nerves have died (at least not in the early stages) unless you knew how many were there before. And this is where the OCT comes in.
OCT and Glaucoma
The OCT can measure the thickness of the nerve fibre layer, which indicates how many nerves are coming into the main optic nerve from all over the retina. This helps in the diagnosis of glaucoma in two ways.
Firstly, it compares the measurement against an average of people of the same age, sex and ethnicity, so it shows the likelihood of glaucoma by virtue of being out of step with average. The, secondly, it stores those measurements. Then, if they are repeated some time later, any deterioration can be picked up.
In one study (Paul 2010) OCT technology was shown to predict field defects that developed on average four years later.
Treatment
People often ask me if anything can be done about glaucoma once it’s been detected. The answer is “Yes, very much so”. The usual treatment is drops, to bring the pressures down, even if they aren’t especially high. But it needs to be detected early, because any vision that’s been lost can’t be regained. Also, once some nerve fibres died, the remaining ones seem to be especially sensitive to even normal pressure.
Closed angle glaucoma
In about 10% of all cases of glaucoma, the iris gets in the way of the drainage system. This is known as closed angle glaucoma because the angle between the drainage system and the iris gets very narrow and can even close completely. When this happens, the pressure builds up very rapidly in an acute attack. The eye becomes red and painful, and vision loss can occur rapidly without treatment. Unfortunately, two-thirds of those with closed angle glaucoma develop it slowly without any symptoms prior to an attack.
Another use of the OCT is that we can use it to see whether the angle is open or close, so we can detect those at risk of an acute glaucoma attack. Often, all that’s required is a little laser treatment to allow fluid to get through and prevent an attack.
Conclusion
OCT is now playing a vital part not only in the early detection of glaucoma, but also in monitoring the effectiveness of glaucoma treatment.
Contact me for more information on Glaucoma and OCT
David Donner
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