Optical coherence tomography is a technique that has been used to measure the thickness of the retinal nerve fiber layer and macular volume in glaucoma and other retinal diseases. These measurements provide information about the thickness of axons that emanate from the retinal ganglion cell bodies, which coalesce in the optic nerve.
Optical coherence tomography (OCT) uses infrared (IR) light to measure the thickness of the nerves in the back of the retina. The light is bounced off the retina; the way it is reflected back to a recorder at a particular point is used to develop images of the thickness of layers of the tissue at the back of the eye.
This is similar to the way ultrasound works. “It's light-based instead of sound-based,” said Dr. Peter Calabresi, director of the Multiple Sclerosis Center at Johns Hopkins University, Baltimore.
The depth of penetrance for the IR light is fairly shallow, which makes it a good technique for looking at structures and tissues right below the surface. With the latest generation of instruments, resolution is down to about 3–4 micrometers, which provides “almost a microscopic picture of the eye,” said Dr. Calabresi.
Dr. Calabresi and his colleagues have been using OCT to assess disease status in patients with multiple sclerosis. Because the retinal nerves coalesce into the optic nerve, “We know with MS that if you injure nerves in one location, that you will see downstream consequences.” Neurologists have long looked at the optic nerve head to see if it looks pale in color to determine whether the patient may have had an optic neuritis attack that led to some injury of the nerves at the back of the eye.
The retinal nerve fiber layer (RNFL) has been described as atrophying in focal/segmental patterns after an optic neuritis attack, which suggests that there were small areas of injury rather than a diffuse global process. OCT allows researchers to quantify these focal/segmental patterns of atrophy in the retinal nerve fiber layer. “This just allows us now to quantify it and to have a reproducible and reliable objective measure.”
In one study published by Dr. Calabresi and his colleagues, they compared OCT findings of the RNFL between a large cohort of MS patients and a group of healthy controls (Neurology 2007;69:2085–92). In this study, patients with MS had abnormal RNFL findings, even if they didn't have a history of optic neuritis. OCT “could then be used as a diagnostic tool to then pick up subclinical disease,” Dr. Calabresi said.
In another study, the researchers were able to show that changes in the RNFL were correlated with global brain atrophy in MS patients (Neurology 2007;69:1603–9). “By looking at this one part of the nervous system, it may be predictive of what's happening in the rest of the brain,” Dr. Calabresi said.
In the third study, they showed that the technique provides intrarater and interrater reproducibility (Arch. Neurol. 2008;65:1218–22), which is important because MS patients produce unique challenges for OCT. “Although other people have reported this in the ophthalmology world, we thought it was important to show [that] you could do this in a neurology office with MS patients,” he said. “I think it was important to show that in a patient with a more diffuse global disease, that we could get them to hold still enough to do the testing.”
There are a few caveats about OCT to consider. “The abnormalities that we look at on OCT are not specific to MS. It's very important to rule out glaucoma and other retinal processes related to hypertension or diabetes,” Dr. Calabresi said. Once a neurologist is fairly certain that the retinal abnormalities are related to MS, “it's a very useful way of tracking the disease,” said Dr. Calabresi.
OCT has some advantages over MRI—it's an office-based technique that takes about 10 minutes to perform and is much cheaper and more user-friendly than MRI. Despite this, OCT will not likely take the place of brain imaging. Dr. Calabresi sees OCT as useful in partnership with MRI.
“I imagine using it frequently in the office when the patient is coming in for a visit to directly assess the eyes and to give you some information about the status of the retinal nerves.
“We know the progressive cases tend to have more [RNFL] damage and we know that it is somewhat predictive of what is happening in the brain.”
The technique could be used in the period between brain imaging scans to assess progression.
As yet, there are no longitudinal data on the use of OCT to track progression, said Dr. Calabresi.