Host – Dan Keller
Hello, and welcome to Episode Ninety-two of Multiple Sclerosis Discovery, the podcast of the MS Discovery Forum. I’m Dan Keller.
Today's interview features a conversation with Dr. Shiv Saidha, an associate professor of neurology in the Division of Neuroimmunology and Neuro-infectious Diseases at Johns Hopkins University in Baltimore, Maryland. His work has focused on the retina in MS, using the technique of optical coherence tomography, or OCT, to follow the disease, assess and monitor therapeutic strategies, and to better understand the pathobiology of MS. I asked him why the retina is of interest in MS and about the utility of OCT.
Interviewee – Shiv Saidha
OCT is the optical analogue of ultrasound B mode imaging. And it's a noninvasive technique that has a lot of utility in quantifying the ultrastructure of various tissues, including the retina. We have a lot of interest in being able to quantify retinal structures specifically in multiple sclerosis because optic nerve pathology, which basically refers to affliction of the optic nerve as part of the MS disease process, is virtually ubiquitous. At the time of postmortem examination of MS patients, 94 to 99% of MS patients are found to have demyelinating plaques within their optic nerves.
So the premise is that demyelination within the optic nerve results in retrograde degeneration of the constituent fibers or axons within the optic nerve. And since those axons or fibers are derived from the retinal nerve fiber layer, which is the innermost layer of the retina, this layer is felt to thin out as part of the MS disease process. Additionally, the neurons – referred to as ganglion cell neurons located in the ganglion cell layer immediately below the retinal nerve fiber layer from which retinal nerve fiber layer axons are derived – are also thought to drop out as part of the MS disease process.
We traditionally conceptualize optic nerve pathology in MS as being an acute phenomenon, namely acute optic neuritis, which does occur in up to, you know, 20 to 70% of MS patients; and in 20 to 25% of cases of MS is the initial hallmark clinical manifestation of the disease process. But beyond acute optic neuritis, there is subclinical optic nerve pathology, which we refer to as subclinical optic neuropathy ongoing within the optic nerves of MS patients.
And so, if we had a technique or an ability to accurately quantify the effects of optic nerve pathology or optic neuropathy – in other words, if we had a way to quantify retinal nerve fiber layer thickness and thickness of ganglion cell…the layer within which ganglion cell neurons are located in the retina – that would provide a substrate or insight into the state or integrity of the optic nerve. And so, optical coherence tomography is a technique which allows us to do this. It allows us to measure thickness of the retinal nerve fiber layer not just around the optic disk which we refer to as the peripapillary retinal nerve fiber layer but also in the macular region.
And with the advent of novel segmentation techniques in OCT – many of which are now commercially available – we now are also afforded the capability of quantifying thickness of other discrete retinal layers such as the combined thickness of the ganglion cell layer and inner plexiform layer, which many of us refer to as GCIP or some also refer to it as GCIPL. Conventionally, peripapillary retinal nerve fiber layer thickness – at least in cross-sectional studies – was found to be associated with high and low contrast visual function, as might be expected since the retina subserves vision as a function.
But interestingly, early studies even found that thickness of the peripapillary retinal nerve fiber layer was associated with disability scores as determined by Expanded Disability Status Scale scores or EDSS scores in MS patients, as well as whole brain volume in MS patients, implying that these metrics derived from OCT somehow provide a window or insight into the global MS disease process. With time, we started to realize that the GCIP thickness might actually be an even more powerful measure of the state of integrity of the optic nerve.
GCIP thickness seems to be more reproducible than that of the retinal nerve fiber layer. It has a intraclass correlation coefficient of about 0.99 with a very tight confidence interval. It has superior structure function correlations with EDSS scores, brain volumes, as well as high and low contrast visual function, as compared to retinal nerve fiber layer thickness. This is really a very interesting and important point about, you know, the potential utility of OCT. Because with this thickness of the GCIP, what we were really getting is a very good estimate of neuronal integrity.
And, one of the factors that has been limited in terms of MRI – or magnetic resonance imaging – is the ability to really accurately and reproducibly quantify collections of axons and neurons. Now in terms of MRI, we often think that the white matter is a very good reflection of axonal integrity, and that gray matter is a good reflection of neuronal integrity. This is not necessarily the case, however. In terms of the white matter, quite a lot of inflammation obviously occurs within the white matter in MS brains. And when that inflammation occurs, white matter volume increases. And then, as that inflammation subsides, the white matter volume drops.
And then, as the next wave of inflammation comes in, again, there's swelling and the white matter volume goes up. And as it resolves, the white matter volume comes down. And so there's this waxing and waning in terms of white matter volume which limits the utility of white matter volume. And in fact, it's for that particular reason that many researchers have found that when you track MS patients over time that the bulk of change is actually seen within the gray matter.
In terms of the gray matter, there is a lot of axons present within the gray matter. And so, gray matter volume is not just a pure measure of neuronal integrity. And the other thing is that the axons within the gray matter are predominantly myelinated similar to within the white matter. And so these brain substructure volumetrics are confounded by myelin too. The retina is an unmyelinated central nervous system structure. And so the measurements that we derive with OCT are not confounded by myelin. And secondly, GCIP thickness does not seem to increase during inflammation of the optic nerve.
There's been a number of studies showing that during acute optic neuritis peripapillary retinal nerve fiber layer thickness increases. There's a number of reasons for that: there's inflammation within the optic nerve, and so there's edema. And so we think that some of that edema may track down to the retinal nerve fiber layer. And there may also be some impaired axonal transport resulting in congested axons within the retinal nerve fiber layer. In addition to that, the retinal nerve fiber layer also contains the bulk of glial cells. And by that, I was mainly referring to astroglia. Now microglia are thought to be present throughout the retina, but there's really no astroglial confound of GCIP thickness as well.
During acute inflammation in the optic nerve, GCIP thickness was not found to increase. And so if you take a patient, as an example, with an acute optic neuritis now, and then you repeat the OCT scan six months later, the GCIP thickness at six months subtracted from that at baseline is felt to be a fairly accurate reflection of net neurodegeneration in terms of net loss of ganglion cell neurons. That absence of edematous or inflammatory or swelling related confound of GCIP thickness yields yet another advantage for this particular measure.
Interviewer – Dan Keller
How does the time course of changes in the GCIP correlate with brain MRI? Can it be predictive or are they in lockstep or how do they relate?
Yeah, so that's a great question. I think one of the things with OCT research has been that the bulk of research to date has been cross-sectional. And so it has really been one of those key things on our mind is does the way that the GCIP atrophies or thins really mirror what's happening in the brain? In other words, are they locked in together? Are the rates of GCIP atrophy and brain atrophy actually associated with one another, or are they a little disconnected?
So in a recent study, which we published in Annals in Neurology, we tracked a little over 100 MS patients for roughly a four-year period, and we did annual MRI scans with a 3-Tesla scanner, and we did six monthly OCT scans. And very importantly and interestingly, we found that the rate of GCIP atrophy was highly correlated with the rate of brain atrophy and a particular rate of gray matter atrophy. Of course, that's a little bit to be expected partly on the basis of what I said earlier that white matter atrophy in itself is not as well detected as gray matter atrophy.
And then when you look by subtypes of MS – meaning, you know, relapsing MS versus progressive MS – we found that the rate of atrophy was even better or more highly correlated in terms of its association with brain atrophy rate. In fact, it appeared that the rate of retinal atrophy could predict 80% of variance in rate of brain atrophy, which is fascinating because it really does imply that what we're seeing within the retina of MS patients is a reflection of global central nervous system pathology.
And the pathobiological changes that we can detect and monitor with OCT appear to very nicely reflect what's happening within the brain. And that this cheap, noninvasive, easily tolerated, easily repeatable technique that's painless can provide so much insight into this disease process is really quite fascinating and really phenomenal when you consider the increasing and growing need for an ability to measure and monitor neurodegeneration in this disease process. We traditionally conceptualize MS as being an inflammatory demyelinating disorder of the central nervous system, and absolutely there's inflammation that occurs as part of the disease process. And when acute inflammation occurs, there's some immediate damage to axons on the form of axonal transection. And then when axons do not have enough myelin around them or are devoid of myelin over sustained periods of time, that normal protective environment for axons is not present, and so we feel that those axons slowly neurodegenerate.
The advent of putatively neuroprotective and putatively remyelinating therapies now more than ever increases our need for an ability to be able to track neurodegeneration. And in fact, it is neurodegeneration that is the principal substrate of disability in MS. And while the inflammation may be at the root cause of this neurodegeneration, the disability that patients have is better associated with the amount of neurodegeneration that's present. So, it's possible – and we postulate – that OCT could be a very useful outcome measure in terms of assessing therapies which are putatively neuroprotective and/or even neurorestorative or remyelinating.
Do you think that there is a common process going on more centrally and in the retina causing the changes in both? Or is it possible that there is more central degeneration, which then is transmitted peripherally causing problems in the retina?
So that's a great question. We think that the bulk of the retinal changes that we're observing are related to pathology within the optic nerve. And because optic nerve pathology is basically ubiquitous as part of the disease process, we think that the changes that we're seeing within the retina are really just a reflection of what's happening throughout the central nervous system.
Now, you do raise a very important point though. Although we think that the bulk of the change that we're seeing within the retina is related to pathology within the optic nerves, that does not exclude the possibility that some of the changes that we're seeing – in terms of thinning of the retinal nerve fiber layer and GCIP, in particular – are actually related to transsynaptic degeneration. Meaning that if you have a distant lesion or distant pathology that as an axon dies that the next neuron and axon, as part of a sequential chain, is not affected. And that's something that we're actively studying at the moment to try to better understand the effects of transsynaptic degeneration on retinal measures. There is some data to suggest that there is transsynaptic effects on retinal measures, but my own view is that longitudinal studies to definitively establish this are currently lacking.
Do you have to watch out for a history of optic neuritis when you look at the OCTs? Does that affect what you're finding?
I think it does. So, if we are kind of going to say that what we're seeing in the retina is a general reflection of what's happening in the brain, we have to at least consider the possibility that a severe inflammatory event with disproportionate local retinal tissue injury might have an affect on the global relationships between OCT derived measures and brain measures. So, when we look at the relationships between rates of GCIP atrophy and rates of brain atrophy, we find that in eyes with a history of optic neuritis that that relationship is not as strong. And we think that that may be the case because immediately following optic neuritis there's an excessive amount of local tissue injury. And that local tissue injury that results in excessive loss of retinal nerve fiber layer and GCIP tissue somehow masks the global information that we're deriving from OCT.
But then what's interesting is that although a history of optic neuritis seems to be relevant at least in the relapsing-remitting subtype it seems to be less relevant in secondary progressive MS. Part of our hypothesis for this – although it needs to be better elucidated and studied – is that brain atrophy continues on following the optic neuritis, and let's just say it carries on, as an example, at the same rate that it did beforehand. Well eventually, the rate of retinal atrophy, although there was initial disproportionate surge in neurodegeneration within the retina, there will be some ongoing neurodegeneration occurring too. And eventually the two rates will become realigned again in the future.
Kind of to get at that point we also looked at rather than just history of optic neuritis we looked at whether baseline GCIP thickness might have an impact on rate of GCIP atrophy to kind of expand a little bit upon that hypothesis. And indeed, what we found was that rate of GCIP thickness at baseline is highly associated with rate of GCIP atrophy. In kind of simplistic terms the way I conceptualize this is that the more retinal tissue that's available the faster the potential rate of retinal atrophy is. And if there's less retinal tissue available, then there's maybe less potential for that rate to be as fast. If the rate was to remain steady the entire time from the day that the disease first begins – and I think this also applies to the brain too – it wouldn't be very long before there's no tissue left.
You had alluded to looking at new potential therapies using OCT as an outcome. Does that also mean that it may have utility in looking at current disease-modifying therapies and being able to compare them?
So that's an excellent question. In fact, that's actually currently no data available that I'm aware of that has assessed the effect of currently available disease-modifying therapies on rates of GCIP or retinal nerve fiber layer thickness atrophy. And I think that's something that a lot of academics and people who do research utilizing the visual system and a particular OCT would be interested in seeing. We have such data – and such data is routinely collected in terms of effects of disease-modifying therapies – on brain volume, and this is something that's now fairly standard to be collected as part of clinical trials. It may be useful to know whether currently available disease-modifying therapies have differential effects on rates of retinal atrophy. Which would imply that maybe in addition to having a role as an outcome measure in trials of putative neuroprotectants, as well as neurorestorative agents, that maybe OCT might actually also have a role in studies of potentially anti-inflammatory treatments or treatments which modulate or suppress the immune system, as do most currently available licensed disease-modifying therapies.
Very good. I appreciate it.
Thank you very much.
Thank you for listening to Episode Ninety-two of Multiple Sclerosis Discovery. This podcast was produced by the MS Discovery Forum, MSDF, the premier source of independent news and information on MS research. Msdiscovery.org is part of the nonprofit Accelerated Cure Project for Multiple Sclerosis. Robert McBurney is our President and CEO, and Hollie Schmidt is Vice President of Scientific Operations.
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