[intro music]

 

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?

 

Dr. Saidha

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 chea