[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