Lecture: Anatomy of the OCT Retinal Nerve Fiber Layer Scan

This live webinar will review the interpretation of an OCT scan for neuro-ophthalmic disorders including glaucoma. It will also include valuable tips for integrating this technology to your clinical practice using a case-based discussion.

Lecturer: Dr. Sowmya Srinivas, OD, MS, ABO, FAAO, Dartmouth-Hitchcock Medical Center, New Hampshire, USA


DR SRINIVAS: Thank you so much for participating in the webinar today. My name is Sowmya Srinivas. I’m in clinical practice at the Dartmouth Hitchcock Medical Center, and I’ll be presenting on anatomy of the OCT retinal nerve fiber layer scan. I have no disclosures. So we’re going to begin this webinar by a few polling questions. The first polling question is: Do you have an OCT device in your office? Yes or no? Great. Thank you. The second polling question: How do you frequently use your OCT? Multiple times a day, once a day, once a week, once a month, or I don’t have an OCT? Thank you. How long have you had an OCT? For many years, within one year, or recent? Great. Thank you. And the final poll question: What OCT do you have? Spectralis, Avanti, 3D OCT, or other? Excellent. Thank you so much. Looks like a good majority of you have an OCT and you use it several times a day, so this is helpful information for this webinar today. So this course will go over interpretation of OCT scan for optic nerve devices/anomalies, including neuro-ophthalmic disorders and glaucoma. OCT has established itself as a technological pillar in eyecare. However, these devices provide sophisticated detail on anatomical status and disease risk, but the sheer volume of complexity in the data can be daunting. So my goal for today is to give you valuable tips and clinical pearls in integrating this technology to your clinical practice, using a case-based discussion. So the outline for the webinar will be an introduction and review of OCT technology, and how you can use an OCT as a diagnostic tool in glaucoma and other neuro-ophthalmic conditions. We’re going to go over a brief overview of optic nerve disorders, so we’re all on the same page. Some cases, conclusion, and end with questions and discussion. So the OCT was first described in 1991, and it became very popular in 2002 with the release of Stratus OCT. This was a time domain technology, validated for retina and glaucoma as a diagnostic tool. The advantages are it’s non-contact, non-invasive, and it reveals a layer of the retina utilizing interference patterns of reflected laser light. There are four commercially available SD OCT devices. Each has a different glaucoma scan pattern, segmentation algorithms, and display outputs, which we will go over. There’s Carl Zeiss, Optovue, Heidelberg, and Topcon. Some of the parameters that are included include the retinal nerve fiber layer, the optic nerve head, the ganglion cell complex, and the numerical values of four representative colors. So you have white, green, yellow, which means less than 5% of your patients are compared to the normative database, and red, less than 1% compared to the normative database. Some of the new innovations are improved resolution, identification and segmentation of individual layers, and better test-retest repeatability. So how do we utilize new technology for test-retest repeatability? So this is with the help of using a technique called Bruch’s membrane opening. This is a recognition of an anatomical landmark. Bruch’s membrane opening is through which the optic nerve passes. So this provides consistent measurement of the optic nerve size and rim area. It’s used to define the borders of an optic disc margin, which then serves as a reference for other and future measurements. This margin is clinically invisible. But it can be identified accurately and repeatedly with OCT, compared to clinicians’ observations of where these disc margins lie. So this is a representation of where the Bruch’s membrane lies. So it is a consistent landmark. It is usually clinically and photographically invisible. So the disc edge is determined by a termination of Bruch’s membrane. This is validated in the literature. The rim width around the circumference of the optic nerve disc is then determined by measuring the amount of neuroretinal rim tissue in the optic nerve. So this technique differs from other methods that determine the cut margin based on its intersection with the plane at a fixed distance above the disc. So in this method, the disc and rim area measurements correspond to the anatomy in the same plane as optic disc. So you can see the Bruch’s membrane opening by the yellow arrows, highlighted in the picture. Here in this diagram it is marked BMO or Bruch’s membrane opening. There are red dots on either side. On the bottom diagram from one end of the Bruch’s membrane opening to the other end of the Bruch’s membrane opening. And so this is how the OCT uses landmarks to have consistency with repeated measurements. So this slide here is a comparison of Cirrus, Topcon, and Heidelberg. Scanning speed, axial resolution, minimum pupil diameter, and focusing diameter. So the difference between time domain and spectral domain OCT is that time domain technology could only acquire 400 scans per second. The current spectral domain OCT models, depending on the manufacturer, can capture between 26,000 and 70,000 axial scans per second. This improvement is beneficial to the clinician, because it minimizes image artifacts and it makes 3D imaging possible, and it increases the image resolution. Today’s SD OCTs or spectral domain OCT can provide an axial resolution of 3 micrometers to 6 micrometers within tissues, compared to the previous time domain technology. Which had a maximum resolution of just 10 micrometers. So this increased speed and resolution provide an enhanced ability to visualize all your retinal layers. So here’s a comparison of the imaging modes and the capabilities by model. So these are imaging modes which each manufacturer can do. Fundus photos and the image field of view. The Cirrus HD OCT’s software features additional layouts that enable the clinician to see the nerve and macula on a single printout and formulate structural and functional relationships between visual fields and the OCT data. Here is a table of scan acquisition and analyzing options per model. So this includes a scanning range, the scan acquisition, and the macula. So recent literature suggests that although reproducible, OCT findings are common among healthy individuals. However, variability can occur when pathology is present, due to their differences in acquisition. And boundary identification between machines. And I have an example coming up in the next few slides. So clinicians can encounter ambiguous findings on non-glaucomatous optic neuropathies and normal optic nerve anatomy, which can lead to false positive and false negative OCT results. Researchers have determined that nerve fiber diameters greater than 4 millimeters squared can mostly affect the accuracy of the nerve fiber layer analysis results. Accuracy is also limited by high refractive error and axial length. Longer eyes — in other words, myopic eyes — can artificially cause thinner peripapillary nerve fiber layer measurements. While shorter eyes may falsely represent higher peripapillary retinal nerve fiber layer values, due to camera magnification. So repeatability of retinal nerve fiber layer and retinal thickness measurements over time must also be considered when monitoring for progression of an ocular disease state. So researchers have determined that spectral domain OCT has an intervisit tolerance limit of 95% for average peripapillary retinal nerve fiber layer. This is equivalent to about 4 micrometers. Cross sectional studies suggest a normal age-related decline in the nerve fiber layer is about 0.2 micrometers per year, and the rate of decline may be higher in eyes with greater baseline retinal nerve fiber layer thickness, based on a few studies. So these considerations must be taken into account when evaluating OCT data and managing ocular conditions. So studies suggest clinicians should be cautious when comparing thickness measurements from one brand of OCT with another brand in the same generation, due to differences in algorithm acquisitions between the machines. This often leads to a difficult decision for the provider looking to make a change from one OCT model, for example, to another company where baseline data has already been obtained on established patients. So this table is a comparison of SD OCT normative databases by manufacturers. On the top row is a number of subjects. So you see there are about 182 to about 284 subjects for the normative database, for each of these top manufacturers. The ages range anywhere from 19 years of age to about 84 years of age. You can also look at the gender distribution. It’s pretty even, I would say, for Carl Zeiss. 133 males, 149 females. Topcon has the disc to macula, 54 males, 92 females. And for the macula, 112 females, 61 males. And the Heidelberg, which is pretty even. 111 males, 90 females. The ethnicity in the Carl Zeiss model includes a varied demographic. Versus if you look at the Heidelberg, it is only a Caucasian ethnicity. The anatomy evaluated — Carl Zeiss includes all these parameters. Topcon has optic disc and macula. And Heidelberg has retinal nerve fiber layer. And the study locations. United States, China. Topcon is United States. Heidelberg, Germany. So in the previous slide, you saw that there are pretty limited normative databases against which scans are compared. And this can never remarkably cover the varied appearance and structure of the optic nerve we see in our patients. And of course, normative databases can sometimes be flawed, relative to the atypical optic nerve head or retinal nerve fiber layer morphologies, and imaging can present artifacts which do not represent true ocular disease, but are secondary to limitations of imaging technology. So what are some of the factors in analyzing OCT scans? So first you have to make sure you have a good signal strength, which is a key indicator of image quality. You have to make sure the images are well centered. You have to make sure that there’s no evidence of movement artifact. You have to make sure that you review all the plots and displays, including thickness and deviation map quadrants, the sector plot, and what they call the TSNIT graphs. So TSNIT means temporal, superior, nasal, inferotemporal circle, with the radius of 1.73 millimeters, and this is established around the disc. And most importantly, always use clinical correlation. So never use OCT as a diagnostic device in and of its own. So you have to make sure you do a clinical exam and obtain other measurements, including visual fields. And I have a lot of cases coming up, which demonstrate how to use OCT in addition to your clinical exam, and other testing. So these are some of the artifacts you can encounter when you do an OCT scan. Acquisition. So if your ring measurement of the nerve fiber layer is not placed concentric with the optic nerve, so for you to kind of get around this, you can look at raw images. You can also get a disease-related artifact, which means, for example, myopes can have peripapillary atrophy, and some scans can fail to identify these nerve fiber layers in those regions. Some of the artifacts which can be instrument-related include a head tilt. So even about an 8-degree head tilt, like so, can result in a difference in thickness rating, and some machines can compensate for this. Some newer machines. And microsaccades. So again, recent technologies have eye tracking mechanisms to reduce microsaccades that can come from use of these devices. So here’s an example of what we might call red disease. So this is when misinterpretation occurs because the normative database is applied in a patient who should not be considered normal. So this example is a patient with high myopia. You can see the green concentric circle is applied at the end of the peripapillary atrophy, as opposed to around the disc. Hopefully you can all see my arrow. My mouse arrow. So again, the green concentric circle is applied around the peripapillary atrophy. Versus the actual disc. So this is making it look like the thinner retinal nerve fiber layer doesn’t conform to the limits identified by the normative database. And thus the entire scan is colored red. Again, because of the ring around the peripapillary atrophy versus the ring around their true optic disc. Here is an example of a green disease, where all your sectors are highlighted green, which means it’s normal. So this occurs in patients who have normal global values, such as average retinal nerve fiber layer thickness, but may have small focal defects which are missed. If you look at your right hand side of the diagram here, there’s a small focal defect highlighted by the red arrow. On the top right diagram. On the bottom right diagram, you can see the red arrow specified as focal loss. So even though both eyes are labeled normal, the red arrow points to a focal loss of the retinal nerve fiber layer, inferotemporal, in the left eye. This is easily detectable on the posterior pole retinal thickness map, and the hemisphere asymmetry analysis, which is on your right. But if you were just to go based on the color pattern, on the right eye and left eye, you might misinterpret this as a normal scan, when in fact there is a focal loss from a disease. So these are representative examples of what each manufacturer has. So this is a very busy slide, but on the top, this is an example from Heidelberg engineering. You have your retinal fundus image with delineation of circular volume scan. You have your intereye symmetry for your average retinal nerve fiber layer thickness, by quadrant. Here. And you have your tomogram here, a color coded table representing the normative database right in the middle. You have your TSNIT retinal nerve fiber layer thickness of the right and left eyes in a graphical format. You have your TSNIT retinal nerve fiber layer thickness of the right and left eye, for symmetry comparisons, in the middle, and there’s a pie graph of quadrant and sector, retinal nerve fiber layer thickness, compared to normative database, which is color coded. Red, green, and yellow. There is a classification whether the scan falls within, borderline, or outside normative database. So in this example here, this is red. Which means the scan is outside borderline limits. This is an example, again, of Heidelberg printout. The next one is an Avanti RTVue XR printout. Again, affecting the same measurements, as we saw in the prior scan. These are examples of a 3D OCT 2000 Topcon and Cirrus. HD-OCT 5000. So there is a great article in the Review of Optometry. I have a link in the reference. Which showed a patient who got an OCT scan of all these four different manufacturers, and the important thing to know from this article is he found that all the four manufacturers had excellent agreement in their analysis, with no statistically significant differences in detecting glaucomatous loss. So your unique needs in your practice should determine what platform you select. And in our office, we have Heidelberg, so the case studies I’ll be going over for the webinar today — we’ll go over the Heidelberg printouts. So we’re talking about a few cases, using OCT as a diagnostic tool. But before we do this, I wanted to give you a quick overview of some of the OCT scans we’ll be using for our cases. So I have several glaucoma scans and glaucoma suspect scans. As you all know, glaucoma is a chronic disease in which the optic nerve undergoes progressive irreversible damage. This is a neurodegenerative disease with variable ocular signs and one underlying theme. So again, this progressive optic neuropathy, characterized by erosion of your neuroretinal rim tissue. And this progressive damage is beyond the normal age-related loss of your retinal ganglion cells. This is the quick overview of unilateral versus bilateral optic nerve edema. So in papilledema, your CSF volume is increased in your subarachnoid space, which causes axoplasmic stasis of both optic nerves. So this leads to an increased intracranial pressure, and the papilledema — to be classified as papilledema, it has to affect both nerves. Most common cases of papilledema include postchiasmal tumor, pseudotumor cerebri, malignant hypertension, infiltrative process, compromised or obstructive venous outflow. So again, papilledema has to affect both nerves. They have to be swollen. When you have unilateral disc edema, when one optic nerve is swollen, you have the presence of prechiasmal disruption of axoplasmic flow, such as either compression or ischemia. Some of the most common causes of unilateral disc edema include AION, optic neuritis, papillophlebitis, compressive lesions, neuroretinitis uveitis, hypotony, CRVO, central retinal vein occlusions, and diabetic papillopathy. When you see a swollen optic nerve, the other diagnosis that comes to mind is idiopathic intracranial hypertension. So this is usually in an overweight female of child bearing age. Some signs and symptoms include a CN6, cranial nerve VI palsy, transient visual obscurations, headaches, visual field defects, papilledema, nausea, and vomiting. Some things that could lead to idiopathic intracranial hypertension include contraceptives, vitamin A, Accutane, Nalidixic acid, tetracycline use, minocycline use, oral steroid, or steroid withdrawal. You should do an MRI, MRV, and CSEF normal composition. You should do a lumbar puncture. So anything above 200 millimeters is normal, for a normal weight individual, is considered high opening pressure. And for an obese individual, anything greater than 250 millimeters would be a high opening pressure on your lumbar puncture. Most importantly, the patient has to be awake, alert, with an otherwise normal neuro-ophthalmic exam. Other than the cranial 6 nerve palsy. And there should not be any other cause of intracranial pressure. Finally, I have a case on optic nerve pallor. When you think of optic nerve pallor, you can think of primary pallor. In other words, primary pallor — this is where a pallor happens without a swelling phase in your optic nerve. So some causes of primary optic nerve pallor include toxic or nutritional neuropathy. This happens in alcoholics. Vitamin deficiencies. And medications. Including some medications for tuberculosis. You can have primary optic nerve pallor in dominant optic atrophy, trauma can cause this, Leber’s optic neuropathy can cause this, radiation, and optic nerve hypoplasia. Secondary optic nerve pallor can happen when a nerve swells in the past and pallor happens later. So some examples here are below. AION, optic neuritis, papillophlebitis, prechiasmal compressions, optic nerve sheath meningioma, neuroretinitis, diabetic papillopathy, and disc edema. Finally, there are optic nerve anomalies, including optic pit, which is a depression of the disc located temporally. This can be associated with a serous retinal detachment in the macula area. Other anomalies include nerve head drusen, optic nerve coloboma, and melanocytoma. I don’t have examples of all of these, but I have some examples of the optic nerve head anomalies. So the first case we’re going to begin with is an example of a patient who I have been following as glaucoma suspect. The fundus images of the right and left eyes show an increased cup to disc ratio, which is why I’ve been following this patient as a glaucoma suspect. This is a representative scan of an OCT of his right eye on your left hand side. And an OCT of the left eye, on your right hand side. On your right hand side are a couple of diagrams with visual field 24-2 Humphrey visual field, which shows stability over the many years I’ve been monitoring him. On first glance, when you look at your OCT, you can see in the red eye in the center there’s a yellow color, saying it’s borderline thinning. In the left eye, there’s a yellow highlight for your inferior quadrant. And both of these right eye and left eye show borderline thinning. However, when you put all the pieces of your puzzle together, from your fundus photos, which show an increased cup to disc ratio, your OCT which shows borderline thinning but visual fields have been normal for many years. So you could misinterpret, saying this patient may have glaucoma, if you were looking at OCT in isolation. But when you put everything together, this appears more to be a physiological cupping, because his imaging has been pretty stable for many years. Including your functional testing or your HVF or Humphrey visual field, 24-2 scans. So this is a second patient I’ve been following as glaucoma suspect. And here please note there are tilted nerves, as this patient is myopic. So both the right eye up top and the left eye down below, the optic nerves are tilted. There is also peripapillary atrophy on both inferiorly — on both optic nerves. Notice how the OCT on the right eye is saying that there’s borderline thinning in the superior quadrant, in the right eye, on your left hand side of the scan. The left eye on the right hand side of your scan is coming up with all green, which is saying normal. So even though both nerves are tilted, one scan is showing some borderline thinning, whereas the other eye has a normal pattern. And when we do additional testing, his 24-2 Humphrey visual field 24-2 is perfectly normal in both of the eyes. Right eye is on your left hand side, and left eye is on your right hand side. So again, this patient was a glaucoma suspect, secondary to large optic nerve cup to disc ratio. His IOP has always been in the mid-teens, and he’s had essentially normal retinal nerve fiber layer and Humphrey 24-2 scans, and these have been documented with photos. But if you had used OCT in isolation, you may have been misled, saying there’s some borderline thinning, when in fact, there’s no reason to treat this patient. But essentially follow as glaucoma suspect. So again, these are a couple of clinical pearls, saying you shouldn’t use an OCT scan, and go based on just the color pattern. It is important to put all your pieces of an exam together, including your clinical exam, your visual field scans, your OCT eye pressures, pachymetry, and for glaucoma suspect or glaucoma diagnosis. So our next case here is a glaucoma suspect due to asymmetric nerves. So his optic nerve in the left eye had a cup to disc ratio which was larger than the optic nerve head cup to disc ratio in the right eye. Here’s an optic nerve scan OCT retinal nerve fiber layer scan of the right and left eye. So right eye is on your left hand side of your screen. Your left eye is on the right hand side. On the right hand scan, there is a green pattern with a little bit of yellow in the inferior temporal quadrant. In the left eye with a larger optic cup to disc ratio, a majority — there’s a definite thinning of the superior quadrant and nasal and inferior quadrant, showing borderline thinning. Here on this page is a Humphrey 24-2 for your right eye, on your left hand side, left eye on your right hand side. Since 2017, you can see that the right hand 24-2 has always been normal. In his left eye, he’s shown some non-specific defects in the left eye. But essentially normal. Compared from 2017 to the 2019 scans. And again, if you were just going based off of the color coded patterns of the OS or left eye, you may think that this patient had more thinning and maybe compelled to start treatment, for example, when the functional test has been normal all along. So this is another patient, for example, I would follow as glaucoma suspect, without treating, based on putting all the information together, including my OCT and my 24-2 scans. So the next case is a patient I saw with sectoral inferior temporal thinning in the left eye. So this is what I was looking at. So these are four representative images. So there’s some possible thinning here in the inferior temporal thinning, in the left eye. On your left hand side is a baseline scan. On your right hand side is a follow-up scan. So clinically, like I said, I noticed inferotemporal thinning, which is why it prompted me to get a follow-up OCT scan. And in this case, the OCT is picking up this borderline inferotemporal thinning in his left eye on the follow-up scan. So inferotemporal thinning on the follow-up scan, compared to the baseline scan on your left hand side. When I did the Humphrey 24-2 visual field, the superior nasal defect that you see on the Humphrey 24-2 are indeed corresponding to the inferior temporal borderline thinning I had seen on clinical exam, and on the OCT retinal nerve fiber layer scan. So this patient is someone I’m very closely going to watch, because not only is my clinical exam picking up some inferotemporal thinning, but the OCT is corresponding to that, as well as my Humphrey 24-2, which is showing some superior nasal early defects, corresponding to the inferior temporal borderline thinning. So again, I’m using OCT as a diagnostic tool, and not just looking at color patterns, but putting all the pieces of the puzzle together, including my Humphrey 24-2 scans, my OCT, my clinical exam, and fundus images. So here’s another case. This is a patient with moderate to advanced primary open-angle glaucoma. His IOP or eye pressures, intraocular pressures, have been stable for many years. He’s had glaucoma surgery over 10 years ago. He has been on timolol and Latanoprost. He has been treated for his glaucoma. And in both eyes, I’m calling this optic nerve cup to disc ratio, 0.85 cup on the top in your fundus image. On the bottom image, again, I would say a 0.85 cup to disc ratio. When we jump to the next slide to look at the OCT retinal nerve fiber layer, even though my cup to disc ratio was pretty similar in both of his eyes, notice how the OCT is so different in his right eye compared to his left eye. So again, if you were going based just on the color patterns, it looks like the right eye is showing much more significant thinning, compared to his left eye. However, on clinical exam, the cup to disc ratio is pretty equivalent in both eyes. So this is a patient I’m treating in both eyes, and not going, again, just based on OCT, where one eye is looking pretty normal, just based on the color pattern, but the other eye is showing significant thinning. So again, I’m utilizing all components of my exam and testing instead of going just based on the OCT pattern. Here’s another case of chronic open-angle glaucoma. So on your right hand side are representative fundus images of this patient. Right eye up top. And left eye down below. On your left hand side, there’s a Heidelberg on the patient’s left eye and right eye. Again, similar to the last scan, if you were just going based on the color patterns, you could look at this and think his left eye was showing more of the glaucoma compared to his right eye. But on clinical exam, the cup to disc ratio is pretty close to the same. So I’ve graded the optic nerve in the right eye, 0.85. Left eye — it does look a bit worse, showing this inferior thinning. So 0.8 cup to disc ratio for the right eye. 0.9 for the left eye. But again, if they were going just based on the OCT, I could be misled, saying all green. So this is a green disease, saying nothing is wrong, when in fact both eyes are looking pretty similar, in terms of the cup to disc ratio. Functionally, when we look at the same patient, we are starting to pick up some defects in the superior nasal quadrant of his right eye. The left eye, on the other hand, showed much more thinning of the optic nerve layer. So we see much more defects here on the left eye and starting to show almost an inferior arcuate in the left eye. Maybe an inferior nasal step to early inferior arcuate, based on these images. So the next case example here is of a Drance heme. In the left eye. I know the fundus is hard to see with the Drance heme. But this is a patient I’ve been following as a glaucoma suspect, secondary to optic nerves, which were larger in the right eye, compared to the left eye. And on the follow-up exam, I saw a new Drance heme in the superior quadrant of his left eye. His eye pressures have always been in the mid-teens. OCT, which you will see on the next slide, shows borderline thinning. But it shows borderline thinning in the right eye. Remember that the left eye is what showed the Drance heme. Usually when we see a Drance heme in a glaucoma suspect, we may think of progression of glaucoma. But here the OCT is not picking that up yet. So that is why it’s important not to be misled, saying that you could be misled, saying the color pattern here is showing all green. However, on clinical exam, the Drance heme might show a possible progression in his left eye. So bringing this patient back for Humphrey visual field 24-2, just to keep a close watch on his possible progression of glaucoma in his left eye. Here’s an example. This is a great photo. Here you can very nicely see the Drance heme in the inferior quadrant of this patient’s right eye. So you have four representative images, showing the Drance heme that was seen on clinical exam. So this is a patient, again, I’ve been following as glaucoma suspect. Stable nerves in the left eye. But a new Drance heme in the right eye. So this is a great example of where the OCT has poor signal strength in the right eye. So as you can see, there’s a huge artifact here in the right eye, and there’s also quite a bit of peripapillary atrophy. So I’m not going to believe the all-green color coded pattern. I’m going to be more careful about watching this patient’s right eye, because of the new Drance hemorrhage that I saw on clinical exam. In his left eye, you can see a significant amount of peripapillary atrophy inferiorly. And in this left eye, the OCT is marking the inferior quadrant thinner, and it’s showing normal color coding patterns on the right eye. So again, normative databases can be different. Each patient is unique. So I’m not going to go based off of the OCT color patterns in this eye, but rather using good clinical judgment, and following this Drance heme hemorrhage closely in the right eye. And here’s the Humphrey 24-2 for the same patient with the Drance heme or hemorrhage in the right eye. Perhaps an enlarged blind spot on the Humphrey 24-2. In the left eye, we’re seeing maybe an early superior arcuate. Again, keeping a close watch on the clinical correlation for this patient. So how do we monitor progression on OCT scan? So there are improvements in the progression analysis for your OCT scans. So you can follow glaucoma patients progressively over many years. So this is just an example of a patient who had… Who I’ve been following since our 2017 for glaucoma. So this OCT is showing the 2017, 2018, and 2019 scans. So you have three scans for the right eye on your left side and again three scans for your left eye on your right hand side. So this is a great way to monitor for progression for glaucoma. His global thickness has been pretty stable. So global thickness is when I’m talking about this center area here. Which shows about 54 micrometers for the right eye. And about 47 micrometers for the left eye. It’s been pretty stable in this range for both eyes since 2017. And just an example of a 24-2 pattern for this patient. So you can see the marked superior arcuate in this patient’s eye. And here’s a case example of a sectoral optic nerve edema, which is on clinical exam. So again, on my clinical exam, I noted a sectoral nerve edema superiorly in the left eye. Notice how this OCT scan is not picking up on the edema. So this could possibly be a non-arteritic ischemic optic neuropathy, so I’m having this patient see a neuro-ophthalmic specialist, just to pick up on this edema superiorly I found on the left eye. But again, the OCT is not picking up on this edema. On the global scan — but if you notice sectorally, on the top portion, I’ve highlighted the superonasal and superotemporal thicknesses. So it was 104 on his baseline exam. 126 on the superotemporal. And on the follow-up scan, even though it’s highlighted green, notice a marked increase in the thickness. So from 104, the superonasal quadrant has jumped to the 125 mark, and the 126 has jumped to 169. So even though it’s highlighting green, there is in fact some edema noted superiorly in the optic nerve. So again, when you pay close attention to your sectoral thickness scans, you can pick up on this edema. And if you see superior edema, it should show up as inferior defects on the Humphrey 24-2. However, this is showing some non-specific defects with poor to low test reliability. So I’m going to watch for the superior optic nerve edema. Here’s a patient I saw recently, to identify the etiology of reduced vision. He came in with reduced vision over the past few months in his right eye. So the vision in his right eye was 21/50, and the vision in his left eye was 20/20. So the purple and the blue represent above normal limits on your OCT scans. Again, the purple and the blue shading represent above normal limits. So even though clinically I had not seen any optic nerve swelling, the OCT is showing some above-normal limit thicknesses for both eyes, most likely because his cup to disc ratio is essentially small, so I don’t think there’s a normative database to pick up on such a small cup to disc ratio, which is why I’m assuming it’s showing that thickness in the optic nerve scan. So I sent it to our neuropathologist. He reported a gradual blurring in his right vision, which started from his right temporal field, and he feels like his left eye is getting cloudy as well. As I said before, the vision in the right eye was 20/400. The vision in the left eye was 20/25. Normal eye pressures. The visual fields here show a bitemporal hemianopia, worse in the right eye, and the sensorimotor exam revealed full extraocular movement, and patient was orthophoric in all positions of gaze. So again, there is vision loss in the right eye, with evidence of bitemporal hemianopia. The MRI of the brain and orbits was ordered to rule out any chiasmal lesion. So this patient had a bitemporal hemianopia, secondary to prolactinoma, causing a supracellular compression. So he was put on cabergoline to treat the high levels of prolactin hormone in his system. You can see on his medication there’s a marked improvement in his bitemporal hemianopia. Again, this is baseline, 24-2, and a follow-up 24-2, revealing improvement in the bitemporal hemianopia, on the prolactin hormone medication. So this is a suppressor for the prolactin hormone. And notice how the OCT showed thickness instead of normal optic nerves, which our neuro-ophthalmologist saw. Here’s another case. We see some drusen, especially more prominent in the left eye, with some irregular margins here, of the optic nerve. The OCT again — I’m not going based off of the color pattern, instead of some thickness, which we should see with drusen. It is showing sort of your green pattern. Not really corresponding to that little bit of drusen here in the left eye. Here’s an example of an optic pit. Temporally. So again, OCT is not the standard of care. You may not choose to do an OCT for your optic nerve pits, but I chose to go ahead and do one, and again, it’s not picking up that optic nerve pit in the left eye. Here’s an example of idiopathic intracranial hypertension, with some swelling of the optic nerve in the left eye. More marked on the image below than the right eye up above. And here on this scan, even though the left eye appears to have more edema than the right eye, both eyes are shown to be above normal limits on the OCT. Visual field is normal. So this patient had a baseline weight of 300 pounds. She does endorse headaches daily, mostly in the evenings. She denies on being on any medications associated with weight gain. Again, normal pressures. The optic nerves had some elevation bilaterally, as I was showing you. So there is mild optic nerve edema. So this is most likely IIH, given the headaches and recent history of weight gain. And imaging was ordered to rule out any other neuro-ophthalmic conditions. This is a case on transient visual obscurations. And this patient presented with headaches, orthostatic hypotension, and transient visual obscuration. BMI of 46. History of OCP use — oral contraceptive pills — use. And there’s not a marked amount of edema in each optic nerve, but it is very suspicious for IIH, or idiopathic intracranial hypertension. Of course, we decided to do an MRI, MRV, and lumbar puncture, paying special attention to the opening pressure. We also discussed this case with neurology. So even though there was no marked edema in the optic nerve, maybe a slight edema in the optic nerve scan on your left hand side, it’s picking up above the borderline, above normal limits, for the optic nerve scans of both of the eyes. And here are her representative 24-2 visual fields, which are essentially normal in both of the eyes. The next example is an example of optic atrophy. And you can see on these fundus images that there’s temporal pallor on each of the right and left optic nerves. So this is a patient who was a heavy alcoholic user and tobacco use in the past. So this caused his optic atrophy or temporal pallor to happen. And we have obtained photos and retinal nerve fiber layer scans. And here correctly the optic nerve scan is picking up on the temporal pallor, by showing some thinning of the optic nerve… Retinal nerve fiber layer scan. So again, OCT of the retinal nerve fiber layer is a diagnostic tool, and it should be used in conjunction with your clinical exam, with other imaging, including your 24-2. So this is something that you should not use going based off of the color patterns that we discussed, including green or red disease, but rather using your clinical judgment in putting all of the clinical pieces together, in making your diagnosis and guiding your treatment and management plan. And I hope I convinced you that we have to use the OCT, but also understand advantages and limitations to this wonderful diagnostic tool. And I thank you for your attention today, and I’m open to any questions you may have.

>> Great, thank you. We have a couple of questions if you want to bring up the Q and A.

DR SRINIVAS: All right, great. So idiopathic intracranial hypertension. Let me go over that. So I had a question about idiopathic intracranial hypertension and changes in the OCT. So remember that idiopathic intracranial hypertension causes swelling of the optic nerve. So we would expect swelling to be noted on the clinical exam, and we would expect to see above normal limits, thickness on the OCT scans, so remember that IIH happens in overweight females of child bearing age, and if I go to the IIH scans…

>> We actually don’t see your slides, if you meant to share them.

DR SRINIVAS: Oh, I’m sorry. Okay. Thank you, Lawrence. So I can explain what we would see in the IIH scan. So you would expect to see either blue or purple shading for your idiopathic intracranial hypertension changes. As I said before, this condition causes swelling of the optic nerve, so you would expect to see either a blue or a purple shaded color coding on your optic nerve scan. So I had a question about explaining what a Drance hemorrhage is. So a Drance hemorrhage happens just on the optic nerve. Just on the edge of the optic nerve. It can be found in progression — glaucoma cases which have been progressing. It can also be found on the hypertension or diabetic vascular changes, if someone is hypertensive or diabetic. You can see Drance hemes then. You can see Drance hemes if a patient has Valsalva. So these are essentially some scenarios where you would see Drance hemorrhages. Again, Drance hemorrhages happen just on the edge of the optic nerve. You’ll see a little bit of bleeding just outside the blood vessels. And in my glaucoma suspects, I would watch for any thinning on the optic nerve layer scans. Again, Drance hemes, progression of glaucoma, I would expect to see thinning on my clinical exam and my optic nerve scan. Great question. In the follow-up of glaucoma suspect, what thinning of the retinal nerve fiber layer do you think is significant? And what vertical cup to disc ratio? So typically average cup to disc ratio is about 0.3. If I see any patient who is above 0.5 cup to disc ratio, I then go ahead and do an optic nerve layer scan on that patient. As far as thinning, we saw the central global thinning of the optic nerve layer scan. So I would consider anywhere from 70s or 80s for global thinning as a glaucoma suspect, and if they’re very thin, advanced glaucoma patients you can see bottoming out anywhere from 40s or 50s even, for your thinning on the optic nerve layer global scan. So that’s what I typically do. Again, if I see a cup to disc ratio of 0.5 or above, I end up doing an optic nerve fiber layer scan. And if I see 70s or 80s, I watch them for glaucoma suspect, and if I start to see that progressively get worse, then I’m more inclined to starting therapy. Great question. So it says: Basically you don’t believe everything you see on OCT? You must always marry with clinical and functional testing. That is indeed absolutely the case. So my presentation today was about how you should not use the OCT nerve fiber layer scan in isolation. You should always use your good clinical judgment. You should always use other testing. Instead of looking at the color pattern. I know we’re all busy doctors, and it’s important to just overlook a scan, saying it’s all green, so this is perfectly normal, or the OCT is all red, so this is perfectly abnormal. That is not the correct approach. It is important to do your additional testing, take a close look at your optic nerve scans, and make a clinical judgment based on everything you have, versus just using an OCT scan. I think we are very fortunate to have amazing advancements in the OCT, but it should be used with other components, instead of relying just on OCT alone. So great point, thank you. So anatomical reason for why visual field nasal steps are observed in early glaucoma and temporal are observed in advanced glaucoma. So as glaucoma is a neurodegenerative disease that causes thinning of the retinal nerve fiber layer, typically when we see a vertical elongation of the optic nerve, we pay close attention to glaucoma. We can see early nasal steps, even arcuates, in glaucoma. So it’s important to keep a watch clinically by looking at your optic nerve in great detail, to see why these defects are happening. Great question about how often do I repeat OCT on Drance heme. I would bring the patient back — this patient I saw, which I showed you a case about — I’m bringing back in about three months, to see if their Drance heme is still there or if it’s resolved. And at that point, I would do an OCT. Since we have an OCT that is readily available in our office, I actually take advantage of it. So when I’m in doubt, I do get an OCT. But I’m not going to rely on it in isolation. So I do use other testing to make a judgment on whether this patient needs to be referred out or treated appropriately. How do you differentiate high myopic disc or glaucoma disc on OCT if clinical exam and Humphrey results are similar? So high myopic disc would have some tilting of the optic nerve. In a couple of slides I showed you, there’s a slight tilt in the optic nerve. You can also see peripapillary atrophy in someone who is myopic. So what the OCT does in these cases is there may not be normative databases on many myopic patients. You would always get maybe a false positive on these scans. So I would look closely at my clinical exam in patients who are myopic. And rely on that instead. And of course, my 24-2, which is a functional test. OCT is your structural finding. 24-2 is a functional test, to see if the structure matches up with the function. So I use both of those things together to see if there’s a correlation between those two tests. Question is: Should we do an OCT scan for every optic disc edema? If there’s no question that the patient has optic disc edema, it’s very clear on clinical exam, there may not be a need to do OCT. But here in our practice, our neuro-ophthalmologist and I, since we have this readily available, we actually end up doing an OCT scan on all patients with optic disc edema, just to have a baseline on file. So in my transient visual obscuration case, the final exam given to the patient — I had this patient with transient visual obscuration follow up with neurology and headache clinic, because her chief complaint was visual obscuration and headache. I had evaluated the patient, I had referred to our neuro-ophthalmologist, who had evaluated that patient, and so we’re sending her to a headache specialist, to hopefully manage the headaches, in hopes of helping her visual symptoms subside. So patients who cannot perform visual fields, do you rely entirely on OCT in glaucoma disc suspects to decide the treatment? Again, great question. I have many elderly patients who cannot go through a visual field 24-2. In those cases, I would say I would use my clinical exam to guide my treatment. I would definitely get an OCT, but I would focus more heavily on my clinical judgment, and if the OCT kind of matches with my clinical judgment, then great. But if the OCT — if they’re myopic and it’s not showing good reliability, I would still use my clinical judgment. So examination of the optic nerve on clinical exam I think is the most important thing to manage pretty much any case, especially your glaucoma cases, to guide your therapy. So role of optic disc edema in AION? So again, this is someone who I would get an OCT on, but I would rely heavily on my clinical exam as well. So the role of macular ganglion cell layer glaucoma… Macula, over the past few years, there’s a good understanding that the macular ganglion cell damage can mean a progression of glaucoma. So this is something that we should keep a watch on. So there was a question about doing a 10-2. So again, because of the role of macular ganglion cell defects and progression of glaucoma, you could absolutely do a 10-2, to see if you were picking up early defects, in addition to the 24-2, if you would like. So I’m happy to review more of the Bruch’s membrane opening. So Bruch’s membrane opening is clinically and photographically invisible. So these are just landmarks that our current OCT technology uses. To make sure that they get repeatable OCT images, time after time. So the disc edge is determined by termination of the Bruch’s membrane. So if you go back on your slides, you can see red dots where the Bruch’s membrane opening starts. And where the Bruch’s membrane opening ends. So this is what newer technology OCT uses to kind of set their boundary layers, and make sure the OCT is repeatable in your cases. So great question about OCT in children and adolescents. So nowadays, obtaining OCT images are pretty rapid. I have used OCT on children and adolescents. And I do feel they do quite well. They have to sit still for a few minutes. But I have gotten good images on children and adolescent patients I have been watching for glaucoma or other optic nerve conditions. So I do find OCT reliable in this age group, from my clinical experience. So great question about… Does OCT help in distinguishing diabetic papillopathy from AION? No. It can’t make your diagnosis for you. You may see optic nerve edema in both of those cases, so it’s important for you to put together whether it’s diabetic papillopathy, versus non-arteritic anterior ischemic optic neuropathy. So again, OCT cannot make the diagnosis. Just because you have optic nerve swelling or optic nerve edema, you won’t know which condition it is. You’ll have to use your case history, patient’s A1C, high blood pressure, things like that, to determine whether it’s caused from diabetes or hypertension. I have a case — I’m in a state of dilemma, whether to treat or not to treat. Absolutely. I’ve had many cases where the clinical tests, my clinical exam, do not correspond. But I’m fortunate to say I work with wonderful colleagues. So if I have questions or things don’t quite add up on clinical exam, I’m fortunate to have colleagues I can discuss with, so we kind of put our heads together to determine whether we should treat or not treat. So Drance heme is in fact a splinter hemorrhage. Someone asked a question about Drance heme. Is it equal to splinter hemorrhage. And question about how do I view a Drance heme? Just based on clinical exam. I have a patient who is dilated and I would see the splinter Drance hemorrhage just on the edge of the optic disc. And I believe there’s no normative databases, as far as children. So as I showed you in the table before, I believe the normative databases include patients from 18 years of age to about 84 years of age. So you could absolutely get OCTs on children and adolescents. But you do have to keep in mind that this may not be in that normative data. So papilledema versus pseudopapilledema on OCT. How do I differentiate? Again, this is something that the OCT is not going to help you differentiate on. So you have to use other components of your exam to determine whether it is pseudopapilledema or true papilledema. So including increased intracranial hypertension, getting neurological studies to rule this out, so an OCT cannot do this for you. So there is a question about… If there is a patient with an OCT, which is thicker than the previous baseline scan, you have to use… Do a clinical exam and see why either the OCT is showing a thicker… So I had an example. If you go back to my slides, about a patient I saw with a sectoral optic disc edema… So this is a patient I’m bringing back to clinic to see if that edema has improved, or whether it’s the same. Again, things like NAION can cause sectoral disc edema. But then it would have to match up with this classic inferior or superior arc on the HVF. So I have to use other testing and not just OCT. So Lawrence, I believe that was the end of the questions?

>> So there’s about 13 left. If you don’t have time to answer them now, I can send them to you via email. So this is a good place to stop, then. All right. Thank you, everyone.

DR SRINIVAS: Thank you so much, everyone, for joining me today.

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Remaining Q&A Questions

Question Answer
Would you please talk about neurological and glaucomatoua changes? Glaucoma causes progressive optic neuropathy and the major risk factor is elevated eye pressure. There are many neuro-ophthalmic diseases which can cause primary swelling of the optic nerve with pallor after damage. In secondary optic atrophy, pallor can happen in the optic nerve without a swelling phase.
Example of use GCC. Ganglion cell complex includes the 3 innermost retinal layers. GCCC ombining ganglion cell analysis with circumpapillary retinal nerve fiber layer thickness measurement may provide a better biomarker for glaucoma management
What is the role of GCC when peripapillary RNFL is normal, in instances of disc suscpets or ocualr hypertensives? Combining ganglion cell analysis with circumpapillary retinal nerve fiber layer thickness measurement may provide a better biomarker for glaucoma management
Please publish articles on reproducibility in 6 sectors measured and how small are the sectors compared (Therefore allowing small sectoral detection of progression). Reference to the article: Danthurebandara VM, Vianna JR, Sharpe GP, Hutchison DM, Belliveau AC, Shuba LM, Nicolela MT, Chauhan BC. Diagnostic Accuracy of Glaucoma With Sector-Based and a New Total Profile-Based Analysis of Neuroretinal Rim and Retinal Nerve Fiber Layer Thickness. Invest Ophthalmol Vis Sci. 2016 Jan 1;57(1):181-7. doi: 10.1167/iovs.15-17820. PubMed PMID: 26795824.
How to diagnose glaucoma suspect if patient unreliable in doing field? Do you give TTT on basis of OCT only? If HVF unreliable, I correlate clinical exam with OCT to determine treatment.
TVD please explain. TVO – transient visual obscurations. The case presentation included a young patient of childbearing age, obesity and we had to rule out idiopathic intracranial hypertension. To complete the testing, OCT RNFL scan was obtained.
What about GCL Scan? Reference: Kim HJ, Lee S-Y, Park KH, Kim DM, Jeoung JW. Glaucoma diagnostic ability of layer-by-layer segmented ganglion cell complex by spectral domain optical coherence tomography. Invest Ophthalmol Vis Sci. 2016;57:4799–4805
What about BMO – peripapillary rnfl – ganglion cells or lamina cribrosa in OCT. BMO is an invisible on photos and clinical exam and is used as areference point. With regards to the rest of your questions, here is a great reference Reference: Kim HJ, Lee S-Y, Park KH, Kim DM, Jeoung JW. Glaucoma diagnostic ability of layer-by-layer segmented ganglion cell complex by spectral domain optical coherence tomography.
OCT’s need to overlay circum presentations taken over time with actual NFL thickness to allow improved comparison, not quadrant colors. Please comment. OCT progression analysis technology has improved greatly.
What can you tell about OCT in children and adolescents? The normative database does not include very young patients so I keep this in mind when interpreting OCT RNFL in children & adolescents.
Please send article(s) on sensitivity and specificity of disc focal or diffuse edema in IIH. Julayanont P, Karukote A, Ruthirago D, Panikkath D, Panikkath R. Idiopathic intracranial hypertension: ongoing clinical challenges and future prospects. J Pain Res. 2016;9:87–99. Published 2016 Feb 19. doi:10.2147/JPR.S60633
Do you think with OCT available there is still a point of doing GDx? Reference: Although a GDx is still a good test, it is fast becoming an outdated instrument. The development of the GDx and other instruments lead to the production of optical coherence tomography (OCT).
I read that in papilledema it is important to do rnfl and gcc scan as follow up in addition of course to vf because if
there is.
I read that in papilledema it is important to do rnfl and gcc scan as follow up in addition of course to vf because if there is any atrophy it will be shown in gcc thinning first do u agree? Thanks. Yes I agree with this and will get a baseline scan in a patient with papilledema.
I had a case of an 8 year old boy with bilateral high cup to disc ratio , normal iop , did the oct rnfl (visual field was not compliant) it showed white areas in all the sectors. For each value, the number appears on a white background if it is within 5% of the best values, and on a green background for 90% of cases. Yellow indicates an anomaly at the < 5% level, and red indicates an anomaly at the < 1% level


Last Updated: October 31, 2022

4 thoughts on “Lecture: Anatomy of the OCT Retinal Nerve Fiber Layer Scan”

  1. Thanks for highlighting some points in OCT optic disc imaging and showing that we still need some more time for this machines to evolve and really help us in a practical way. This OCT give us some more clues to dignosis but it is still needs to be completed by classical visual fields screening.

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