Navigating Monocular Visual Loss: Distinguishing Ophthalmologic and Neurologic Causes

Diagnosing and differentiating among the many possible localizations and causes of vision loss is an essential skill for neurologists. The approach to vision loss should include a history and examination geared toward localization, followed by a differential diagnosis based on the likely location of the pathophysiologic process. 

In this episode, Aaron Berkowitz, MD, PhD, FAAN speaks with Nancy J. Newman, MD, FAAN, author of the article “Approach to Vision Loss” in the Continuum® April 2025 Neuro-ophthalmology issue. 

Dr. Berkowitz is a Continuum® Audio interviewer and a professor of neurology at the University of California San Francisco in the Department of Neurology and a neurohospitalist, general neurologist, and clinician educator at the San Francisco VA Medical Center at the San Francisco General Hospital in San Francisco, California. 

Dr. Newman is a professor of ophthalmology and neurology at the Emory University School of Medicine in Atlanta, Georgia. 

Additional Resources

Read the article: Approach to Vision Loss

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Full episode transcript available here

Dr Jones: This is Dr Lyell Jones, Editor-in-Chief of Continuum. Thank you for listening to Continuum Audio. Be sure to visit the links in the episode notes for information about earning CME subscribing to the journal, and exclusive access to interviews not featured on the podcast.

Dr Berkowitz: This is Dr Aaron Berkowitz, and today I'm interviewing Dr Nancy Newman about her article on the approach to visual loss, which she wrote with Dr Valerie Biousse. This article appears in the April 2025 Continuum issue on neuro-ophthalmology. Welcome to the podcast, Dr Newman. I know you need no introduction, but if you wouldn't mind introducing yourself to our listeners.

Dr Newman: Sure. My name's Nancy Newman. I am a neurologist and neuro-ophthalmologist, professor of ophthalmology and neurology at the Emory University School of Medicine in Atlanta, Georgia.

Dr Berkowitz: You and your colleague Dr Biousse have written a comprehensive and practical article on the approach to visual loss here. It's fantastic to have this article by two of the world's leading experts and best-known teachers in neuro-ophthalmology. And so, readers of this article will find extremely helpful flow charts, tables and very nuanced clinical discussion about how to make a bedside diagnosis of the cause of visual loss based on the history exam and ancillary testing. We'll talk today about that important topic, and excited to learn from you and for our listeners to learn from you. To begin, let's start broad. Let's say you have a patient presenting with visual loss. What's your framework for the approach to this common chief concern that has such a broad differential diagnosis of localizations and of causes? Where do you start when you hear of visual loss? How do you think about this chief concern?

Dr Newman: Well, it's very fun because this is the heart of being a neurologist, isn't it? Nowhere in the nervous system is localization as important as the complaint of vision loss. And so, the key, as any neurologist knows, is to first of all figure out where the problem is. And then you can figure out what it is based on the where, because that will limit the number of possibilities. So, the visual system is quite beautiful in that regard because you really can exquisitely localize based on figuring out where things are. And that starts with the history and then goes to the exam, in particular the first localization. So, you can whittle it down to the more power-for-your-buck question is, is the vision lost in one eye or in two eyes? Because if the vision loss clearly, whether it's transient or persistent, is in only one eye, then you only have to think about the eyeball and the optic nerve on that side. So, think about that. Why would you ever get a brain MRI? I know I'm jumping ahead here, but this is the importance of localization. Because what you really want to know, once you know for sure it's in one eye, is, is it an eyeball problem---which could be anything from the cornea, the lens, the vitreous, the retina---or is it an optic nerve problem? The only caveat is that every once in a while, although we trust our patients, a patient may insist that a homonymous hemianopia, especially when it's transient, is only in the eye with the temporal defect. So that's the only caveat. But if it's in only one eye, it has to be in that side eyeball or optic nerve. And if it's in two eyes, it's either in both eyeballs or optic nerves, or it's chiasmal or retrochiasmal. So that's the initial approach and everything about the history should first be guided by that. Then you can move on to the more nuanced questions that help you with the whats. Once you have your where, you can then figure out what the whats are that fit that particular where.

Dr Berkowitz: Fantastic. And your article with Dr Biousse has this very helpful framework, which you alluded to there, that first we figure out, is it monocular or binocular? And we figure out if it's a transient or fixed or permanent deficit. So, you have transient monocular, transient binocular, fixed monocular, fixed binocular. And I encourage our listeners to seek out this article where you have a table for each of those, a flow chart for each of those, that are definitely things people want to have printed out and at their desk or on their phone to use at the bedside. Very helpful. So, we won't be able to go through all of those different clinical presentations in this interview, but let's focus on monocular visual loss. As you just mentioned, this can be an eye problem or an optic nerve problem. So, this could be an ophthalmologic problem or a neurologic problem, right? And sometimes this can be hard to distinguish. So, you mentioned the importance of the history. When you hear a monocular visual loss- and with the caveat, I said you're convinced that this is a monocular visual problem and not a visual field defect that may appear. So, the patient has a monocular deficit, how do you approach the history at trying to get at whether this is an eye problem or an optic nerve problem and what the cause may be?

Dr Newman: Absolutely. So, the history at that point tends not to be as helpful as the examination. My mentor used to say if you haven't figured out the answer to the problem after your history, you're in trouble, because that 90% of it is history and 10% is the exam. In the visual system, the exam actually may have even more importance than anywhere else in the neurologic examination. And we need as neurologists to not have too much hubris in this. Because there's a whole specialty on the eyeball. And the ophthalmologists, although a lot of their training is surgical training that that we don't need to have, they also have a lot of expertise in recognizing when it's not a neurologic problem, when it's not an optic neuropathy. And they have all sorts of toys and equipment that can very much help them with that. And as neurologists, we tend not to be as versed in what those toys are and how to use them. So, we have to do what we can do.

Your directive thalmoscope, I wouldn't throw it in the garbage, because it's actually helpful to look at the eyeball itself, not just the back of the eye, the optic nerve and retina. And we'll come back to that, but we have in our armamentarium things we can do as neurologists without having an eye doctor's office. These include things like visual acuity and color vision, confrontation, visual fields. Although again, you have to be very humble. Sometimes you're lucky; 30% of the time it's going to show you a defect. It has to be pretty big to pick it up on confrontation fields. And then as we say, looking at the fundus. And you probably know that myself and Dr Biousse have been on somewhat of a crusade to allow the emperor's new clothes to be recognized, which is- most neurologists aren't very comfortable using the direct ophthalmoscope and aren't so comfortable, even if they can use it, seeing what they need to see. It's hard. It's really, really hard. And it's particularly hard without pupillary dilation. And technology has allowed us now with non-mydriatic cameras, cameras that are incredible, even through a small pupil can take magnificent pictures of the back of the eye. And who wouldn't rather have that? And as their cost and availability- the cost goes down and their availability goes up. These cameras should be part of every neurology office and every emergency department. And this isn't futuristic. This is happening already and will continue to happen. But over the next five years or so… well, we're transitioning into that.

I think knowing what you can do with the direct ophthalmoscope is important. First of all, if you dial in plus lenses, you can't be an ophthalmologist, but you can see media opacities. If you can't see into the back of the eye, that may be the reason the patient can't see out. And then just seeing if someone has central vision loss in one eye, it's got to be localized either to the media in the axis of vision; or it's in the macula, the very center of the retina; or it's in the optic nerve. So, if you get good at looking at the optic nerve and then try to curb your excitement when you saw it and actually move a little temporally and take a look at the macula, you're looking at the two areas. Again, a lot of ophthalmologists these days don't do much looking with the naked eye. They actually do photography, and they do what's called OCT, optical coherence tomography, which especially for maculopathies, problems in the macula are showing us the pathology so beautifully, things that used to be considered subtle like central serous retinopathy and other macula. So, I think having a real healthy respect for what an eye care provider can do for you to help screen away the ophthalmic causes, it's very, very important to have a patient complaining of central vision loss, even if they have a diagnosis like multiple sclerosis, you expect that they might have an optic neuritis… they can have retinal detachments and other things also. And so, I think every one of these patients should be seen by an eye care provider as well.

Dr Berkowitz: Thank you for that overview. And I feel certainly as guilty as charged here as one of many neurologists, I imagine, who wish we were much better and more comfortable with fundoscopy and being confident on what we see. But as you said, it's hard with the direct ophthalmoscope and a non-dilated exam. And it's great that, as you said, these fundus photography techniques and tools are becoming more widely available so that we can get a good look at the fundus. And then we're going to have to learn a lot more about how to interpret those images, right? If we haven't been so confident in our ability to see the fundus and analyze some of the subtle abnormalities that you and your colleagues and our ophthalmology colleagues are more familiar with. So, I appreciate you acknowledging that. And I'm glad to hear that coming down the pipeline, there are going to be some tools to help us there. So, you mentioned some of the things you do at the bedside to try to distinguish between eye and optic nerve. Could you go into those in a little bit more detail here? How do you check the visual fields? For example, some people count fingers, some people wiggle fingers, see when the patient can see. How should we be checking visual fields? And what are some of the other bedside tasks you use to decide this is probably going to end up being in the optic nerve or this seems more like an eye?

Dr Newman: Of course. Again, central visual acuity is very important. If somebody is older than fifty, they clearly will need some form of reading glasses. So, keeping a set of plus three glasses from cheapo drugstore in your pocket is very helpful. Have them put on their glasses and have them read an ear card. It's one of the few things you can actually measure and examine. And so that's important. The strongest reflex in the body and I can have it duke it out with the peripheral neurologists if they want to, it's not the knee jerk, it's looking for a relative afferent pupillary defect. Extremely important for neurologists to feel comfortable with that. Remember, you cut an optic nerve, you're not going to have anisocoria. It's not going to cause a big pupil. The pupils are always equal because this is not an efferent problem, it's an afferent problem, an input problem.

So basically, if the eye has been injured in the optic nerve and it can't get that information about light back into the brain, well, the endoresfol nuclei, both of them are going to reset at a bigger size. And then when you swing over and shine that light in the good optic nerve, the good eye, then the brain gets all this light and both endoresfol nuclei equally set those pupils back at a smaller size. So that's the test for the relative afferent pupillary defect. When you swing back and forth. Of course, when the light falls on the eye, that's not transmitting light as well to the brain, you're going to see the pupil dilate up. But it's not that that pupil is dilating alone. They both are getting bigger. It's an extremely powerful reflex for a unilateral or asymmetric bilateral optic neuropathy. But what you have to remember, extremely important, is, where does our optic nerve come from? Well, it comes from the retinal ganglion cells. It's the axons of the retinal ganglion cells, which is in the inner retina. And therefore inner retinal disorders such as central retinal artery occlusion, ophthalmic artery occlusion, branch retinal artery occlusion, they will also give a relative afferent pupillary defect because you're affecting the source. And this is extremely important. A retinal detachment will give a relative afferent pupillary defect. So, you can't just assume that it's optic nerve. Luckily for us, those things that also give a relative afferent pupillary defect from a retinal problem cause really bad-looking retinal disease. And you should be able to see it with your direct ophthalmoscope. And if you can't, you definitely will be able to see it with a picture, a photograph, or having an ophthalmologist or optometrist take a look for you.

That's really the bedside. You mentioned confrontation visual fields. I still do them, but I am very, very aware that they are not very sensitive. And I have an extremely low threshold to- again, I have something in my office. But if I were a general neurologist, to partner with an eye care specialist who has an automated visual field perimeter in their office because it is much more likely to pick up a deficit. Confrontation fields. Just remember, one eye at a time. Never two eyes at the same time. They overlap with each other. You're going to miss something if you do two eyes open, so one eye at a time. You check their field against your field, so you better be sure your field in that eye is normal. You probably ought to have an automated perimetry test yourself at some point during your career if you're doing that. And remember that the central thirty degrees is subserved by 90% of our fibers neurologically, so really just testing in the four quadrants around fixation within the central 30% is sufficient. You can present fingers, you don't have to wiggle in the periphery unless you want to pick up a retinal detachment.

Dr Berkowitz: You mentioned perimetry. You've also mentioned ocular coherence tomography, OCT, other tests. Sometimes we think about it in these cases, is MRI one of the orbits? When do you decide to pursue one or more of those tests based on your history and exam?

Dr Newman: So again, it sort of depends on what's available to you, right? Most neurologists don't have a perimeter and don't have an OCT machine. I think if you're worried that you have an optic neuropathy, since we're just speaking about monocular vision loss at this point, again, these are tests that you should get at an office of an eye care specialist if you can. OCT is very helpful specifically in investigating for a macular cause of central vision loss as opposed to an optic nerve cause. It's very, very good at picking up macular problems that would be bad enough to cause a vision problem. In addition, it can give you a look at the thickness of the axons that are about to become the optic nerve. We call it the peripapillary retinal nerve fiber layer. And it actually can look at the thickness of the layer of the retinal ganglion cells without any axons on them in that central area because the axons, the nerve fiber layer, bends away from central vision. So, we can see the best we can see. And remember these are anatomical measurements. So, they will lag, for the ganglion cell layer, three to four weeks behind an injury, and for the retinal nerve fiber, layer usually about six weeks behind an entry. Whereas the functional measurements, such as visual acuity, color vision, visual fields, will be immediate on an injury. So, it's that combination of function and anatomy examination that makes you all-powerful. You're very much helped by the two together and understanding where one will be more helpful than the other.

Dr Berkowitz: Let's say we've gotten to the optic nerve as our localization. Many people jump to the assumption it's the optic nerve, it's optic neuritis, because maybe that's the most common diagnosis we learn in medical school. And of course, we have to sometimes, when we're teaching our students or trainees,  say, well, actually, not all optic nerve disease, optic neuritis, we have to remember there's a broader bucket of optic neuropathy. And I remember, probably I didn't hear that term until residency and thought, oh, that's right. I learned optic neuritis. Didn't really learn any of the other causes of optic nerve pathology in medical school. And so, you sort of assume that's the only one. And so you realize, no, optic neuropathy has a differential diagnosis beyond optic neuritis. Neuritis is a common cause. So how do you think about the “what” once you've localized to the optic nerve, how do you think about that? Figure out what the cause of the optic neuropathy is?

Dr Newman: Absolutely. And we've been trying to convince neuro-radiologists when they see evidence of optic nerve T2 hyperintensity, that just means damage to the optic nerve from any cause. It's just old damage, and they should not put in their read consistent with optic neuritis. But that's a pet peeve. Anyway, yes, the piece of tissue called the optic nerve can be affected by any category of pathophysiology of disease. And I always suggest that you run your categories in your head so you don't leave one out. Some are going to be more common to be bilateral involvement like toxic or metabolic causes. Others will be more likely unilateral. And so, you just run those guys. So, in my mind, my categories always are compressive-slash-infiltrative, which can be neoplastic or non-neoplastic. For example, an ophthalmic artery aneurysm pressing on an optic nerve, or a thyroid, an enlarged thyroid eye muscle pressing on the optic nerve. So, I have compressive infiltrative, which could be neoplastic or not neoplastic. I have inflammatory, which can be infectious. Some of the ones that can involve the optic nerve are syphilis, cat scratch disease. Or noninfectious, and these are usually your autoimmune such as idiopathic optic neuritis associated with multiple sclerosis, or MOG, or NMO, or even sarcoidosis and inflammation.

Next category for me would be vascular, and you can have arterial versus venous in the optic nerve, probably all arterial if we're talking about causes of optic neuropathy. Or you could have arteritic versus nonarteritic with the vascular, the arteritic usually being giant cell arteritis. And the way the optic nerve circulation is, you can have an anterior ischemic optic neuropathy or a posterior ischemic optic neuropathy defined by the presence of disc edema suggesting it’s anterior, the front of the optic nerve, or not, suggesting that it's retrobulbar or posterior optic nerve.

So what category am I- we mentioned toxic, metabolic nutritional. And there are many causes in those categories of optic neuropathy, usually bilateral. You can have degenerative or inherited. And there are causes of inherited optic neuropathies such as Leber hereditary optic neuropathy and dominant optic atrophy. And then there's a group I call the mechanical optic neuropathies. The obvious one is traumatic, and that can happen in any piece of tissue. And then the other two relate to the particular anatomy of the eyeball and the optic nerve, and the fact that the optic nerve is a card-carrying member of the central nervous system. So, it's not really a nerve by the way, it's a tract. Think about it. Anyway, white matter tract. It is covered by the same fluid and meninges that the rest of the brain. So, what mechanically can happen? Well, you could have an elevated intraocular pressure where that nerve inserts. That's called glaucoma, and that would affect the front of the optic nerve. Or you can have elevated intracranial pressure. And if that's transmitted along the optic nerve, it can make the front of the optic nerve swell. And we call that specifically papilledema, optic disk edema due specifically to raised intracranial pressure. We actually even can have low intraocular pressure cause something called hypotony, and that can actually even give an optic neuropathy the swelling of the optic nerve. So, these are the mechanical. And if you were to just take that list and use it for any piece of tissue anywhere, like the heart or the kidney, you can come up with your own mechanical categories for those, like pericarditis or something like that. And then all those other categories would fit. But of course, the specific causes within that pathophysiology are going to be different based on the piece of tissue that you have. In this case, the optic nerve.

Dr Berkowitz: In our final moments here, we've talked a lot about the approach to monocular visual loss. I think most neurologists, once we find a visual field defect, we breathe a sigh of relief that we know we're in our home territory here, somewhere in the visual task base that we've studied very well. I'm not trying to distinguish ocular causes amongst themselves or ocular from optic nerve, which can be very challenging at the bedside. But one topic you cover in your article, which I realized I don't really have a great approach to, is transient binocular visual loss. Briefly here, since we're running out of time, what's your approach to transient binocular visual loss? 

Dr Newman: We assume with transient binocular vision loss that we are not dealing with a different experience in each eye, because if you have a different experience in each eye, then you're dealing with bilateral eyeball or optic nerve. But if you're having the same experience in the two eyes, it's equal in the two eyes, then you're located. You're located, usually, retro chiasmally, or even chiasm if you have pituitary apoplexy or something. So, all of these things require imaging, and I want to take one minute to talk about that. If you are sure that you have monocular vision loss, please don't get a brain MRI without contrast. It's really useless. Get a orbital MRI with contrast and fat suppression techniques if you really want to look at the optic nerve. Now, let's say you you're convinced that this is chiasmal or retrochiasmal. Well then, we all know we want to get a brain MRI---again, with and without contrast---to look specifically where we could see something. And so, if it's persistent and you have a homonymous hemianopia, it's easy, you know where to look. Be careful though, optic track can fool you. It's such a small little piece, you may miss it on the MRI, especially in someone with MS. So really look hard. There's very few things that are homonymous hemianopias MRI negative. It may just be that you didn't look carefully enough. And as far as the transient binocular vision loss, again, remember, even if it's persistent, it has to be equal vision in the two eyes. If there's inequality, then you have a superimposed anterior visual pathway problem, meaning in front of the chiasm on the side that's worse.

The most common cause of transient binocular vision loss would be a form of migraine. The visual aura of migraine usually is a positive phenomenon, but sometimes you can have a homonymous hemianopic persistent defect that then ebbs and flows and goes away. Usually there's buildup, lasts maybe fifteen minutes and then it goes away, not always followed by a headache. Other things to think of would be transient ischemic attack in the vertebra Basler system, either a homonymous hemianopia or cerebral blindness, what we call cortical blindness. It can be any degree of vision loss, complete or any degree, as long as the two eyes are equal. That should last only minutes. It should be maximum at onset. There should be no buildup the way migraine has it. And it should be gone within less than ten minutes, typically. After fifteen, that's really pushing it. And then you could have seizures. Seizures can actually be the aura of a seizure, the actual ictal phenomenon of a seizure, or a postictal, almost like a todd's paralysis after a seizure. These events are typically bright colors and flashing, and they last usually seconds or just a couple of minutes at most. So, you can probably differentiate them. And then there are the more- less common but more interesting things like hyperglycemia, non-ketonic hyperglycemia can give you transient vision loss from cerebral origin, and other less common things like that.

Dr Berkowitz: Fantastic. Although we've talked about many pearls of clinical wisdom here with you today, Dr Newman, this is only a fraction of what we can find in your article with Dr Biousse. We focused here on monocular visual loss and a little bit at the end here on binocular visual loss, transient binocular visual loss. But thank you very much for your article, and thank you very much for taking the time to speak with us today. Again, today I've been interviewing Dr Nancy Newman about her article with Dr Valerie Biousse on the approach to visual loss, which appears in the most recent issue of Continuum on neuro-ophthalmology. Be sure to check out Continuum audio episodes from this and other issues. Thank you so much to our listeners for joining us today.

Dr Monteith: This is Dr Teshamae Monteith, Associate Editor of Continuum Audio. If you've enjoyed this episode, you'll love the journal, which is full of in-depth and clinically relevant information important for neurology practitioners. Use this link in the episode notes to learn more and subscribe. AAN members, you can get CME for listening to this interview by completing the evaluation at continpub.com/audioCME. Thank you for listening to Continuum Audio.