Navigating Antibody Testing in Autoimmune Movement Disorders

Autoimmune cerebellar ataxia and other autoimmune movement disorders encompass a broad spectrum of different clinical syndromes, antibodies, and immunopathophysiologic mechanisms. Given the overlap between phenotypes and antibodies, panel testing in serum and CSF is recommended.

In this episode, Gordon Smith, MD, FAAN, speaks with Bettina Balint, MD, author of the article "Autoimmune Movement Disorders," in the Continuum August 2024 Autoimmune Neurology issue.

Dr. Smith is a Continuum® Audio interviewer and professor and chair of neurology at Kenneth and Dianne Wright Distinguished Chair in Clinical and Translational Research at Virginia Commonwealth University in Richmond, Virginia.

Dr. Balint is an assistant professor for clinical research on complex movement disorders and Parkinson's diseases, a consultant neurologist, the head of the Department of Movement Disorders, and co-lead for the Centre for Movement Disorders and Functional Neurosurgery in the Department of Neurology at the University Hospital Zurich in Zurich, Switzerland.

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Host: @gordonsmithMD

Full episode transcript available here

Dr Jones: This is Dr Lyell Jones, Editor-in-Chief of Continuum, the premier topic-based neurology clinical review and CME journal from the American Academy of Neurology. Thank you for joining us on Continuum Audio, which features conversations with Continuum's guest editors and authors who are the leading experts in their fields. Subscribers to the Continuum journal can read the full article or listen to verbatim recordings of the article and have access to exclusive interviews not featured on the podcast. Please visit the link in the episode notes for more information on the article, subscribing to the journal, and how to get CME.

Dr Smith: This is Dr Gordon Smith. Today, I'm interviewing Dr Bettina Balint about her article on ataxia and other autoimmune movement disorders, which appears in the August 2024 Continuum issue on autoimmune neurology, which is a highly anticipated and exciting issue. Dr Balint, welcome to the podcast, and, perhaps, you can just introduce yourself to our audience and tell us a little bit about your practice and how you became interested in this topic.

Dr Balint: Thank you, Gordon, for having me. I am an assistant professor for clinical research in complex movement disorders and Parkinson's disease at the University of Zurich and the Head of the Movement Disorders Department at the University Hospital in Zurich. So while I'm originally German (from Heidelberg), I have now been to Switzerland since end of 2021.

Dr Smith: So, you know, how many movement disorder chiefs have a focus on autoimmune movement disorders? I found that really interesting. Most of the movement disorder folks I interact with, their primary interest is in neurodegeneration.

Dr Balint: Very good question. Even so, I never asked myself that question, really, but I think I'm the only one with this designated focus as such. Many people come from the neurology angle - most of them. Even so, movement-disorder people really welcome this field and are interested, but I think somebody who has dedicated their interest and time to it? I think I can't actually think of many other people.

Dr Smith: Yeah, I think it's really cool, and, of course, autoimmune neurology is the flavor of the day these days, right? I mean, I remember when I was at the University of Utah, we were recruiting Stacy Clardy (who I think many of our listeners will know). I remember thinking, you know, she's never going to be busy. How many of these autoimmune problems are there, really? And she was, like, deluged when she came. These are really common problems. I guess that was one question I had for you. You know, we think of these as rare disorders, and when we look at the article, you have these tables of these antibodies, and a lot of them are pretty uncommon – but, cumulatively, how common are autoimmune movement disorders?

Dr Balint: It's a very difficult question, because we don't have good epidemiological data. And if you look at series, I mean, most papers addressing this issue come actually from the ataxia field. And then, depending on where you look at, you might find varying numbers, and they might be also influenced by the fact that they come from ataxia centers with own certain biases. Even so, it's very close to my heart, but, I also still think it's overall very rare. So, in my practice, I see all sorts of movement disorders, and overall, they're still quite rare, but the point is that they are treatable and have important management implications, so you want to be sure not to miss any of them.

Dr Smith: Well, maybe we can go to that next. Part of the challenge here, of course, is there's just so many of these different syndromes and antibodies. Are there pearls that you can provide our listeners that would help them guide when they should be thinking about these disorders when they confront a patient with a particular phenotype? Like ataxia, for instance - you know, there are certain aspects of the clinical scenario that should trigger, "Wow, this might be an autoimmune problem".

Dr Balint: So, in general, I would say there are certain scenarios where you would want to think of an autoimmune etiology in your differential. One is a very characteristic phenotype. So, speaking broadly in terms of movement disorders, stiff-person spectrum disorders have a very characteristic phenotype which you need to recognize, and then you will be able to see it when a patient enters. Important phenotypes to know which are very characteristic are faciobrachial dystonic seizures, for example, with anti-GA1 antibodies, or pseudofinalistic movements in non-REM sleep is IgLON5 antibodies, leg myoclonus is CASPR2 antibodies. I don't want to necessarily enumerate all the scenarios. The point here is there are some characteristic phenotypes where you would think of autoimmune neurology. Another scenario where you would think of autoimmune, for example, the context of late-onset paroxysmal movement disorders. So, classically, when we think of paroxysmal dyskinesia, we think of a group of genetic disorders, but if somebody develops a paroxysmal movement disorder later in life in adulthood, then you would think of autoimmune neurology, and this applies also in the context of episodic ataxias. Another red flag might be a propensity to autoimmunity. For example, somebody with type one diabetes and vitiligo coming in for cerebellar ataxia, of course, you would think of anti-GAD ataxia. And, similarly, if somebody has recently been diagnosed with a cancer and develops a rapidly disabling syndrome, of course, then you would think of a paraneoplastic autoimmune disorder. And with autoimmune syndrome, there are some symptoms which are also like tell-tale signs. So, for example, somebody with a stiff-person spectrum disorder, an ataxia with long-lasting diarrhea over months, losing weight - investigations haven't found anything, then you would think of DPPX antibodies or celiac disease. Or, if you have, like, a neuropathic pain which is otherwise not explained, then you might think of CASPR2 antibodies in somebody with a cerebellar ataxia. So, there are some features of some antibodies. (Again, I will not now list all of them which might point you to a diagnosis.) Then, of course, another scenario which is important, I think, is if you have a hemisyndrome without a structural lesion on imaging. Classically, neurologists are trained to think of a hemisyndrome - we look for a lesion on the contralateral side. But if you have, like, for example, a hemichorea without a lesion or a hemiataxia without a lesion, one should also think of an autoimmune disorder with antibodies. And then, more generally, of course, if you have changes on brain MRI or information on CSF, of course, if the clinical cause is more rapidly progressive - and last, but not least, if somebody does not really fit into our categories of the degenerative symptoms or metabolic syndromes or functionality disorders, then, of course, one should just take a step back and think, could it be something autoimmune? Having said that, if I may, I just want to say that, I mentioned that rapid disease course, and on the other hand, it's important to stress that a slowly progressive disease cause does not exclude an autoimmune etiology.

Dr Smith: So, that was a great summary. Thank you. I don't know if you're familiar with the term "Aunt Minnie" (something I learned in medical school and radiology). There are certain findings that are "Aunt Minnie", you know what "Aunt Minnie" looks like, and if you see these particular findings, you should really think about a specific disease - and I think you gave a lot of pearls in that answer, so I appreciate that. This may seem like a bit of a random question, but it's interesting that there are some of these phenotypes that do replicate genetic phenotypes, and you used episodic ataxia, which, in a younger individual, we think of a spectrum of various genetic disorders. Is that random, or are there instances where the underlying mutation in a genetic disorder actually serves as a target for autoimmunity in a later-onset autoimmune problem? Not that the mutation causes autoimmunity, but are there shared targets - in one disease it's the mutation, and another, there's an antibody that binds to the protein, for instance?

Dr Balint: That's an excellent topic, and even though it's not addressed in the Continuum article, I actually covered this in an article in Brain from 2018, where we also discuss parallels (immunogenetic parallels) with targets seen in genetic disease or in autoimmune disease, and there are actually some examples for cerebellar ataxia, and some of the targets are, indeed, the same for the antibodies and mutation. And some targets are a little bit more difficult, because for those, the antibodies would probably not be pathogenic, but it's more like an autoimmune overall target but it's T-cell mediated. But, for example, water-gated, um, calcium channels - we have antibodies and we have mutations. Or, another example would be glycine receptor antibodies give you acquired hyperekplexia, whereas the mutations give you hereditary hyperekplexia. So, there is, indeed, a bit of an overlap between autoimmune and genetic disorders, but often, also, like, the age at onset (because that might be the next question, the age at onset), and maybe family history and associated features, should help to distinguish the two. I think more from the pathophysiological point interesting, rather than clinically too confusing.

Dr Smith: Wow, that's really cool. So, another question I have is regarding antibody panels, right? And so, I think, oftentimes (at least around here), folks confronting an unusual phenotype will send the Mayo panel - they'll send autoimmune encephalitis or a paraneoplastic panel – and, you know, I think one of the challenges I have thinking about the spectrum of phenotypes that you described, I mean, if you recognize "Aunt Minnie", then you know where to go, but it seems to me that there's a lot of these that maybe folks don't recognize "Aunt Minnie". What is the diagnostic utility and pearls and pitfalls of ordering these panels when you're not really certain? In other words, is there a risk of a false positive if the pretest probability is low? So, I guess that's a long question, but do you have guidance about when we should and maybe when we should not be ordering these panels? So, you know, undifferentiated ataxia that's chronically progressive - should we be sending a panel or not? Patients who are later-onset acute, maybe so. So, what's the guidance on when to order the panel?

Dr Balint: It's a tricky topic also for many people in our practice, because, of course, as you said, we don't want to miss something, but, indeed, with any test which you order with a low pretest probability and which is not quite appropriate, you might have false positives, and that might cause much additional trouble in security, or maybe unnecessary and invasive immunotherapy with adverse effects – so, it's really important to think well about antibody testing. And, generally speaking, like always in medicine, we shouldn't order random tests, and antibody panels and neuronal antibodies are not designed as a screening test, so you need to have a phenotype and a reasonable suspicion - and clinical acumen is really key, and that's why also the article is so much focused on the phenotype. It's clearly not that any movement disorder patient who enters the outpatient clinic should get a blood test for antibodies that will likely cause harm, and it has been shown that these antibodies can be falsely positive, both in other diseases but also in healthy controls, and much depends also on which tests you use (but, let's not go into too much detail over here) - so, generally speaking, I would say if you have a suspicion of an autoimmune disease clinically (I mentioned some scenarios where you would think of an autoimmune disorder). And then, ataxias are, of course, a bit tricky, because often, we don't have too many other handles there, and there's still also a significant number of acquired late-onset ataxia where we don't know what the cause is. I think in the ataxia scenario, if I don't have a good answer or explanation, I would order antibody tests a bit more freely - I mean, if you do it properly, you do the serum and the CSF, and that also increases your sensitivity but also the specificities, so I wouldn't then just do the serum, but then go for serum and CSF. In other movement disorders, it depends also a little bit on the phenotype. So, somebody with a phenotype fitting well with Parkinson's disease, I wouldn't do any testing. Somebody with clear PSP phenotype without any red flags or not-fitting features, it is very unlikely to have an antibody finding, and this has been shown also in cohorts. But, if you have something which is not fitting in the phenotypes - for example, you have somebody where you think it might be a PSP phenotype with predominantly axial Parkinsonism falls, but you notice that the oculomotor disturbance is not a vertical gaze palsy, but a horizontal gaze palsy – so, it's not really fitting phenotype as you know it. That's a scenario where would probably think of antibody testing. Then, if you do the testing theorem - and CSF, in general, is gold standard - there are some antibodies where theorem is good enough (like, for example, with aquaporin-4 antibodies), but the reason why we do serum and CSF, as I mentioned, is the increased sensitivity and specificity. And nowadays, in the antibody world, we have something similar to the genetics - we have the variant of unknown significance and in the neurology world, we coin the term "antibody of unknown significance" to also give a name to the problem that, sometimes, we get a test result and it is difficult to interpret. Another handle over there would be to try to confirm the test result in another test method. So for example, if you have a cell-based assay with an antibody finding, you would like to confirm that on immunohistochemistry - the staining pattern is in keeping with that.

Dr Smith: So, Bettina, that was a really great and comprehensive answer to the question with a lot of pearls packed into it, and I think the idea that, you know, oftentimes, it's helpful to do both serum and CSF testing is important - also looking for staining to further confirm the diagnosis. And, I think one of the things that I was struck by in your response was the example of a PSP patient who instead of vertical gaze palsy had horizontal gaze palsy as a red flag, and I think a lot of our listeners are probably familiar with the idea that maybe hyperkinetic movement disorders might be autoimmune, or certainly rapidly progressive ataxia, but at least I don't think of Parkinsonian syndromes as often. I know there are some that we need to consider. Maybe you can give us some pearls about when we should consider antibody testing in a patient who has a Parkinson syndrome?

Dr Balint: So, I will not cover now the paraneoplastic Parkinsonian syndromes (because they typically develop as rapidly that you would anyway think about it, hopefully), but go more into those conditions which might mimic degenerative disease - and one of the most interesting antibodies in this regard is IgLON5, and you will be aware that it has been discovered in 2014 in patients who shared a characteristic sleep movement disorder (non-REM parasomnia). The spectrum has broadened a lot, and one possible manifestation is that it could come into the differential of Parkinsonian syndromes - so, for example, if you have axial Parkinsonism and a gaze palsy, you are in a PSP phenotype, but the red flag would be maybe if the eye movement disorders are not really fitting with the PSP phenotype. Also, in PSP patients, we don't expect parasomnias at night. If the bed partner is, for example, complaining that the patient is moving in his sleep and doing movements, then this would be a red flag, and in this context, you would think of IgLON5. IgLON5 could also give you Parkinsonism and cerebellar ataxia, and they might have dysautonomia, and, of course, with a sleep movement disorder, you are now in the ballpark of MSA phenotypes; however, if there are additional features (like, for example, fasciculations) which you don't expect in MSA, that would be, again, the red flag. So, typically, even in those differentials, there are some red flags on handles which would point you to the diagnosis - it is not that it completely mimics the phenotype of our default degenerative disease, but, sometimes, you need to hunt a little bit for those handles.

Dr Smith: So, Bettina, that's really interesting. I wanted to ask you about IgLON5, and in particular, the sleep phenotype, but, you know, I wonder whether there's a risk of just confusing this with REM sleep behavior disorder and a chronic Parkinsonian syndrome - what's the time course of this, and any other wisdom in terms of how to differentiate it from, you know, a more common neurodegenerative problem?

Dr Balint: So, the spectrum of sleep disorders in IgLON5 is actually a bit broad. The characteristic thing is the non-REM sleep parasomnia with the finalistic fine movements, but classic REM sleep behavior disorder has also been reported in these patients. And one of the tricky things is IgLON5 is a slowly progressive disease (some patients had symptoms for a decade prior to diagnosis), so it's really an important differential of autoimmune disease - but as mentioned, the features not fitting in, and they are typically also the cardinal features. So, gaze palsies are very frequent, ptosis, bulbar symptoms, vocal cord palsy, sleep movement disorders which might not fit to the original phenotype, and breathing problems (for example) so severe that they require a tracheostoma – so, these are some red flags which would alert you to this diagnosis of anti-IgLON5 disease.

Dr Smith: I'm curious, Bettina, how do you keep up on all of this and keep it all straight? Right, there's a lot of information, and as I was reading your article, you've got these wonderful tables - and in fact, this whole issue for our listeners feels that way. I've read several of these articles now, and I'm just curious what your strategy is to stay up to date and stay organized. You have to be very organized to be an autoimmune neurologist, it seems to me.

Dr Balint: And having a little bit of OCD helps clearly, as always, in neurology. I think it is just that I started to be interested in this area for a while and I have in my head the clinical phenotype to most important associated antibodies, and as the field continues, I just add up on that panel. But, I don't want people to be discouraged - you're right, many antibodies, but I think the point is not to know each and every antibody but to know in which scenario to think of an autoimmune syndrome and then to know where to look it up.

Dr Smith: Well, I think that's a great way of ending our conversation, Bettina. I think your article does a great job of that, and one of the things I love about Continuum is these articles serve as point-of-care tools. I think our conversation will also serve as a useful framework, because I think you've talked a lot about how to organize your thinking, and, you know, pearls for when we should be thinking about these disorders which are uncommon, but you certainly don't want to miss one because the therapy can be very effective. So, Bettina, thank you so much for joining me. This has been a really great conversation.

Dr Balint: Thank you so much, Gordon. Thank you very much for your good questions.

Dr Smith: So, again, today, I've had the great pleasure of interviewing Dr Bettina Balint, whose article on ataxia and other autoimmune movement disorders appears in the most recent issue of Continuum, which is on autoimmune neurology. Be sure to check out Continuum Audio episodes from this and other issues. And thanks 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.