Guest introduction and focus on inclusion body myositis

Inclusion body myositis (IBM), the most common myopathy in adults, is a disease of aging characterized by slowly progressive weakness. Diagnosis of IBM requires the integration of historical, clinical, and laboratory data, while management consists of a multidisciplinary approach to address comorbidities and potential complications.

In this episode, Aaron Berkowitz, MD, PhD, FAAN speaks with Elie Naddaf, MD, author of the article "Inclusion Body Myositis" in the Continuum® October 2025 Muscle and Neuromuscular Junction Disorders 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 in San Francisco, California.

Dr. Naddaf is an associate professor of neurology at the Mayo Clinic College of Medicine in Rochester, Minnesota.

Additional Resources

Read the article: Inclusion Body Myositis

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Host: @AaronLBerkowitz

Guest: @ElieNaddaf3

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 Elie Naddaf about his article on inclusion body myositis, which appears in the October 2025 Continuum issue on muscle and neuromuscular junction disorders. Welcome to the podcast, Dr Naddaf, and would you please introduce yourself to our audience?

Dr Naddaf: Thank you for having me. I am Elie Naddaf, a neuromuscular neurologist at Mayo Clinic in Rochester, Minnesota, and one of my main focus research-wise is about inclusion body myositis.

Dr Berkowitz: Fantastic. Well, this is a great article on inclusion body myositis, or IBM, as we may refer to it today. It has a lot of clinically practical tips for examining patients at the bedside and a lot of important updates for us on how to diagnose this condition. So, I encourage our listeners to check out the article, and I look forward to discussing some of the key aspects here and learning from your expertise. First, tell us about the classic presentation of IBM, when the disease is pretty easy to recognize at the bedside based on the history and exam.

Dr Naddaf: Luckily, IBM is one of those diseases that has a very particular pattern of weakness that makes it easier to diagnose. However, in real life it can be very challenging for several reasons, which we will cover in this podcast and in the article. However, a typical presentation of IBM is that of an older patient, more likely to be a male---twice more likely to be a male---,presenting with slowly progressive weakness over a couple years or so. And the weakness predominantly affects the deep finger flexors in the hand. So, they most commonly present with hand grip weakness, or the quadriceps muscle presenting with some lower limb weakness. However, some patients can present with one or the other, not necessarily with both at the same time. It is usually a painless disease,, and because of the lingering course, patients tend to present within two to three years from their symptom onset. So, with that, on examination, if the patient is presenting with the hand weakness, they would demonstrate weakness in the deep finger flexors, which are the muscles that we use to flex the distal intralaryngeal joints. This weakness is often asymmetric and can be in only one hand; and also, even within the same hand, you can have a variable severity from one finger to the other. And that's one reason, although it sounds a classic phenotype, if you can imagine a patient just presenting with hand weakness, a lot of other things come to mind, whether it's a compressive neuropathy or whether it's a radiculopathy or motor neuron disease.

Similarly, in the leg, the quadriceps is a big and strong muscle. So, it's often that patients' symptoms originally or in the beginning get dismissed because the physician did not demonstrate any weakness on manual motor testing. Because it's a strong muscle, it needs to lose a certain amount of strength to be able to demonstrate that weakness by just pushing against the patient trying to extend their knee from about a 90 degree or so. That's why another way in those cases would be to examine them more functionally whether they're kneeling or squatting. It is usually a pure motor disease, although patients can commonly have a peripheral neuropathy. But typically, on exam, you mainly see muscle weakness.

Dr Berkowitz: Perfect. So, if we see a patient who's an older man with progressive painless weakness of the finger flexors, quadriceps, this is sort of the classic presentation where we would consider IBM. But you mentioned in your article that some patients can present somewhat atypically when we might not immediately think of this diagnosis. What are some of the atypical presentations we should be aware of that should lead us to think about IBM as a possible etiology for the patient's pattern of weakness?

Dr Naddaf: So, IBM indeed can present in any muscle group. That's why about 14% of patients may have the weakness onset beyond the finger flexors or knee extensions. And there are very particular phenotypes that stand out. Especially the most common in that scenario would be patients presenting with pure difficulty swallowing, isolated dysphasia that can sometimes precede the limb weakness by several years, and that's especially common in females. Other phenotypes including just a proximal weakness, like a limb-girdle weakness; an axial weakness, for example, head drop or camptocormia or a foot drop. And because it's an asymmetric disease, you can see the challenge there---if someone just presenting with a foot drop on one side, that it could be challenging to just think of IBM. So, those are the main phenotype. One particular phenotype that is super interesting is, patients present with severe facial weakness as if they have severe bilateral Bell's palsy. And that's the, usually, the most common first misdiagnosis. And all these patients reported so far in the literature are only females. This has not been reported in any male patients. So yes, the finger flexor quadriceps weakness is the most common typical presentation. However, IBM can present, technically, in any other muscle in the body.

Dr Berkowitz: Great. Well that's very helpful information that especially comes from experts like you who see a lot of these patients and are able to make these diagnoses of these rare phenotypes. Whereas many of us general neurologists, like myself, might think of IBM only with finger flexor weakness and quadriceps weakness, perhaps with some foot drop dysphagia associated. Sounds like this is a diagnosis to consider in atypical presentations or atypical presentations of myopathy that aren't fitting other phenotypes or aren't yielding diagnostic results for other phenotypes. And you have in your article a very helpful table that goes through some of the common sites of weakness in IBM and the differential diagnosis for myopathies and other conditions to consider in patients who have the classic and less typical presenting features. So, let's say that we see a patient with clinical features suggestive of IBM. How do we go about confirming the diagnosis? What are the main diagnostic tests we would use to try to make a firm diagnosis here?

Dr Naddaf: So, the gold standard so far for diagnosis for inclusion body myositis, as it is an acquired disease, has been a muscle biopsy. So, muscle biopsy is the probably most important tool in the diagnostic approach to IBM. Even in patients with a classical phenotype, that all like in any other test, it depends on your pretest probability and how sure you are the patient has IBM. But even with the classic phenotype, it is characteristic of IBM, but not pathognomic of IBM. Because if we see a high number of patients with similar phenotypes, we will run into a lot of other disorder that present similarly. And some patients---especially for instance, with myotonic dystrophies, specially type two---may be very difficult to distinguish from IBM, especially those that present in adulthood that they don't have the classic picture of a myotonic dystrophy patient you would think of. And some of them may not even have percussion myotonia. Because of that, the biopsy is very important to confirm your diagnosis in that regard. And on the biopsy, you want to see evidence of inflammation, basically, and the mesial inflammation, without going into a lot of details, to set it apart from those genetic ones. But in IBM it's not a pure inflammatory disease.

There are other features on the biopsy that are very particular to IBM, two main other things we need to find. One is that of the accumulation of autophagic vacuole and protein aggregates and that of mitochondrial dysfunction. So, the other test for patients, presenting with weakness---again that depends on your clinical suspicion---would be first to establish that the underlying process is a myopathy; and hence, the EMG. And also, that's particularly important in patient with symptoms in one limb to differentiate it from compressive neuropathies or from a motor neuron disorder or other. So, the EMG tells you it's a myopathy with fibrillation potential, helps you also choose a muscle for biopsy. So as far as blood tests, the main blood test is that of the cytosolic nucleosidase 1A, or people call them IBM antibody. That's present in about half of the patients with IBM, 50%. Specificity originally reported to be high in the 90s, but in real-life practice with commercial lab it's probably lower in the 70s. There is a growing interest in the use of muscle imaging as well and IBM is one of those diseases similar to the clinical phenotype that has some distinctive pattern on imaging on MRI. But again, like the clinical phenotype, it is helpful, it is characteristic. It's not diagnostic on its own. Other blood tests in IBM include creatine kinase level. As long as you're not seeing creatine kinase in the 10,000 or 20,000, that would be very unusual in IBM and other routine blood tests that are discussed in the article.

Dr Berkowitz: Right. That's a very helpful overview of the testing we would consider when we're trying to make this diagnosis that can include, as you mentioned, EMG, muscle MRI, and this five-prime cytosolic nucleosidase 1A antibody. As you said, many of these have variable sensitivity and specificity. We can diagnose a myopathy generally, or muscle inflammation more specifically, using EMG and muscle MRI respectively. It's interesting to hear that that antibody, I know there was a lot of excitement about that originally. Sounds like less sensitive and less specific than one would have hoped. In your article, you discussed sort of how to use some of these tools to try to meet the diagnostic criteria that were updated in 2024 for diagnosing IBM. And correct me if I'm wrong, this is my first time reading about these. I want to make sure I understand that essentially, you always require a biopsy for definitive diagnosis of this disease. Is that right?

Dr Naddaf: That's correct. That's because of the issues that you raised with the antibody and because, even with people with the classic phenotype, you still could have other diseases that can mimic IBM that are associated with different complications, comorbidity profiles. For instance, myotonic dystrophy requires very close monitoring of cardiac rhythm, which is usually not affected in IBM. Hence, there is still the need for a biopsy, as we don't have a better test so far. The antibody has its own challenging, but I still find it very helpful in certain situations, and that's also reflected in the new criteria. It's definitely not a standalone diagnostic test. I don't think you can just draw the antibody, if it's positive, tell the patient they have IBM.

To me, it's helpful in two ways, at least in my practice. One way is, it makes me think of IBM in situations where I was not considering IBM. In the end IBM is the most common disease in adulthood, so it's not a bad habit to just check the antibodies in any unusual case. Or where inconclusive case, it's just me saying, huh, that could be IBM that I missed. Then I go and try to confirm it. The other scenario that it helps me, there are a lot of cases that remain inconclusive. For instance, you just saw inflammation on biopsy, you have some pattern on MRI. You just need another piece of information to further make the case for the diagnosis. And that's where I find the antibodies helpful. And that's reflected in the new criteria.

Dr Berkowitz: I see. So, if you- Yeah, so just to pick up on the antibody here, since I'm sure people be interested in when or whether to send this. You sort of use this in atypical cases where you know it's a myopathy or not. You haven't yet made a diagnosis. And you think, well, if I send this and it's positive, that's not 100% specific, but going to tip me a little more towards thinking this could be an atypical IBM case. But if it's negative, it doesn't sort of tell you either way, since you said it's only about 50% sensitive, right? Am I understanding correctly how you use the antibody?

Dr Naddaf: Yes, this is correct. So basically, you don't have to check it if you think it is IBM; that also depends on your style of practice. To me, I think at some stage, with all the progress we're seeing in research in systems biology, so far there's no indication that the antibody predicts severity, and I don't think it does. But there could be at some time down the road that there are different underlying pathways or disease mechanisms that are associated with people that have the antibodies. So, it's more like for the future, it's good to know if the patient is positive or not.

Dr Berkowitz: Okay. So, if biopsy is really necessary to make this diagnosis, if you see a classic presentation with an older man, as we've said before, with slowly progressive painless finger flexor weakness and quadriceps weakness, do we even need EMG or muscle MRI or this antibody or do you just go straight to biopsy in those cases when the clinical phenotype looks relatively certain? Do you still think one or more of those tests are helpful in the classic presentation? Or are those EMG, muscle MRI, and the antibody ones used more when you think, this looks sort of like IBM but it's either it's maybe early and it's not sort of fully kind of developed into the classic phenotype or it's very asymmetric, which you can see in IBM, or whether it's sort of has more atypical features or only one of the classic features? How do you use EMG muscle MRI in the antibodies when you know you're going to need biopsy ultimately, if this is the diagnosis you have in mind?

Dr Naddaf: That's a great question. And all of the above would be correct depending on the setting you practice in. If I were practicing in a community where I have the privilege to do things step by step, I might approach it a little bit differently. Being at the referral center with many patients coming with a one-stop shop and they need to do everything possible rather than returning, that's also another different practice scenario. But in general, you want the EMG if you're not sure it's a myopathy, especially that these patients are asymmetric and sometimes the findings are in a single limb. So, you would want the EMG when you're not sure it's a myopathy, or also you need a little bit more information on what muscle to biopsy, you could use the EMG. If those questions have already been answered, you don't have to do the EMG. Now regarding the biopsy, if it is the classic phenotype and you think it's going to be a bingo with the biopsy, you might not need to do any antibody or MRI or any further testing. Now with one caveat that the biopsy might not give you the full picture or all the features, then you could go back and do the rest. And that's actually all integrated in the new criteria.

So, in the new 2024 revised European Neuromuscular Center diagnostic criteria for IBM, the main differences are the following: now you can use additional test measures to support the diagnosis in addition to the biopsy. So that will probably reduce the need to have a repeated muscle biopsy. Two, you could have an atypical form and still have the diagnosis. You don't have to wait until your finger flexor or knee extensors are involved. And that's very important also, because criteria generally are made for clinical trials where you want to know for sure who you are treating. But at the same time, you don't want to wait till advanced stages of the disease where treatments might not work. Diagnostic criteria are not made to be used in clinical practice, but at the same time they offer a good framework, a good suggestion for people to use in their clinical practice. And just always remember to tailor it to your particular patients that you're seeing. In those criteria, basically, we go by scenario depending on your level of suspicion. There is no possible probable. You either make the diagnosis or you don't make it. And as you said, if you have a classic phenotype patient with deep finger flexor weakness in the upper limb and knee extends her weakness in the lower limb, your pretest probability is very high, and the chances that you're wrong are low. They're not zero, they're low. That's why I was just showing in the mesial inflammation on the biopsy, which would rule out your genetic mimicor. You're pretty confident of the diagnosis.

And the other scenarios, then let's say you have only one limb, then you need a little bit more support. And you have an atypical form where the differential is very wide. You want to provide more supportive criteria, and the supportive criteria could be other features in the biopsy, whether it's the CCO negative fibers or derma vacuoles, or you could use MRI and antibodies if they're available to you. In many countries, these are still not available, but if they're available, that's good. You could use them as supportive criteria. That was the logic behind those revised criteria that are discussed by scenario in detail in the article.

Dr Berkowitz: Fantastic. That's very helpful. Yeah, as you mentioned this- the criteria reviewed in the article, you have a very helpful figure there with all the branch points of how to get to the diagnosis, but very helpful to hear how you think about combining these tests in practice in different scenarios to get to the final diagnosis. So, IBM is considered an inflammatory myopathy, right? Like dermatomyositis and antisynthetase syndrome and immune-mediated necrotizing myopathy. And yet it gets its own separate article each time we do a Continuum issue on muscle disorders, right? Because it's unique in that it doesn't respond to any immune- immunomodulatory or immunosuppressive therapy like these other inflammatory myopathies. So why do you think, or why is it thought by experts in the field like yourself, that this is an inflammatory myopathy but it, at least from a treatment perspective, appears to behave so differently from the other inflammatory myopathies and doesn't respond to immunomodulatory therapy---or at least we haven't found the right key in the right block of immunomodulatory therapy to treat this disease?

Dr Naddaf: That's a great question, and it's an area that I'm particularly interested in from a research standpoint. So, all these entities were classified as idiopathic inflammatory myopathies because they had inflammation on muscle biopsy. And IBM does have inflammation on muscle biopsy. Even when we study gene expression, all the immunoglobulin, interferon-related genes, the data are very, very highly expressed in IBM. However, IBM histopathology is not just inflammation. There are other features of IBM that are shared with other diseases of aging, and IBM only affects the aging population. So, by definition it is a disease of aging, and it shares a lot of similarities with other diseases of aging such as Alzheimer's disease or Parkinson's disease or those others that occur with aging. And IBM, unlike any other idiopathic inflammatory myopathy, it is twice more common in males and does not have a juvenile form.

Taking all of this together, we could argue that IBM is a disease of aging, and those diseases usually are very complex and have involvement of different pathways---including but not restricted to inflammatory pathways, disrupted protein homeostasis, mitochondrial abnormalities and so on and so forth. And that's why so far we have not had any luck with any immunosuppressive or immunomodulatory therapy in IBM. Now the type of inflammation in IBM is also different, that's mediated by highly differentiated cytotoxic CD8 cells that may not respond to the other treatment. But again, IBM is not just inflammation, and unfortunately in these complicated disorders, we're still at early stages of understanding them, let alone intervene on them. There have been a lot of progress in the last few years in the area of aging in general, and hopefully we would see some new developments in IBM and in other diseases that would help at least slow down the progression.

Dr Berkowitz: Fantastic. Well, we've only gotten an opportunity to scratch the surface of your article today talking about the clinical presentation, diagnosis, and a little bit about some of the challenges in treatment. But there's a lot more for our listeners to refer to in your article related to some of the aspects of management, not yet a disease-modifying therapy, but other important aspects of management and many other clinical pearls in this article. So, I encourage our listeners to take a look at your article in this issue.

Dr Naddaf: Thanks a lot.

Dr Berkowitz: Again, today I've been interviewing Dr Elie Naddaf about his article on inclusion body myositis, which appears in the October 2025 Continuum issue on muscle and neuromuscular junction disorders. Be sure to check out Continuum Audio episodes from this and other issues, and thank you again 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 the 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.