Robin (00:01):

Hello and welcome to today's installment of Diagnostics In Practice, where we discuss unique diagnostic offerings from Mayo Clinic Laboratories. I'm Robin Huiras, a senior marketing specialist with Mayo Clinic Labs and strong believer in the power of advanced laboratory testing to improve patients' lives. I'm excited to be here today with our guest, Dr. Margherita Milone, a neuromuscular neurologist and director of the Muscle Pathology Laboratory at Mayo Clinic, to talk about muscle biopsy testing and how it can help patients with neuromuscular disorders. Among muscle biopsy testing labs in the United States, the laboratory at Mayo Clinic is one of the oldest and most well-established. Thanks for joining us today, Dr. Milone.

Dr. Milone (00:42):

Thank you so much for having me, Robin.

Robin (00:45):

Now, before we get into our discussion, I'm wondering if you can share with our listeners a little bit about yourself and your experience as both a neuromuscular neurologist and a pathologist at Mayo Clinic.

Dr. Milone (00:57):

Of course. So I'm a neurologist specialized in neuromuscular medicine and I'm a professor of neurology at Mayo Clinic Rochester, where I serve also as a medical director of the diagnostic Muscle Pathology Laboratory. Muscle diseases, which can be both genetic or acquired, have been the focus of my clinical experience and research activity for many years, since I was a neurology resident. My initial exposure to muscle pathology began actually more than 30 years ago. When I started at Mayo Clinic, my fellowship (was) under the mentorship of Dr. Andrew Engel, who was the founder of our Muscle Pathology Lab. And since then, I have been fortunate to continue enriching my experience in muscle pathology, thanks to the patients I see in the clinic and the large volume of muscle biopsy specimens that our lab receives for interpretation.

Robin (01:52):

Great. Thanks so much for sharing your background and experience, Dr. Milone. Your expertise in this testing space really does underscore the skill and experience that goes into test development at Mayo Clinic. Now to kick off our discussion, I'm curious to know, because you care for patients with suspected muscle diseases every day, why does muscle biopsy testing remain such a critical diagnostic tool in the evaluation of neuromuscular disorders?

Dr. Milone (02:17):

So there is a huge variety of muscle diseases, and they can be acquired or genetic. Most muscle diseases are classified as rare diseases. This by itself limits the physician's clinical experience and ability to recognize them. The main problem is that many of these diseases, although they have different causes, they can manifest with similar symptoms. These include muscle weakness, muscle pain, muscle wasting, fatigue. Now for genetic muscle diseases, genetic testing has replaced the muscle biopsy in many cases, but muscle biopsy remains critical when evaluating patients with acquired muscle disease, and it is often the only way to establish the diagnosis.

Robin (03:08):

Ah, I see. It really does sound like muscle biopsy testing is just as relevant today as it has always been. I'm wondering, are there specific muscle diseases where biopsy is absolutely required?

Dr. Milone (03:20):

Definitely. A clear example of this is inclusion body myositis. This is a muscle disease that is one of the most common acquired muscle diseases we see in the neuromuscular clinic. There is no blood test or imaging study that can diagnose inclusion body myositis. There are antibodies that can accompany this muscle disease, but they are not diagnostic. So muscle biopsy is the only way to make that diagnosis. Of relevance, although inclusion body myositis has very peculiar clinical features, such as weakness of the quadriceps or finger flexors, there are other muscle diseases that can have similar clinical features. And therefore, the muscle biopsy is the only instrument to make a precise diagnosis for our patients. There are obviously other types of myopathies where a muscle biopsy is required for diagnosis. For example, dermatomyositis is an inflammatory muscle disease accompanied by skin inflammation, but sometimes the skin inflammation does not occur. The only way to make the diagnosis of dermatomyositis sine dermatitis, so dermatomyositis without skin involvement, muscle biopsy is a must. Amyloid myopathy where there is accumulation of amyloid in muscle tissue, sporatic late-onset nemaline myopathy, toxic myopathies such as hydroxychloroquine-induced myopathy, are other examples of acquired muscle disease that absolutely require a biopsy for diagnosis.

Robin (05:00):

Thanks so much for highlighting all of those disease states and the importance of muscle biopsy testing for diagnosing these particular cases. Now you mentioned nonspecific symptoms and how those nonspecific symptoms make muscle disease challenging to diagnose in some cases. So I'm wondering, where does muscle biopsy testing typically fall in the diagnostic workup when muscle disease is suspected?

Dr. Milone (05:23):

So a muscle biopsy follows a careful clinical evaluation. The clinical history is critical because understanding the disease evolution helps in narrowing the differential diagnosis. Assessment of muscle weakness distribution is equally relevant because it may suggest one versus another muscle disease. Initial testing usually includes blood tests, such as measurement of CK, which are muscle enzymes, and often an EMG study. When the combination of clinical history findings and these other laboratory studies suggest a muscle disease, the key question is whether the condition is genetic or acquired. So if the disease is suspected to be acquired, or if there is uncertainty about its nature, muscle biopsy is required to reliably establish the diagnosis. This obviously has a huge impact on patient care and treatment.

Robin (06:23):

Thanks so much, Dr. Milone. And I'm curious, what about the role of antibody testing? Does this type of testing have a role when diagnosing muscle disorders?

Dr. Milone (06:32):

Regarding muscle disorders, along the years, numerous antibodies have been discovered in immune-mediated myopathies. Many of them, however, may have low specificity or sensitivity, and the pathogenicity has not been proven for many of these antibodies. Even the anti-HMGCR antibodies, which are the most robust marker for immune-mediated myopathy, have been detected in patients with different neuromuscular diseases. So specific antibodies can help characterize the muscle disease, but in the majority of the cases in adults with acquired or suspected acquired muscle disease, the muscle biopsy is needed for diagnosis.

Robin (07:13):

Got it. Thanks for breaking that down and sharing your perspective on that, Dr. Milone. Now let's switch gears a little bit. Given how definitive muscle biopsy results can be for muscle disorders and the rarity of these diseases, experience clearly matters. Can you talk a little bit about muscle biopsy testing specifically at Mayo Clinic? I'm wondering, can you provide a little bit of the history of our lab here and why this is important when considering sending a patient sample in for testing?

Dr. Milone (07:41):

Of course, Robin. The Mayo Clinic Muscle Pathology Laboratory was founded in the mid-60s by Dr. Andrew Engel, a pioneer of myology. It is one of the oldest diagnostic muscle pathology labs in the United States. When it opened, the lab was a combined clinical-research operation that later evolved into a dedicated diagnostic muscle pathology lab. Of course, it continues to benefit from insight revealed through Mayo Clinic research focused on muscle disorders. Today, we process approximately 1,200 to 1,250 muscle biopsies per year, with about two-thirds referred from outside institutions. That level of experience, also generated by the volume, is critical when dealing with rare diseases and allows us to provide exceptionally accurate diagnostic service to the patient and to the referring physicians.

Robin (08:40):

Thanks so much for providing that historical perspective, Dr. Milone. This level of experience naturally raises expectations for technical quality, and so I'm wondering, how does Mayo Clinic's approach to tissue handling and staining help ensure reliable, high-quality biopsy interpretation?

Dr. Milone (08:57):

Muscle biopsy is technically demanding because the stains are performed on frozen tissue sections. At Mayo Clinic in the muscle lab, we have three full-time histotechnologists who specialize exclusively in fresh muscle tissue handling. They have been trained and have a longstanding experience with not only handling fresh tissue, but freezing, storing samples in liquid nitrogen, mounting the specimens properly, cutting with the microtome, and staining the sections. We routinely perform approximately 15 histochemical stains, typically at two different sectioning levels, often to address the patchy nature of many muscle diseases and at the same time reduce sampling errors. Of relevance, Congo red stain is routinely performed on all samples, and the stained sections are reviewed under Rhodamine optics, the most sensitive way to detect amyloid in skeletal muscle. Additional histochemical stains or immunohistochemical studies are performed at the muscle pathologist’s discretion, or upon patient's physician’s diagnostic clinical suspicion or result of a genetic test. The quality of the tissue handling is paramount. Accurate handling and mounting of the specimen are crucial to prevent artifact and to maintain fiber orientation for diagnostic accuracy.

Robin (10:28):

Got it. Thanks so much, Dr. Milone, for describing the technical rigor that goes behind the slides themselves. I'm curious to know who interprets the staining patterns at our lab?

Dr. Milone (10:41):

At Mayo Clinic, muscle biopsies are interpreted by neuromuscular neurologists who were trained at Mayo Clinic also in muscle pathology. Essentially, they are neuromuscular neurologists who see patients in the clinic and interpret muscle biopsies. Therefore, they bring to the microscope their clinical expertise in addition to their pathology expertise. This is an exceptional diagnostic setting for our patients.

Robin (11:08):

Yeah, it sure sounds like it. I'm wondering, do you think Mayo Clinic's clinician-pathologist model influences the way that muscle biopsies are read and reported?

Dr. Milone (11:18):

Absolutely. Because we are clinicians, we can directly correlate the histological findings we see on the muscle biopsy with the clinical history, the neurological examination findings, EMG results, serological data, and genetic testing findings. That clinical-pathological integration is a major advantage in the interpretation of muscle biopsies and directly impact diagnostic accuracy.

Robin (11:44):

Yeah, definitely. Integration is hallmark of Mayo Clinic and it's so great to hear that as it applied in this setting as well. As a follow-up question, I'm wondering, how does the clinical integration of our muscle pathologists tie into genetic testing? I'm wondering how does muscle biopsy complement genetic results, particularly when variants of unknown significance are identified?

Dr. Milone (12:08):

That's a great question, Robin. Genetic testing has replaced muscle biopsy as a diagnostic test when a genetic muscle disease is clinically suspected, especially in the pediatric population. Genetic testing, however, is not always positive or negative. It may identify variants of uncertain significance, what we call VUS. Muscle biopsy can help in interpreting those findings by showing whether the pathology supports or not the possible deleterious role of a specific VUS. For example, a patient had an ACTA1 VUS. We know that ACTA1 pathogenic variants can cause nemaline myopathy, which is characterized by the presence of nemaline rods on muscle biopsy. The patient underwent a muscle biopsy that showed nemaline rods. These findings supported the deleterious role of the ACTA1 VUS. In other cases, the muscle biopsy has expanded the previously known spectrum of genetic muscle diseases. In additional cases, the abnormalities as seen on muscle biopsy have guided the genetic testing, allowing identification of the underlying gene defect. And all this has led to a variety of scientific publications in the field of myology.

Robin (13:27):

Wow. That is really cool to hear that your lab has been able to characterize genetic disorders based on pathology results. I'm wondering, when it comes to reporting results, whether genetic or acquired, how does that information come together? For instance, what should referring neurologists expect from a Mayo Clinic muscle biopsy report and the consultation process that follows?

Dr. Milone (13:50):

Our muscle biopsy report contains a description of all findings for each specific stain. Then it has a section diagnosis, where there are bullet point diagnoses. This section has the main diagnostic points that we like to transfer to the requesting physician. There is also a comment section where we include comments based on what we see on muscle biopsy slides and the patient's information provided to us. In this section, when possible, we make some suggestions on patient care. We offer a unique combination of technical expertise — professional expertise that can link pathology to the clinical information directly to the patient. We believe that this is extremely important for the referring physicians who evaluated the patient, especially if they are not a muscle diseases expert. Most importantly, we are always available for direct clinical consultation. Especially for rare or complex cases, a referring physician can contact us to discuss the findings, which can directly influence patient care. Additionally, if desired by the referring physician, we welcome a referral to our muscle clinic for further evaluation.

Robin (15:03):

Thanks for detailing the result report, Dr. Milone. As we conclude our talk today, I'd like to take a step back and think about the full picture from diagnosis through interpretation. What does this level of expertise provided at Mayo Clinic ultimately mean for patients navigating rare or complex muscle diseases?

Dr. Milone (15:22):

Patients with muscle diseases can count on a state-of-the-art diagnostic service at Mayo Clinic. This can mean the identification of a treatable muscle disease for some, while for others, it can be reaching a precise diagnosis, not amenable for treatment, but able to provide clarity and peace of mind. For example, sometimes patients may be started on immunosuppressive drugs and exposed to potentially related side effects only because they have elevated CK, the muscle enzyme. A muscle biopsy showing the features of a nontreatable muscle disease can stop exposure to unneeded drugs. A diagnosis may not always change treatment. It provides understanding, direction, and peace of mind, and that is a meaningful outcome for patients and family.

Robin (15:57):

Thanks, Dr. Milone. I also believe that providing meaningful outcomes for patients remains a top goal at Mayo Clinic and Mayo Clinic Laboratories. Thank you so much for sharing your time with us today. I've really enjoyed our talk and hope our listeners have as well. And to our listeners, thanks for joining. We hope you'll tune in again for the next installment of Diagnostics in Practice.