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Idiopathic inflammatory myopathies (IIM) are rare, acquired muscle diseases; their diagnosis of is based on clinical, serological, and histological criteria. MHC-I-positive immunostaining, although non-specific, is used as a marker for IIM diagnosis; however, the significance of major histocompatibility complex (MHC)-II immunostaining in IIM remains debated. We investigated patterns of MHC-II immunostaining in myofibers and capillaries in muscle biopsies from 103 patients with dermatomyositis ([DM], n = 31), inclusion body myositis ([IBM], n = 24), anti-synthetase syndrome ([ASyS], n = 10), immune-mediated necrotizing myopathy ([IMNM], n = 18), or overlap myositis ([OM], n = 20). MHC-II immunostaining of myofibers was abnormal in 63/103 of patients (61%) but the patterns differed according to the IIM subgroup. They were diffuse in IBM (96%), negative in IMNM (83%), perifascicular in ASyS (70%), negative (61%) or perifascicular (32%) in DM, and either clustered (40%), perifascicular (30%), or diffuse heterogeneous (15%) in OM. Capillary MHC-II immunostaining also identified quantitative (capillary dropout, n = 47/88, 53%) and qualitative abnormalities, that is, architectural abnormalities, including dilated and leaky capillaries, (n = 79/98, 81%) in all IIM subgroups. Thus, MHC-II myofiber expression patterns allow distinguishing among IIM subgroups. We suggest the addition of MHC-II immunostaining to routine histological panels for IIM diagnosis.

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OBJECTIVES: The aim of this study was to determine the association between different histological patterns and prognosis in patients with SSc and histologically proven muscle involvement. METHODS: A multicentre retrospective study was conducted of a cohort of scleroderma patients who had undergone muscle biopsy. The biopsies were reviewed in a coordinated manner to classify patients based on histological findings. Three different patterns were observed: fibrosing myopathy (FM), inflammatory myopathy (IM) and necrotizing myopathy (NM). Rates of survival, muscle relapse, and cardiac and pulmonary events were compared between these three groups. RESULTS: Among 71 scleroderma patients with muscle biopsy specimens available for review, 33 (46.5%) were classified in the FM group, 18 (25.5%) in the IM group, and 20 (28%) in the NM group. The median follow-up time was 6.4 years (interquartile range, 2.2-10.9 years) and 21 patients died during follow-up, primarily from heart disease and infections. The 10-year survival rate after the first non-Raynaud's disease symptom was 80% and the cumulative incidence of muscle relapse was 25%. Neither factor differed significantly between the three groups. The risk of pulmonary events was lowest in the OM group, significantly lower than in the FM group (hazard ratio, 0.17; 95% CI, 0.04-0.67) and non-significantly lower than in the IMNM group (hazard ratio, 0.28; 95% CI, 0.06-1.24). The risk of cardiac events did not differ significantly between the three groups. CONCLUSION: The mortality rate of scleroderma patients with muscle involvement was not associated with their histological patterns.

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Dystrophinopathies are a group of neuromuscular disorders, inherited in an X-linked recessive manner, caused by pathogenic variants in the DMD gene. Copy number variation detection and next generation sequencing allow the detection of around 99 % of the pathogenic variants. However, some patients require mRNA studies from muscle biopsies to identify deep intronic pathogenic variants. Here, we report a child suspected of having Duchenne muscular dystrophy, with a muscle biopsy showing dystrophin deficiency, and negative molecular testing for deletions, duplications, and small variants. mRNA analysis from muscle biopsy revealed a pseudoexon activation that introduce a premature stop codon into the reading frame. gDNA sequencing allowed to identified a novel variant, c.832-186 T>G, which creates a cryptic donor splice site, recognizing the underlying mechanism causing the pseudoexon insertion. This case highlights the usefulness of the mRNA analysis from muscle biopsy when routine genetic testing is negative and clinical suspicion of dystrophinopathies remains the main clinical diagnosis suspicion.

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OBJECTIVES: TO DETERMINE WHETHER MAGNETIC RESONANCE IMAGING (MRI) FINDINGS REFLECT THE PATHOLOGICAL FEATURES OF INFLAMMATORY MYOPATHIES: Methods: Patients with idiopathic inflammatory myopathies (IIMs) diagnosed using the 2017 EULAR/ACR classification criteria in our university between 2005 and 2020 were retrospectively reviewed. IIMs were subclassified into the anti-ARS syndrome (ASSD), immune-mediated necrotizing myositis (IMNM), Dermatomyositis DM and others. Fat-suppressed T2-weighted MRI and muscle biopsy specimens were assessed in IIMs followed by the comparison among the four subgroups. RESULTS: MRI findings were available for 62 patients and histopathological findings were available for 27 patients. Perifascicular atrophy or necrosis in the muscle tissues from the patients with IIM was more frequently observed in patients with subcutaneous and fascial high signal intensity (HSI) on MRI than those without. Four-group comparison among ASSD, IMNM, DM and others revealed HSI in fasciae on MRI was more frequently observed in patients with ASSD and DM than others. Perifascicular atrophy or necrosis in muscle tissues was more frequently observed in patients with ASSD than in others. CONCLUSION: Patients with ASSD had distinct MRI features compared with anti-ARS negative patients. The fascial high signal intensity on MRI may reflect distinctive pathological features of muscles.

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PURPOSE: To gather information on practices and perceptions of high-performance experts regarding their athletes' muscle fiber-type composition (MFTC) and its estimation. METHODS: A questionnaire on the noninvasive versus invasive estimation of MFTC was completed by 446 experts including coaches and sport-science/sports-medicine staff. Moreover, the perceived importance of MFTC for training and performance optimization was assessed. Differences between sport types (individual and team sports) were analyzed using chi-square tests. RESULTS: Forty percent of the experts implemented MFTC assessment in pursuit of performance optimization, while 50% did not know their athletes' MFTC but expressed a desire to implement it if they would be able to assess MFTC. Ten percent did not perceive value in MFTC assessment. Only 18% of experts believed that their athletes would undergo a muscle biopsy, leading to the adoption of alternative noninvasive techniques. Experts primarily relied on their experience to estimate MFTC (65%), with experts working in individual sports using their experience more frequently than those working in team sports (68% vs 51%; P = .009). Jump tests emerged as the second-most commonly employed method for estimating MFTC (56%). When only considering experts who are currently using MFTC, 87% use MFTC to individualize training volume and 84% to individualize training intensity. CONCLUSIONS: Experts value MFTC assessment primarily to individualize training but mainly rely on noninvasive methods to estimate MFTC. Some of these methods lack scientific validity, suggesting a continuing need for education and further research in this area.

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PURPOSE: to investigate the early consequences of type 1 diabetes (T1D) on the neural strategies of muscle force production. METHODS: motor unit (MU) activity was recorded from the vastus lateralis muscle with High-Density surface Electromyography during isometric knee extension at 20 and 40% of maximum voluntary contraction (MVC) in 8 T1D (4 males, 4 females, 30.5 ± 3.6 years) and 8 matched control (4 males, 4 females, 27.3 ± 5.9 years) participants. Muscle biopsies were also collected from vastus lateralis for fiber type analysis, including myosin heavy chain (MyHC) isoform content via protein and mRNA expression. RESULTS: MVC was comparable between groups as well as MU conduction velocity, action potentials' amplitude and proportions of MyHC protein isoforms. Nonetheless, MU discharge rate, relative derecruitment thresholds and mRNA expression of MyHC isoform I were lower in T1D. CONCLUSIONS: young people with uncomplicated T1D present a different neural control of muscle force production. Furthermore, differences are detectable non-invasively in absence of any functional manifestation (i.e., force production and fiber type distribution). These novel findings suggest that T1D has early consequences on the neuromuscular system and highlights the necessity of a better characterization of neural control in this population.

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AIMS: We describe the varied clinical presentations, barriers in diagnosis and outcomes of anti-HMGCR myopathy in a large national cohort. METHODS: Adults found positive for serum anti-HMGCR autoantibodies via line blot or enzyme-immunoassay followed by immunoprecipitation were included in the study. RESULTS: Of 75 patients identified, the records of 72 (96 %) described weakness as the presenting symptom. The records of 65 gave a reliable description of proximal weakness. In 22/65 (33.8 %) the weakness was described as predominantly or solely lower limb weakness. Forty-five of 75 (60 %) presented with a subacute onset (duration of symptoms >4 weeks -≤6 months), whilst 22/75 (29.3 %) presented with a more indolent chronic onset (duration of symptoms >6 months). Eighteen of 75 (24 %) suffered falls and 2/75 (2.7 %) had "general decline". In three patients no weakness was described: two presented with myalgia and one with a skin rash characterized as Jessner lymphocytic skin rash. Median creatine kinase at presentation was 7337 U/L (range 1050-25,500). Muscle biopsy was performed in 38 (50.7 %). Associated malignancy was infrequent. Four patients recovered without immunosuppression. Five-year and 10-year survival was 92.7 % (95 % CI 80.6-97.4 %), and 82.5 % (95 % CI 61.2-92.8 %) respectively. CONCLUSION: Recurrent falls, a long prodrome and dominant lower limb proximal weakness were common in this anti-HMGCR myopathy cohort. These features overlap with frailty syndrome and sporadic inclusion body myositis emphasizing the importance of considering anti-HMGCR myopathy in that clinical context. A minority of patients recover after statin withdrawal alone.

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BACKGROUND: Muscle biopsy is an important test in the evaluation of individuals with suspected myopathy, including those with suspected idiopathic inflammatory myopathy (IIM). Various approaches, including open surgical biopsy, needle biopsy and conchotome forceps, have been reported. However the real-world utilisation of these approaches remains unclear. There are no established guidelines for the use of muscle biopsy, or selection of biopsy technique, in investigating IIM and international practices are not well-documented. This study describes current approaches to muscle biopsy amongst clinicians with expertise in IIM. METHODS: A survey regarding muscle biopsy practices was disseminated among members of the International Myositis Assessment and Clinical Studies (IMACS) group. Data were analysed using descriptive statistics. RESULTS: One-hundred and sixteen clinicians completed the survey, primarily rheumatologists. Open surgical biopsy was the most commonly employed technique (74.5 %), followed by needle (11.3 %) and conchotome (9.4 %) approaches. Clinical examination was the most common method of muscle selection, with 85.2 % of respondents reporting they 'always or almost always' relied on it. MRI and electromyography were also frequently utilised for muscle selection (51.9 %, 45.4 % respectively). There was variability in the perceived utility of muscle biopsy in certain clinical contexts, such as presence of myositis specific antibodies or cutaneous manifestations of dermatomyositis. While respondents generally reported low complication rates following muscle biopsy, non-diagnostic histopathology was commonly reported, regardless of procedural approach. CONCLUSION: Clinicians managing IIM report muscle biopsy to be well tolerated however, non-diagnostic results are common. Substantial heterogeneity regarding perceived indications for biopsy, procedural approaches, and muscle selection strategies were observed within this expert group. Future research is needed to establish best practice and determine the role of muscle biopsy in the context of continued advancements in serological profiling of IIM.

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Extraocular muscle (EOM) enlargement may be due to a variety of causes. These causes can be classified in three ways: according to pathogenesis and histopathological features, according to the site, and according to the clinical features. Diagnosis of the cause is dependent upon history, clinical examination, and investigations. Imaging with computed tomography or magnetic resonance imaging and muscle biopsy is typically necessary to make the correct diagnosis. Treatment of the patient must be directed toward the specific cause. This review emphasizes important clinical and pathological guidelines for appropriate diagnosis and treatment of patients with EOM enlargement.

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BACKGROUND: Early diagnosis of neosporosis in dogs is challenging. OBJECTIVES: To evaluate the feasibility of a compound multimodal testing approach for diagnosing in dogs neuromuscular and combined forms of neosporosis. ANIMALS: A total of 16 dogs diagnosed with solely neuromuscular neosporosis or with a combination of neuromuscular and central nervous system neosporosis. METHODS: Retrospective review of clinical signs, laboratory findings, treatment, and outcome with focus on the diagnostic utility of different tests. Development of a chromogenic in situ hybridization (ISH) assay for the identification of Neospora caninum in paraffin-embedded muscle samples. RESULTS: 13/16 dogs had only neuromuscular signs of neosporosis, 3/16 had disease signs with concomitant central nervous system (CNS) involvement. Serology was performed in 15/16, with 10/15 showing titers >1 : 160 at admission. PCR on muscle samples detected N. caninum DNA in 11/16. Immunohistochemistry (IHC) detected N. caninum in 9/16 and ISH in 9/16. Histopathology revealed inflammatory myopathy in 10/16, necrotizing myopathy in 5/16, borderline changes in 1/16 and tachyzoites in 9/16. In 4 cases, N. caninum infection was confirmed with all 5 diagnostic methods, 3 cases with 4, 2 with 3, 6 with 2, and 1 animal with 1. CONCLUSIONS AND CLINICAL IMPORTANCE: Diagnosis of N. caninum infection should rely on a multimodal diagnostic approach and negativity of 1 single test should not allow for exclusion. Serology in combination with direct parasite identification via histopathology, DNA via PCR, or both modalities, appears a reliable diagnostic approach.

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As the most frequent cause of acquired myopathy, toxic myopathies are characterised by clinicopathological features that vary depending on the mode of action of the drugs or toxins involved. Although a large number of substances can induce myotoxicity, the main culprits are statins, alcohol, and corticosteroids. A rigorous, well-organised diagnostic approach is necessary to obtain a rapid diagnosis. For early diagnosis and management, it is important for clinicians to be aware that most toxic myopathies are potentially reversible, and the goal of treatment should be to avoid serious muscle damage.

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Pompe disease is a rare genetic disorder characterized by a deficiency of acid α-glucosidase (GAA), leading to the accumulation of glycogen in various tissues, especially in skeletal muscles. The disease manifests as a large spectrum of phenotypes from infantile-onset Pompe disease (IOPD) to late-onset Pompe disease (LOPD), depending on the age of symptoms onset. Quantifying GAA activity and glycogen content in skeletal muscle provides important information about the disease severity. However, the distribution of GAA and glycogen levels in skeletal muscles from healthy individuals and those impacted by Pompe disease remains poorly understood, and there is currently no universally accepted standard assay for GAA activity measurement. This systematic literature review aims to provide an overview of the available information on GAA activity and glycogen content levels in skeletal muscle biopsies from patients with Pompe disease. A structured review of PubMed and Google Scholar literature (with the latter used to check that no additional publications were identified) was conducted to identify peer-reviewed publications on glycogen storage disease type II [MeSH term] + GAA, protein human (supplementary concept), Pompe, muscle; and muscle, acid alpha-glucosidase. A limit of English language was applied. Results were grouped by methodologies used to quantify GAA activity and glycogen content in skeletal muscle. The search and selection strategy were devised and carried out in line with Preferred Reporting of Items in Systematic Reviews and Meta-Analysis guidelines and documented using a flowchart. Bibliographies of papers included in the analysis were reviewed and applicable publications not already identified in the search were included. Of the 158 articles retrieved, 24 (comprising >100 muscle biopsies from >100 patients) were included in the analysis, with four different assays. Analysis revealed that patients with IOPD exhibited markedly lower GAA activity in skeletal muscles than those with LOPD, regardless of the measurement method employed. Additionally, patients with IOPD had notably higher glycogen content levels in skeletal muscles than those with LOPD. In general, however, it was difficult to fully characterize GAA activity because of the different methods used. The findings underscore the challenges in the interpretation and comparison of the results across studies because of the substantial methodological variations. There is a need to establish standardized reference ranges of GAA activity and glycogen content in healthy individuals and in Pompe disease patients based on globally standardized methods to improve comparability and reliability in assessing this rare disease.

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BACKGROUND: Exome sequencing (ES) is a useful tool in diagnosing suspected mitochondrial disease but can miss pathogenic variants for several reasons. Additional testing, such as muscle biopsy or biochemical testing, can be helpful in exome-negative cases. METHODS: We report a patient who presented with repeated episodes of lactic acidosis and failure to thrive. RESULTS: ES and mitochondrial sequencing were initially negative but clinical suspicion for mitochondrial disease remained high. After muscle biopsy showed evidence of mitochondrial dysfunction, the ES was reanalyzed and revealed novel variants in AARS2. CONCLUSION: This case demonstrates the importance of muscle biopsy and biochemical testing in evaluating patients with a high suspicion of mitochondrial disease, even in the genomics era. Closed-loop communication between molecular genetics laboratories and clinical geneticists is an important step to help establish diagnosis in unsolved cases.

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Anti-Ku autoantibodies are associated with several autoimmune inflammatory diseases. We aimed to review our anti-Ku positive pediatric patients in this study. Four pediatric patients (all female) who had anti-Ku positivity were included (Patients 1-2-3 with idiopathic inflammatory myopathy (IIM); Patient 4 with chronic urticaria). Patient 1 (onset:10.5 years) had proximal muscle weakness, Raynaud phenomenon, sclerodactyly, hyperpigmentation, joint contracture, and tenosynovitis. The disease course was progressive despite treatment with corticosteroids, intravenous immunoglobulin (IVIG), plasma exchange, and 11 different immunosuppressive drugs. Patient 2 (onset:15 years) presented with proximal muscle weakness, fatigue, weight loss. She recovered normal muscle strength after treatment with corticosteroids, IVIG, methotrexate, cyclosporine A, mycophenolate mofetil. Patient 3 (onset:10 years) had juvenile dermatomyositis with proximal muscle weakness, Gottron's papules, and calcinosis. She also had anti-NXP2 positivity. Remission was achieved with corticosteroids, methotrexate, azathioprine, and infliximab. Muscle biopsy findings revealed a variable spectrum of necrosis, regeneration, perifascicular pattern, and inflammation. Patient 4 had only chronic urticaria (onset: 6.5 years). The striking features of this series were heterogeneity in clinical presentations including solely chronic urticaria and IIM; variable response to immunosuppressive treatments; and histopathology revealing a spectrum of necrosis, regeneration and inflammatory infiltration. Expanding the spectrum of anti-Ku positivity will allow better understanding of anti-Ku-associated phenotype clusters.

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Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease, pathologically characterized by TDP-43 aggregates. Recent evidence has been indicated that phosphorylated TDP-43 (pTDP-43) is present not only in motor neurons but also in muscle tissues. However, it is unclear whether testing pTDP-43 aggregation in muscle tissue would assist in the diagnosis of ALS. We propose three key questions: (i) Is aggregation of pTDP-43 detectable in routine biopsied muscles? (ii) Can detection of pTDP-43 aggregation discriminate between ALS and non-ALS patients? (iii) Can pTDP-43 aggregation be observed in the early stages of ALS? We conducted a diagnostic study comprising 2 groups: an ALS group in which 18 cases underwent muscle biopsy screened from a registered ALS cohort consisting of 802 patients and a non-ALS control group, in which we randomly selected 54 muscle samples from a biospecimen bank of 684 patients. Among the 18 ALS patients, 3 patients carried pathological GGGGCC repeats in the C9ORF72 gene, 2 patients carried SOD1 mutations, and 7 patients were at an early stage with only one body region clinically affected. The pTDP-43 accumulation could be detected in routine biopsied muscles, including biceps brachii, deltoid, tibialis anterior, and quadriceps. Abnormal aggregation of pTDP-43 was present in 94.4% of ALS patients (17/18) compared to 29.6% of non-ALS controls (16/54; p < 0.001). The pTDP-43 aggregates were mainly close to the sarcolemma. Using a semi-quantified pTDP-43 aggregates score, we applied a cut-off value of 3 as a diagnostic biomarker, resulting in a sensitivity of 94.4% and a specificity of 83.3%. Moreover, we observed that accumulation of pTDP-43 occurred in muscle tissues prior to clinical symptoms and electromyographic lesions. Our study provides proof-of-concept for the detection of pTDP-43 accumulation via routine muscle biopsy which may serve as a novel biomarker for diagnosis of ALS.

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We report the case of a 46-year-old female patient who developed a subacute progression of axial and proximal muscle weakness. Laboratory findings revealed mildly elevated serum creatine kinase levels. No monoclonal gammopathy was detected. A muscle biopsy revealed that she had nemaline myopathy. Serological tests and a lip biopsy revealed Sjögren's syndrome (SjS). We diagnosed her as having sporadic late-onset nemaline myopathy without monoclonal gammopathy of undetermined significance associated with SjS. Her symptoms improved after methylprednisolone pulse therapy followed by intravenous immunoglobulin therapy. A good response to immunotherapy demonstrates the necessity of making a correct diagnosis, for which a muscle biopsy is required.

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OBJECTIVE: X-linked myopathy with excessive autophagy (XMEA) linked to the VMA21 gene leads to autophagy failure with progressive vacuolation and atrophy of skeletal muscles. Current knowledge of this rare disease is limited. Our objective was to define the clinical, radiological, and natural history of XMEA. METHODS: We conducted a retrospective study collecting clinical, genetic, muscle imaging, and biopsy data of XMEA patients followed in France and reviewed the literature for additional cases. RESULTS: Eighteen males had genetically confirmed XMEA in France, carrying four different VMA21 variants. Mean age at disease onset was 9.4 ± 9.9 (range 1-40) years. In 14/18 patients (77.8%), onset occurred during childhood (< 15 years); however in four patients, the disease started in adulthood. Patients had anterior and medial compartment thigh muscle weakness, distal contractures (56.3%), elevated CK levels (1287.9 ± 757.8 U/l) and autophagic vacuoles with sarcolemmal features on muscle histopathology. Muscle MRI (n = 10) showed a characteristic pattern of lower limb muscle involvement. In 11 patients, outcome measures were available for an average follow-up period of 10.6 ± 9.8 years and six of them show disease progression. Mean change of functional outcomes was 0.5 ± 1.2 points for Brooke and 2.2 ± 2.5 points for Vignos score, 7/16 patients (43.8%) needed a walking aid and 3/16 (18.8%) were wheelchair-bound (median age of 40 years old, range 39-48). The variant c.164-7 T > G was associated with a later onset of symptoms. Respiratory insufficiency was common (57.1%) but cardiac involvement rare (12.5%). INTERPRETATION: XMEA has variable age of onset, but a characteristic clinical, histopathological, and muscle imaging presentation, guiding the diagnosis. Although slowly, motor disability progresses with time, and relevant genotype-phenotype correlations will help design future clinical trials.

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BACKGROUND: The significance of muscle biopsy as a diagnostic tool in idiopathic inflammatory myopathies (IIM) remains elusive. We aimed to determine the diagnostic weight that has been given to muscle biopsy in patients with suspected IIM, particularly in terms of clinical diagnosis and therapeutic decisions. MATERIAL AND METHODS: In this retrospective multicentric study, we analyzed muscle biopsy results of adult patients with suspected IIM referred to a tertiary center between January 1, 2007, and October 31, 2021. Information regarding referral department, suspected diagnosis, biopsy site, demographic, clinical, laboratory data, and imaging results were extracted. Statistical analyses included the level of agreement between suspected and histological diagnosis and calculation of diagnostic performance (positive and negative predictive values, positive and negative likelihood ratios, sensitivity, and specificity of muscle biopsy in relation to clinical diagnosis and/or treatment initiation). Performance was tested in different strata based on clinical pre-test probability. RESULTS: Among 758 muscle biopsies, IIM was histologically compatible in 357/758 (47.1%) cases. Proportion of IIM was higher if there was a solid clinical pre-test probability (64.3% vs. 42.4% vs. 48% for high, medium and low pre-test probability). Sensitivity and specificity of muscle biopsy were highest (82%) when the diagnosis by the clinician was used as outcome scenario. Negative predictive value was only moderate (between 63% and 80%) and lowest if autoantibodies were positive (35%). CONCLUSION: In patients with clinically suspected IIM, approximately 50% of biopsies revealed features indicative of IIM. Diagnostic performance of muscle biopsy was moderate to high depending on clinical pre-test probability.

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Myofibrillar myopathies (MFMs) are a group of genetically heterogeneous diseases affecting the skeletal and cardiac muscles. Myofibrillar myopathies are characterized by focal lysis of myogenic fibers and integration of degraded myogenic fiber products into inclusion bodies, which are typically rich in desmin and many other proteins. Herein, we report a case of a 54-year-old woman who experienced bilateral thigh weakness for over three years. She was diagnosed with MFMs based on muscle biopsy findings and the presence of a novel mutation in exon 8 of the LDB3 gene. Myofibrillar myopathies caused by a mutation in the LDB3 gene are extremely uncommon and often lack distinct clinical characteristics and typically exhibit a slow disease progression. When considering a diagnosis of MFMs, particularly in complex instances of autosomal dominant myopathies where muscle biopsies do not clearly indicate MFMs, it becomes crucial for clinicians to utilize genetic test as a diagnostic tool.

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INTRODUCTION/AIMS: Myxovirus resistance protein A (MxA) is a type I interferon (IFN1) pathway activation marker and MxA sarcoplasmic expression is currently recognized as a highly specific marker for dermatomyositis (DM). However, we have frequently observed endothelial tubuloreticular inclusions (TRI), another surrogate IFN1 activation marker, in a variety of overlap myositides. The aim of this study was to examine MxA expression in those myositides. METHODS: We retrospectively performed MxA immunostaining on a wide range of myositides. RESULTS: MxA sarcoplasmic expression was present in DM (94.4%, 17/18), active lupus myositis (LM, 80%,16/20), inactive LM (36%, 4/11), antisynthetase syndrome (ASyS, 20%, 2/10), systemic sclerosis (13%, 2/15), Sjogren's syndrome (7.7%, 1/13), and human immunodeficiency virus (HIV) myositis (5.6%, 1/18) and was absent in immune-mediated necrotizing myopathy (IMNM, 0/16) and hydroxychloroquine myopathy (0/5). The sensitivity and specificity of MxA sarcoplasmic expression for LM and DM combined compared with all other myositides were 84.6% (95% CI: 69.5-94.1) and 92.1 (95% CI: 83.6-97.0), respectively, and superior to TRIs. MxA capillary expression was nonspecific. Histologically, 35% of LM cases demonstrated a unique panfascicular necrotizing myopathy pattern. The remainder of the LM cases had significant morphological overlap with DM/ASyS (20%), IMNM (20%), or polymyositis (15%). DISCUSSION: MxA sarcoplasmic expression is highly prevalent in LM and DM and is a useful marker in differentiating DM and LM from other myositides. LM can manifest in various pathology patterns that need to be differentiated from DM, IMNM, ASyS, and polymyositis.

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