Immune-mediated necrotizing myopathy: Unusual presentations of a treatable disease

Immune-mediated necrotizing myopathy: A comprehensive review of the pathogenesis, clinical features, and treatments

Immune-mediated necrotizing myopathy (IMNM) is a rare and newly recognized autoimmune disease within the spectrum of idiopathic inflammatory myopathies. It is characterized by myositis-specific autoantibodies, elevated serum creatine kinase levels, inflammatory infiltrate, and weakness. IMNM can be classified into three subtypes based on the presence or absence of specific autoantibodies: anti-signal recognition particle myositis, anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase myositis, and seronegative IMNM. In recent years, IMNM has gained increasing attention and emerged as a research hotspot. Recent studies have suggested that the pathogenesis of IMNM is linked to aberrant activation of immune system, including immune responses mediated by antibodies, complement, and immune cells, particularly macrophages, as well as abnormal release of inflammatory factors. Non-immune mechanisms such as autophagy and endoplasmic reticulum stress also participate in this process. Additionally, genetic variations associated with IMNM have been identified, providing new insights into the genetic mechanisms of the disease. Progress has also been made in IMNM treatment research, including the use of immunosuppressants and the development of biologics. Despite the challenges in understanding the etiology and treatment of IMNM, the latest research findings offer important guidance and insights for delving deeper into the disease's pathogenic mechanisms and identifying new therapeutic strategies.

Secondary mitochondrial dysfunction across the spectrum of hereditary and acquired muscle disorders

Mitochondria form a dynamic network within skeletal muscle. This network is not only responsible for producing adenosine triphosphate (ATP) through oxidative phosphorylation, but also responds through fission, fusion and mitophagy to various factors, such as increased energy demands, oxidative stress, inflammation, and calcium dysregulation. Mitochondrial dysfunction in skeletal muscle not only occurs in primary mitochondrial myopathies, but also other hereditary and acquired myopathies. As such, this review attempts to highlight the clinical and histopathologic aspects of mitochondrial dysfunction seen in hereditary and acquired myopathies, as well as discuss potential mechanisms leading to mitochondrial dysfunction and therapies to restore mitochondrial function.

Ophthalmoparesis as an unusual manifestation of anti-3‑hydroxy-3-methyl-glutaryl-coenzyme A reductase antibody-associated myopathies

We describe two anti-3‑hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR) antibody-positive patients with treatment-responsive ophthalmoparesis. Patient 1 was a 53-year-old male with progressive proximal limb weakness, dysphagia, ptosis, and diplopia over 6 weeks and creatine kinase (CK) of 3,512 units/L. Patient 2 was a 55-year-old female with progressive proximal weakness, dysarthria, ptosis, diplopia, and dyspnea over 2 weeks with CK of 31,998 units/L. Both patients had normal thyroid studies and repetitive nerve stimulation, myopathic electromyography with fibrillation potentials, magnetic resonance imaging demonstrating abnormal enhancement of extraocular muscles, muscle biopsy showing necrotic myofibers, and positive anti-HMGCR antibodies. Patient 1 also had weakly positive anti-PM/Scl antibodies. Immunomodulatory therapies led to resolution of oculobulbar weakness and normalization of CK levels in both patients, while limb weakness resolved completely in patient 1 and partially in patient 2. These cases expand the phenotypic spectrum of anti-HMGCR antibody-associated myopathies to include subacute ophthalmoparesis with limb-girdle weakness and markedly elevated CK.

Granulomatous myopathy co-existent immune-mediated necrotizing myopathy: A case report

Granulomatous myopathy (GM) is a rare disease characterized by non-caseating inflammation of the skeletal muscle, with sarcoidosis as a common cause. Here, we report a case of GM co-existent immune-mediated necrotizing myopathy (IMNM) in which an anti-signal recognition particle (SRP) antibody was positive and a muscle biopsy showed a non-caseating granulomatous structure, along with myofiber necrosis and inflammatory cell infiltration.

Symptomatic myopathies in sarcoidosis: disease spectrum and myxovirus resistance protein A expression

OBJECTIVES

Symptomatic myopathy in sarcoidosis patients is not always due to sarcoid myopathy (ScM). We investigated the clinical and pathological spectrum including myxovirus resistance protein A (MxA) expression among sarcoidosis patients.

METHODS

We reviewed the Mayo Clinic database (May 1980-December 2020) to identify sarcoidosis patients with myopathic symptoms and pathological evidence of myopathy.

RESULTS

Among 5885 sarcoidosis patients, 21 had symptomatic myopathy. Eight carried a diagnosis of sarcoidosis 5.5 years (median) prior to myopathy onset. Eleven patients had ScM. The remaining had non-sarcoid myopathies (five IBM, one immune-mediated necrotizing myopathy, one non-specific myositis, two non-specific myopathy and one steroid myopathy). Estimated frequency of IBM is 85 per 100 000 sarcoidosis patients. The following features were associated with non-sarcoid myopathies (P < 0.05): (i) predominant finger flexor and quadriceps weakness, (ii) modified Rankin scale (mRS) >2 at time of diagnosis, (iii) creatine kinase >500 U/l, and (iv) absence of intramuscular granulomas. Sarcoplasmic MxA expression was observed in scattered myofibres in three patients, two of whom were tested for DM-specific autoantibodies and were negative. Immunosuppressive therapy led to improvement in mRS ≥1 in 5/10 ScM, none of the five IBM, and 3/3 remaining patients with non-sarcoid myopathies.

DISCUSSION

Symptomatic myopathy occurred in 0.36% of sarcoidosis. IBM was the second most common cause of myopathies after ScM. Frequency of IBM in sarcoidosis is higher than in the general population. Recognition of features suggestive of alternative aetiologies can guide proper treatment. Our findings of abnormal MxA expression warrant a larger study.

A Rare Case of PL-7-Associated Immune-Mediated Necrotizing Myopathy With Isolated Dysphagia as the Presenting Symptom

Immune-mediated necrotizing myopathy (IMNM) is a rare, progressive disease that accounts for about 19% of all inflammatory myopathies. Dysphagia occurs in about 20%-30% of IMNM patients. This case results in the third presumptive instance of IMNMwith dysphagia as the initial symptom. Given that isolated dysphagia in IMNM is atypical to the conventional symptoms in the late stage of the disease, it is critical for clinicians to have a high degree of suspicion for IMNM due to the aggressive nature of the disease and its refractoriness to treatment. Additionally, this case also highlights an atypical autoantibody, PL-7, being positive in an IMNM patient who presents with dysphagia as an initial symptom.

Granulomatous myopathy: Sarcoidosis and beyond

Non-necrotizing granulomatous inflammation is a rare but easily recognized histopathological finding in skeletal muscle biopsy. A limited number of diseases are known to be associated with non-necrotizing granulomatous myopathy. Once identified, a careful evaluation for evidence of extramuscular granulomatosis and other signs suggestive of sarcoidosis is warranted as about half of the patients have sarcoid myopathy. In addition, the presence of granulomatous myopathy should trigger a search for clinical and pathological clues of inclusion body myositis (IBM), which accounts for most of the remaining patients and can coexist with sarcoidosis. Recognizing the features of IBM in patients with granulomatous myopathy can potentially spare the patients from unnecessary exposure to immunosuppressive therapies. In patients whose granulomatous myopathy remain unexplained, further investigations should aim at identifying myasthenia gravis and other autoimmune disorders, especially those known to cause granulomatous inflammation in other organs. Laboratory investigations should include acetylcholine receptor, antimitochondrial, antineutrophil cytoplasmic, thyroglobulin, and thyroid peroxidase autoantibodies. In the appropriate clinical context, exposure to immune checkpoint inhibitors and chronic graft-vs-host disease can be causes of granulomatous myopathy. In cases of unexplained granulomatous myopathy, natural killer/T-cell lymphoma should be considered and careful histopathological examination for atypical cells and appropriate immunostaining is crucial. Identifying the etiology of granulomatous myopathy in each patient can guide appropriate treatment.

Leflunomide-Induced Immune-Mediated Necrotizing Myopathy in a Patient With Rheumatoid Arthritis: A Case Report

Immune-mediated necrotizing myopathy (IMNM) is a subtype of inflammatory myopathy that is characterized by proximal muscle weakness, markedly elevated serum creatine kinase, myopathic electromyographic findings, and muscle biopsies revealing necrosis or regeneration with sparse inflammatory infiltrate. IMNM tends to be idiopathic but has been associated with certain medications. This supports the possibility for other pharmacotherapies to induce IMNM-particularly leflunomide. Leflunomide is used in the treatment for rheumatoid arthritis and has been shown to induce autoimmune diseases-including autoimmune hepatitis and polymyositis. After an extensive review of history and workup of muscle weakness, we conclude that leflunomide induced an IMNM in our patient. As this is the first case of leflunomide-induced IMNM, it is important for clinicians to suspect an inflammatory myopathy in the setting of myositis while on leflunomide.

Anti-HMGCR myopathy: barriers to prompt recognition

Anti-HMGCR (3-hydroxy-3-methylglutaryl coenzyme A reductase) myopathy is an immune-mediated necrotising myopathy. Atypical presentations hinder its recognition and its prompt treatment. We present two patients with atypical clinical or pathological features. A 45-year-old woman had an asymptomatic serum creatine kinase (CK) of ~10 000 IU/L and muscle biopsy showing minimal changes. She then developed slowly progressive proximal weakness, diagnosed as limb-girdle muscular dystrophy but with negative genetics. Twelve years later, now with severe proximal weakness, her MR scan of muscle showed diffuse asymmetrical fatty degeneration, with conspicuous hyperintense STIR signal abnormalities. HMGCR antibodies were positive and she partially improved with immunosuppression. The second patient developed slowly progressive proximal limb weakness with a high serum CK (~4000 IU/L); muscle biopsy showed a lymphocyte infiltrate with angiocentric distribution suggesting vasculitis. Serum HMGCR antibodies were positive. Anti-HMGCR myopathy can present as a slowly progressive myopathy with atypical pathology. HMGCR antibody screening is indicated for people with suspected limb-girdle muscular dystrophy or atypical inflammatory muscle conditions.

Anti-SRP immune-mediated necrotizing myopathy: A critical review of current concepts

Purpose of review

This review aims to describe clinical and histological features, treatment, and prognosis in patients with anti-signal recognition particle (SRP) autoantibodies positive immune-mediated necrotizing myopathy (SRP-IMNM) based on previous findings.

Previous findings

Anti-SRP autoantibodies are specific in IMNM. Humoral autoimmune and inflammatory responses are the main autoimmune characteristics of SRP-IMNM. SRP-IMNM is clinically characterized by acute or subacute, moderately severe, symmetrical proximal weakness. Younger patients with SRP-IMNM tend to have more severe clinical symptoms. Patients with SRP-IMNM may be vulnerable to cardiac involvement, which ought to be regularly monitored and cardiac magnetic resonance imaging is the recommended detection method. The pathological features of SRP-IMNM are patchy or diffuse myonecrosis and myoregeneration accompanied by a paucity of inflammatory infiltrates. Endoplasmic reticulum stress-induced autophagy pathway and necroptosis are activated in skeletal muscle of SRP-IMNM. Treatment of refractory SRP-IMNM encounters resistance and warrants further investigation.

Summary

Anti-SRP autoantibodies define a unique population of IMNM patients. The immune and non-immune pathophysiological mechanisms are involved in SRP-IMNM.

Incidence and Prevalence of Immune-mediated Necrotizing Myopathy in Adults in Olmsted County, Minnesota

INTRODUCTION/AIMS

Immune-mediated necrotizing myopathy (IMNM) is considered a rare subtype of the immune-mediated myopathies, but its incidence and prevalence are unknown. This study aimed to determine the incidence and prevalence of IMNM in the adults in Olmsted County, Minnesota.

METHODS

We identified adult patients with IMNM defined by the 2016 European Neuromuscular Centre diagnostic criteria among Olmsted County, Minnesota, residents over a 20-year period RESULTS: Seven patients fulfilled the inclusion criteria. Six patients were tested for IMNM antibodies: 4 were anti 3-Hydroxy-3-Methylglutaryl-CoA Reductase (HMGCR)-positive, 1 was anti-signal recognition particle (SRP)-positive and 1 was seronegative. The incidence of IMNM during 2010-2019 was 8.3 per million person-years. The prevalence of IMNM in 2010 was 1.85 per 100,000 people ≥50 years. Median age at symptom onset was 64 years (range: 52-86) and median time from symptom onset to diagnosis was 3 months (range <1-156). Statin use among anti-HMGCR IMNM patients, but not the entire IMNM cohort, was higher than in controls (P=0.024). Two IMNM patients developed cancers. The incidence of malignancy in IMNM was not higher than that of the general population. Treatment outcome was favorable in all patients except for 1 with delayed treatment and one with insufficient therapy. Among 3 deceased patients, 1 died from cancer while 2 died from IMNM-related cardiorespiratory complications.

DISCUSSION

IMNM is a rare disease. Its prevalence is one tenth that of inclusion body myositis in Olmsted County, Minnesota. IMNM patients in our cohort were not at higher risk for developing cancer.