Immune Mediated Necrotizing Myopathy: Where do we Stand?

What Is in the Myopathy Literature?

ABSTRACT

We cover intensive care unit-acquired neuromuscular disorders associated with coronavirus disease 2019. Outcomes may be worse than expected in these patients, and there is some evidence that coronavirus disease 2019 causes myopathy directly. Corticosteroid regimens in Duchenne muscular dystrophy are addressed including outcomes in pulmonary and cardiac function. A recent article notes a continued diagnostic delay in Duchenne muscular dystrophy. An interesting report of a Canary Islands cohort of patients with oculopharyngeal muscular dystrophy is discussed. Features and clinical pearls related to a series of patients with limb-girdle muscle dystrophy R12 (anoctaminopathy) and a misdiagnosis of idiopathic inflammatory myopathy are provided. The last section on autoimmune myopathy includes articles on clinical and pathologic features associated with myositis-specific antibodies and dermatomyositis, the epidemiology of immune-mediated necrotizing myopathies (IMNMs) in Olmsted County, Minnesota, and features of a German cohort of hydroxy-3-methylglutaryl coenzyme A reductase-associated IMNM. A recent article proposes the benefit of early intravenous immunoglobulin use for adults with IMNM. We also highlight a report of 2 unusual cases of antisignal recognition particle myopathy presenting with asymmetric distal weakness.

Immune-mediated necrotizing myopathy which showed deposition of C5b-9 in the necrotic muscle fibers and was successfully treated with intensive combined therapy with high-dose glucocorticoids, tacrolimus, and intravenous immunoglobulins

Currently, no standard treatment strategy has been established for immune-mediated necrotizing myopathy (IMNM). Here we present a case of IMNM which was successfully treated with intensive combined therapy with high-dose glucocorticoids, tacrolimus, and intravenous immunoglobulins. Her muscle weakness was rapidly progressive and severe so that she became bedridden one week after admission. She was complicated with dysphagia and had serum myogenic enzymes elevation, ventricular diastolic dysfunction, and interstitial lung disease. Serum anti-SRP antibody was positive and her muscle biopsy revealed many necrotic fibers with minimal inflammation. Further histological analysis demonstrated infiltration of phagocytic macrophages with deposition of membrane attack complex (C5b-9) in the necrotic muscle fibers, suggesting activation of complement pathway and macrophages as a pathomechanism of this disease. She was diagnosed as IMNM and was immediately initiated a combination therapy described above, which led to dramatic clinical improvements. Recent studies suggest that intravenous immunoglobulins and tacrolimus can inhibit the activation of complement pathway and macrophages. Our present case suggests that early initiation of intensive combined therapy including intravenous immunoglobulins and tacrolimus might be effective for preventing irreversible muscle damages by disrupting a pathogenic activation of complement and macrophages in IMNM.

Role of Intravenous Immunoglobulin in Necrotizing Autoimmune Myopathy

BACKGROUND/OBJECTIVE

Immune-mediated necrotizing myopathy (IMNM) is a subtype of myositis that is associated with a refractory phenotype and poorer prognosis. The aim of the study was to provide single large center experience of outcomes of intravenous immunoglobulin (IVIg) for patients with IMNM using longitudinally collected data.

METHODS

This case series longitudinally evaluated 4 of the 6 myositis core set measures at baseline and at 3 and 6 months after IVIg on 20 adult IMNM patients from 2014 to 2019 at the University of Pittsburgh. We assessed patients for improvement in core set measures, prednisone dose, adverse effects, and by the "limited" ACR/EULAR 2016 myositis response criteria. The mean differences in CK and manual muscle testing (MMT-8) were compared using a paired t test. A clinically significant response was defined as a >10% absolute improvement in the MMT-8 and a >50% absolute reduction in serum CK at 6 months of IVIg.

RESULTS

Intravenous immunoglobulin treatment was associated with marked improvement in IMNM patients, with 85% of patient meeting clinically significant response. The median (interquartile range) relative percent improvement in CK level was 96% (85%-98%) and in MMT was 29% (14%-36%) at 6 months.There was a significant reduction in the mean (SD) dose of prednisone at 6 months and had minimal adverse effects. In addition, with IVIg, most (13/14) patients had at least minimal improvement as per ACR/EULAR 2016 myositis response criteria.

CONCLUSIONS

Based on objective, meaningful improvement in MMT-8 and CK as well as marked reduction in prednisone doses with acceptable tolerability, early implementation of IVIg should be considered in adult IMNM.

TGM2 positively regulates myoblast differentiation via enhancing the mTOR signaling

Myoblast differentiation is an essential process for the control of muscle regeneration. However, the intrinsic mechanisms underlying this dynamic process are still not well clarified. Herein, we identified transglutaminase type 2 (TGM2) as a novel regulator of muscle differentiation and regeneration in vitro and in vivo. Specifically, knockdown of TGM2 suppresses whereas overexpression of TGM2 promotes myoblast differentiation in differentiating C2C12 cells. Mechanistic studies revealed that TGM2 promotes C2C12 myoblast differentiation via enhancing GPR56 mediated activation of the mTOR signaling. Additionally, lentivirus mediated knockdown of TGM2 hinders the regeneration of muscles in a BaCl₂ induced skeletal muscle injury model of mice. Finally, we found that both TGM2 and activation of the mTOR signaling are up-regulated in muscles of patients with immune-mediated necrotizing myopathy (IMNM), especially in the regenerating myofibers. Collectively, our research demonstrates that TGM2 positively regulates muscle differentiation and regeneration through facilitating the myogenic mTOR signaling, which might be a potential target of therapy for skeletal muscle injury.

Atorvastatin-Induced Necrotizing Myopathy and its Response to Combination Therapy

Atorvastatin is the most commonly used statin medication to decrease cholesterol levels and prevent atherosclerosis. Myopathy is a reported side effect of atorvastatin which can happen even after more than six months after starting the medication. The side effect on the muscle tissue can range from simple reversible myalgia to respiratory muscle compromise. Here we present a 46-year-old male who presented with myopathy after taking atorvastatin for two years. Biopsy proved immune-mediated necrotizing myopathy which responded to a combination of Rituximab and intravenous immunoglobulin therapy.

Influenza A (H3N2) infection followed by anti-signal recognition particle antibody-positive necrotizing myopathy: A case report

A 60-year-old Japanese woman presented with subacute progressive muscle pain and weakness in her proximal extremities. She was diagnosed with influenza A (H3N2) infection a week before the onset of muscle pain. At the time of admission, she exhibited weakness in the proximal muscles of the upper and lower limbs, elevated serum liver enzymes and creatinine kinase, and myoglobinuria. She did not manifest renal failure and cardiac abnormalities, indicating myocarditis. Electromyography revealed myogenic changes, and magnetic resonance imaging of the upper limb showed abnormal signal intensities in the muscles, suggestive of myopathy. Muscle biopsy of the biceps revealed numerous necrotic regeneration fibers and mild inflammatory cell infiltration, suggesting immune-mediated necrotizing myopathy (IMNM). Necrotized muscle cells were positive for human influenza A (H3N2). Autoantibody analysis showed the presence of antibodies against the signal recognition particle (SRP), and the patient was diagnosed with anti-SRP-associated IMNM. She was resistant to intravenous methylprednisolone pulse therapy but recovered after administration of oral systemic corticosteroids and immunoglobulins. We speculate that the influenza A (H3N2) infection might have triggered her IMNM. Thus, IMNM should be considered as a differential diagnosis in patients with proximal muscle weakness that persists after viral infections.

Necrotizing myositis in a rectus muscle arising in the setting of long-standing Langerhans cell histiocystosis and recent dabrafenib treatment

Purpose

to describe an unusual case of necrotizing myositis in a rectus muscle, possibly related to BRAF inhibitor therapy.

Observations

An 18-year old man with neurodegenerative Langerhans cell histiocytosis (LCH), recently started on the BRAF inhibitor dabrafenib, presented with right eye pain. Magnetic resonance imaging (MRI) orbits revealed a rectus muscle mass concerning for LCH recurrence or malignancy. Dabrafenib was stopped, and incisional biopsy of the mass was performed. The mass was absent on post-operative MRI, so no further treatment was pursued. Histopathologic evaluation was initially concerning for sarcoma, but on further analysis, appeared more consistent with necrotizing myositis. The mass did not recur, nor did the patient develop other signs or symptoms concerning for myositis or malignancy over a 24-month follow-up period.

Conclusions

Necrotizing myositis has not been previously described in a rectus muscle or with BRAF inhibitor use, though myalgias and malignancies are established side effects. Necrotizing myositis may masquerade as sarcoma and should be on the differential diagnosis for a new mass in the setting of dabrafenib therapy.

Report of a case of necrotizing autoimmune myopathy with thymoma-associated myasthenia gravis

Purpose: Myasthenia gravis (MG) is an autoimmune disease and closely related to thymoma. Inflammatory myopathy may accompany with other autoimmune diseases. However, concurrence of inflammatory myopathy and MG is very rare. Necrotizing autoimmune myopathy (NAM), a rare form of inflammatory myopathy, is characterized by necrosis and regeneration of myocytes in proximal muscles without significant inflammation. The aim of the study was to report a rare case of NAM and concomitant thymoma-associated MG after thymectomy.Materials and methods/results: A 27-year-old female patient presented with muscle soreness and weakness in four limbs. Eyelid fatigue and neostigmine tests were negative, and no ptosis was found but the electromyographic examination (EMG) showed myogenic damage and a gradual decrease in the amplitude (20%) of EMG activities evoked by repetitive electrical stimulation. Antibodies against AChR and increased titer of creatine kinase were detected and plaque-like signals in both legs were found in magnetic resonance imaging. Myositis-related antibodies were negative but necrotic myocytes without inflammatory cell infiltration, and MHC-1 positive muscle fibers were found in muscle biopsy. Pathological examination confirmed anterior mediastinal B2 type thymoma. Five weeks after thymectomy, she started to show typical MG symptoms. No recurrence of thymoma was found but immunoassay showed a higher titer of AChR-Ab. Myositis-related antibodies negative necrotizing autoimmune myopathy (NAM) was reported to be associated with thymoma-associated MG.Conclusions: The patient showed symptoms related NAM but developed MG-related symptoms only after thymectomy. The mechanisms for the phenomena may be related to immune dysfunction associated with thymoma.