Inclusion Body Myositis

Inclusion body myositis (IBM) is the most common acquired myopathy in adults over age 50, with an estimated prevalence of 77 per million in individuals older than 50 years. Males are affected approximately twice as often as females. IBM is classified among the idiopathic inflammatory myopathies due to the presence of endomysial inflammation on muscle biopsy; however, it is distinguished by its insidious onset, characteristic pattern of weakness involving the deep finger flexors and quadriceps, resistance to immunosuppressive therapy, and prominent degenerative features on histopathology. Unlike other inflammatory myopathies, IBM is fundamentally a disease of aging—sharing pathomechanisms with neurodegenerative disorders such as Alzheimer disease and Parkinson disease, including disrupted protein homeostasis, mitochondrial dysfunction, and accumulation of abnormal protein aggregates.

Epidemiology

IBM predominantly affects individuals in their fifth decade or older, with a peak onset between ages 50 and 70. The male predominance (~2:1 ratio) distinguishes it from other inflammatory myopathies, which generally show female predominance. Prevalence estimates vary by population and study methodology but converge on approximately 46–77 per million in individuals over 50 years. IBM is likely underdiagnosed given its indolent onset, frequent misdiagnosis as polymyositis, and the ~5-year average delay from symptom onset to correct diagnosis. Historically, many patients diagnosed with “treatment-refractory polymyositis” were in fact harboring IBM.

IBM is associated with modestly decreased survival. In a population-based study, patients with IBM had a mean age at death of 79.3 years compared with 83.6 years in age-matched and sex-matched controls. The leading causes of death are complications of dysphagia (aspiration pneumonia) and respiratory insufficiency. Patients with IBM may have comorbid autoimmune conditions, particularly associations with HLA-DRB1 alleles that influence disease risk and clinical phenotype. Presentation in childhood or adolescence, rapidly progressive weakness, or a family history of affected individuals is inconsistent with IBM and should prompt evaluation for alternative diagnoses, particularly hereditary myopathies. Familial cases of IBM are exceedingly rare.

Clinical Features

Characteristic Pattern of Weakness

The hallmark of IBM is slowly progressive, asymmetric weakness with selective involvement of the deep finger flexors and quadriceps muscles. This distinctive pattern develops insidiously over months to years:

• Deep finger flexors (FDP/FPL): Weakness of the flexor digitorum profundus and flexor pollicis longus impairs grip strength and fine motor tasks such as buttoning, turning keys, and opening jars. Weakness may vary markedly between individual fingers in the same hand—for example, complete paralysis of the flexor pollicis longus with preserved strength of the fifth finger flexor. The “finger-flexor sign” is elicited by asking the patient to supinate the forearm and make a fist, revealing incomplete finger flexion
• Quadriceps: Because the quadriceps is the largest and strongest muscle group, mild early weakness may be missed on manual testing. Functional testing—such as asking the patient to kneel on one knee and stand without arm support—may reveal weakness earlier. Progressive quadriceps weakness leads to early falls, difficulty rising from chairs, and eventual loss of ambulation (typically within 10 years of onset)
• Other commonly affected muscles: Ankle dorsiflexors (foot drop), elbow flexors and extensors (triceps), hip flexors, neck flexors, and facial muscles

Dysphagia

Dysphagia occurs in up to 60% of patients with IBM and can develop at any disease stage—even as the initial presenting symptom. Oropharyngeal weakness causes difficulty initiating swallowing, with food “getting stuck” in the throat, nasal regurgitation, and choking episodes. A cricopharyngeal bar (prominent cricopharyngeus muscle) may contribute to obstructive dysphagia and is potentially amenable to dilation or cricopharyngeal myotomy. Barium swallow evaluation may reveal a prominent cricopharyngeal bar with marked luminal obstruction. Dysphagia can markedly affect quality of life and may result in social withdrawal, weight loss, and nutritional deficiency. Aspiration pneumonia secondary to dysphagia is a major cause of morbidity and mortality in IBM.

Importantly, patients with IBM may initially present with dysphagia alone, without overt limb weakness. In such cases, searching for subclinical muscle involvement on EMG or muscle imaging is highly valuable for establishing the diagnosis. Dysphagia as the presenting symptom is more common in female patients and may precede limb weakness by several years.

Atypical Presentations

Sex-Specific Differences

Males tend to have more severe finger extension and knee extension weakness. Females exhibit a slower rate of strength decline and are more likely to develop dysphagia. Isolated dysphagia as the presenting feature is most commonly seen in females, and facial diplegia has been reported exclusively in females. Variation by race, ethnicity, and geographic location is not well established; however, one study reported more severe proximal limb weakness in Black patients compared with White patients. These sex-specific and demographic differences remain areas requiring further investigation.

Pathogenesis

Dual Pathogenesis: Autoimmune and Degenerative

IBM occupies a unique position at the intersection of autoimmunity and neurodegeneration. The coexistence of inflammatory and degenerative features has generated decades of debate about whether inflammation drives degeneration or vice versa. Current evidence supports a model in which both processes are intertwined and self-reinforcing:

One hypothesis for IBM’s refractoriness to immunotherapy is that the highly differentiated cytotoxic CD8+ T cells driving muscle damage are resistant to conventional immunosuppressants, which preferentially target proliferating and less differentiated lymphocytes. These terminally differentiated T cells express killer cell lectin-like receptor G1 (KLRG1), which has become a therapeutic target in ongoing clinical trials. Critically, in a xenograft model of IBM, T-cell depletion failed to reverse TDP-43 pathology and rimmed vacuole formation, suggesting that the degenerative cascade, once initiated, may be self-sustaining and independent of the inflammatory component.

The interplay between aging and IBM pathogenesis is increasingly recognized. Like Alzheimer disease and Parkinson disease, IBM shares hallmarks of cellular aging: mitochondrial dysfunction, disrupted protein homeostasis (proteostasis), epigenetic alterations, and impaired autophagy. Recent work has identified NLRP3 inflammasome activation and altered mitophagy as key pathways in IBM, linking the inflammatory and mitochondrial components. Sex-specific metabolomic alterations centered on mitochondrial pathways have also been identified, highlighting the biological basis for the male predominance and sex-based differences in clinical presentation.

Diagnostic Workup

Laboratory Testing

• Creatine kinase (CK): Elevated in most patients but typically <10–15 times the upper limit of normal; normal CK does not exclude IBM, particularly in patients with insidious onset or advanced disease with reduced muscle mass; very high CK (>15× ULN) is atypical and should prompt consideration of alternatives such as immune-mediated necrotizing myopathy or muscular dystrophy
• Anti-cN1A (anti-NT5C1A) antibody: Positive in approximately 50% of IBM patients; commercially available ELISA and line immunoassay formats exist with variable sensitivity (33–89%) and specificity (80–96%); supports the diagnosis but should not be used as a standalone test because it may be positive in connective tissue diseases including systemic lupus erythematosus, Sjögren syndrome, and other inflammatory myopathies; some studies suggest anti-cN1A seropositivity is associated with more severe dysphagia, although a clear association with overall disease severity or clinical phenotype has not been established
• T-cell large granular lymphocyte (LGL) clone: Detectable by flow cytometry or T-cell receptor gene rearrangement; present in a subset of IBM patients; usually a benign phenomenon, though cases of T-cell LGL leukemia have rarely been reported; should raise suspicion for IBM when detected in the appropriate clinical context; T-cell LGL clones may also occur in Sjögren syndrome and rheumatoid arthritis
• Myositis-specific antibodies: Standard myositis panels (anti-Jo-1, anti-Mi-2, anti-SRP, anti-HMGCR, anti-MDA-5, anti-TIF1-γ) should be checked to exclude other inflammatory myopathies; they are characteristically negative in IBM

Electrodiagnostic Testing

Nerve conduction studies and EMG are essential for confirming the myopathic nature of the process and excluding mimics such as compressive mononeuropathy, radiculopathy, motor neuron disease, or neuromuscular junction disorders. In IBM, EMG typically shows a characteristic mixed pattern of both short-duration (“myopathic”) and long-duration (“neuropathic”) motor unit potentials, reflecting the complex remodeling of muscle fibers. Fibrillation potentials and occasionally myotonic discharges are present, indicating active muscle fiber instability. This mixed pattern is a hallmark of IBM on electrodiagnostic testing and may lead to the erroneous diagnosis of amyotrophic lateral sclerosis if the myopathic units are overlooked.

Strategies to increase electrodiagnostic accuracy include: examining the flexor pollicis longus (less commonly affected by entrapment neuropathies), contrasting the severity of EMG findings in specific muscles (e.g., quadriceps) with the severity of structural abnormalities at the corresponding spinal level (L3/4), and analyzing motor unit potentials at low activation where individual units can be more easily isolated. EMG also helps select an appropriate target for muscle biopsy.

Muscle Imaging

MRI shows a characteristic pattern of fatty replacement in IBM that, while not pathognomonic, strongly supports the diagnosis:

• Prominent involvement of the quadriceps with a distal-to-proximal gradient, most severely affecting the distal vastus medialis and lateralis
• Relative sparing of the rectus femoris
• Sparing of the posterior thigh and hip girdle muscles
• In the leg, prominent involvement of the medial gastrocnemius
• T2/STIR hyperintensity may indicate active muscle edema

Muscle ultrasound shows increased echogenicity of the vastus medialis and lateralis, medial gastrocnemius, and deep finger flexor muscles, and can help differentiate IBM from disease mimics.

Muscle Biopsy

Muscle biopsy remains the cornerstone of IBM diagnosis. The three canonical histopathologic features are:

2024 ENMC Diagnostic Criteria

In 2024, the European Neuromuscular Centre (ENMC) revised the 2013 IBM diagnostic criteria to improve sensitivity and incorporate advances in diagnostic tools. Major changes include the recognition of atypical presentations, addition of mitochondrial abnormalities as a supportive histopathologic feature, incorporation of anti-cN1A antibodies and muscle imaging as supportive criteria, and elimination of the hierarchical diagnostic categories. Strict age and CK cutoffs are no longer required.

Supportive criteria include: (1) rimmed vacuoles and/or protein aggregates on biopsy; (2) mitochondrial abnormalities (COX-negative fibers, ragged red/blue fibers); (3) positive anti-cN1A antibody; (4) typical muscle MRI or ultrasound pattern.

The 2024 revision represents a significant advance over the 2013 criteria by eliminating the hierarchical diagnostic categories (clinicopathologically defined, clinically defined, probable) that generated confusion. In the new framework, the level of diagnostic evidence required scales with the degree of clinical certainty—the more typical the presentation, the less additional evidence is needed beyond the mandatory biopsy criterion. Notably, the sensitivity and specificity of the 2024 criteria remain to be formally validated in prospective cohorts.

Differential Diagnosis

The pattern of deep finger flexor and quadriceps weakness is characteristic but not pathognomonic for IBM. Several conditions may present with similar distribution:

Red flags suggesting an alternative diagnosis: Rapid progression (IBM typically takes 5–6 years before requiring gait assistance), onset before age 45, positive family history of myopathy, cardiomyopathy, autonomic dysfunction, CK >15 times the upper limit of normal, and orthopnea at presentation (respiratory involvement in IBM usually occurs at advanced stages). A detailed motor examination keeping in mind the commonly and uncommonly affected muscles in IBM, gauging the rate of progression, obtaining a comprehensive family history, and screening for subtle action or percussion myotonia are critical steps in guiding the diagnostic workup.

Management

Pharmacologic Treatment: Current Limitations

There is no evidence-based pharmacologic treatment for IBM. All immunosuppressive and immunomodulatory agents trialed to date have been ineffective, including corticosteroids, methotrexate, azathioprine, mycophenolate mofetil, IVIg, anti-T lymphocyte globulin, alemtuzumab, and beta-interferon. Moreover, corticosteroids and other immunosuppressants may be associated with worse outcomes, potentially through catabolic muscle effects compounding IBM weakness, metabolic complications, and failure to address the degenerative component of disease while exposing patients to treatment-related adverse effects. The refractoriness of IBM to immunotherapy is now understood as reflecting the fundamental nature of the disease: the pathogenic highly differentiated T cells are resistant to standard immunosuppression, and the degenerative cascade (protein aggregation, mitochondrial dysfunction) appears to be self-sustaining even when inflammation is reduced.

Dysphagia Management

Given that aspiration pneumonia is a major cause of death, proactive dysphagia screening and management are essential:

• Regular screening with structured questionnaires (e.g., the Eating Assessment Tool-10) and referral to speech-language pathology, including for asymptomatic patients
• Modified barium swallow study or fiberoptic endoscopic evaluation to characterize the swallowing deficit
• Diet modification (texture, consistency), swallowing techniques, and postural adjustments
• Cricopharyngeal myotomy or dilation: Indicated when a prominent cricopharyngeal bar contributes to obstructive dysphagia; both endoscopic and transcervical approaches are available
• Percutaneous gastrostomy (PEG) tube placement for severe dysphagia with recurrent aspiration or significant weight loss

Respiratory Management

Respiratory involvement usually occurs at advanced disease stages in association with severe limb weakness, although it may appear earlier in patients with prominent craniobulbar involvement. Symptoms include orthopnea, daytime somnolence, morning headaches, and snoring—asking only about shortness of breath may miss respiratory muscle weakness. Evaluation includes overnight oximetry, pulmonary function tests (particularly forced vital capacity in upright and supine positions), and referral to sleep medicine for consideration of noninvasive ventilation (BiPAP).

Exercise and Rehabilitation

A physical medicine and rehabilitation team plays a key role in IBM management at all disease stages. Tailored exercise programs are safe and beneficial, improving or maintaining strength, endurance, and quality of life. Importantly, exercise does not exacerbate muscle inflammation in IBM. Programs should be individualized to the patient’s abilities and limitations, incorporating both resistance and aerobic components when tolerable. As the disease progresses, emphasis shifts to adaptive strategies, fall prevention, and assistive device prescription.

Multidisciplinary Care

A comprehensive care team should include neurology, speech-language pathology, physical medicine and rehabilitation, occupational therapy, sleep medicine, and pulmonology. Key interventions include:

• Gait assistive devices (cane, walker, wheelchair)—most patients require a gait aid 5–6 years after symptom onset
• Home and workplace modifications for accessibility and safety
• Adaptive equipment for hand weakness (built-up utensils, jar openers, button hooks)
• Fall prevention strategies addressing quadriceps weakness and impaired balance
• Nutritional counseling, particularly for patients with dysphagia and weight loss
• Psychosocial support for the impact of progressive disability on quality of life and social participation

Emerging Therapies and Clinical Trials

While the clinical trial landscape for IBM has historically been limited, two major trials are providing cautious optimism:

The rationale for ulviprubart addresses a fundamental limitation of prior immunotherapy: conventional immunosuppressants target proliferating lymphocytes but fail to deplete the terminally differentiated KLRG1-expressing CD8+ T cells that drive muscle damage in IBM. By selectively targeting this pathogenic population, ulviprubart aims to reduce autoimmune injury while preserving protective immunity.

The rationale for sirolimus addresses the degenerative component of IBM. By activating autophagy through mTOR inhibition, sirolimus may promote clearance of protein aggregates and improve mitochondrial function—both central to IBM pathogenesis.

Additional areas of investigation include the NLRP3 inflammasome pathway (linked to altered mitophagy in IBM), sex-specific metabolomic alterations, and advances in understanding age-related pathways common to IBM and other neurodegenerative diseases. The ongoing progress in the broader field of aging research and neurodegenerative disease therapeutics—including strategies targeting mitochondrial dysfunction, disrupted proteostasis, and epigenetic alterations—offers potential pathways for IBM drug development.

Natural History and Prognosis

IBM follows a slowly progressive course with significant individual variability. In a 12-year follow-up study, the majority of patients required a wheelchair for mobility and had lost meaningful grip strength. Key milestones in disease progression include:

• 5–6 years: Most patients require a gait assistive device (cane or walker)
• ~10 years: Complete loss of independent ambulation in most patients
• Progressive hand disability: Loss of deep finger flexor and grip strength leads to inability to perform fine motor tasks, write, or open containers
• Dysphagia progression: May develop at any stage; causes social withdrawal, weight loss, and risk of aspiration pneumonia
• Respiratory decline: Typically occurs at advanced stages; may require noninvasive ventilatory support

IBM modestly decreases longevity, with a mean age at death of approximately 79 years compared with 84 years in age-matched controls. Complications of dysphagia (aspiration pneumonia) and respiratory involvement are the most common causes of death. IBM is not associated with cardiomyopathy, autonomic failure, or cognitive impairment—the presence of these features should prompt consideration of alternative diagnoses. There is no evidence that IBM undergoes spontaneous remission.

The diagnosis of IBM should prompt early engagement with rehabilitation services, proactive screening for dysphagia and respiratory compromise, and longitudinal monitoring of functional status. The IBM Functional Rating Scale (IBMFRS) provides a standardized tool for tracking disease progression and is used as a primary outcome measure in clinical trials. Patient education about the disease course, available resources, and participation in clinical trials is an essential component of management.

References

1. Naddaf E. Inclusion body myositis. Continuum (Minneap Minn) 2025;31(5):1372–1384.
2. Callan A, Capkun G, Vasanthaprasad V, Freitas R, Needham M. A systematic review and meta-analysis of prevalence studies of sporadic inclusion body myositis. J Neuromuscul Dis 2017;4(2):127–137.
3. Naddaf E, Shelly S, Mandrekar J, et al. Survival and associated comorbidities in inclusion body myositis. Rheumatology (Oxford) 2022;61(5):2016–2024.
4. Lilleker JB, Naddaf E, Saris CGJ, et al. 272nd ENMC International Workshop: 10 years of progress—revision of the ENMC 2013 diagnostic criteria for inclusion body myositis. Neuromuscul Disord 2024;37:36–51.
5. Greenberg SA, Pinkus JL, Kong SW, et al. Highly differentiated cytotoxic T cells in inclusion body myositis. Brain 2019;142(9):2590–2604.
6. Salam S, Dimachkie MM, Hanna MG, Machado PM. Diagnostic and prognostic value of anti-cN1A antibodies in inclusion body myositis. Clin Exp Rheumatol 2022;40(2):384–393.
7. Ikenaga C, Findlay AR, Goyal NA, et al. Clinical utility of anti-cytosolic 5’-nucleotidase 1A antibody in idiopathic inflammatory myopathies. Ann Clin Transl Neurol 2021;8(3):571–578.
8. Britson KA, Ling JP, Braunstein KE, et al. Loss of TDP-43 function and rimmed vacuoles persist after T cell depletion in a xenograft model of sporadic inclusion body myositis. Sci Transl Med 2022;14(628):eabi9196.
9. Benveniste O, Hogrel JY, Belin L, et al. Sirolimus for treatment of patients with inclusion body myositis: a randomised, double-blind, placebo-controlled, proof-of-concept, phase 2b trial. Lancet Rheumatol 2021;3(1):e40–e48.
10. Machado PM, Dimachkie MM, Engel AG, et al. Safety and efficacy of arimoclomol for inclusion body myositis: a multicentre, randomised, double-blind, placebo-controlled trial. Lancet Neurol 2023;22(11):1020–1029.
11. Cox FM, Titulaer MJ, Sont JK, et al. A 12-year follow-up in sporadic inclusion body myositis: an end stage with major disabilities. Brain 2011;134(Pt 11):3167–3175.
12. Greenberg SA. Inclusion body myositis: clinical features and pathogenesis. Nat Rev Rheumatol 2019;15(5):257–272.
13. Naddaf E, Nguyen TKO, Watzlawik JO, et al. NLRP3 inflammasome activation and altered mitophagy are key pathways in inclusion body myositis. J Cachexia Sarcopenia Muscle 2025;16(1):e13672.
14. Alamr M, Pinto MV, Naddaf E. Atypical presentations of inclusion body myositis: clinical characteristics and long-term outcomes. Muscle Nerve 2022;66(6):686–693.
15. Tasca G, Monforte M, De Fino C, et al. Magnetic resonance imaging pattern recognition in sporadic inclusion-body myositis. Muscle Nerve 2015;52(6):956–962.
16. Roy B, Dimachkie MM, Naddaf E. Phenotypic spectrum of inclusion body myositis. Clin Exp Rheumatol 2024;42(2):445–453.
17. Skolka MP, Naddaf E. Exploring challenges in the management and treatment of inclusion body myositis. Curr Opin Rheumatol 2023;35(6):404–413.
18. Mastaglia FL, Needham M, Scott A, et al. Sporadic inclusion body myositis: HLA-DRB1 allele interactions influence disease risk and clinical phenotype. Neuromuscul Disord 2009;19(11):763–765.