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# Necrotizing myopathy, inclusion body myositis and polymyositis

- Lesion

Some muscle autoimmunity kills fibres with little inflammation, sometimes after statins; inclusion body myositis slowly weakens thighs and fingers in old age.

Updated Nov 12, 2026

## Key points

- IMNM: anti-SRP or anti-HMGCR; CK very high; necrosis with scant inflammation; HMGCR form linked to statins and persists after stopping them.
- sIBM: over 50, men more; quadriceps and deep finger flexors, falls, dysphagia; CK mildly raised.
- sIBM biopsy: rimmed vacuoles, eosinophilic inclusions (p62, TDP-43, amyloid), CD8 T cells invading non-necrotic fibres, mitochondrial changes; anti-cN1A antibodies in some.
- sIBM is refractory to steroids and other immunosuppression.
- Polymyositis: endomysial CD8 T cells surrounding and invading healthy fibres that express MHC class I; exclude IBM, IMNM and dystrophies first.
- Statin myopathy is usually self-limited toxic injury; IMNM is the rare autoimmune form.

## What it is

Beyond dermatomyositis, three inflammatory myopathies matter. Immune-mediated necrotizing myopathy causes subacute, severe proximal weakness with very high CK; the biopsy shows many necrotic and regenerating fibres with little inflammation, and antibodies target SRP or HMG-CoA reductase (HMGCR), the latter often after statin exposure. Sporadic inclusion body myositis is the commonest myopathy after 50: slowly progressive, asymmetric weakness of quadriceps and finger flexors with dysphagia, rimmed vacuoles, protein aggregates and CD8 T cells invading fibres, and poor response to immunosuppression. Polymyositis, once common, is now rare and a diagnosis of exclusion: endomysial CD8 T cells attacking MHC class I-expressing fibres without skin disease.

## Structure

- IMNM: scattered necrotic fibres at different stages with macrophages, regenerating fibres, MAC on the sarcolemma and few lymphocytes.
- sIBM: rimmed vacuoles lined by basophilic granules, cytoplasmic inclusions, invading CD8 cells, COX-negative fibres.
- Polymyositis: endomysial lymphocytes, mostly CD8, surrounding and invading non-necrotic fibres; widespread MHC class I upregulation.

## Step by step

- Statin exposure raises HMGCR expression in muscle and may break tolerance in susceptible people (HLA-DRB1*11:01).
- Anti-HMGCR or anti-SRP antibodies bind regenerating fibres and fix complement, causing necrosis.
- In sIBM, cytotoxic CD8 T cells and degenerative protein aggregation act together in aging muscle.
- Aggregates of misfolded proteins and failed autophagy produce rimmed vacuoles.
- The degenerative component explains why sIBM does not respond to immunosuppression.

## The inflammatory myopathies compared

| Feature | Dermatomyositis | IMNM | Inclusion body myositis | Polymyositis |
| --- | --- | --- | --- | --- |
| Age | Children and adults | Adults | Over 50 | Adults |
| Weakness | Proximal, symmetric; skin rash | Proximal, severe, subacute | Quadriceps, finger flexors; asymmetric; slow | Proximal, symmetric |
| CK | Raised | Very high | Normal or mildly raised | Raised |
| Inflammation | Perimysial, perivascular | Scant | Endomysial CD8 | Endomysial CD8 |
| Key biopsy finding | Perifascicular atrophy | Necrotic fibres | Rimmed vacuoles, inclusions | CD8 cells invading non-necrotic fibres |
| Response to immunosuppression | Good | Usually, sometimes hard | Poor | Good |

## Clinical notes

- Weakness after starting a statin that keeps worsening after the drug is stopped: test for anti-HMGCR.
- An older man who trips on his knees and cannot grip well, with steroid-resistant 'polymyositis': reconsider inclusion body myositis.
- Dysphagia in sIBM can cause aspiration.

## Memory aid

Statins can switch on HMGCR immunity; IBM likes knees, knuckles and the aging, and laughs at steroids.

## Test yourself

Think of the answer, then tap the question to see it.

## Which autoantibody links statins to immune-mediated necrotizing myopathy?

Anti-HMG-CoA reductase (anti-HMGCR).

## Which muscles are typically weak in inclusion body myositis?

Quadriceps and deep finger flexors, often asymmetrically.

## What are rimmed vacuoles?

Vacuoles lined by basophilic granular material in muscle fibres, typical of inclusion body myositis.

## Why is polymyositis now a diagnosis of exclusion?

Many cases once called polymyositis are IBM, IMNM, antisynthetase syndrome or dystrophies with inflammation.

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## Elsewhere in DurgiPedia

- [CD8⁺ cytotoxic T cell](https://durgical.com/en/immunology/cd8-cytotoxic-t-cell/)
- [Autoantibodies: the table to know](https://durgical.com/en/immunology/autoantibodies-the-table-to-know/)

## Connections

## Related 4

- [Injury by therapeutic drugs: the classic examples](https://durgical.com/en/general-pathology/injury-by-therapeutic-drugs-the-classic-examples/)
- [Dermatomyositis and antisynthetase syndrome](https://durgical.com/en/general-pathology/dermatomyositis-and-antisynthetase-syndrome/)
- [Toxic and endocrine myopathies and rhabdomyolysis](https://durgical.com/en/general-pathology/toxic-and-endocrine-myopathies-and-rhabdomyolysis/)
- [Reading a muscle biopsy: neurogenic vs myopathic](https://durgical.com/en/general-pathology/reading-a-muscle-biopsy-neurogenic-vs-myopathic/)

## Related conditions 2

- [Inflammatory myopathies (dermatomyositis, polymyositis)](https://durgical.com/en/conditions/inflammatory-myopathies/)
- [Inclusion body myositis](https://durgical.com/en/conditions/inclusion-body-myositis/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Myositis | Polymyositis | Dermatomyositis | MedlinePlus](https://medlineplus.gov/myositis.html) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/necrotizing-myopathy-inclusion-body-myositis-and-polymyositis/