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Inflammatory Myositis for MRCP Part 1

TL;DR

Rheum: Inflammatory Myositis (Polymyositis/DM) is a core MRCP Part 1 topic centred on proximal muscle weakness, characteristic dermatological signs, and autoantibody profiles. Distinguishing polymyositis, dermatomyositis, and inclusion body myositis is essential for scoring well. Focus on CK elevation, malignancy associations, and antibody patterns. Expect clinically oriented MCQs testing subtle differentiators.


Why this matters

Inflammatory myopathies are consistently examined in MRCP Part 1 because they test clinical reasoning across rheumatology, neurology, and immunology. Candidates must recognise hallmark features, interpret laboratory findings, and identify systemic associations such as malignancy and interstitial lung disease.

For a structured revision approach, start with the MRCP Part 1 overview and reinforce learning through Free MRCP MCQs.


Core sections

1. Definition and classification

Idiopathic inflammatory myopathies are autoimmune disorders characterised by muscle inflammation and weakness. The key subtypes tested are:

  • Polymyositis (PM) – T-cell mediated muscle fibre injury

  • Dermatomyositis (DM) – Humoral-mediated microangiopathy with skin involvement

  • Inclusion body myositis (IBM) – degenerative + inflammatory, often a distractor in exams

2. Clinical features (pattern recognition)

Feature

Polymyositis

Dermatomyositis

Weakness

Symmetrical proximal

Symmetrical proximal

Skin involvement

Absent

Present

CK levels

Elevated

Elevated

Malignancy risk

Moderate

High

Dysphagia

Common

Common

Key exam clue: Difficulty climbing stairs, rising from a chair, or lifting arms overhead indicates proximal muscle weakness.

3. Dermatological features in Dermatomyositis

Dermatomyositis is frequently tested via visual clues:

  • Heliotrope rash – violaceous discolouration of eyelids

  • Gottron’s papules – erythematous lesions over MCP/PIP joints

  • Shawl sign – rash over shoulders and upper back

  • Mechanic’s hands – hyperkeratotic fissured skin

4. Autoantibodies (high-yield associations)

Understanding antibody profiles is essential:

  • Anti-Jo-1 → antisynthetase syndrome (ILD, arthritis, Raynaud’s)

  • Anti-Mi-2 → classic dermatomyositis, favourable prognosis

  • Anti-SRP → severe polymyositis

  • Anti-TIF1-γ → malignancy-associated dermatomyositis

5. Investigations

Typical MRCP Part 1 findings:

  • Creatine kinase (CK) markedly elevated

  • AST/ALT elevated due to muscle breakdown

  • EMG showing myopathic changes

  • Muscle biopsy:

    • PM → endomysial inflammation

    • DM → perifascicular atrophy

Authoritative reference: UpToDate: Idiopathic inflammatory myopathies

6. Malignancy association

Dermatomyositis is strongly associated with malignancy, especially:

  • Ovarian cancer

  • Lung cancer

  • Pancreatic cancer

  • Gastric cancer

Clinical guidance from British Society for Rheumatology emphasises malignancy screening in adults with dermatomyositis.

7. Interstitial lung disease (ILD)

Seen in antisynthetase syndrome:

  • Progressive dyspnoea

  • Dry cough

  • Restrictive lung function

  • HRCT: ground-glass opacities

8. Management principles

  • First-line: corticosteroids

  • Steroid-sparing agents: methotrexate, azathioprine

  • Refractory disease: IVIG, rituximab

IBM typically shows poor response to steroids, a key exam distinction.


High-yield summary (must-know points)

  1. Proximal muscle weakness is the hallmark

  2. Dermatomyositis includes characteristic skin findings

  3. CK is markedly elevated

  4. Anti-Jo-1 is associated with ILD

  5. Dermatomyositis is linked to malignancy

  6. Muscle biopsy differentiates PM vs DM

  7. IBM presents with distal weakness

  8. Steroids are first-line treatment

  9. Raised AST/ALT may reflect muscle damage

  10. Dysphagia suggests severe disease


Practical examples / mini-cases

MCQ Example

A 58-year-old woman presents with progressive proximal muscle weakness and a violaceous rash around her eyes. CK is elevated. What is the most appropriate next step?

A. Start statinsB. Screen for malignancyC. Prescribe NSAIDsD. Check thyroid function

Answer: B. Screen for malignancy

Explanation: This presentation is classic for dermatomyositis. A key MRCP Part 1 concept is its association with malignancy, requiring appropriate cancer screening.


MRCP Part 1 study setup with notes on inflammatory myositis and autoimmune diseases

Common pitfalls (5 bullets)

  • Confusing inclusion body myositis with polymyositis

  • Missing malignancy screening in dermatomyositis

  • Misinterpreting elevated AST/ALT as liver pathology

  • Forgetting ILD association with anti-Jo-1

  • Assuming normal CK excludes inflammatory myopathy


Study-tip checklist (exam-focused)

Cross-link suggestion: Combine this topic with systemic autoimmune conditions such as SLE for integrated revision.


FAQs

1. What is the key difference between polymyositis and dermatomyositis?

Dermatomyositis has characteristic skin findings such as heliotrope rash and Gottron’s papules, whereas polymyositis involves muscle only.

2. Which antibody is associated with interstitial lung disease?

Anti-Jo-1 is strongly associated with antisynthetase syndrome, which includes ILD.

3. Why is malignancy screening important in dermatomyositis?

Dermatomyositis can be a paraneoplastic condition, particularly in older adults, making cancer screening essential.

4. What distinguishes inclusion body myositis?

IBM causes distal muscle weakness and is typically resistant to corticosteroids.

5. Why are liver enzymes elevated in inflammatory myopathy?

AST and ALT are present in muscle tissue, so muscle inflammation can elevate these enzymes.


Ready to start?

Strengthen your preparation for MRCP Part 1 by consolidating high-yield topics like inflammatory myositis. Begin with the MRCP Part 1 overview and practise regularly using Free MRCP MCQs.


Sources

 
 
 

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