Idiopathic inflammatory myopathies

Definition

• Dermatomyositis (DM): an inflammatory myopathy characterized by progressive symmetrical proximal muscle weakness and distinctive skin findings
• Polymyositis (PM): an inflammatory myopathy affecting the proximal skeletal muscles, with evidence of elevated CK and myositis on EMG and biopsy in the absence of any of the characteristic findings of the other IIM (a rare diagnosis of exclusion)

---

Epidemiology

---

Etiology

• Malignancy Association: strong association with occult malignancy (esp. adenocarcinomas of ovary, lung, pancreas, stomach) in Dermatomyositis. c

---

Pathophysiology

• Polymyositis:
  • T-cell mediated cytotoxicity against skeletal muscle fibers.
  • CD8+ T-cells infiltrate the endomysium (inner layer of muscle).
• Dermatomyositis:
  • Humoral (antibody) and complement-mediated damage to microvasculature surrounding muscle.
  • CD4+ T-cells infiltrate the perimysium (sheath around muscle fascicles).

---

Clinical features

Muscle weakness

• Limb weakness
  • Symmetrical proximal muscle weakness and atrophy c
  • Commonly affects pelvic and shoulder girdle muscles, leading to difficulties combing hair, standing up from a sitting position, and climbing stairs
• Axial muscle weakness
  • Oropharyngeal muscle weakness leading to dysphagia

Cutaneous features

Cutaneous features are characteristic of dermatomyositis but may be found in other subtypes.

• Symmetric erythematous rash on the:
  • Extensor surfaces of the hand joints, elbows, and knees (Gottron sign); scaly papules may form (Gottron papules)
  • Upper eyelids (heliotrope rash); often associated with periorbital edema
  • Mid-face
  • Upper back, posterior neck, and shoulders (shawl sign)
  • Upper chest and anterior neck (V sign)
• Mechanic’s hands: thickened and cracked skin on the sides of the fingers and palms (horizontal fissures may appear darkened or dirty, hence the name)

Tip

The clinical features of PM is similar to DM except no cutaneous features.

---

Diagnostics

• ↑ CK
• Antinuclear antibodies (ANAs)
• Myositis-specific antibodies
  • Anti-Jo-1 antibodies (anti–histidyl-tRNA synthetase)
• Muscle biopsy: Gold standard for diagnosis of IIM
  • Typical findings
    • Muscle fiber damage
      • Necrosis, degeneration, and regeneration
    • Inflammatory cellular infiltrates
      • DM: antibody-mediated inflammatory infiltrates that predominantly involve CD4+ T cells, plasmacytoid dendritic cells, and B lymphocytes in the perimysium (the connective tissue surrounding the fascicle); can lead to perifascicular atrophy
      • IBM and PM: cell-mediated inflammatory infiltrates that predominantly involve cytotoxic CD8+ T cells in the endomysium (the connective tissue surrounding each muscle fiber), patchy necrosis

Differential diagnostics

---

Feature	Fibromyalgia (FM)	Polymyositis (PM)	Polymyalgia Rheumatica (PMR)
Demographics	F, 20–55 yo	F > M, > 40 yo	Age > 50 yo (peak 70s)
Core Symptom	Widespread chronic pain, fatigue, insomnia	Painless proximal muscle weakness	Proximal morning stiffness/pain (shoulders, hip girdle)
Strength	Normal	Decreased (proximal)	Normal (pain-limited ROM)
CK / Aldolase	Normal	Markedly elevated (↑↑)	Normal
ESR / CRP	Normal	Normal or mild ↑	Markedly elevated (↑↑, ESR > 50)
Diagnostic Test	Clinical	Muscle biopsy (CD8+ endomysial)	Clinical + rapid steroid response
1st-Line Tx	Exercise, Duloxetine, TCAs, Pregabalin	High-dose prednisone (± MTX/AZA)	Low-dose prednisone (10–20 mg)
Key Association	IBS, tension headaches, depression	Interstitial lung disease, malignancy	Giant Cell Arteritis (GCA)

Link to original

Dermatomyositis vs. Systemic Sclerosis

Feature	Dermatomyositis (DM)	Systemic Sclerosis (SSc)
Primary Issue	Muscle & skin inflammation (capillary damage)	Widespread fibrosis & vasculopathy
Key Skin	Gottron’s papules, Heliotrope rash, Shawl/V-sign, Mechanic’s hands	Skin thickening/tightening (scleroderma), Sclerodactyly, Telangiectasias, Calcinosis, Raynaud’s (often first)
Muscle	Symmetric proximal weakness (prominent), myalgias	Often mild or late; arthralgias/tendon rubs more common
Key Organ	Interstitial Lung Disease (ILD), dysphagia (skeletal muscle)	ILD, Pulmonary Arterial Hypertension (PAH), Esophageal dysmotility (smooth muscle), Scleroderma Renal Crisis
Antibodies	Anti-Jo-1 (ILD), Anti-Mi-2 (classic skin), Anti-MDA5 (amyopathic, severe ILD), Anti-TIF1-γ (cancer)	Anti-Scl-70 (diffuse, ILD), Anti-centromere (limited/CREST, PAH), Anti-RNA Pol III (renal crisis)
Malignancy	High risk (ovary, lung, etc.)	Slight increase (e.g., lung with ILD)
Tx Pearl	Corticosteroids, immunosuppressants; Cancer screening crucial	Organ-based; ACE Inhibitors for Renal Crisis (VITAL!)
Subtype/Note	Amyopathic DM (skin, no weakness)	Limited (CREST) vs. Diffuse variants

Treatment

---