Inflammatory myopathies

Classification - The Muscle Hustle

• Dermatomyositis (DM): Proximal muscle weakness accompanied by pathognomonic skin findings (Gottron's papules, heliotrope rash). Key autoantibodies include anti-Mi-2 and anti-Jo-1.
• Polymyositis (PM): Proximal muscle weakness without the cutaneous features of DM. Histology shows endomysial inflammation. A diagnosis of exclusion.
• Inclusion Body Myositis (IBM): Typically affects individuals > 50 years old. Presents with insidious, asymmetric weakness that involves distal muscles (e.g., finger flexors) early.
• Necrotizing Autoimmune Myopathy (NAM): Characterized by severe proximal weakness and markedly elevated CK levels. Often associated with statin use (anti-HMGCR antibodies).

⭐ Dermatomyositis in adults carries a significant association with underlying malignancy (e.g., ovarian, lung, gastric), often developing within a few years of the myositis diagnosis.

Dermatomyositis (DM) - Rash & Aches

• Pathognomonic Skin Findings (Gottron's & Heliotrope):
  • Gottron's Papules: Violaceous papules over knuckles/joints.
  • Heliotrope Rash: Lilac-colored rash on the upper eyelids, often with edema.
  • Other rashes: "Shawl sign" (shoulders/upper back), "V-sign" (anterior neck/chest).

• Musculoskeletal:

  • Symmetrical, proximal muscle weakness (e.g., difficulty rising from a chair, combing hair).
  • Dysphagia due to esophageal muscle involvement.

• Diagnosis & Labs:

  • ↑ CK, ↑ Aldolase, ↑ AST/ALT.
  • Autoantibodies: Anti-Mi-2 (highly specific), Anti-Jo-1 (associated with interstitial lung disease), Anti-p155/140 (associated with malignancy).
  • Definitive Dx: Muscle biopsy shows perifascicular atrophy.

⭐ Associated with malignancy in up to 25% of adult cases, often ovarian, lung, or GI cancers. Screen for underlying cancer!

Polymyositis & IBM - Proximal vs Distal

⭐ Inclusion Body Myositis (IBM) is the most common inflammatory myopathy in patients over age 50. It's characterized by insidious onset and notable weakness in the quadriceps and forearm flexors, leading to falls and difficulty with fine motor tasks.

Diagnosis & Treatment - The Workup Flow

• Labs: ↑ CK (often >50x ULN), ↑ aldolase, + myositis-specific antibodies (e.g., Anti-Jo-1, Anti-Mi-2).
• Procedures:
  • EMG: Shows myopathic irritability (short, small, polyphasic MUAPs).
  • Muscle Biopsy: Definitive diagnosis. Distinguishes PM (endomysial CD8+) vs. DM (perifascicular/perivascular CD4+).
• Management:
  • Initial: High-dose corticosteroids (e.g., Prednisone 1 mg/kg/day).
  • Long-term: Steroid-sparing agents (e.g., methotrexate, azathioprine).

⭐ In adults, dermatomyositis carries a high risk of underlying malignancy (especially ovarian, lung, gastric). Mandates age-appropriate cancer screening.

High‑Yield Points - ⚡ Biggest Takeaways

• Dermatomyositis is defined by Gottron's papules and heliotrope rash, and carries a high risk of internal malignancy.
• Polymyositis causes symmetric proximal weakness from endomysial inflammation (CD8+ T cells).
• Inclusion Body Myositis is unique for asymmetric weakness, distal muscle involvement, and rimmed vacuoles on biopsy.
• Anti-Jo-1 antibody is linked to interstitial lung disease; anti-Mi-2 is highly specific for dermatomyositis.
• All show elevated creatine kinase (CK); muscle biopsy is the definitive diagnostic test.