A 58-year-old man from Mumbai presents with progressive muscle weakness, particularly affecting the lower limbs, and difficulty climbing stairs for the past 3 months. He also complains of myalgia and fatigue. On examination, proximal muscle weakness is noted. Serum creatine kinase is markedly elevated at 3200 U/L. Chest X-ray reveals a 4 cm mass in the left hilum. What is the most likely paraneoplastic syndrome associated with this presentation?

Explanation

## Diagnosis: Paraneoplastic Dermatomyositis ### Clinical Presentation Analysis The patient presents with: - Progressive proximal muscle weakness (lower limbs > upper limbs) - Myalgia and fatigue - Markedly elevated creatine kinase (3200 U/L) - Lung mass on imaging **Key Point:** Paraneoplastic dermatomyositis (PM/DM) is a systemic inflammatory myopathy associated with malignancy in 15–25% of adult-onset cases, with lung cancer being the most common underlying malignancy in men. ### Pathophysiology Paraneoplastic dermatomyositis results from immune-mediated muscle inflammation triggered by tumor-associated antigens. The mechanism involves: 1. Aberrant expression of tumor antigens in skeletal muscle 2. Cross-reactive autoimmune response (CD8+ T cells and complement-mediated injury) 3. Deposition of immune complexes in muscle and skin ### Diagnostic Features | Feature | Paraneoplastic DM | LEMS | MG | Neuropathy | |---------|-------------------|------|----|-----------| | **Proximal weakness** | Yes (prominent) | Yes | No (ocular/bulbar) | Distal predominant | | **Elevated CK** | Marked (>1000) | Normal | Normal | Normal | | **Skin involvement** | Yes (rash, heliotrope) | No | No | No | | **Associated malignancy** | Lung, ovary, gastric | Small cell lung | Thymoma | Various | | **Antibodies** | Anti-Mi-2, anti-Jo-1 | Anti-VGCC | Anti-AChR | Anti-Hu, anti-VGCC | **High-Yield:** The combination of **proximal myopathy + markedly elevated CK + lung mass** is pathognomonic for paraneoplastic dermatomyositis until proven otherwise. ### Clinical Pearl Although this patient does not mention skin findings (heliotrope rash, Gottron papules), up to 25% of PM/DM cases present with myositis alone (seronegative polymyositis), especially in paraneoplastic forms. Skin manifestations may develop later or be subtle. ### Management Approach 1. **Confirm diagnosis:** Muscle biopsy (shows perivascular and interfascicular inflammation), EMG/NCS 2. **Screen for malignancy:** CT chest/abdomen, age-appropriate cancer screening 3. **Treat myositis:** Corticosteroids (first-line), immunosuppressants (methotrexate, azathioprine) 4. **Treat underlying cancer:** Chemotherapy or surgery may improve myositis **Mnemonic:** **DERM** = **D**ermatitis, **E**levated **CK**, **R**esponse to steroids, **M**alignancy association