A 58-year-old man from Delhi presents with a 3-month history of progressive proximal muscle weakness, myalgia, and difficulty climbing stairs. On examination, he has symmetric proximal muscle weakness (grade 3/5 in hip and shoulder girdles) and a violaceous rash over the knuckles and eyelids. Serum creatine kinase is elevated at 2800 IU/L. Chest X-ray reveals a 4 cm left hilar mass. Which paraneoplastic syndrome is most likely responsible for his clinical presentation?

Explanation

## Clinical Diagnosis: Paraneoplastic Dermatomyositis ### Key Clinical Features **Key Point:** Dermatomyositis (DM) is a paraneoplastic syndrome characterized by the triad of proximal muscle weakness, elevated creatine kinase, and a pathognomonic violaceous (heliotrope) rash over the eyelids and Gottron's papules over the knuckles. ### Cancer Association **High-Yield:** Lung cancer is the most common malignancy associated with paraneoplastic dermatomyositis in adults (especially in smokers from India). Other common associations include gastric, ovarian, and breast cancers. ### Pathophysiology 1. Autoimmune destruction of muscle fibers mediated by CD8+ T cells and antibodies against myositis-associated antigens (e.g., anti-Mi-2, anti-Jo-1) 2. Vasculitis of small blood vessels in muscle and skin 3. Perifascicular atrophy on muscle biopsy ### Diagnostic Criteria | Feature | Dermatomyositis | LEMS | Osteoarthropathy | |---------|-----------------|------|------------------| | **Muscle weakness** | Proximal, symmetric | Proximal, with autonomic signs | None | | **Rash** | Violaceous, pathognomonic | Absent | Absent | | **CK elevation** | Marked (>1000 IU/L) | Normal or mild | Normal | | **EMG findings** | Myopathic | Facilitation with high-frequency stimulation | N/A | | **Associated cancer** | Lung, gastric, ovarian | Small cell lung cancer | Lung, gastric | ### Clinical Pearl **Clinical Pearl:** Paraneoplastic DM in adults has a cancer association rate of 15–30%, making malignancy screening mandatory. The presence of anti-p155/140 antibodies (TIF1-γ) is highly specific for malignancy-associated DM. ### Management Approach ```mermaid flowchart TD A[Suspected Paraneoplastic DM]:::outcome --> B{Confirm diagnosis}:::decision B -->|Elevated CK + EMG myopathy| C[Screen for malignancy]:::action C --> D[Chest CT, abdominal imaging]:::action D --> E{Cancer found?}:::decision E -->|Yes| F[Treat underlying cancer + immunosuppression]:::action E -->|No| G[Repeat imaging in 6 months]:::action F --> H[Corticosteroids + steroid-sparing agents]:::action H --> I[Clinical and biochemical improvement]:::outcome ``` ### Mnemonic for Paraneoplastic Myositis **Mnemonic:** **DM-LUNG** = Dermatomyositis–Lung cancer (most common), Gastric, Ovarian, Breast [cite:Robbins 10e Ch 7]