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) and a violaceous rash over the knuckles, eyelids, and chest. Serum creatine kinase is 3200 U/L, and anti-Jo-1 antibodies are positive. Chest X-ray reveals a 4 cm left hilar mass. What is the most likely paraneoplastic syndrome associated with this patient's underlying malignancy?

Explanation

## Clinical Presentation Analysis This patient presents with classic features of **dermatomyositis (DM)** with a strong paraneoplastic association: ### Key Clinical Features | Feature | Finding in This Case | |---------|----------------------| | **Muscle involvement** | Symmetric proximal weakness (characteristic of myositis) | | **Cutaneous findings** | Violaceous (heliotrope) rash on eyelids, Gottron papules on knuckles | | **Serum markers** | Elevated CK (3200 U/L), anti-Jo-1 antibodies positive | | **Malignancy association** | Lung mass on imaging | ### Paraneoplastic Significance **Key Point:** Dermatomyositis has a **40–60% association with underlying malignancy**, particularly in adults >40 years. The most common associated malignancies are: - Lung cancer (most common in India and worldwide) - Gastric cancer - Breast cancer - Ovarian cancer **High-Yield:** The presence of **anti-Jo-1 antibodies** (anti-histidyl-tRNA synthetase) is highly specific for myositis and is associated with the "myositis-specific antibody" (MSA) panel. Jo-1 positivity correlates with interstitial lung disease and myositis. ### Mechanism of Paraneoplastic Dermatomyositis The syndrome arises through: 1. **Molecular mimicry** — tumor antigens cross-react with muscle and skin autoantigens 2. **Immune complex deposition** — IgG and complement deposits in muscle and dermal–epidermal junction 3. **Cytotoxic T-cell infiltration** — CD8+ T cells target muscle fibers **Clinical Pearl:** Dermatomyositis (especially with cutaneous involvement) is MORE likely to be paraneoplastic than polymyositis alone. The rash is the clinical hallmark that raises suspicion for underlying malignancy. ### Why Lung Cancer? In this case, the chest X-ray finding of a **4 cm left hilar mass** strongly suggests **lung adenocarcinoma** or squamous cell carcinoma, which are the most common lung malignancies associated with dermatomyositis in Indian populations. **Mnemonic:** **DERM** = **D**ermatomyositis **E**levated **R**isk **M**alignancy (40–60% of adult cases).