A 58-year-old man from rural Maharashtra 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 skin examination reveals a violaceous rash over the knuckles, elbows, and knees. Serum creatine kinase is markedly elevated at 3200 IU/L. Chest X-ray shows a 4 cm nodular opacity in the left upper lobe. Which paraneoplastic syndrome is most likely responsible for his clinical presentation?

Explanation

## Clinical Diagnosis: Paraneoplastic Dermatomyositis **Key Point:** Dermatomyositis (DM) is a paraneoplastic syndrome in 15–30% of adult cases, with lung cancer being the most common underlying malignancy in Indian populations. ### Diagnostic Features Present | Feature | Finding in This Case | Significance | | --- | --- | --- | | **Rash** | Violaceous (heliotrope) over knuckles, elbows, knees | Pathognomonic for DM | | **Muscle involvement** | Symmetric proximal weakness (hip, shoulder) | Classic pattern in myositis | | **CK elevation** | 3200 IU/L (markedly elevated) | Indicates muscle necrosis | | **Malignancy** | Lung nodule on CXR | DM–lung cancer association is strong | | **Age & demographics** | 58-year-old from rural India | Peak incidence 40–60 years; smoking history likely | **High-Yield:** In adult-onset dermatomyositis, **malignancy screening is mandatory**. The lung nodule here is the culprit malignancy, not coincidental. ### Pathophysiology 1. Tumor expresses autoantigens (e.g., Mi-2, TIF1-γ, MDA5) that cross-react with muscle and skin epitopes 2. CD8^+^ T-cell infiltration of muscle → myonecrosis and CK elevation 3. Immune complex deposition in skin → characteristic rash **Clinical Pearl:** The presence of **cutaneous findings (rash) + myositis + elevated CK + malignancy** is the gold standard for paraneoplastic DM. Skin biopsy would show interface dermatitis; muscle biopsy would show perifascicular atrophy and CD8^+^ infiltration. ### Screening & Management - **Malignancy workup:** CT chest/abdomen, age-appropriate cancer screening (colonoscopy, mammography if female) - **Treatment:** Corticosteroids (first-line), azathioprine or mycophenolate for steroid-sparing effect - **Prognosis:** Linked to underlying cancer; treatment of malignancy may improve myositis **Mnemonic: RASH + WEAK + CK = DM (Rash, Weakness, elevated CK = Dermatomyositis)**