A 58-year-old man with a 40-pack-year smoking history presents with progressive proximal muscle weakness, difficulty climbing stairs, and myalgia. Serum creatine kinase is elevated at 2800 U/L. Chest X-ray reveals a 3 cm left hilar mass. Which investigation is most appropriate to confirm the suspected paraneoplastic syndrome?
A. Thyroid function tests and thyroid antibodies
B. Muscle biopsy with histopathology
C. Serum anti-Jo-1 antibody and myositis panel
D. Electromyography (EMG) and nerve conduction studies
Explanation
Paraneoplastic Myositis and Autoantibody Detection
Clinical Context
The patient presents with:
Proximal muscle weakness (myositis pattern)
Elevated CK (muscle necrosis)
Lung malignancy (smoking history + hilar mass)
This constellation suggests paraneoplastic inflammatory myositis
Why Serum Autoantibody Panel Is Correct
Key Point
Anti-Jo-1 (histidyl-tRNA synthetase) antibodies are the most common myositis-specific antibodies, found in ~30% of paraneoplastic myositis cases, particularly associated with lung cancer.
High-YieldNEET PG
The myositis panel includes:
Anti-Jo-1 (most frequent)
Anti-PL-7, Anti-PL-12
Anti-Mi-2
Anti-SRP (signal recognition particle)
Anti-HMGCR
These are:
1.
Specific — directly diagnostic of myositis
2.
Sensitive — positive in 50–80% of inflammatory myositis
3.
Prognostic — certain antibodies (e.g., anti-SRP) correlate with malignancy risk
Diagnostic Algorithm
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Clinical Pearl
Anti-Jo-1 positivity in a patient with lung cancer and myositis has ~90% specificity for paraneoplastic myositis and guides both diagnosis and prognosis.
Why Serology Before Biopsy?
Non-invasive
High specificity
Guides biopsy interpretation if needed
Faster turnaround than biopsy
Robbins 10e Ch 7
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