A 65-year-old man with squamous cell carcinoma of the lung presents with progressive ataxia, nystagmus, and dysarthria over 3 months. Serum calcium is 11.2 mg/dL (normal). CSF analysis shows mild lymphocytic pleocytosis. Which investigation is most appropriate to confirm the paraneoplastic etiology?

## Paraneoplastic Cerebellar Degeneration (PCD) in Lung Cancer ### Clinical Presentation Decoding The patient presents with: - **Cerebellar syndrome** (ataxia, nystagmus, dysarthria) - **Squamous cell lung cancer** (most common lung histology associated with PCD) - **Progressive course over months** — typical for paraneoplastic degeneration - **CSF pleocytosis** — inflammatory, not metabolic - **Normal serum calcium** — excludes hypercalcemia-related ataxia ### Why Anti-Hu Serology (Serum + CSF) Is Most Appropriate **Key Point:** Anti-Hu (anti-PNHP, Hu antigen) antibodies are the most common paraneoplastic antibodies in lung cancer, present in ~80% of SCLC-associated paraneoplastic neurological syndromes (PNS) and ~20% of non-SCLC lung cancers. **High-Yield:** Anti-Hu antibodies are associated with: 1. **Paraneoplastic cerebellar degeneration** (especially with SCLC and squamous cell lung cancer) 2. Paraneoplastic encephalomyelitis 3. Sensory neuronopathy 4. Autonomic dysfunction ### Diagnostic Significance of Serum + CSF Testing ```mermaid flowchart TD A[Cerebellar syndrome + lung cancer + CSF pleocytosis] --> B{Suspect paraneoplastic PCD} B --> C[Serum anti-Hu antibody] C -->|Positive| D[Highly specific for paraneoplastic syndrome] D --> E[CSF anti-Hu antibody] E -->|Positive| F[Confirms intrathecal immune response] F --> G[Paraneoplastic PCD confirmed] C -->|Negative| H[Consider anti-Yo, anti-CRMP5] ``` **Clinical Pearl:** Detection of anti-Hu antibodies in **both serum and CSF** is highly specific (~95%) for paraneoplastic neurological disease and indicates active intrathecal antibody production against tumor-associated antigens. ### Paraneoplastic Antibody-Syndrome Associations in Lung Cancer | Antibody | Lung Histology | Syndrome | Frequency | |----------|----------------|----------|----------| | **Anti-Hu** | SCLC, SCC | **Cerebellar degeneration, encephalomyelitis** | **Most common (80% SCLC)** | | Anti-Yo | Adenocarcinoma | Cerebellar degeneration | Rare in lung (more common in ovarian) | | Anti-VGCC | SCLC | Lambert-Eaton syndrome | ~85% LEMS | | Anti-CRMP5 | SCLC, SCC | Encephalomyelitis, optic neuritis | ~10–15% | | Anti-GAD | Various | Stiff-person syndrome | Rare | **Mnemonic:** **HU-SCLC** — Anti-**Hu** is the most common paraneoplastic antibody in **SCLC** and associated lung cancers. ### Why Serum AND CSF? 1. **Serum anti-Hu** — indicates systemic immune response to tumor antigen 2. **CSF anti-Hu** — indicates intrathecal synthesis, confirming CNS involvement 3. **Both positive** — diagnostic specificity >95% for paraneoplastic PCD [cite:Harrison 21e Ch 390; Robbins 10e Ch 7]