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?

Question

Accepted Answer

A. Serum anti-Hu (anti-PNHP) antibody and CSF anti-Hu antibody. ## 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]

Answer

B. CSF oligoclonal bands and IgG index

Answer

C. Serum anti-VGCC (voltage-gated calcium channel) antibody

Answer

D. Serum anti-Yo (anti-PCA-1) antibody

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?

Explanation

Paraneoplastic Cerebellar Degeneration (PCD) in Lung Cancer

Clinical Presentation Decoding

Why Anti-Hu Serology (Serum + CSF) Is Most Appropriate

Diagnostic Significance of Serum + CSF Testing

Paraneoplastic Antibody-Syndrome Associations in Lung Cancer

Why Serum AND CSF?

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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