A 62-year-old woman from Delhi presents with a 2-month history of progressive confusion, memory loss, and personality changes. She has also developed jerky involuntary movements and gait ataxia. MRI brain shows cerebellar atrophy. CSF analysis reveals mild lymphocytic pleocytosis. Serum and CSF anti-Hu antibodies are positive. Subsequent imaging reveals a small-cell lung cancer in the right upper lobe. What is the most likely paraneoplastic neurological syndrome?

Explanation

## Diagnosis: Paraneoplastic Encephalomyelitis (PEM) ### Clinical Presentation Analysis The patient presents with: - Cognitive dysfunction (confusion, memory loss, personality changes) - Movement disorder (jerky involuntary movements) - Gait ataxia - CSF lymphocytic pleocytosis - **Positive anti-Hu antibodies** (serum and CSF) - Small-cell lung cancer (SCLC) **Key Point:** Anti-Hu antibodies are highly specific for paraneoplastic encephalomyelitis and are found in 10–15% of SCLC patients. The presence of multifocal CNS involvement (cognitive + cerebellar + movement disorder) with anti-Hu seropositivity is diagnostic. ### Pathophysiology Paraneoplastic encephalomyelitis results from: 1. **Tumor expression of neuronal antigens** (HuD, HuC proteins in SCLC) 2. **Cross-reactive immune response:** CD8+ T cells infiltrate the brain and spinal cord 3. **Neuronal loss and gliosis** in multiple brain regions (cortex, cerebellum, brainstem, spinal cord) 4. **Antibody-mediated and cellular immunity** both contribute ### Comparison of Paraneoplastic Syndromes | Syndrome | Antibody | Presentation | Imaging | Associated Cancer | |----------|----------|--------------|---------|-------------------| | **PEM** | Anti-Hu | Multifocal: cognitive + ataxia + myoclonus | Cerebellar atrophy, cortical changes | SCLC (most common) | | **PCD** | Anti-Yo, anti-VGCC | Pure cerebellar: ataxia, dysarthria | Cerebellar atrophy | Ovary, breast, lung | | **Opsoclonus-myoclonus** | Anti-Ri | Rapid eye movements + myoclonus | Normal or minimal changes | Breast, SCLC, neuroblastoma | | **Stiff-person** | Anti-GAD65 | Rigidity, spasms, hyperreflexia | Normal | Breast, lung (30% paraneoplastic) | **High-Yield:** **Anti-Hu = PEM** is a high-yield association. Anti-Hu is the most common paraneoplastic antibody in SCLC and is virtually pathognomonic for PEM when present with appropriate clinical features. ### Why This Is PEM and Not PCD Although cerebellar atrophy is present, the **multifocal CNS involvement** (cognitive dysfunction + movement disorder + ataxia) and **anti-Hu positivity** distinguish PEM from pure paraneoplastic cerebellar degeneration (PCD), which typically presents with isolated progressive ataxia and is associated with anti-Yo or anti-VGCC antibodies in breast and ovarian cancers. ### Clinical Pearl Paraneoplastic encephalomyelitis often precedes cancer diagnosis by months to years. Patients with anti-Hu seropositivity should undergo urgent malignancy screening, particularly for SCLC. The presence of CSF lymphocytic pleocytosis supports inflammatory CNS involvement and helps exclude other causes of dementia. ### Management 1. **Confirm diagnosis:** CSF analysis (anti-Hu, elevated protein), brain MRI, PET-CT for malignancy 2. **Treat underlying cancer:** Chemotherapy (most important intervention) 3. **Immunotherapy:** Corticosteroids, IVIG, plasmapheresis (limited efficacy if neuronal loss is advanced) 4. **Symptomatic management:** Anticonvulsants for myoclonus, cognitive support **Mnemonic:** **HU** = **H**uD/HuC antigen, **U**sually SCLC; **PEM** = **P**araneoplastic **E**ncephalo**M**yelitis