## Paraneoplastic Encephalomyelitis (PEM) with Anti-Hu Antibodies ### Clinical Presentation The patient exhibits: - Cognitive dysfunction (memory loss, confusion) - Personality changes - Possible seizures, ataxia, or autonomic dysfunction - CSF lymphocytic pleocytosis (indicating CNS inflammation) ### Antibody-Malignancy Association | Antibody | Target Antigen | Most Common Malignancy | Frequency | |---|---|---|---| | Anti-Hu (ANNA-1) | HuD protein (neuronal) | SCLC | 80–90% | | Anti-Yo (PCA-1) | Purkinje cell antigen | Breast/ovarian | 90% | | Anti-Ri (ANNA-2) | RNA-binding protein | SCLC, breast | 50% | | Anti-VGCC | P/Q-type calcium channel | SCLC | 50–60% | **High-Yield:** Anti-Hu antibodies are present in 80–90% of SCLC cases with paraneoplastic neurological syndrome. The positive anti-Hu serology is virtually diagnostic of an underlying malignancy. ### Pathophysiology Anti-Hu antibodies cross-react with HuD protein expressed in both tumor cells and neurons. This triggers T-cell-mediated cytotoxic destruction of neurons, causing encephalomyelitis. ### Clinical Pearl **Key Point:** Anti-Hu seropositivity is a paraneoplastic marker with >95% specificity for malignancy. In a smoker with anti-Hu+ encephalomyelitis, SCLC is present in >80% of cases. ### Diagnostic Approach 1. **Serology:** Anti-Hu (ANNA-1), anti-Yo, anti-Ri, anti-VGCC 2. **Imaging:** Chest CT (SCLC screening), brain MRI (T2/FLAIR hyperintensities) 3. **CSF:** Lymphocytic pleocytosis, elevated protein, oligoclonal bands 4. **Oncology:** Urgent malignancy screening ### Management 1. Treat underlying malignancy (chemotherapy for SCLC) 2. Immunotherapy: IVIg, plasmapheresis, corticosteroids 3. Symptomatic seizure control **Mnemonic:** **SCLC-Hu** = Small Cell Lung Cancer is the most common source of anti-Hu paraneoplastic neurological syndromes.
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