A 62-year-old woman from Mumbai presents with a 2-month history of progressive confusion, memory loss, and personality changes. She also reports vertigo, ataxia, and involuntary jerking movements of the limbs. MRI brain shows no acute findings. Serum and CSF analysis reveal positive anti-CRMP5 antibodies. A subsequent CT chest shows a 3 cm nodule in the right upper lobe. What is the most likely paraneoplastic neurological syndrome in this patient?

Explanation

## Diagnostic Framework This patient presents with a constellation of neurological symptoms strongly suggestive of **paraneoplastic encephalomyelitis (PEM)** with an underlying **small cell lung cancer (SCLC)**. ### Clinical Presentation Breakdown | Symptom | Localization | Significance | |---------|--------------|---------------| | **Confusion, memory loss, personality change** | Limbic system + cortex | Encephalitis | | **Vertigo, ataxia** | Cerebellum | Cerebellar involvement | | **Involuntary jerking (myoclonus)** | Motor cortex + brainstem | Encephalomyelitis | | **Anti-CRMP5 antibodies** | Paraneoplastic marker | Highly specific for PEM | | **Lung nodule on CT** | Thorax | Primary malignancy | ### Anti-CRMP5 Antibodies: The Diagnostic Clue **Key Point:** **Anti-CRMP5 (collapsin response mediator protein-5)** antibodies are highly specific paraneoplastic markers associated with: - **Small cell lung cancer (SCLC)** — most common (70–80% of cases) - Non-small cell lung cancer - Thymoma - Other malignancies (rare) **High-Yield:** Anti-CRMP5 is part of the **onconeural antibody panel** and is considered a **definite paraneoplastic antibody** (tier 1) by the Autoimmune Encephalitis Society. ### Paraneoplastic Encephalomyelitis (PEM) Pathophysiology 1. **Molecular mimicry** — tumor expresses CRMP5 antigen 2. **Immune response** — B cells produce anti-CRMP5 IgG; T cells infiltrate CNS 3. **Multi-system involvement** — limbic system, cerebellum, brainstem, and spinal cord affected 4. **Result** — broad neurological syndrome with cognitive, cerebellar, and motor signs ### Why SCLC? SCLC is the **most common malignancy associated with anti-CRMP5 antibodies**. The 3 cm right upper lobe nodule on CT is consistent with SCLC presentation. SCLC is also the leading cause of paraneoplastic neurological syndromes overall. **Mnemonic:** **CRMP5-SCLC** = **C**RMPN5 antibodies → **S**mall **C**ell **L**ung **C**ancer (70–80% association). ### Differential Consideration: Why Not the Other Options? **Paraneoplastic cerebellar degeneration (PCD)** would present with **isolated cerebellar ataxia** (gait, limb, and speech ataxia) WITHOUT significant cognitive or myoclonic features. Anti-Yo and anti-Hu are more common markers. **Paraneoplastic limbic encephalitis (PLE)** would show **prominent memory loss and psychiatric symptoms** with **MRI evidence of medial temporal lobe T2/FLAIR hyperintensity** (not present here). Anti-Hu and anti-NMDA are typical markers. **Paraneoplastic opsoclonus-myoclonus** presents with **chaotic eye movements (opsoclonus)** and **myoclonus**, often in children with neuroblastoma, not in a 62-year-old with lung cancer.