What is the appropriate workup and management for a patient with suspected paraneoplastic syndrome?

Paraneoplastic Syndrome: Workup and Management

Direct Recommendation

For suspected paraneoplastic syndrome, immediately initiate comprehensive cancer screening with CT chest/abdomen/pelvis while simultaneously performing CSF analysis and neuronal antibody testing in both serum and CSF, then start empiric immunotherapy without waiting for antibody results if clinical suspicion is high and infection is excluded. 1

Diagnostic Workup Algorithm

Step 1: Establish Clinical Syndrome (Within 12 weeks of onset)

Identify if the patient presents with a high-risk or intermediate-risk phenotype per 2021 PNS-CARE criteria 2:

High-risk phenotypes include:

• Rapidly progressive cerebellar syndrome (most common at 29%)
• Limbic encephalitis
• Brainstem encephalitis
• Opsoclonus-myoclonus syndrome

Key clinical features to document:

• Acute/subacute onset (<12 weeks)
• Preceding viral prodrome or febrile illness
• History of autoimmune disorders
• Smoking history or advanced age (increases cancer suspicion)

Step 2: Obtain CSF Analysis Immediately

This is the most critical diagnostic test 1. Do not delay lumbar puncture unless herniation risk exists. If brain MRI is delayed due to patient agitation, proceed with LP after screening head CT.

Essential CSF studies:

• Cell count with differential (expect lymphocytic pleocytosis 20-200 cells, can reach 900)
• Protein (expect hyperproteinorrachia)
• Glucose with CSF/serum ratio
• IgG index and synthesis rate
• Oligoclonal bands (unmatched in serum)
• HSV1/2 and VZV PCR
• Bacterial/fungal cultures
• Cytology and flow cytometry
• Neuronal antibody panel in CSF AND serum 1

Critical caveat: Some antibodies are more sensitive in CSF (NMDAR, GFAP), while others are more sensitive in serum (onconeural antibodies, LGI1, AQP4) 1

Step 3: Neuronal Antibody Testing Strategy

Test both serum and CSF simultaneously for:

• NMDAR, LGI1, CASPR2, AMPAR, GABA-B receptor, GlyR
• Onconeural antibodies (high-risk): Hu, Yo (PCA1), CRMP5, amphiphysin
• GAD antibodies
• KLHL11 (associated with testicular tumors and brainstem encephalitis) 2

Confirmation requirements:

• Intracellular antigens: immunohistochemistry confirmed by immunoblot
• Surface antigens: cell-based assay (CBA)
• Positive results by only one method should be interpreted cautiously 3

Step 4: Cancer Screening Protocol

Initiate immediately, do not wait for antibody results 1

Primary screening:

• CT chest/abdomen/pelvis with contrast (first-line due to cost-effectiveness and structural detail for biopsy planning)

Common associated malignancies to target:

• Small cell lung cancer (most common)
• Thymic neoplasm
• Breast cancer
• Ovarian teratoma/carcinoma (especially NMDAR-antibody encephalitis)
• Testicular teratoma/seminoma
• Neuroblastoma
• Lymphoma

Additional sex-specific screening:

For females:

• Mammogram if initial CT negative (CT has low sensitivity for early breast cancer) 1
• Breast MRI if strong family history or high suspicion
• Pelvic ultrasound or MRI (preferred over pelvic CT in childbearing age)

For males:

• Testicular ultrasound (CT has low sensitivity for testicular cancer) 1

Special considerations:

• Avoid CT in children and pregnant women
• CT iodine contrast is safer in pregnancy than MRI gadolinium 1
• Consider FDG-PET if CT negative and suspicion remains high
• Repeat screening every 3-6 months if initial screen negative with onconeural antibodies present 3

Management Algorithm

Immediate Actions (Do Not Delay)

Start immunotherapy empirically if:

• High-risk or intermediate-risk phenotype present
• Infection excluded
• Inflammatory CSF findings present
• Do not wait for antibody results or cancer identification 4, 5

First-Line Immunotherapy

Choose one or combine:

• High-dose IV methylprednisolone followed by oral taper
• IVIG (0.4 g/kg/day × 5 days)
• Plasma exchange (5-7 exchanges; perform BEFORE IVIG if combining)

Expected response:

• Most patients respond within weeks
• NMDAR-antibody encephalitis may have slower response (weeks to months) 4

Second-Line Immunotherapy

Initiate if no response to first-line within 2-4 weeks:

• Rituximab (375 mg/m² weekly × 4 doses or 1000 mg × 2 doses)
• Cyclophosphamide (750 mg/m² monthly)
• Consider combination therapy 4, 5

Tumor Treatment

Tumor control is the first therapeutic priority 5:

• Surgical resection when feasible
• Chemotherapy/radiation per oncology
• In rare cases, tumor treatment alone can reverse neurologic symptoms
• Prevents disease progression even if neurologic recovery incomplete

Antibody-Specific Considerations

Surface antibody syndromes (NMDAR, LGI1, CASPR2, AMPAR, GABA-B):

• Better immunotherapy response
• More aggressive immunosuppression justified
• Consider maintenance therapy to prevent relapse

Intracellular antibody syndromes (Hu, Yo, CRMP5):

• T-cell mediated, poorer immunotherapy response
• Focus on tumor control
• Immunotherapy still warranted but temper expectations 6, 7

Critical Pitfalls to Avoid

1. Do not wait for antibody results to start treatment - neurologic damage can be irreversible 4, 5

2. Do not stop cancer screening after negative initial CT - repeat every 3-6 months for up to 4 years if onconeural antibodies present 3

3. Do not order antibody panels indiscriminately - only test when high-risk or intermediate-risk phenotype present; overutilization leads to false positives and overtreatment 3, 7

4. Do not assume PNS-CARE score <6 excludes PNS - 15 patients in one cohort had likely PNS despite intermediate scores, particularly KLHL11-IgG brainstem encephalitis with burned-out testicular tumors 2

5. Do not use plasma exchange after IVIG - PLEX will remove the immunoglobulins; always perform PLEX first if combining 4

6. Do not overlook sex-specific cancers - add mammogram for women and testicular ultrasound for men beyond standard CT 1

Special Population: Immune Checkpoint Inhibitor-Associated PNS

For ICI-related neurologic syndromes:

• Follow ASCO guidelines for immune-related adverse events
• Hold ICI immediately
• Higher threshold for aggressive immunosuppression
• Consider permanent ICI discontinuation for severe cases 5

Monitoring and Long-Term Management

• Serial antibody titers in serum and CSF guide treatment weaning 4
• Wean immunosuppression very carefully - abrupt cessation increases relapse risk
• NMDAR-antibody encephalitis has highest relapse risk; consider maintenance therapy 4
• Monitor for drug toxicity, especially in ICU settings with multiple medications 1