Does Amphiphysin IgG indicate paraneoplastic neurologic autoimmunity?

Amphiphysin IgG is a clinically important onconeural marker, but its meaning is probabilistic and context-dependent—not a stand-alone diagnosis of either cancer or paraneoplastic neurologic syndrome (PNS).

Clinical significance

• Amphiphysin is a high-risk PNS antibody in the 2021 PNS-Care framework, a category associated with cancer in >70% of relevant PNS contexts.
• Compatible neurologic phenotypes include stiff-person spectrum disorder, encephalomyelitis/myelopathy, limbic encephalitis, sensory neuronopathy, and other neuropathies.
• Principal tumor associations are breast cancer and small-cell lung cancer (SCLC). The phenotype and coexisting antibodies can refine this inference—for example, stiff-person syndrome with isolated amphiphysin IgG has a classic breast-cancer association, while ANNA-1/Hu co-positivity favors SCLC.

Interpretation, testing, and cancer assessment

• A positive result does not establish active, recurrent, or occult cancer. PNS-Care diagnostic certainty integrates the neurologic syndrome, antibody risk category, tumor evidence, and follow-up; a compatible cancer is generally needed for definite PNS classification.
• Isolated commercial line-blot positivity can be clinically irrelevant or false-positive, particularly with an atypical syndrome or absent CSF corroboration. Confirmation with tissue-based immunofluorescence/immunohistochemistry plus antigen-specific immunoblot, ideally assessing serum and CSF, is important.
• With a confirmed result and compatible syndrome, screening should target thoracic malignancy (CT; FDG-PET/PET-CT if suspicion persists) and breast cancer where appropriate. If initial testing is negative but both antibody and phenotype are high risk, repeat screening every 4–6 months for 2 years is supported.

Mechanistic context

• Amphiphysin is an intracellular synaptic protein; IgG more likely marks tumor-driven antigen-specific cellular autoimmunity than proves direct antibody-mediated neuronal injury.

Bottom line

• Amphiphysin IgG warrants validated testing and focused malignancy evaluation, but only concordance among antibody, neurologic phenotype, and cancer evidence supports a paraneoplastic attribution.