Convergent Positivity of Anti-Gad Antibodies on Complementary Platforms: A Case Highlighting Autoimmune Neurological Phenotypes

Paraneoplastic neurological syndromes (PNS) are immune mediated disorders triggered by antibodies targeting shared neuronal and tumour antigens.Anti-glutamic acid decarboxylase (GAD) antibodies are typically linked to autoimmune conditions such as stiff person syndrome and cerebellar ataxia, but their occurrence in paraneoplastic contexts is rare.The authors describe a 62-year-old man with progressive limb stiffness, truncal rigidity and gait impairment.Magnetic resonance imaging was unremarkable while cerebrospinal fluid showed mild pleocytosis.Antibody screening by indirect immunofluorescence (IIF) and line immunoassay confirmed strong anti-GAD positivity, whereas other paraneoplastic antibodies were negative.A systemic workup revealed small cell lung carcinoma.High dose corticosteroids and oncological therapy provided partial improvement though significant neurological disability persisted.This case emphasises the diagnostic relevance of confirming anti GAD antibodies across complementary platforms and the importance of considering paraneoplastic etiologies in atypical anti-GAD neurological syndromes.