Anti-Hu-Associated Paraneoplastic Myeloneuropathy as the Initial Presentation of Early-Onset Colon Adenocarcinoma

Anti-Hu paraneoplastic neurological syndromes are most commonly associated with small-cell lung cancer and are rarely reported in colorectal malignancy. A 43-year-old man presented with progressive sensory neuropathy and weight loss. Evaluation revealed positive serum anti-Hu antibodies, markedly elevated cerebrospinal fluid protein, and imaging demonstrating an ascending colon mass. Right hemicolectomy confirmed Stage IIIA (pT3N1aM0) moderately differentiated adenocarcinoma, with mismatch repair-deficient (loss of MLH1 and PMS2) and B-Raf proto-oncogene, serine/threonine kinase wild-type tumor profile. Despite a significant family history, the patient had not undergone recommended early colorectal screening. Postoperative circulating tumor DNA was negative, and the patient was placed on active surveillance; however, neurological symptoms persisted, and immunomodulatory therapy access was initially limited. This case highlights a rare association between anti-Hu-associated myeloneuropathy and early-onset colon cancer and underscores the importance of malignancy evaluation in unexplained sensory neuronopathy.