What happened, in summary

A man in his late 70s developed progressive unsteadiness and numbness over 2 weeks. The symptoms began in both feet and later affected his hands. Examination found weakness, reduced reflexes, loss of sensation and proprioception, and a broad-based, high-stepping gait with marked sensory ataxia. He had a 20 pack-year smoking history but had quit 20 years earlier, and he also lived with several chronic heart and kidney conditions. Because he had experienced a mild COVID-19 infection 1 month earlier, post-COVID neuropathy and Guillain–Barré syndrome were initially considered.

Nerve-conduction studies instead showed a length-dependent axonal sensorimotor polyneuropathy rather than the demyelinating pattern expected with Guillain–Barré syndrome. Testing found a weakly positive anti-CV2 antibody, raising concern for a paraneoplastic neurological syndrome. Further investigation identified a spiculated mass in the right lung, which was confirmed as primary lung adenocarcinoma. The anti-CV2 finding supported a paraneoplastic neurological syndrome rather than serving as a tumour biomarker.

His lower-limb strength improved within 2 weeks, and by discharge he could walk 60 m with a rollator frame. He was referred to medical and radiation oncology and later began alectinib, which he tolerated well. Follow-up CT showed stable lung cancer with no metastases identified. Repeat nerve-conduction studies also showed improvement in the sensorimotor polyneuropathy.

The neurological symptoms therefore became the clue that led to the lung-cancer diagnosis. At the latest follow-up, both the cancer and the associated neuropathy were stable or improving. He continued coordinated neurological and oncology follow-up.