Introduction:
Systemic lupus erythematosus (SLE) is a complex autoimmune disease. Its diagnosis is based on clinical and laboratory criteria, with mandatory exclusion of other differential diagnoses. Paraneoplastic syndrome is a rare disorder in which there is an underlying malignancy, causing symptoms not directly related to tumour metastasis or local invasions. Paraneoplastic rheumatic syndrome closely resembles the rheumatic disease and may be accompanied by the presence of autoantibodies.

Case description:
A 45-year-old woman was admitted to the Rheumatology Ward following a previous suspicion of granulomatosis with polyangiitis (GPA) and an inconclusive diagnostic evaluation during outpatient treatment. Patient’s medical history included generalised joint pain with oedema, weakness, occasional low-grade fever, and recurrent epistaxis with nasal perforation. Histopathological examination of the nasal mucosa excluded findings characteristic of GPA. The patient has been treated with methotrexate and glucocorticosteroids. During the current hospitalisation, physical examination revealed no joint or upper respiratory tract inflammation. However, supraclavicular lymph nodes were enlarged, and the patient reported newly onset of dysphagia. Laboratory tests showed mildly elevated C-reactive protein, lymphopenia, positive antinuclear antibody with a homogeneous pattern and a titre of 1 : 3,200, positive anti-dsDNA and mildly elevated carcinoembryonic antigen. The patient was referred for a lymph node biopsy, which concluded third--grade adenocarcinoma. Computed tomography showed gastric wall thickening and multiple enlarged lymph nodes in the abdominal area, followed by diffuse nodules in the lungs. Gastroscopy revealed a pathological, cauliflower-like infiltrative lesion involving a greater part of the gastric body. Histopathological examination of gastric biopsy specimens confirmed gastric adenocarcinoma. Due to the advanced stage of cancer, the patient was qualified for palliative treatment.

Conclusions:
Paraneoplastic rheumatic syndromes pose a diagnostic challenge because they can closely mimic SLE while presenting with few unusual features. Therefore, the exclusion of other differential diagnoses is crucial. This case highlights the necessity to reconsider the diagnosis in patients with atypical presentations, even when other diagnostic criteria are met.