# Paraneoplastic Syndromes Cancer Associations and Clinical Clues > Endocrine, neurological, haematological and skin paraneoplastic syndromes matched to specific cancers, with bedside clues for NEET-PG Medicine exam preparation. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/paraneoplastic-syndromes - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-08 - Last updated: 2026-10-08 - Reference basis: Harrison's Principles of Internal Medicine; DeVita's Principles and Practice of Oncology; Adams and Victor's Principles of Neurology - How to cite: "Paraneoplastic Syndromes Cancer Associations and Clinical Clues", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/paraneoplastic-syndromes ## Direct answer Paraneoplastic syndromes are clinical disorders caused by tumour-secreted hormones, cytokines or immune cross-reactions rather than by the local mass, and they often appear before the cancer is found. Each is linked to particular tumours: small cell lung cancer with SIADH, Lambert-Eaton syndrome and anti-Hu encephalitis; thymoma with myasthenia gravis and pure red cell aplasia; ovarian teratoma with anti-NMDA receptor encephalitis. Recognising the pairing guides the search for the underlying tumour. ## What you must remember - **Endocrine:** Small cell lung cancer causes SIADH and ectopic ACTH causing Cushing syndrome; squamous lung and renal cancers cause hypercalcaemia through PTHrP, while lymphomas do so mainly through calcitriol; hepatocellular carcinoma can cause hypoglycaemia through IGF-2. - **Neurological:** Anti-Hu with small cell lung cancer; anti-Yo with ovarian or breast cancer causing cerebellar degeneration; anti-NMDA receptor with ovarian teratoma in young women. - **Lambert-Eaton syndrome:** Antibodies to voltage-gated calcium channels, usually with small cell lung cancer; weakness improves with repeated use, unlike myasthenia gravis. - **Haematological:** Erythrocytosis from renal cell carcinoma or hepatocellular carcinoma through erythropoietin; migratory thrombophlebitis (Trousseau syndrome) with pancreatic cancer; pure red cell aplasia with thymoma. - **Dermatological:** Dermatomyositis with ovarian and lung cancer; acanthosis nigricans and tripe palms with gastric adenocarcinoma; Sweet syndrome with acute myeloid leukaemia; sudden multiple seborrhoeic keratoses (Leser-Trélat sign) with gastrointestinal adenocarcinoma. - **Musculoskeletal:** Hypertrophic pulmonary osteoarthropathy with clubbing and periosteal new bone, mostly with lung cancer. - **Carcinoid:** Flushing, diarrhoea and bronchospasm from serotonin, with niacin deficiency when tryptophan is diverted. ## Worked scenario: hyponatraemia in a smoker A 60-year-old smoker presents with confusion and a serum sodium of 118 mmol/L. The reasoning follows the chain from hormone to tumour rather than treating the sodium in isolation. 1. **Confirm hypotonic hyponatraemia:** Check that the patient is euvolaemic, with normal thyroid and adrenal function, low serum osmolality and inappropriately concentrated urine. 2. **Interpret the urine:** A raised urine osmolality with urine sodium above about 30 mmol/L suggests SIADH, which in an older smoker raises the question of a small cell lung cancer. 3. **Image the chest:** A chest CT is the next investigation, and a central mass with mediastinal nodes directs the team to biopsy. 4. **Treat while seeking the tumour:** Fluid restriction and careful correction of the sodium are started, and the correction is slow because rapid rises risk osmotic demyelination. 5. **Judge the link:** Once the tumour is found, the paraneoplastic syndrome often improves with treatment of the cancer, which supports the link between them. ## Where the exam tests the pairing Pairing questions hinge on a few distinctions. - **Trap:** confusing Lambert-Eaton syndrome with myasthenia gravis. — **Correct:** strength improves briefly after exercise and reflexes are reduced in Lambert-Eaton, whereas myasthenia worsens with repeated use and has normal reflexes. - **Trap:** assuming a neurological syndrome means a brain tumour. — **Correct:** anti-Hu and anti-Yo show that the tumour is elsewhere, often in the chest or pelvis. - **Trap:** overlooking thymoma in pure red cell aplasia. — **Correct:** it should prompt a search for thymoma even when the patient has no myasthenia. ## Frequently asked questions ### Which small cell lung cancer paraneoplastic syndrome is most often tested? SIADH is the most often tested paraneoplastic syndrome of small cell lung cancer, producing hypotonic hyponatraemia with inappropriately concentrated urine. Lambert-Eaton syndrome and anti-Hu encephalitis are the other classic small cell associations. Recognising SIADH is the first step to finding the tumour. ### How is Lambert-Eaton syndrome distinguished from myasthenia gravis clinically? Lambert-Eaton syndrome shows proximal limb weakness that improves briefly with repeated effort and reduced reflexes, while myasthenia gravis shows fatigable weakness that worsens with use and normal reflexes. Lambert-Eaton is linked most often to small cell lung cancer, whereas myasthenia is linked to thymoma. Antibody testing for voltage-gated calcium channels or acetylcholine receptors confirms the diagnosis. ### Which tumour is associated with anti-NMDA receptor encephalitis in young women? Ovarian teratoma is the tumour most associated with anti-NMDA receptor encephalitis in young women, and removing it is central to recovery. The syndrome presents with psychiatric change, seizures and movement disorders. A pelvic imaging search is part of the workup even when the initial presentation is psychiatric. ### What skin sign suggests an underlying gastric adenocarcinoma? Acanthosis nigricans, a velvety hyperpigmentation of skin folds, can signal an underlying gastric adenocarcinoma, especially when it appears suddenly in an adult. Tripe palms, which are thickened palmar skin, may accompany it. Gastroscopy is indicated when such skin changes appear without another explanation. ### Why is migratory thrombophlebitis called Trousseau syndrome? Migratory thrombophlebitis is called Trousseau syndrome because the French physician Armand Trousseau described it in association with occult cancer, most often pancreatic adenocarcinoma. It reflects a hypercoagulable state driven by the tumour. Recurrent venous thromboses in an older adult should prompt a search for malignancy.