Paraneoplastic syndromes are one of those Step 1 topics that feel “random” until you realize they’re actually pattern-recognition gold: a tumor causes symptoms at a distance, often via ectopic hormone production or immune cross-reactivity, and those symptoms can be the first clue to an occult malignancy. If you can link the syndrome to the cancer, you’ll pick up easy points—and sometimes save a patient in real life.
What Are Paraneoplastic Syndromes?
Definition: A symptom complex caused by cancer but not due to:
- direct tumor invasion,
- mass effect/obstruction,
- metastasis, or
- infection/therapy side effects.
Instead, it’s typically due to:
- Ectopic production of hormones/cytokines
- Immune-mediated (autoimmune) cross-reactivity between tumor antigens and normal tissues
High-yield framing:
If the clinical finding seems “out of proportion” to tumor burden, or appears before the cancer is diagnosed, think paraneoplastic.
Pathophysiology (Step 1-Style)
1) Ectopic Hormone/Cytokine Production
Tumor cells (often poorly differentiated) express genes they normally wouldn’t → secrete biologically active substances.
Common categories:
- ADH → SIADH (hyponatremia)
- ACTH → Cushing syndrome
- PTHrP → hypercalcemia
- EPO → polycythemia
- TGF-α/EGF-like factors → skin changes (e.g., acanthosis nigricans)
- Mucin/thrombogenic factors → hypercoagulability
2) Immune Cross-Reactivity (Autoimmune Paraneoplastic)
Tumor expresses antigens similar to neural/muscle proteins → immune system attacks both.
Classic examples:
- Lambert-Eaton myasthenic syndrome (LEMS) in small cell lung cancer
- Subacute cerebellar degeneration (anti-Yo, anti-Hu, etc.)
- Paraneoplastic encephalitis/neuropathy
Board clue: neurologic paraneoplastic syndromes often have anti-neuronal antibodies and can precede cancer diagnosis.
Why It Matters Clinically (Big Picture)
Paraneoplastic syndromes can:
- be the earliest manifestation of cancer,
- cause significant morbidity (e.g., arrhythmias from hypercalcemia, seizures from hyponatremia),
- provide a diagnostic shortcut to the primary tumor,
- and sometimes improve with tumor treatment.
High-Yield Paraneoplastic Syndromes by Tumor (Must-Know Table)
The Core Associations (memorize these)
| Tumor | Paraneoplastic syndrome | Mediator/Mechanism | Key Step clue |
|---|---|---|---|
| Small cell lung carcinoma | SIADH | ectopic ADH | euvolemic hyponatremia, low serum osm, inappropriately concentrated urine |
| Cushing syndrome | ectopic ACTH | hypertension, hyperglycemia, metabolic alkalosis ± hypokalemia | |
| LEMS | Ab vs presynaptic Ca channels | proximal weakness improves with use, autonomic sx | |
| Squamous cell carcinoma (lung) | Hypercalcemia | PTHrP | high Ca, low PTH; kidney stones, constipation |
| Renal cell carcinoma | Polycythemia | EPO | high Hgb/Hct; can also cause hypercalcemia, HTN (renin) |
| Hepatocellular carcinoma | Polycythemia, hypoglycemia | EPO; IGF-like | big liver mass + systemic metabolic findings |
| Ovarian (esp. clear cell), endometrial, GI adenocarcinomas | Migratory thrombophlebitis (Trousseau) | hypercoagulable state | recurrent superficial clots in different sites |
| Pancreatic adenocarcinoma | Trousseau | mucin → thrombosis | unexplained VTEs + weight loss/jaundice |
| Thymoma | Myasthenia gravis | Ab vs postsynaptic AChR | fluctuating fatigable weakness, ptosis |
| Hodgkin lymphoma | Pruritus, alcohol-induced pain | cytokines | painless LAD + B symptoms |
| Acute promyelocytic leukemia (APL) | DIC | procoagulants | bleeding + Auer rods; treat ATRA |
| Breast, ovarian carcinoma | Subacute cerebellar degeneration | anti-Yo (classically) | ataxia, dysmetria; precede diagnosis |
| Lung (small cell), neuroblastoma | Opsoclonus-myoclonus | anti-neuronal | “dancing eyes” + jerks |
Classic Paraneoplastic Syndromes (Clinical Presentation + Diagnosis + Treatment)
1) SIADH (Small Cell Lung Cancer)
Presentation
- Nausea, confusion, seizures (severe)
- Euvolemic hyponatremia
- Low serum osmolality, inappropriately high urine osmolality
Diagnosis
- Hyponatremia with low plasma osm
- Urine Na usually elevated (not “trying” to conserve sodium)
Treatment (Step-style priorities)
- Treat underlying malignancy
- Fluid restriction
- Hypertonic saline for severe symptoms
- Consider demeclocycline or vasopressin receptor antagonists (e.g., tolvaptan)
First Aid cross-ref: Endocrine (SIADH), Pulmonary (small cell lung carcinoma), Renal electrolytes.
2) Hypercalcemia of Malignancy (Squamous Cell Lung Cancer → PTHrP)
Presentation (stones, bones, groans, psychiatric overtones)
- Constipation, polyuria/polydipsia, confusion
- Short QT on ECG
- Nephrolithiasis
Pathophys
- PTHrP mimics PTH → ↑ osteoclast activity (via osteoblast signaling), ↑ renal Ca reabsorption, ↓ phosphate
Diagnosis
- ↑ Ca, low PTH
- Often ↑ PTHrP (if tested)
- Phosphate tends to be low/normal (similar direction as PTH physiology)
Treatment
- IV fluids
- Bisphosphonates (e.g., zoledronic acid)
- Calcitonin for rapid temporary effect
- Treat the cancer
First Aid cross-ref: Endocrine calcium regulation; Pulmonary (lung cancers).
3) Cushing Syndrome from Ectopic ACTH (Small Cell Lung Cancer)
Presentation
- Hypertension, hyperglycemia, muscle weakness
- Hypokalemic metabolic alkalosis can occur (ACTH → cortisol can stimulate mineralocorticoid receptors)
Diagnosis
- Elevated cortisol with failure to suppress appropriately
- Context clues: smoker + lung mass + rapid onset Cushingoid features
Treatment
- Treat cancer
- Consider steroidogenesis inhibitors in select cases (Step 2-level nuance)
First Aid cross-ref: Endocrine (Cushing), Pulmonary oncology.
4) Lambert-Eaton Myasthenic Syndrome (LEMS) (Small Cell Lung Cancer)
Key distinguishing features vs Myasthenia gravis
- Presynaptic Ca channel antibodies → ↓ ACh release
- Proximal muscle weakness that improves with repeated use
- Autonomic symptoms: dry mouth, impotence
- Decreased reflexes
Diagnosis
- Clinical pattern + antibodies
- EMG: incremental response with repetitive stimulation
Treatment
- Treat underlying cancer
- 3,4-diaminopyridine (increases ACh release) ± immunotherapy (conceptual)
First Aid cross-ref: Neuro (NMJ disorders), Pulmonary (small cell).
5) Myasthenia Gravis (Thymoma)
Presentation
- Ptosis, diplopia, fatigable weakness worse with use
- Bulbar symptoms possible
Path
- Postsynaptic ACh receptor antibodies
Diagnosis
- AChR antibodies, EMG decrement, ice pack test (classic vignette)
Treatment
- AChE inhibitors (pyridostigmine)
- Immunotherapy
- Evaluate for thymoma (CT chest)
First Aid cross-ref: Neuro (MG), Immunology (type II hypersensitivity), Mediastinal masses.
6) Trousseau Syndrome (Migratory Thrombophlebitis) (Pancreatic/GI Adenocarcinoma)
Presentation
- Recurrent, migratory superficial thrombophlebitis
- Unexplained VTEs, sometimes resistant to standard therapy
Mechanism
- Tumor mucin and procoagulants → hypercoagulability
Diagnosis
- Clinical pattern + search for occult malignancy (especially pancreas)
Treatment
- Treat underlying cancer
- Anticoagulation as indicated (Step 2 oriented)
First Aid cross-ref: Hematology (hypercoagulability), GI oncology.
7) Acanthosis Nigricans (Gastric Adenocarcinoma and Others)
Presentation
- Velvety, hyperpigmented plaques in skin folds (neck, axilla)
- Can be benign (insulin resistance) or malignant (rapid onset, extensive, older patient)
Mechanism
- Growth factor signaling (e.g., TGF-α)
First Aid cross-ref: Dermatology + GI malignancy associations.
Diagnostic Approach: How Vignettes Want You to Think
Step-by-step pattern recognition
- Identify the syndrome (e.g., hyponatremia + confusion → SIADH)
- Ask: “Is this explained by local tumor effects?”
- If no → paraneoplastic
- Match to the tumor
- SIADH/ACTH/LEMS → small cell lung
- PTHrP hypercalcemia → squamous cell lung
- EPO polycythemia → RCC/HCC
- Migratory thrombophlebitis → pancreatic/GI adenocarcinoma
- Confirm with targeted tests
- Osmolality studies, PTH level, antibodies, imaging for primary
Common testable lab patterns
- Hypercalcemia of malignancy: ↑ Ca, ↓ PTH
- SIADH: ↓ serum osm, ↑ urine osm, euvolemia
- Ectopic ACTH: ↑ cortisol ± hypokalemia/alkalosis
- EPO secretion: ↑ Hgb/Hct
Treatment Principles (What Boards Emphasize)
Core rule: Manage the acute dangerous physiology and treat the underlying tumor.
- Stabilize life-threatening derangements
- Severe hyponatremia seizures → hypertonic saline
- Severe hypercalcemia → fluids + bisphosphonate ± calcitonin
- Definitive management is often oncologic treatment
- surgery/chemo/radiation depending on cancer
- Immune-mediated syndromes
- may need immunotherapy (steroids, IVIG, plasmapheresis) but tumor control is key
High-Yield “Don’t Mix These Up” (Rapid Review)
Small cell vs squamous lung cancer
- Small cell: neuroendocrine → ADH, ACTH, LEMS
- Squamous: PTHrP → hypercalcemia
LEMS vs Myasthenia gravis
| Feature | LEMS | Myasthenia gravis |
|---|---|---|
| Target | Presynaptic Ca channels | Postsynaptic ACh receptor |
| Weakness with use | Improves | Worsens |
| Reflexes | Decreased | Normal |
| Assoc tumor | Small cell lung | Thymoma |
First Aid Cross-References (Where This Lives)
Use these as your quick map while flipping through First Aid:
- General Pathology: neoplasia overview; paraneoplastic syndromes concept
- Pulmonary: lung cancer subtypes (small cell vs squamous) + key paraneoplastic links
- Endocrine: SIADH, Cushing syndrome, calcium regulation (PTH/PTHrP)
- Neurology: NMJ disorders (MG vs LEMS), paraneoplastic neurologic syndromes
- Hematology/Oncology: hypercoagulable states (Trousseau), malignancy-associated DIC
Ultra–High-Yield One-Liners (Exam Day Memory Hooks)
- Small cell lung: “Small cell = SIADH, Steroids (ACTH), Synaptic issue (LEMS).”
- Squamous lung: “Squamous squeezes out Ca” (PTHrP).
- RCC: “Kidney tumor makes EPO” → polycythemia.
- Pancreatic adenocarcinoma: “Thrombosis that travels” (Trousseau).
- Thymoma: think myasthenia gravis.