Paraneoplastic Syndromes

• Ectopic hormone production (endocrine paraneoplastic syndromes): Malignant cells acquire dysregulated expression of genes encoding peptide hormones normally produced by neuroendocrine tissues. Small cell lung cancer (SCLC), the most common culprit, frequently expresses atrial natriuretic peptide (ANP), adrenocorticotropic hormone (ACTH), vasopressin (ADH), and calcitonin. Squamous cell lung cancers and renal cell carcinomas produce parathyroid hormone-related protein (PTHrP) through constitutive promoter activation and loss of normal transcriptional repression. These ectopic hormones circulate at pathologic concentrations, activate cognate receptors on distant tissues, and trigger secondary physiologic cascades. For example, ACTH stimulates the adrenal cortex to produce excess cortisol (causing Cushing syndrome), PTHrP activates PTH1R on osteoblasts to increase bone resorption (causing hypercalcemia), and ADH leads to aquaporin-2 upregulation in collecting duct principal cells (causing hyponatremia). The molecular basis involves epigenetic modifications, loss of differentiation-inducing signals, and activation of normally silenced developmental programs.

• Antibody-mediated autoimmune paraneoplastic syndromes: Tumors express or release antigens that trigger B and T cell responses, generating pathogenic autoantibodies and autoreactive T cell clones that cross-react with normal tissues expressing the same epitope. This "molecular mimicry" mechanism is central to anti-Hu (ANNA-1), anti-Yo (PCA-1), anti-CV2/CRMP5, and anti-CRMP5 antibodies found in SCLC and ovarian cancer patients with encephalomyelitis. The putative tumor antigen shares sequence identity or structural homology with neuronal proteins; immune activation generates high-affinity IgG antibodies that penetrate the blood-brain barrier (particularly when it is disrupted by inflammation) and fix complement in neuronal synapses and axons. This complement-mediated cytotoxicity, enhanced by antibody-dependent cellular cytotoxicity (ADCC) via Fc receptor-bearing NK cells and macrophages, causes irreversible neuronal loss. Anti-NMDA receptor antibodies in ovarian teratomas directly block ionotropic glutamate signaling, producing a distinctive psychiatric and seizure phenotype. Anti-voltage-gated calcium channel (VGCC) antibodies in SCLC patients with Lambert-Eaton myasthenic syndrome (LEMS) impair presynaptic calcium influx, reducing acetylcholine release at the neuromuscular junction through both antibody-mediated calcium channel internalization and complement activation.

• T cell-mediated paraneoplastic neurological syndromes: CD8+ cytotoxic T lymphocytes recognize tumor-associated peptides presented on MHC class I and infiltrate the central nervous system, causing direct neuronal cytotoxicity. This mechanism underlies some cases of paraneoplastic cerebellar degeneration (PCD) and encephalomyelitis, where tumor-infiltrating lymphocytes and peritumoral T cells share TCR clonotypes with CSF T cell populations. The T cell response is often directed against intracellular tumor-associated antigens (e.g., CDK4 in SCLC), explaining why immunosuppressive treatment is less effective than in antibody-mediated syndromes.

• Growth factor and cytokine-mediated syndromes: Tumors produce paraneoplastic growth factors that alter host physiology. Paraneoplastic erythrocytosis results from erythropoietin (EPO) or EPO-like substances produced by renal cell carcinoma, hepatocellular carcinoma, and cerebellar hemangioblastomas, which activate EPOR-JAK2 signaling in bone marrow erythroid progenitors. Tumor necrosis factor-α (TNF-α) and IL-6 produced by lymphomas and other tumors induce fever, cachexia, and systemic inflammation through hypothalamic temperature set point elevation and hepatic acute-phase protein synthesis.

• Prothrombotic and coagulation abnormalities: Tumor cells express tissue factor (TF/CD142) and cancer procoagulant, directly activating the extrinsic pathway and Factor X, respectively. Tumor-associated macrophages and neutrophils produce additional TF. Circulating microparticles shed from apoptotic tumor and endothelial cells expose phosphatidylserine and TF, propagating thrombin generation. Malignant cells also produce cancer-associated fibrinogen-like protein 1 (FAP) and other prothrombotic molecules. This hypercoagulable state—particularly pronounced in adenocarcinomas (lung, pancreas, ovary)—manifests as venous thromboembolism, arterial thrombosis, and disseminated intravascular coagulation (DIC).

• Paraneoplastic renal disease mechanisms: Some PNS directly target the kidneys. Membranous nephropathy associated with malignancy results from in situ immune complex deposition and antibodies against phospholipase A2 receptor (PLA2R) or thrombospondin type-1 domain-containing 7A (THSD7A) expressed by tumor cells or shared epitopes. Minimal change disease in Hodgkin lymphoma involves T cell dysfunction and release of lymphokines (IL-13, TNF-α) that directly damage glomerular podocytes.

• Small cell lung cancer (SCLC): The highest frequency of paraneoplastic syndromes, occurring in 15-50% of cases. SCLC cells are neuroendocrine in origin and hence intrinsically predisposed to hormone production (ACTH, ADH, calcitonin). SCLC also associates with the majority of anti-Hu (60-70% of anti-Hu seropositive patients), anti-VGCC/LEMS (85% of LEMS cases), and paraneoplastic cerebellar degeneration cases. The small cell phenotype reflects derivation from neuroendocrine (Kulchitsky) cells of the bronchial epithelium.

• Non-small cell lung cancer (NSCLC): Squamous cell carcinoma accounts for most PTHrP-mediated hypercalcemia (up to 80% of paraneoplastic hypercalcemia cases overall). Adenocarcinoma may produce various hormones but is less commonly associated with classic paraneoplastic syndromes than SCLC; however, it accounts for paraneoplastic thrombosis and some cases of hypertrophic osteoarthropathy.

• Ovarian cancer: Particularly commonly associated with paraneoplastic neurological syndromes. Epithelial ovarian cancer associates with anti-Yo antibodies in paraneoplastic cerebellar degeneration and anti-CV2/CRMP5. Immature teratomas produce anti-NMDA receptor antibodies causing encephalitis with prominent psychiatric and movement disorder features. The association is so strong that any woman with anti-Yo seropositivity or anti-NMDA receptor antibodies should undergo transvaginal ultrasound and consideration of tumor markers (CA-125).

• Lymphoproliferative malignancies (Hodgkin lymphoma, B-cell lymphomas): Hodgkin lymphoma associates with minimal change glomerulonephritis, immune complex-mediated vasculitis, and dermatomyositis-like syndromes. B-cell non-Hodgkin lymphomas (particularly MALT and lymphoplasmacytic lymphomas) produce monoclonal immunoglobulins that cause cryoglobulinemia, cold agglutinin disease, and hyperviscosity syndrome.

• Gastric and other adenocarcinomas: Notable for association with dermatomyositis and other connective tissue disease-like syndromes. Gastric cancer ranks second only to ovarian cancer as a culprit malignancy in paraneoplastic dermatomyositis cases (up to 30% of DM patients have occult malignancy, with gastric cancer most common in Asian populations).

• Renal cell carcinoma: Beyond PTHrP (though less common than in NSCLC), RCC produces erythropoietin, causing paraneoplastic erythrocytosis in up to 10% of cases. RCC also associates with paraneoplastic nephrotic syndrome (membranous nephropathy) and hypertrophic osteoarthropathy.

• Hepatocellular carcinoma and other hepatic tumors: EPO-producing and causes erythrocytosis; also associated with hypoglycemia (via insulin-like growth factor II production), hypercholesterolemia, and thrombosis.

• Other malignancies: Breast cancer occasionally presents with paraneoplastic encephalitis (particularly involving limbic structures); testicular cancers and other germ cell tumors produce anti-NMDA receptor antibodies; pancreatic and biliary cancers are prothrombotic.

• Risk factors: Advanced stage disease increases PNS frequency, though PNS may precede malignancy detection. Smoking history increases SCLC risk (main culprit for ACTH syndrome, ADH syndrome, and LEMS). Genetic predisposition to autoimmune phenomena may underlie susceptibility to antibody-mediated PNS.

• Hypercalcemia (PTHrP-mediated, most common endocrine PNS): Presents with polyuria, polydipsia, nausea, vomiting, constipation, confusion, and altered mental status. Severe hypercalcemia (>14 mg/dL) causes dehydration, renal insufficiency, and cardiac arrhythmias. Typically associated with squamous cell lung cancer or renal cell carcinoma. Biochemically characterized by elevated calcium, suppressed PTH, and elevated PTHrP levels; normal 1,25-dihydroxyvitamin D (in contrast to sarcoidosis or lymphoma-associated hypercalcemia which produce excess calcitriol).

• SIADH (ADH-mediated hyponatremia, second most common endocrine PNS): Hyponatremia (<130 mEq/L) causes headache, confusion, seizures, cerebral edema, and coma if severe or acute. SCLC accounts for 50% of all malignancy-associated SIADH. Diagnosis requires hyponatremia, low serum osmolality, inappropriately elevated urine osmolality, and euvolemia (without edema, ascites, or orthostasis). Urine sodium typically exceeds 40 mEq/L. PTH, TSH, and cortisol are normal, excluding these alternative causes.

• Cushing syndrome (ACTH-mediated): SCLC produces ACTH ectopically in 12-16% of cases, though most (50-60%) do not have classic cushingoid features due to rapid disease progression. However, severe hypokalemia (often <3.0 mEq/L), metabolic alkalosis, hyperglycemia, hypertension, and muscle weakness are hallmark features. Diagnosis confirmed by 24-hour urinary free cortisol >4× upper limit normal and elevated plasma ACTH. Distinguishing ectopic ACTH from pituitary adenoma requires high-dose dexamethasone suppression test (ACTH from pituitary suppresses 17-OH corticosteroids by >50%; ectopic ACTH does not) and imaging.

• Paraneoplastic neurological syndromes: Present with subacute onset (weeks to months) of progressive neurological deficits. Paraneoplastic cerebellar degeneration (PCD) causes progressive ataxia, dysarthria, and nystagmus; associated with anti-Yo (ovarian, breast cancer) and anti-Hu (SCLC). Paraneoplastic encephalomyelitis (PEM) presents with cognitive changes, mood disturbance, seizures, memory loss, and myelitis; reflects CD8+ T cell-mediated inflammation of gray matter structures. Anti-NMDA receptor encephalitis (ovarian teratoma, testicular cancer) presents with prominent neuropsychiatric symptoms (personality change, psychosis, paranoia), movement disorders (orofacial dyskinesias, limb dystonia), autonomic instability, and seizures, often with CSF pleocytosis and EEG showing extreme delta brush pattern. Peripheral neuropathy (anti-Hu, anti-CV2) causes sensory predominant or sensorimotor axonal neuropathy. Lambert-Eaton myasthenic syndrome (LEMS) presents with proximal lower limb weakness, diminished reflexes that potentiate with brief exercise, and autonomic symptoms (dry mouth, constipation, impotence); 85% associated with SCLC.

• Skin manifestations: Dermatomyositis (DM) associates with malignancy in 25-30% of cases (gastric cancer most common); characterized by violaceous heliotrope rash on eyelids, erythematous papules over joints (Gottron's papules), facial erythema, and proximal muscle weakness. Acanthosis nigricans presents as velvety hyperpigmentation in intertriginous areas, associated with gastric, lung, and breast cancers. Tripe palms (exaggerated ridging of palms) and tylosis (thickened soles) occur predominantly with gastric cancer. Paraneoplastic pemphigus causes stomatitis, conjunctivitis, alopecia, and severe erosive lesions; presents with antibodies against desmoplakin I and III.

• Hematologic manifestations: Paraneoplastic erythrocytosis (RCC, hepatocellular carcinoma, cerebellar hemangioblastoma) presents with plethora, headache, thrombosis, and elevated hemoglobin (>16 g/dL in men, >14 g/dL in women). Microangiopathic hemolytic anemia manifests as anemia, reticulocytosis, schistocytes on blood smear, elevated LDH, low haptoglobin, and thrombocytopenia (simulating TTP or HUS) but associated with malignancy (especially adenocarcinomas). Paraneoplastic thrombosis presents with unprovoked DVT, PE, or arterial thrombosis; DIC manifests with bleeding, thrombosis, and coagulopathy simultaneously.

• Rheumatologic manifestations: Hypertrophic osteoarthropathy (HOA) presents with clubbing, periosteal new bone formation on long bones (visible on X-ray as parallel "railroad track" appearance), arthralgia, and arthritis; strongly associated with lung cancer (both SCLC and NSCLC). Polymyalgia rheumatica-like syndrome causes proximal muscle aching; associated with Hodgkin lymphoma. Vasculitis manifests as palpable purpura, glomerulonephritis, and peripheral neuropathy; seen with lymphoproliferative malignancies producing cryoglobulins or immune complexes.

• Paraneoplastic renal disease: Nephrotic syndrome (>3.5 g/24-hour proteinuria) from membranous nephropathy or minimal change disease; may precede malignancy

Step 1 — confirm the syndrome biochemically or electrophysiologically

• Hypercalcemia of malignancy: ionized or albumin-corrected calcium, then intact PTH — suppression of PTH is the pivotal finding. Follow with PTHrP (elevated in humoral hypercalcemia) and 1,25-dihydroxyvitamin D (elevated only in lymphoma/granulomatous calcitriol excess). Skeletal metastases raise calcium with suppressed PTH and normal PTHrP.
• SIADH: serum osmolality low, urine osmolality inappropriately concentrated, urine sodium typically >40 mEq/L, clinical euvolemia, with normal thyroid and adrenal function required before the label is applied (US hyponatremia expert panel recommendations, Verbalis et al.).
• Ectopic ACTH: Endocrine Society Cushing's syndrome guideline sequence — screen with late-night salivary cortisol, 24-hour urinary free cortisol, or 1 mg overnight dexamethasone suppression; confirm hypercortisolism, then measure plasma ACTH (non-suppressed). Inferior petrosal sinus sampling is the gold standard separating pituitary from ectopic source when imaging is equivocal.
• LEMS: serum P/Q-type voltage-gated calcium channel antibodies plus repetitive nerve stimulation showing low baseline CMAP amplitude, decrement at low-frequency stimulation, and marked post-exercise facilitation — the electrophysiologic mirror image of myasthenia gravis.

Step 2 — apply named criteria for neurologic syndromes

• PNS-Care Score (2021 updated diagnostic criteria for paraneoplastic neurologic syndromes) grades cases as possible, probable, or definite by combining phenotype (high- vs intermediate-risk), antibody class, and presence of cancer within a defined follow-up window.
• Test paired serum and CSF; CSF shows lymphocytic pleocytosis and oligoclonal bands in antibody-mediated encephalitides.

Immediate stabilisation

• Severe symptomatic hyponatremia (seizure, obtundation): hypertonic 3% saline as small boluses, targeting a rapid few-mEq/L rise to abort herniation, then strict limitation of total correction — US expert panel recommendations cap correction at roughly 8 mEq/L in 24 hours to prevent osmotic demyelination. Re-lowering with desmopressin plus free water is appropriate if overcorrection occurs.
• Hypercalcemic crisis: isotonic saline volume repletion first (these patients are profoundly nephrogenic-diabetes-insipidus dry), then calcitonin for rapid but tachyphylactic effect, plus an IV bisphosphonate (zoledronic acid) or RANKL inhibitor (denosumab) for durable control, per the Endocrine Society hypercalcemia of malignancy guideline. Denosumab is preferred in significant renal impairment or bisphosphonate-refractory disease.

Syndrome-directed therapy

• SIADH: fluid restriction first-line; add oral salt/urea or a loop diuretic; vasopressin receptor antagonists (tolvaptan) are reserved and time-limited given FDA hepatotoxicity labeling. Demeclocycline is a legacy option.
• Ectopic ACTH: steroidogenesis inhibitors (ketoconazole, metyrapone, or etomidate infusion when oral route fails) control cortisol while the tumor is treated; add Pneumocystis prophylaxis for profound hypercortisolism.
• LEMS: potassium channel blocker amifampridine (3,4-diaminopyridine) is FDA-approved first-line symptomatic therapy; pyridostigmine is adjunctive; IVIG, prednisone, or azathioprine for refractory weakness.
• Antibody-mediated encephalitis: first-line high-dose corticosteroids with IVIG or plasma exchange; escalate to rituximab and/or cyclophosphamide.

Disease-related — emergencies flagged

• Cerebral edema from acute hyponatremia (EMERGENCY): free water shifts intracellularly across an osmotic gradient; signaled by seizure, vomiting, or declining GCS. Requires hypertonic saline, not fluid restriction alone.
• Hypercalcemic crisis (EMERGENCY): calcium-induced nephrogenic diabetes insipidus causes volume depletion, which worsens calcium reabsorption in a vicious cycle; signaled by obtundation, AKI, and a shortened QT interval with bradyarrhythmia.
• Trousseau syndrome and DIC (EMERGENCY): tumor tissue factor drives systemic thrombin generation; signaled by migratory superficial thrombophlebitis, unprovoked VTE, or simultaneous bleeding with a falling fibrinogen and rising D-dimer. Nonbacterial thrombotic (marantic) endocarditis may embolize to brain.
• Neuromuscular respiratory failure (EMERGENCY): LEMS or paraneoplastic encephalomyelitis involving brainstem; signaled by a falling forced vital capacity, not by oxygen saturation.
• Autonomic instability and status epilepticus in anti-NMDA receptor encephalitis; central hypoventilation frequently mandates ICU care.
• Irreversible neuronal loss in anti-Hu/anti-Yo syndromes: cytotoxic T cell killing leaves fixed ataxia or sensory neuronopathy even after tumor cure.
• Metabolic sequelae of ectopic ACTH: profound hypokalemic metabolic alkalosis (cortisol saturates 11β-HSD2 and stimulates renal mineralocorticoid receptors), hyperglycemia, proximal myopathy, and opportunistic infection — Pneumocystis pneumonia is the classic lethal one.

Treatment-related

• Osmotic demyelination syndrome (EMERGENCY): overly rapid sodium correction dehydrates oligodendrocytes; signaled by a delayed (days later) spastic quadriparesis, dysarthria, and locked-in syndrome.
• Bisphosphonate/denosumab toxicity: hypocalcemia (worse with vitamin D deficiency or renal failure), osteonecrosis of the jaw, and atypical femoral fracture; zoledronic acid can precipitate AKI.
• Adrenal insufficiency from over-suppression with ketoconazole or metyrapone; ketoconazole additionally causes hepatotoxicity and QT prolongation.
• Tumor lysis syndrome after chemotherapy for bulky SCLC or lymphoma: hyperkalemia, hyperphosphatemia, hypocalcemia, urate nephropathy.
• Immunotherapy paradox: checkpoint inhibitors can unmask or worsen paraneoplastic neurologic syndromes.

• SCLC is the answer for three syndromes: SIADH, ectopic ACTH, and LEMS. Squamous cell lung carcinoma is the answer for PTHrP hypercalcemia. Examiners rotate these four constantly — anchor on histology, not on "lung cancer."
• LEMS strengthens with use: reflexes and power improve after brief exercise (post-tetanic facilitation), the opposite of myasthenia gravis, which fatigues. Autonomic features (dry mouth, constipation) and sparing of prominent ocular findings further separate LEMS from MG. Antibody target is the P/Q-type voltage-gated calcium channel.
• Suppressed PTH with hypercalcemia in a cancer patient = malignancy until proven otherwise. Best next step is isotonic saline, then calcitonin plus zoledronic acid or denosumab (Endocrine Society). Loop diuretics up front is the classic distractor.
• Hypercalcemia in lymphoma is calcitriol-mediated, not PTHrP-mediated — 1,25-dihydroxyvitamin D is elevated and glucocorticoids are the targeted therapy. Do not reflexively answer bisphosphonate here.
• High-dose dexamethasone fails to suppress cortisol in ectopic ACTH but suppresses in pituitary Cushing disease; inferior petrosal sinus sampling settles ambiguous cases. Marked hypokalemic alkalosis in a smoker with weight loss and hyperpigmentation is the stem.
• Young woman with psychosis, orofacial dyskinesias, seizures, and autonomic instability = anti-NMDA receptor encephalitis; the single best next step is pelvic imaging for ovarian teratoma, and resection plus immunotherapy is curative in many. Extreme delta brush is the EEG buzzword.
• Anti-Yo → ovarian/breast cerebellar degeneration; anti-Hu → SCLC sensory neuronopathy and encephalomyelitis. These target intracellular antigens, are T cell-mediated, and respond poorly to immunotherapy — treating the tumor is the priority. Surface-antigen antibodies (NMDAR, VGCC) respond well.
• Migratory superficial thrombophlebitis (Trousseau sign of malignancy) points to pancreatic or other adenocarcinoma; treat with LMWH or a DOAC per ASCO, not warfarin.
• Correct hyponatremia slowly — overcorrection causes osmotic demyelination, presenting days later with quadriparesis.