Paraneoplastic syndrome

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"paraneoplastic syndrome" AND diagnosis

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I now have comprehensive material from Harrison's 22E (2025). Let me compile the full, authoritative answer:

Paraneoplastic Syndromes

Paraneoplastic syndromes (PNS) refer to disorders that accompany benign or malignant tumors but are not directly related to mass effects or invasion — they arise from tumor-secreted substances (hormones, cytokines, growth factors) or tumor-triggered immune responses. They affect up to 15% of cancer patients and, critically, precede the cancer diagnosis in ~60–65% of cases, making recognition essential for early tumor detection.

Pathogenesis

Two major mechanisms operate:

1. Ectopic Hormone/Substance Production

Tumor cells (often poorly differentiated or dedifferentiated) express peptide hormones normally produced only by specific endocrine tissues. The resulting hormone excess is:

Unregulated — defective feedback control (e.g., ACTH from SCLC is not suppressed by dexamethasone in standard doses)
Aberrantly processed — large, unprocessed precursors (e.g., POMC instead of mature ACTH)
Driven by epigenetic derepression, translocation, or cellular dedifferentiation

2. Autoimmune/Immune-Mediated (Paraneoplastic Neurologic Disorders)

Tumors ectopically express neural antigens → immune response generated against the tumor cross-reacts with the nervous system. Two mechanistic subtypes:

Target Antigen Location	Mechanism	Antibody Examples	Response to Treatment
Intracellular	CD4+/CD8+ T-cell cytotoxicity predominates; antibodies cannot access the epitope	Anti-Hu, Anti-Yo, Anti-Ri	Poor — irreversible neuronal loss
Cell-surface / synaptic	Antibodies directly alter receptor function/structure	Anti-NMDAR, Anti-AMPAR, Anti-LGI1, Anti-CASPR2	Better — often reversible

Pathogenesis of paraneoplastic encephalitis: intracellular vs. cell-surface antigen mechanisms, with immunofluorescence and histopathology panels

(Harrison's 22E, Fig. 99-1: T-cell–mediated neuronal death with intracellular antigens [left] vs. antibody-mediated receptor dysfunction with surface antigens [right])

I. Endocrine Paraneoplastic Syndromes

The most common and clinically significant:

Hypercalcemia of Malignancy

Most common paraneoplastic endocrine syndrome
Mediator: PTHrP (parathyroid hormone-related protein) → activates PTH receptor
Tumors: squamous cell carcinoma (lung, head/neck, esophagus), renal cell, breast, bladder
Also caused by: ectopic 1,25-(OH)₂D (lymphoma), direct bone resorption via cytokines (myeloma), rarely ectopic PTH

Ectopic ACTH Syndrome (Cushing's Syndrome)

Mediator: ectopic ACTH or CRH production
Tumors: SCLC (#1), carcinoid tumors, medullary thyroid cancer, pheochromocytoma, pancreatic islet cell tumors
Key features: rapid onset, profound hypokalemia, muscle wasting, hyperglycemia; classic cushingoid habitus often absent because tumor progression is too fast
Biochemistry: very high cortisol, high ACTH, no suppression with high-dose dexamethasone

SIADH (Syndrome of Inappropriate ADH)

Mediator: ectopic vasopressin secretion
Tumors: SCLC (#1), head/neck squamous cell, primary brain tumors
Presents: euvolemic hyponatremia, urine osmolality inappropriately high relative to serum
Treatment: fluid restriction; demeclocycline for long-term control; tumor treatment is definitive

Other Endocrine Syndromes

Syndrome	Mediator	Tumors
Non-islet cell hypoglycemia	IGF-2 (large retroperitoneal sarcomas, hepatocellular Ca) or insulin (islet cell)	Sarcomas, HCC, islet cell
Gynaecomastia / hCG excess	hCG, estrogens	Testicular germ cell, lung
Acromegaly	Ectopic GH or GHRH	Carcinoid, pancreatic islet cell
Hyperthyroidism	hCG (TSH-like activity)	Choriocarcinoma, gestational trophoblastic disease
Erythrocytosis	Ectopic EPO	RCC, cerebellar hemangioblastoma, uterine fibroids, HCC

II. Neurologic Paraneoplastic Disorders (PNDs)

Clinically disabling PNDs occur in 0.5–1% of all cancer patients but 2–3% of SCLC/neuroblastoma patients and 30–50% of thymoma patients.

High-Risk Antibodies (>70% probability of underlying cancer)

Antibody	Syndrome	Tumor
Anti-Hu (ANNA-1)	Encephalomyelitis, sensory neuronopathy	SCLC
Anti-Yo (PCA-1)	Rapidly progressive cerebellar degeneration	Ovary, breast
Anti-Ri (ANNA-2)	Cerebellar degeneration, opsoclonus, brainstem encephalitis	Breast, gynecologic, SCLC
Anti-CRMP5 (CV2)	Encephalomyelitis, chorea, optic neuritis, peripheral neuropathy	SCLC, thymoma
Anti-Tr (DNER)	Rapidly progressive cerebellar degeneration	Hodgkin's lymphoma
Anti-Ma2	Limbic/hypothalamic/brainstem encephalitis	Testicular (young men)
Anti-Kelch-like protein 11	Brainstem encephalitis, ataxia, sensorineural hearing loss	Seminoma, germ cell

Intermediate-Risk Antibodies (30–70% probability of cancer)

Anti-NMDAR, Anti-AMPAR, Anti-GABA-B-R, Anti-LGI1, Anti-CASPR2

Specific PND Syndromes

Paraneoplastic Cerebellar Degeneration (PCD)

Subacute onset of dysarthria, limb and gait ataxia, oculomotor dysfunction
Pathology: severe Purkinje cell loss
Antibodies: Anti-Yo (breast/ovarian), Anti-Tr (Hodgkin's), Anti-Hu (SCLC)

Paraneoplastic Encephalomyelitis / Limbic Encephalitis

Limbic: subacute amnesia, personality change, temporal lobe seizures, psychiatric symptoms
MRI: bilateral medial temporal lobe FLAIR hyperintensity
Antibodies: Anti-Hu, Anti-Ma2, Anti-LGI1 (SCLC, testicular)

Paraneoplastic Sensory Neuronopathy

Asymmetric numbness/burning paresthesias, loss of proprioception and vibration (posterior column)
Anti-Hu (SCLC)

Lambert-Eaton Myasthenic Syndrome (LEMS)

Proximal lower limb weakness; strength improves with sustained contraction (opposite to myasthenia gravis)
Antibody: Anti-VGCC (voltage-gated calcium channels); >80% with SCLC also have Anti-SOX1
EMG: incremental response at high-frequency (>10 Hz) stimulation
Autonomic features common (dry mouth, erectile dysfunction)

Paraneoplastic Opsoclonus-Myoclonus

Chaotic conjugate eye movements + myoclonus; in adults usually SCLC or breast (Anti-Ri)
In children: neuroblastoma (most common cause)

Paraneoplastic Stiff-Person Syndrome

Progressive rigidity, painful spasms of axial muscles; Anti-amphiphysin or Anti-GAD65
Tumors: breast cancer (Anti-amphiphysin), SCLC

III. Hematologic Paraneoplastic Syndromes

Syndrome	Mechanism	Associated Tumors
Erythrocytosis	Ectopic EPO	RCC, cerebellar hemangioblastoma, HCC
Granulocytosis / leukocytosis	Tumor production of G-CSF, GM-CSF, IL-6	Lung, GI, GU cancers
Thrombocytosis	Ectopic thrombopoietin or IL-6	Various carcinomas
Pure red cell aplasia	Autoimmune	Thymoma (#1)
Trousseau's syndrome	Hypercoagulability (mucin-secreting adenocarcinomas activate clotting)	Pancreatic, GI, lung adenocarcinoma

IV. Dermatologic Paraneoplastic Syndromes

Sign	Features	Tumor
Acanthosis nigricans	Velvety hyperpigmentation in skin folds	Gastric, GI adenocarcinoma
Dermatomyositis	Heliotrope rash, Gottron's papules, proximal myopathy	Lung, ovarian, breast, GI
Leser-Trélat sign	Sudden eruption of multiple seborrheic keratoses	GI, lymphoma
Paraneoplastic pemphigus	Severe mucocutaneous blistering	Lymphoma (NHL, CLL), thymoma
Necrolytic migratory erythema	Migratory, blistering rash in skin folds	Glucagonoma
Sweet's syndrome	Tender red plaques + fever + neutrophilia	AML, myelodysplastic syndrome

V. Rheumatologic & Other

Hypertrophic osteoarthropathy — periosteal new bone formation, clubbing; lung carcinoma
Palmar fasciitis/polyarthritis syndrome — rapidly progressive joint contractures; ovarian cancer
Paraneoplastic glomerulonephritis — membranous nephropathy (solid tumors, especially lung/GI), minimal change disease (lymphoma, NSAID-like)

Diagnosis

The Goldman-Cecil approach to evaluating a suspected PNS:

Characterize the abnormality; obtain labs and biopsy as needed
Eliminate common causes first
If no obvious etiology → consider PNS
If consistent with a known PNS → screen for occult malignancy (CT chest/abdomen/pelvis, PET-CT, tumor markers, testicular US in young men)
Antibody panel (serum and CSF for neurologic PNDs)
Course of the syndrome should parallel tumor course — a hallmark of PNS

Treatment

Principle	Details
Treat the underlying tumor	Definitive treatment; resolution of PNS often follows tumor response
Immunosuppression (neurologic PNDs)	High-dose corticosteroids, IVIG, plasmapheresis, rituximab
Prognosis	Surface-antigen PNDs (anti-NMDAR, LGI1) respond better than intracellular-antigen PNDs (anti-Hu, anti-Yo)
Endocrine syndromes	Symptomatic management (bisphosphonates for hypercalcemia, fluid restriction + tolvaptan for SIADH, metyrapone/ketoconazole for ectopic ACTH) while treating the primary cancer

Sources: Harrison's Principles of Internal Medicine 22E (2025), Ch. 98–99; Goldman-Cecil Medicine, Ch. 164; Bradley and Daroff's Neurology in Clinical Practice

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