Syndrome of Inappropriate Antidiuretic Hormone Secretion

Elderly pts

Nursing home residents

Planned major operations, especially neurosurgical procedures

Pts receiving exogenous hormone therapy, especially desmopressin

CNS disorders including psychiatric diseases

Cancer, especially small-cell lung cancer

Lung disease

Hyponatremia

Cerebral edema causing altered mentation, seizures, and coma

Acute water intoxication and fluid overload

Other causes of hyponatremia, such as heart failure, liver failure, renal failure, or pseudohyponatremia (e.g., hyperglycemia) (see Hyponatremia).

Acuity and magnitude of hyponatremia influences the risk of CNS complications.

Osmotic demyelination syndrome is caused by rapid correction of hyponatremia.

Hyponatremia is the most common electrolyte disorder in hospitalized pts (affects 15%), and SIADH is the most frequent cause of hyponatremia, but other causes of hyponatremia should be excluded before making a Dx of SIADH.

Normally, increased serum osmolarity, hypovolemia, or hypotension triggers thirst and ADH release. ADH increases aquaporin-2 channels on the luminal surface of the distal tubules and collecting duct and acts to promote free water reabsorption. Thirst, free water intake, or hypotonic fluid administration combined with ADH-induced free water retention causes hyponatremia.

Dx of SIADH: Symptoms include serum osmolarity less than 275 mOsm/L, urine osmolarity >100 mOsm/L, urine sodium >40 mEq/L, euvolemia, normal thyroid and adrenal function, and absence of diuretic therapy.

SIADH can be classified as follows: Type A is unregulated secretion of ADH, type B is elevated basal secretion, type C is reset osmostat, and type D is undetectable ADH.

Malignant diseases causing ectopic ADH secretion: Lung cancer (especially small-cell and mesothelioma), brain tumors, cancer of the duodenum, pancreas, head and neck, GU tract, lymphoma, and sarcomas.

Pulm disorders: Infections, asthma, cystic fibrosis.

CNS disorders: Infection, masses, head trauma, intracranial bleed, MS, Guillain-Barré syndrome, Shy-Drager syndrome, delirium tremens, and acute intermittent porphyria.

Immune compromised states like HIV with associated pulm infections or malignancies.

Drugs include, but are not limited to, chlorpropamide, carbamazepine, cyclophosphamide, SSRIs, TCAs, clofibrate, nicotine, NSAIDs, antipsychotics, narcotics, arginine vasopressin analogues (DDAVP, oxytocin, and vasopressin).

Major surgery: Pain, stress, general anesthesia, PPV, neurosurgery.

SIADH may be hereditary, with a mutation of gene for renal vasopressin-2 receptor and a mutation for gene affecting osmolarity sensing in the hypothalamus.