Synonyms
Syndrome of inadequate ADH secretion (SIADH), ADH excess, ADH overproduction
Definition
Schwartz-Bartter syndrome is a disorder of the regulation of the water and electrolyte balance, in which an inappropriately (inadequately) high secretion of the antidiuretic hormone (ADH – hormone, also: vasopressin) leads to a reduced excretion of water (water retention) and a loss of sodium (hyponatremia).
Frequency
It is believed that almost all patients may experience temporary inadequate secretion of ADH after surgery.
History
The Schwartz-Bartter syndrome is named after the American internists William Benjamin Schwartz (* 1922) and Frederic Crosby Bartter (1914-1983).
Causes
There are several causes of Schwartz-Bartter syndrome. In 80% of cases it occurs as a paraneoplastic syndrome in small cell lung carcinomas. A paraneoplastic syndrome describes accompanying symptoms of a cancer disease that are neither directly caused by the tumor nor by metastases, but rather by the body’s defensive reactions against the tumor or by the release of messenger substances such as hormones by the tumor.
Other rarer causes can be disorders of the central nervous system (CNS), such as meningitis, encephalitis, tumors, or traumatic brain injury. Pneumonia, tuberculosis and certain drugs (e.g. cytostatic drugs such as vincristine, cyclophosphamide; indomethacin, carbamazepine, tricyclic antidepressants, morphine, nicotine, barbiturates) can also lead to this clinical picture. It is also believed that almost all patients may experience temporary inadequate secretion of ADH after surgery.
These processes or substances lead to a decoupling of the regulatory circuit and thus to a disinhibition of ADH secretion from its place of formation, the posterior lobe of the pituitary gland (neurohypophysis). The resulting excess of ADH causes free water to be retained in the kidney, thus reducing the volume of urine and increasing body weight. This is often accompanied by an increased feeling of thirst.
After distribution in the body, the excess free water first leads to an expansion of the fluid space outside the cells (extracellular) and then, due to the concentration gradient of the fluids in the body, to an increase in the fluid in the intracellular space. This happens, however, without water retention in the tissue (edema). As a counter-regulation to this volume expansion, there is an increase in the excretion of sodium in the urine, which is supposed to draw the excess water into the urine with it.
The excretion of sodium (natriuresis) continues until a new equilibrium is reached; the sodium excretion then corresponds to the sodium intake. In the absence of sodium intake, sodium excretion thus also decreases, which increases water retention and decreases the amount of urine excreted. The regulation of sodium excretion by the kidneys is maintained when sodium in blood serum is low. Although the concentration of ADH in the blood is within its normal range at this time, it is increased in relation to the low concentrations of other substances in the blood due to the dilution of the blood (low plasma osmolarity). Inadequate ADH secretion is biochemically characterized by blood dilution (low plasma osmolarity), lack of urine fluid (high urinary osmolarity) (urine to plasma ratio >1) and low sodium levels in the blood (hyponatremia).
