Syndrome of Inappropriate ADH Secretion
Definition: levels of ADH are inappropriately elevated compared to body’s low osmolality, and ADH levels are not suppressed by further decreases in blood osmolality.

SIADH: causes
Irritation of CNS:  meningitis, encephalitis, brain tumors, brain hemorrhage, hypoxic insult, trauma, brain abscess, Guillain Barre, hydrocephalus
Pulmonary disorders:  pneumonia, asthma, positive end expiratory pressure ventilation, CF, TB, pneumothorax
Unregulated tumor production of ADH-like peptides:  oat cell lung carcinoma for example, Ewings sarcoma, carcinoma of duodenum, pancreas, thymus

Function of ADH
ADH is made in the supra-optic nuclei in the hypothalamus, stored in the posterior pituitary
Normally released into the bloodstream when osmo-receptors detect high plasma osmolality
At the kidney, attaches to receptors in the collecting ducts, opens up water channels
Water is passively reabsorbed along the kidney’s medullary concentration gradient

SIADH: signs and symptoms
Signs of hyponatremia:  lethargy, apathy, disorientation, muscle cramps, anorexia, agitation
Signs of water toxicity:  nausea, vomiting, personality changes, confused, combative

Diabetes Insipidus
Definition:  inability to effectively conserve urinary water
Central:  ADH not made or not released in the hypothalamic-pituitary axis
Nephrogenic:  ADH is released but not detected by the receptors in the kidney collecting ducts, often a sex-linked recessive condition, also due to renal pathology, electrolyte disorders, drugs...

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