The important difference between normal physiology and what occurs in SIADH is the lack of an effective negative feedback mechanism. An illustration of how serum osmolality is regulated in healthy individuals. This results in both a decrease in volume and an increase in osmolality (concentration) of the urine excreted.Ħ. The extra water that has been reabsorbed re-enters the circulatory system, reducing the serum osmolality.ħ. This reduction in serum osmolality is detected by the hypothalamus and results in decreased production of ADH. These aquaporin-2 channels allow water to be reabsorbed out of the collecting ducts and back into the bloodstream. The binding of ADH to these receptors causes aquaporin-2 channels to move from the cytoplasm, into the apical membrane of the tubules.
ADH then travels to the kidneys, where it binds to ADH receptors on the distal convoluted tubules.ĥ. ADH is released into the circulatory system via the posterior pituitary gland.Ĥ. ADH is transported from the hypothalamus to the posterior pituitary gland.ģ. ADH (also known as vasopressin) is produced by the hypothalamus in response to increased serum osmolality.Ģ. In all instances, identifying the cause of hyponatremia remains an integral part of the treatment plan.You might also be interested in our medical flashcard collection which contains over 1000 flashcards that cover key medical topics.ġ. Loop diuretics are useful in managing edematous hyponatremic states and chronic SIADH. Rapid correction should be avoided to reduce the risk of central pontine myelinolysis.
In patients with chronic hyponatremia, fluid restriction is the mainstay of treatment, with demeclocycline therapy reserved for use in persistent cases. Management includes instituting immediate treatment in patients with acute severe hyponatremia because of the risk of cerebral edema and hyponatremic encephalopathy. Low urinary sodium concentration is caused by severe burns, gastrointestinal losses, and acute water overload. High urinary sodium concentration in the presence of low plasma osmolality can be caused by renal disorders, endocrine deficiencies, reset osmostat syndrome, SIADH, and medications. The urinary sodium concentration helps in diagnosing patients with low plasma osmolality. Hyponatremia with a high plasma osmolality is caused by hyperglycemia, while a normal plasma osmolality indicates pseudohyponatremia or the post-transurethral prostatic resection syndrome. Differentiating between euvolemia and hypovolemia can be clinically difficult, but a useful investigative aid is measurement of plasma osmolality. Hypervolemic hyponatremia may be caused by congestive heart failure, liver cirrhosis, and renal disease. Hyponatremia can be classified according to the volume status of the patient as hypovolemic, hypervolemic, or euvolemic.
Common causes include medications and the syndrome of inappropriate antidiuretic hormone (SIADH) secretion. Hyponatremia is an important electrolyte abnormality with the potential for significant morbidity and mortality.
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