What is the role of water restriction in the treatment of acute syndrome of inappropriate antidiuretic hormone secretion (SIADH)?

Updated: Aug 16, 2019
  • Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FASN  more...
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The degree of water restriction depends on the prior water intake, the expected ongoing fluid losses, and the degree of hyponatremia. Water restriction to about 500-1500 mL/d (or even lower in some cases) is usually prescribed. Although easier to maintain in the hospital setting, this becomes difficult for patients to follow in an outpatient setting. 

One of the functions of the kidneys is to excrete solutes in varying amounts of water. In persons with SIADH, urine osmolality is fixed at a certain value; for the kidneys to eliminate an "X" amount of solutes, a certain volume of water must be excreted. If water intake is lowered below total obligatory fluid losses (insensible losses plus volume of urine required to excrete the osmolar load), then serum osmolality rises because a net loss of water occurs. The insensible losses of relatively hypotonic fluids also contribute to net water loss. The key is sufficient restriction of water intake so that the excretion of free water from all sources is in excess of that taken in.

For example, consider a patient who has a net solute load of 900 mOsm/kg/day that must be excreted, and, because of SIADH, his or her urine osmolality is fixed at 600 mOsm/kg. This patient then excretes the solute load in 1.5 L of urine. On the other hand, if the urine osmolarity is fixed at 300 mOsm/kg, then 3 L of urine is required to excrete the same osmolar load. When water intake is restricted, the body mobilizes the free water already present to excrete this load. Thus, if urine output (plus insensible losses) exceeds water intake, a net water loss occurs and the serum Na+level returns towards normal.  For that reason when the sum of urinary Naand K+ is greater than serum Na+ concentration, fluid restriction alone is unlikely to be effective. [31]

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