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A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma. Syndrome of inappropriate ADH secretion occurs when excessive release of antidiuretic hormone results in inappropriately elevated urine osmolality (>100 mOsmol/L) relative to the blood plasma, leading to hyponatraemia.
Hypoosmolar hyponatremia is a condition where hyponatremia is associated with a low plasma osmolality. [1] The term "hypotonic hyponatremia" is also sometimes used.[2]When the plasma osmolarity is low, the extracellular fluid volume status may be in one of three states: low volume, normal volume, or high volume.
Diagnosis is based on clinical and laboratory findings of low serum osmolality and low serum sodium. [13] Urinalysis reveals a highly concentrated urine with a high fractional excretion of sodium (high sodium urine content compared to the serum sodium). [14] A suspected diagnosis is based on a serum sodium under 138.
The calculated osmolality is then subtracted from the measured osmolality to provide the osmol gap, or the difference between these two values. If this gap falls within an acceptable range,(<10) then it is assumed that sodium, glucose, BUN are indeed the major dissolved ions and molecules in the serum.
Similarly, it is said to be hypoossmolar if the osmolarity, or osmatic concentration, is too low. For example, if the osmolarity of parenteral nutrition is too high, it can cause severe tissue damage. [7] One example of a condition caused by hypoosmolarity is water intoxication. [8]
High serum osmolarity stimulates polydipsia in an attempt to dilute the serum back to normal and provide free water for excreting the excess serum solutes. However, since the patient is unable to concentrate urine to excrete the excess solutes, the resulting urine fails to decrease serum osmolarity and the cycle repeats itself, hence polyuria.
Hyponatremia or hyponatraemia is a low concentration of sodium in the blood. [4] It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L. [3] [8] Symptoms can be absent, mild or severe.
In contrast to DKA, serum glucose levels in HHS are extremely high, usually greater than 40-50 mmol/L (600 mg/dL). [6] Metabolic acidosis is absent or mild. [6] A temporary state of confusion (delirium) is also more common in HHS than DKA. HHS also tends to affect older people more. DKA may have fruity breath, and rapid and deep breathing. [6]