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HYPONATREMIA is a topic covered in the Harrison's Manual of Medicine.

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This is defined as a serum [Na+] <135 mmol/L and is among the most common electrolyte abnormalities encountered in hospitalized pts. Symptoms include nausea, vomiting, confusion, lethargy, and disorientation; if severe (<120 mmol/L) and/or abrupt, seizures, central herniation, coma, or death may result (see Acute Symptomatic Hyponatremia, below). Hyponatremia is almost always the result of an increase in circulating AVP and/or increased renal sensitivity to AVP; a notable exception is in the setting of low solute intake (“beer potomania”), wherein a markedly reduced urinary solute excretion is inadequate to support the excretion of sufficient free H2O. The serum [Na+] by itself does not yield diagnostic information regarding total-body Na+ content; hyponatremia is primarily a disorder of H2O homeostasis. Pts with hyponatremia are thus categorized diagnostically into three groups, depending on their clinical volume status: hypovolemic, euvolemic, and hypervolemic hyponatremia (Fig. 1-1). All three forms of hyponatremia share an exaggerated, “nonosmotic” increase in circulating AVP, in the setting of reduced serum osmolality. Notably, hyponatremia is often multifactorial; clinically important nonosmotic stimuli that can cause a release of AVP and increase the risk of hyponatremia include drugs, pain, nausea, and strenuous exercise.


The diagnostic approach to hyponatremia. See text for details. (From S Kumar, T Berl: Diseases of water metabolism, in Atlas of Diseases of the Kidney, RW Schrier [ed]. Philadelphia, Current Medicine, Inc, 1999; with permission.)

Laboratory investigation of a pt with hyponatremia should include a measurement of serum osmolality to exclude “pseudohyponatremia” due to hyperlipidemia or hyperproteinemia. Serum glucose also should be measured; serum Na+ falls by 1.4 mM for every 100-mg/dL increase in glucose, due to glucose-induced H2O efflux from cells. Hyperkalemia may suggest adrenal insufficiency or hypoaldosteronism; increased blood urea nitrogen (BUN) and creatinine may suggest a renal cause. Urine electrolytes and osmolality are also critical tests in the initial evaluation of hyponatremia. In particular, a urine Na+ <20 meq/L is consistent with hypovolemic hyponatremia in the clinical absence of a “hypervolemic,” Na+-avid syndrome such as congestive heart failure (CHF) (Fig. 1-1). Urine osmolality <100 mosmol/kg is suggestive of polydipsia or, in rare cases, of decreased solute intake; urine osmolality >400 mosmol/kg suggests that AVP excess is playing a more dominant role, whereas intermediate values are more consistent with multifactorial pathophysiology (e.g., AVP excess with a component of polydipsia). Finally, in the right clinical setting, thyroid, adrenal, and pituitary function should also be tested.

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Loscalzo, Joseph, et al., editors. "HYPONATREMIA." Harrison's Manual of Medicine, 19th ed., McGraw Hill Inc., 2017. harrisons.unboundmedicine.com/harrisons/view/Harrisons-Manual-of-Medicine/623466/all/HYPONATREMIA.
HYPONATREMIA. In: Loscalzo J, Jameson JL, Longo DL, et al, eds. Harrison's Manual of Medicine. 19th ed. McGraw Hill Inc.; 2017. https://harrisons.unboundmedicine.com/harrisons/view/Harrisons-Manual-of-Medicine/623466/all/HYPONATREMIA. Accessed April 23, 2019.
HYPONATREMIA. (2017). In Loscalzo, J., Jameson, J. L., Longo, D. L., Hauser, S. L., Fauci, A. S., & Kasper, D. L. (Eds.), Harrison's Manual of Medicine. Available from https://harrisons.unboundmedicine.com/harrisons/view/Harrisons-Manual-of-Medicine/623466/all/HYPONATREMIA
HYPONATREMIA [Internet]. In: Loscalzo J, Jameson JL, Longo DL, Hauser SL, Fauci AS, Kasper DL, editors. Harrison's Manual of Medicine. McGraw Hill Inc.; 2017. [cited 2019 April 23]. Available from: https://harrisons.unboundmedicine.com/harrisons/view/Harrisons-Manual-of-Medicine/623466/all/HYPONATREMIA.
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TY - ELEC T1 - HYPONATREMIA ID - 623466 ED - Loscalzo,Joseph, ED - Jameson,J Larry, ED - Longo,Dan L, ED - Hauser,Stephen L, ED - Fauci,Anthony S, ED - Kasper,Dennis L, BT - Harrison's Manual of Medicine UR - https://harrisons.unboundmedicine.com/harrisons/view/Harrisons-Manual-of-Medicine/623466/all/HYPONATREMIA PB - McGraw Hill Inc. ET - 19 DB - Harrison's Manual of Medicine DP - Unbound Medicine ER -