Hyponatremia
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Contents |
More Specific Terms
Etiology
- edema-forming states
- mechanism
- decreased effective arterial blood volume
- sodium & water retention by the kidney
- excessive total body sodium
- excessive extravascular fluid ( 3rd spacing)
- mechanism
- renal salt loss
- extrarenal salt loss
- replacement with hypotonic fluid
- stimulation of ADH despite hypotonicity ( hypovolemia overrides hyponatremia)
- gastrointestinal loss from vomiting or diarrhea
- 3rd space loss
- skin losses
- lung & respiratory tract losses
- hyponatremia associated with normal extracellular volume
-
- malignancy ( carcinoma causing ectopic production of ADH)
- CNS disease
- pulmonary disorders
- pharmacologic agents
- pharmacologic agents that potentiate renal effect of ADH
- pharmacologic agents that produce ADH-like effect
- oxytoxin
- desmopressin
- administration of hypotonic fluids ( urine Na+ < 10 meq/L)
- severe hypothyroidism
- cortisol deficiency or panhypopituitarism
- hyponatremia with hypertonicity
- mechanism:
- osmotically active substance causes movement of intracellular fluid extracellularly
- relic of days past when concentration of sodium was measured using flame photometry
- occurred because of non- aqueous volume occupied by lipid in hypertriglyceridemia & perhaps protein in monoclonal & polyclonal gammopathies
- ion-specific electrodes used in most modern instruments measure activity not concentration, thus making the issue of pseudohyponatremia obsolete
Epidemiology
- most common electrolyte abnormality in hospitalized patients
Clinical-manifestations
- symptoms related to etiology of hyponatremia
- orthostasis associated with volume depletion, but not edema-forming states
- symptoms related to degree of hyponatremia & acute vs chronic nature of the disorder
- symptoms do not appear until:
- serum sodium drops below 125 meq/L suddenly
- much lower if hyponatremia is chronic
- neurologic manifestations predominate
- headache
- lethargy, apathy, muscle weakness, muscle cramps
- agitation/ irritability
- nausea/vomiting
- dysgeusia [6]
- cognitive impairment
- disorientation
- confusion
- decreased level of consciousness
- decreased deep tendon reflexes
- muscle twitching
- grand mal seizures
- Cheyne-Stokes respirations
- coma & death may occur with [Na+] < 110 meq/L
Laboratory
-
- serum osmolality is decreased except in hyperosmolar conditions, i.e. diabetes
- urine osmolality is increased except in primary polydipsia
-
-
- hypokalemia if volume depletion with fluid loss secondary to renal or GI etiology
- normal in SIADH
- serum bicarbonate: normal in SIADH
- serum chloride: normal in SIADH
- BUN/creatinine ratio > 20 with volume depletion
-
-
- urine sodium < 20 meq/L in:
- edema-forming states
- hypovolemia of extrarenal origin
- urine sodium > 20 meq/L in:
- renal failure
- hypovolemia of renal origin
- SIADH
- fractional excretion of sodium ( FENA)
- thyroid function tests if indicated
- adrenal function tests if indicated
Complications
- increased risk of myocardial infarction & death (mild hyponatremia, serum Na+ <136 meq/L) [7]
Management
- Establish urgency of treatment
- Urgent treatment
- 3-5% saline
- increase sodium < 2 meq/L/hour & < 20 meq/L/24 hours
- < 10-12 meq/L/24 hours, < 18 meq/L/48 [4]
- central pontine myelinolysis is danger of too rapidly correcting sodium
- furosemide may be given for volume overload
- conivaptan IV for life-threatening euvolemic & hypervolemic hyponatremia in hospitalized patients [4,5]
- Asymptomatic patient
- water restriction unless patient is volume contracted
- 500 to 1000 mL/day
- free water restriction alone is not sufficient
- demeclocycline
- do NOT use in patients with cirrhosis
- address underlying disease processes
More General Terms
Additional Terms
- drugs associated with hyponatremia
- fractional excretion of sodium (FENA)
- hypernatremia
- hypoaldosteronism; mineralocorticoid deficiency
- sodium (Na+) in serum
- syndrome of inappropriate antidiuretic hormone; SIADH; nephrogenic syndrome of inappropriate antidiuresis; NSIAD
References
- Saunders Manual of Medical Practice, Rakel (ed), WB Saunders, Philadelphia, 1996, pg 679-681
- Harrison's Principles of Internal Medicine, 14th ed. Fauci et al (eds), McGraw-Hill Inc. NY, 1998, pg 268-69
- Mayo Internal Medicine Board Review, 1998-99, Prakash UBS (ed) Lippincott-Raven, Philadelphia, 1998, pg 599-600
- Medical Knowledge Self Assessment Program (MKSAP) 11, 14, 15 American College of Physicians, Philadelphia 1998, 2006, 2009
- Internal Medicine World Report 2006; 21(2)
- Ellison DH and Berl T Clinical Practice: The syndrome of inappropriate antidiuresis N Engl J Med 2007, 356:2064 PMID: [1]
- Sajadieh A et al Mild hyponatremia carries a poor prognosis in community subjects. Am J Med 2009 Jul; 122:679. PMID: [2]
