Hyponatremia
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Synonyms and keywords: Hyponatraemia
Overview
The electrolyte disturbance hyponatremia exists in humans when the sodium (Natrium in Latin) concentration in the plasma falls below 130 mmol/L. At lower levels water intoxication may result, an urgently dangerous condition. Hyponatremia is an abnormality that can occur in isolation or, as most often is the case, as a complication of other medical illnesses.
Pseudohyponatremia
Certain conditions that interfere with laboratory tests of serum sodium concentration (such as extraordinarily high blood levels of lipid or protein) may lead to an erroneously low measurement of sodium. This is called pseudohyponatremia, and can occur when laboratories use the flame-photometric and indirect (but not direct) ion-selective electrode assays.[1][2] This is distinct from a true dilutional hyponatremia that can be caused by an osmotic shift of water from cells to the bloodstream after large infusions on mannitol or intravenous immunoglobulin.
Hypoosmolar hyponatremia
When the plasma osmolarity is low, the extracellular fluid volume status may be in one of three states:
- Low volume. Loss of water is accompanied by loss of sodium.
- Excessive sweating
- Burns
- Vomiting
- Diarrhea
- Urinary loss
- Diuretic drugs (especially thiazides)
- Addison's disease
- Cerebral salt-wasting syndrome
- Other salt-wasting kidney diseases
Treat underlying cause and give IV isotonic saline. It is important to note that sudden restoration of blood volume to normal will turn off the stimulus for continued ADH secretion. Hence, a prompt water diuresis will occur. This can cause a sudden and dramatic increase the serum sodium concentration and place the patient at risk for so-called "central pontine myelinolysis" (CPM). That disorder is characterized by major neurologic damage, often of a permanent nature.
Because of the risk of CPM, patients with low volume hyponatremia may eventually require water infusion as well as volume replacement. Doing so lessens the chance of a too rapid increase of the serum sodium level as blood volume rises and ADH levels fall.
- Normal volume.
- SIADH (syndrome of inappropriate antidiuretic hormone)
- Some cases of psychogenic polydipsia
The cornerstone of therapy for SIADH is reduction of water intake. If hyponatremia persists, then demeclocycline (an antibiotic with the side effect of inhibiting ADH) can be used. SIADH can also be treated with specific antagonists of the ADH receptors, such as conivaptan or tolvaptan.
- High volume. There is retention of water.
Placing the patient on water restriction can also help in these cases.
Severe hyponatremia may result from a few hours of heavy exercise in high temperature conditions, such as hiking in desert areas, or from endurance athletic events when electrolytes are not supplied. (Such an incident notably happened to long-distance athlete Craig Barrett in 1998).
Diagnosis
Laboratory Findings
- Serum osmolality
- Blood urea nitrogen (BUN)/creatinine
- Calcium
- Magnesium
- Urine sodium
- Thyroid stimulating hormone (TSH)
- Serum glucose
Related Chapters
References
Template:Endocrine, nutritional and metabolic pathology