Hyponatremia resident survival guide

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Priyamvada Singh, M.D. [2]

Definition

Hyponatremia is defined as a serum sodium concentration < 135 meq/L.

Hyponatremia with low osmolality (hypotonic hyponatremia) is defined as hyponatremia with a serum osmolality below 280 mOsm/kg.

Hyponatremia with high osmolality (hypertonic hyponatremia) is defined as hyponatremia with a serum osmolality greater than 295 mOsm/kg.

Hyponatremia with normal osmolality (Isotonic hyponatremia) is defined as hyponatremia with a serum osmolality ranging between 280-295 mOsm/kg.

Causes

Each of the causes listed below can be life threatening when Na⁺ levels are severely low. (Below 125 meq/L)

Common Causes

Hyponatremia causes can be classified into 3 types based on calculating serum osmolality, which is calculated as follows:
Sosm(mmol/kg) = (2 x serum Na⁺ conc.) + (serum glucose conc./18) + (blood urea nitrogen/2.8)
Where Ssom is Serum osmolality.

♦ Hyponatremia with low serum osmolality

• Appropriate ADH secretion (Primary polydipsia, advanced renal failure, low dietary intake)
• Arterial blood volume depletion

1. True blood volume depletion (Diarrhea, vomiting, bleeding, use of diuretics)
2. Thiazide diuretic induced
3. Heart failure, cirrhosis

• Ecstasy consumption.
• Endocrine disorders such as hypothyroidism and adrenal failure.
• Syndrome of inappropriate antidiuretic hormone secretion(SIADH)

♦ Hyponatremia with high serum osmolality

• Marked hyperglycemia
• Mannitol infusion

♦ Hyponatremia with normal serum osmolality

• Pseudohyponatremia (hyperlipidemia, hyperproteinemia)

Do's

1) Cerebral salt wasting syndrome causes hypo-osmolar hyponatremia with lab parameters similar to that seen in SIADH. It is associated with conditions like hypovolemia, hypotension, neurosurgical procedure, and subarachnoid hemorrhage (within previous 10 days). It is treated as hypoosmolar hyponatremia.

2) While deciding the rate of normal saline for hypovolemia hyponatremia, consideration of the following factors are helpful: patient's BMI (faster rates for higher BMI), cardiac (slower rate for low ejection fraction) and renal function (slower rates for low GFRs).

3) Common causes of SIADH are, small cell lung cancer, intracranial pathology, increased intrathoracic pathology, medications (thiazides, SSRI, tricyclic antidepressant, narcotics, phenothiazine, carbamazepine)

4) Vaptans such as conivaptan (intravenous), and tolvaptans (oral preparations) can be used for resistant euvolemic, and hypervolemic hyponatremia. These are very expensive and should be initiated in hospital.

Don'ts

1) Serum sodium shouldn't be corrected faster than 0.5 meq/h, as faster correction increases the risk for central pontine myelinolysis.

References

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