Hyponatremia is a common electrolyte disorder that affects around 20% of patients in the hospital. It is defined as a serum sodium level of less than 135 mEq/L. It may occur in patients with normal intravascular volume (euvolemia), depleted intravascular volume (hypovolemia) and increased intravascular volume (hypervolemia).
Hyponatremia may be classified as mild (130 – 135mEq/L), moderate (121-129mEq/L) or severe/profound (<120mEq/L).
Low sodium intake
Cerebral salt wasting
Renal tubular acidosis
Third space shift (e.g. bowel obstruction, burns)
Hyperproteinemia (e.g. monoclonal gammopathies)
SIGNS & SYMPTOMS:
The reduction of the serum sodium can be asymptomatic or cause brain swelling and neurological symptoms.
Patients may present with acute hyponatremia may have malaise, nausea, vomiting, headache or confusion. In severe cases, cardiovascular distress, coma and seizures may occur. Chronic hyponatremia may present with nausea, fatigue, confusion and muscle cramps.
In a patient with suspected hyponatremia the serum sodium, other electrolytes and glucose should be measured. If hyponatremia is confirmed and the patient does not need immediate treatment (severe symptoms) the investigation should continue.
If there is no obvious cause of pseudohyponatremia (e.g. hyperglycemia) serum osmolality should be measured. Hyponatremia with normal or high serum osmolality is rare but may occur in patients with pseudohyponatremia or patients that received alcohols (e.g. irrigation solutions during surgeries or procedures). Most cases will have low osmolality (<280mOsmol/kg), a situation called hypotonic hyponatremia.
The volume status should be assessed (edema, jugular venous pressure, skin turgor). That will give clues about the hypovolemic, euvolemic or hypervolemic state.
Urine sodium and urine osmolality should be obtained. Urine osmolality <= 100mOsm/kg may be due to primary polydipsia, low solute intake or beer potomania. Urine osmolality > 100mOsm/kg can be due to many reasons (and that`s why urine sodium is important).
A basic workup would include CBC, CMP, urine sodium and osmolality, TSH, cortisol AM and uric acid. Imaging may be helpful to identify cancers in patients with SIADH.
The urinary sodium will help to identify the cause. Example:
Hypovolemic status with urinary sodium <20mEq/L – Extrarenal losses (e.g. GI losses, third space, bowel obstruction).
Hypovolemic status with urinary sodium >20mEq/L – Renal losses (e.g. diuretics, mineralocorticoid deficiency).
Euvolemic status (urinary sodium usually >20mEq/L) and urine osmolality >100mOsml/kg – SIADH, adrenal insufficiency, drug use, hypothyroidism.
Euvolemic status (urinary sodium usually >20mEq/L) and urine osmolality <100mOsml/kg – Primary polydipsia, low solute intake, beer potomania.
Hypervolemic status with urinary sodium < 20mEq/L – Heart failure, liver failure
Hypervolemic status with urinary sodium > 20mEq/L – Renal failure
Serum Osmolality (mmol/kg) = (2 x serum [Na]) + (serum [glucose]/18) + (blood urea nitrogen/2.8)
Corrected sodium in hyperglycemia: [Na+] falls by about 2 meq/L for each 100 mg/100 mL increase in glucose concentration.
Sodium deficit = Total body water (TBW) x (desired SNa – actual SNa)
Total body water (TBW) = 60% (0.6) of body weight for men, 50% (0.5) of body weight for women, 45% (0.45) of body weight for elderly.
Increase in Serum Na with 1 liter of the solution = (Solution [Na] – Serum Na) ÷ (TBW + 1)
Needless to say that the underlying cause, when identified, should be corrected (e.g. medications suspended, diseases treated).
The hyponatremia itself may need emergency correction (if severe symptoms are present or if the hyponatremia is acute (<24h)) or slow correction (chronic (>24-48h hyponatremia without severe symptoms).
During the correction the provider should avoid sodium elevations of more than 8-9mEq/L per day (24h). Whatever the treatment choice is, the healthcare provider should predict the rate of correction with the formulas provided above.
For emergency therapy hypertonic saline (3%) can be used (100mL bolus over 10-15 minutes). Patients should have their sodium measured every two hours.
Slow correction can also be done with hypertonic saline, but given slowly (15-30mL/h).
Hypovolemic patients may receive isotonic saline.
Euvolemic patients may need fluid restriction or vasopressin antagonists (tolvaptan). Euvolemic patients with SIADH may also receive salt tablets (1g = 17mEq).
Hypervolemic patients may need diuresis and fluid restriction.
Rapid correction should be avoided because it may cause osmotic demyelination (central pontine myelinolysis). Osmotic demyelination usually shows clinical manifestations (altered mental status, coma and extrapyramidal symptoms) after 2-6 days of the quick correction (so the patient may improve from the hyponatremia symptoms on the first days and then deteriorate again due to the demyelination). If osmotic demyelination occurs, the patient should have his sodium lowered again (D5w 6mL/kg over two hours). dDAVP (2mcg IV or SC q6-8h for 24-48h) may also be used to stop free water losses. This strategy may improve the neurological status of patients with osmotic demyelination syndrome (the recovery may take many weeks though).
Content of sodium in different solutions:
• 5% dextrose in water (D 5W): 0 mmol/L
• 0.2% sodium chloride in 5% dextrose in water (D 52NS): 34 mmol/L
• 0.45% sodium chloride in water (0.45NS): 77 mmol/L
• Ringer’s lactate solution: 130 mmol/L
• 0.9% sodium chloride in water (0.9NS): 154 mmol/L
• 3% saline (hypertonic saline): 513 mmol/L
SOURCES & FURTHER READING:
- Spasovski G et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol March 1, 2014 170G1-G47.
- Stellarishealth. Hyponatremia Algorithm.
- Braun MM et al. Diagnosis and Management of Sodium Disorders: Hyponatremia and Hypernatremia. Am Fam Physician. 2015 Mar 1;91(5):299-307.
- Sterns RH et al. Treatment of hyponatremia. Current Opinion in Nephrology and Hypertension 2010, 19:493–498.