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

• Arterial blood volume depletion

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

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

• Appropriate ADH secretion (Primary polydipsia, advanced renal failure, low dietary intake)
• Ecstasy consumption.

♦ Hyponatremia with high or normal serum osmolality

• Marked hyperglycemia (High osmolality)
• Pseudohyponatremia (normal osmolality)

Management

Diagnosis

Characterize the symptoms: Nausea and vomiting Headache Confusion Lethargy, fatigue, loss of appetite Restlessness and irritability Muscle weakness/spasms/cramps Seizures Decreased consciousness or coma. Na+ < 115 meq/L: Hyponatremic encephalopathy Symptoms mentioned above plus Brain stem compression Respiratory arrest Non-cardiogenic pulmonary edema

Examine the patient: Orthostatic vital signs - orthostatic hypotension Mental status examination - low score Jugular venous pressure ↑/↓ Peripheral edema

Check labs: Plasma osmolality Urine osmolality Urinary sodium concentration Serum uric acid/Creatinine TSH (Thyroid stimulating hormone) Serum cortisol level Serum proteins/triglyceride Random blood sugar

Consider alternative diagnosis: Adrenal Crisis Alcoholism Hypothyroidism Pulmonary/cardiogenic edema

Plasma osmolality

< 280 mOsm/kg - Hypotonic hyponatremia

280-295 mOsm/kg - Isotonic

> 295 mOsm/kg - Hypertonic hypernatremia

Assess volume status

Increase in osmotically active compounds; glucose, protein, lipid, mannitol, sorbitol

Hypovolemia

Hypervolemia

Euvolemia

Treat etiology

Spot urine Na < 10, BUN/Creatinine > 20:1, Urine osmolality > 450

Same as hypovolemia, Spot urine Na < 10, BUN/Creatinine > 20:1, Urine osmolality > 450

Spot urine Na > 20, BUN/Creatinine < 20:1, Urine osmolality > 300

GI, renal losses, dehydration, diuretics, adrenal insufficiency, cerebral salt wasting syndrome

heart failure cirrhosis renal failure

Urine osmolality > 300

Urine osmolality 50-100

(a) Normal saline; (b) If neurological sign/symptoms 3% hypertonic saline, and furosemide

Fluid restriction, Diuretics, Treat etiology

SIADH, Hypothyroidism

Compulsive water drinking

(a) Fluid restriction (b) Demeclocycline (if fluid restriction fails) (c) Vaptans; Conivaptan, Tolvaptan for resistant cases

Treat etiology

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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