Hyponatremia

● definition:serum[Na+]<135mEq/L.

Common etiologies:

○ SIADH: hypotonic hyponatremia (efective serum osmol <275 mOsm/L) within appropriately high urinary concentration (urine osmol>100 mOsm/L) and euvolemia or hypervolemia.

○ Cerebral salt wasting (CSW): similar to SIADH but with extracellular fluid volume depletion due to renal sodium loss (urinary Na > 20 mEq/L).

● minimum W/U: serum [Na+], serum osmolality, urine osmolality, clinical assessment of volume status. If volume status is high or low: urinary [Na+] TSH (to R/O hypothyroidism).

● treatment: based on acuity, severity, symptoms & etiology; as appropriate.

● risk of overly rapid correction: osmotic demyelination (including central pontine myelinolysis).

Hyponatremia, is low sodium concentration in the blood, the most common electrolyte abnormality in clinical medicine ¹⁾.

Generally defined as a serum sodium level of less than 135 mEq/L and is considered severe when the serum sodium level is below 125 mEq/L.

1. isotonic hyponatremia (effective serum osmolality: 275–295 mOsm/kg):

a) pseudohyponatremia: an artifact of indirect lab techniques. Unusually high levels of lipids (e.g., hypertriglyceridemia) or proteins (e.g., immunoglobulins as can occur in multiple myeloma) reduce the aqueous fraction of plasma and produce artifactually low sodium lab values. This error does not occur with direct measurement methods

b) nonconductive irrigants, e.g., as used in cystoscopy to allow coagulation, when large volumes are inadvertently absorbed through a severed vein (“TURP syndrome”)

[Na+] <135 mEq/L=mild, <130=moderate, <125=severe hyponatremia.

Syndrome of inappropriate antidiuretic hormone secretion is the most common type of hyponatremia (dilutional hyponatremia) ²⁾.

Cerebral salt wasting syndrome.

Work-up requires assessment of:

1. serum sodium: must be <135 mEq/Lto qualify as hyponatremia.

1. urine osmolality: values>100mOsm/kg are inappropriately high if serum tonicity is <275 mOsm/kg

2. volume status: differentiates SIADH from CSW

a) clinical assessment: better for hypervolemia (edema, upward trend in patient weights) but is insensitive in identifying extracellular fluid depletion as an etiology of hyponatremia8 (look for dry mucous membranes, loss of skin turgor, orthostatic hypotension)

b) normal saline infusion test used in uncertain cases. If the baseline urine osmolality is <500 mOsm/kg, it is usually safe to infuse 2 Lof 0.9%saline over 24–48 hours. Correction of the hyponatremia suggests extracellular fluid volume depletion was the cause

c) central venous pressure (CVP) may be used: CVP<5–6 cm H2O suggests hypovolemia in patients with normal cardiac function

3. check urinary [Na+] if volume status is high or low

4. determine duration of hyponatremia:

a) duration documented as <48 hours is considered acute

b) hyponatremia of>48 hours duration or of unknown duration is chronic

c) hyponatremia that occurs outside the hospital is usually chronic and asymptomatic except in marathoners and MDMA(“ecstasy”) drug users

Hyponatremia Etiology

Due to slow compensatory mechanisms in the brain, a gradual decline in serum sodium is better tolerated than a rapid drop. Symptoms of mild ([Na]<130 mEq/L) or gradual hyponatremia include: anorexia, headache, difficulty concentrating, irritability, dysgeusia and muscle weakness. Severe hyponatremia (<125 mEq/L) or a rapid drop (>0.5 mEq/hr) can cause neuromuscular excitability, cerebral edema, muscle twitching and cramps, nausea/vomiting, confusion, seizures, respiratory arrest and possibly permanent neurologic injury, coma or death.

Data suggest that acute mild hyponatremia is associated with a reduction in bone formation activity ³⁾.