Hyponatremia causes

Etiology:

• • Pseudohyponatremia: refers to lab phenomenon by which ▲ plasma proteins as in multiple myeloma, ▲ TG expand the nonaqueous portion of the plasma sample leading to an errant report of low ECF, Na+. Can be prevented by using Na+ sensitive electrodes and the normal ECF Na+ can be confirmed with a normal serum osmolality.

  • Hyperosmolar hyponatremia: Osmotically active solute other than Na accumulates in the ECF, drawing water into the ECF and diluting the Na content.

    • Hyperglycemia > ▼ in plasma Na by 1.6 - 2.4 mEq/L for every 100 mg/dL ▲ in plasma glucose.

    • Post-TURP, bladder irrigation with solutes like glycine, mannitol, or sorbitol leading to absorption into ECF. Prompt renal excretion and metabolism of these solutes usually corrects hyponatremia rapidly, however, if the patient has renal insufficiency the hyponatremic status lingers longer.

    • Water intoxication, psychogenic polydipsia, beer potomonia, "tea-toast" diet. Fact: There is a limit to renal water clearance. Urine cannot be diluted to an osmolality less than ~50 mOsm/L, meaning that a small amount of solute is needed in even the most dilute urine. Ingestion of a high volume of water can thus exceed the capacity for excretion, particularly in those with solute poor diet, as the solute load required to generate urinary water loss is quickly depleted. Excess water is retained, Na+ conc. falls, and hyponatremia results.

  • Appropriate" ADH secretion occurs with a fall in effective circulating volume. In these conditions, thirst and water retention is stimulated, protecting volume status at the cost of osmolar status. This category is classically subdivided based on the associated assessment of ECF status.

    • • • Hypovolemic hyponatremia: from any causes of net Na+ loss.

        • Hypervolemic hyponatremia: from edematous states such as CHF, hepatic cirrhosis, and severe nephrotic syndrome. Despite the expanded interstitial space, the circulating volume is reduced.

• "Inappropriate" secretion of ADH (SIADH).

  • Reset osmotat is a phenomenon in which the set point of plasma osmolality is reduced. ADH and thirst responses, maintain osmolality at this new, lower level. Occurs in almost all pregnant women (perhaps in response to changes in the hormonal mileiu), and occasionally in those with a chronic decreased effective circulating volume.

Causes of hyponatremia:

• 1. Pseudophyponatremia

     1. Normal plasma osmolality

        1. Hyperlipidemia

        2. Hyperporteinemia

        3. Post TURP

     2. Increased plasma osmolality

• • • 1. Hyperglycemia

      2. Mannitol

• 2. Hyposomolal hyponatremia

• • 1. Primary Na+ loss (secondary water gain)

       1. Integumentary loss sweating, burns

       2. GI loss, vomiting, tube drainage, fistula, obstruction, diarrhea

       3. Renal loss diuretics, osmotic diuresis, hypoaldosteronism, salt-wasting nephropathy, postobstructive diuresis, nonoliguric acute tubular necrosis.

    2. Primary water gain (secondary Na+ loss)

• • • 1. Primary polydipsia

      2. Decreased solute intake (e.g., beer potomania)

      3. AVP release due to pain, nausea, drugs

      4. SIADH (SIAVP secretion)

      5. Glucocorticoid deficiency, Adrenal insufficiency.

      6. Hypothyroidism

      7. Chronic renal insufficiency

• • 3. Primary Na+ gain (exceeded by secondary water gain)

• • • 1. Heart failure

      2. Hepatic cirrhosis

      3. Nephrotic syndrome