Hyponatremia pathophysiology

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

Overview

The exact pathogenesis of [disease name] is not fully understood.

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Pathophysiology

Sodium is the main cation in the extracellular fluid, thus the plasma concentration of sodium is determinant of tonicity and serum osmolality.

The osmotic gradient of solutes that do not cross cell membranes constitutes serum Tonicity which determines the distribution of water in the body.

Plasma tonicity = (Extracellular solute + Intracellular solute) / TBW

Serum or plasma osmolality measures different solutes in plasma. It helps to evaluate the etiology of hyponatremia and screen other solutes in serum.

Serum Osmolality = (2 x (Na + K)) + (BUN (mg/dL) / 2.8) + (glucose (mg/dL) / 18) + (Ethanol (mg/dL) /3.7)

Normal Range= 275–295 mosm /kg (mmol /kg)

Normal range		Osmolality versus Osmolarity
Sodium ‡	135-145 mEq/L	Osmolality is a measure of the osmoles (Osm) of solute per kilogram of solvent (osmol/kg or Osm/kg) Osmolarity is defined as the number of osmoles of solute per liter (L) of solution (osmol/L or Osm/L) (one liter of plasma equals to one kilogram of plasma thus plasma osmolarity and plasma osmolality would be the same but osmolality is independent of temperature and pressure so it's the more stable unite of measurment)
Potassium ‡	3.5-5.1 mEq/L
Blood Urea Nitrogen	7-20 mg/dL (2.5-7.1 mmol/L)
Glucose	70-100 mg/dL ( 3.9-5.5 mmol/L)
Serum osmolality	275–295 mosm /kg (mmol /kg) †

‡ Mmol and Meq are the same for univalent ions like sodium, potassium

† mOsmol/kg = n x mmol/L, for Na⁺, Cl^-, Ca²⁺, urea, and glucose, 1 mmol/L equals 1 mOsmol/kg because n=1 , for NaCl n=2

Plasma water is regulated by sensory organs (baroreceptors and hypothalamus osmoreceptors), antidiuretic hormone (ADH or vasopressin, AVP), and the kidney.

Osmoreceptors in the hypothalamus are sensitive to the increased or decreased tonicity of serum. The primary brain osmoreceptors are located outside the blood-brain barrier in the lamina terminalis. Primary osmoreceptors are connected to brain areas responsible for ADH secretion and thirst by neuronal projections. Osmoreceptors can both stimulate and inhibit ADH secretion and thirst in response to hyper-and hypotonicity of serum, respectively .

ADH secretion from hypothalamus through posterior pituitary is increased by :

• ↑ Angiotensin II (activation of Renin-Angiotensin-Activation System)
• ↑ Sympathetic stimulation
• ↑ Effective osmoles (Hypertonicity)
• ↓ Baroreceptor firing ( ↓effective intravascular volume)
• ↓ Right atrium stretching

ADH increases renal free water reabsorption from the collecting tubules which results in correction of plasma sodium toward the normal range. The vasopressin type 2 (V₂) receptor in the basolateral membrane of the collecting tubule acts as the antidiuretic effect of ADH. Binding of ADH to V2 receptor intensifies the action of intracellular cyclic adenosine monophosphate (cAMP) which results in insertion of water channel (aquaporin 2) into the luminal membrane and increasing the numbers of aquaporin-2 mRNA level .As plasma water increases, plasma sodium concentration, osmolality, and ADH secretion decrease and the collecting tubule becomes impermeable to water.

Mechanism of action of ADH , (ɔ) Image courtesy of WikiDoc.org, by "Saeedeh Kowsarnia M.D"

Hyponatremia is defined as serum sodium less than 135 mEq/L (mmol/L). Hyponatremia is a water balance disorder which represents an imbalance in a ratio where total body water is more than total body solutes ( total body sodium and total body potassium).

Pathogenesis

Hyponatremia occurs when the release of ADH (AVP) is increased either physiologically appropriate due to decreased effective circulating volume, or inappropriately due to no physiologic reason. In response to the release of ADH, urine volume decreases and hyponatremia will develop especially when water intake exceeds urinary and insensible losses of water. Patients are typically classified based on their total body sodium as hypovolemic, euvolemic, and hypervolemia.

Hypovolemic hyponatremia

• Volume loss: GI loss, bleeding and insensible loss cause solute and water loss simultaneously which leads to the rise in ADH secretion. A considerable reduction in effective arterial blood volume increase release of ADH by baroreceptors rather than osmoreceptors. There is a marked release in ADH secretion by acute hypovolemia compared to the response that is caused by hypertonicity . ADH increases free water reabsorption from collecting tubules by V2 receptors and vascular resistance by V1 receptors. Replacement of losses with hypotonic fluid may cause further hyponatremia in addition to ADH effect. Hypovolemia caused by diarrhea induces sodium absorption from urine, results in low urine sodium. Vomiting caused hyponatremic hypovolemia which results in high urine sodium and low urine chloride due to bicarbonaturia and metabolic alkalosis.

• Third spacing of fluid: Causes decreased intravascular volume which increases ADH secretion and water reabsorption. Decreased vascular volume induces the activity of the renin-angiotensin-aldosterone system. Aldosterone increases water and sodium absorption by the kidney. As a net result of ADH and aldosterone actions, water is absorbed more than sodium which causes hyponatremia.

• Diuretics: Thiazides increase water reabsorption and water permeability of medullary part of collecting tubules which is independent of ADH action. Excretion of sodium and potassium to the urine causes further hyponatremia . In contrast, loop diuretics decrease the action of ADH on medullary part of collecting tubules by impairing medullary gradient .
• Renal loss: Inappropriately loss of sodium in urine causes hypovolemia and ADH secretion.
  • Salt-wasting nephropathy: Renal tubular dysfunction causes hyponatremia especially when sodium intake is reduced. The causes include Interstitial nephropathies, reflux nephropathies, recovery phase of ATN, medullary cystic disease and post-obstructive uropathies.
  • Bicarbonaturia: Renal tubular acidosis and metabolic alkalosis cause loss of bicarbonate and sodium in urine to balance charges in urine.
  • Cerebral salt-wasting syndrome: Subarachnoid hemorrhage, ischemic stroke, spontaneous intracerebral hemorrhage (ICH), craniotomy, encephalitis, meningitis and head trauma are considered the most common etiologies. The pathophysiology of CSW syndrome is not completely understood. Decreased sympathetic activity to the kidneys and Increased brain natriuretic peptide are the most accepted hypothesis. Renin-aldosterone is decreased due to reduced sympathetic stimulation . Increased natriuretic peptides causes sodium loss( natriuresis )by increasing GFR and preventing sodium reabsorption in the collecting duct .Natriuretic peptides inhibit aldosterone and ADH action. Reducing sympathetic outflow by natriuretic peptides decreases aldosterone secretion which lowers sodium level further .
  • Osmotic diuresis: Urinary excretion of osmotically active solutes causes volume depletion and hyponatremia. Glucosuria and ketonuria in DKA and HHS obligate sodium and water loss even in the presence of hypovolemia.
  • Mineralocorticoid deficiency: Adrenal insufficiency ( primary, aldosterone deficiency) leads to renal sodium loss and volume depletion, causing release of ADH due to hypovolemia. Patients typically present with hypovolemia, hyponatremia, hyperkalemia, decreased bicarbonate and increased urine sodium and inappropriate renal response in the setting of volume depletion.

Hypervolemic hyponatremia

• Renal disease: Chronic or acute renal failure results in reduced functional nephron mass, decreased glomerular filtration rate (GFR) and therefore decreased capacity for water excretion. Nephrotic syndrome causes reduced effective circulatory volume and edema which results in increased ADH effect.

• Clinical disorders: In Congestive heart failure( CHF) and cirrhosis , the reduction in effective arterial blood volume, resulting in persistent ADH activity despite hypoosmolar plasma. The ability to excrete water is also limited when the posterior pituitary continues to secrete ADH despite a low serum osmolality and plasma sodium concentration. Decreased effective arterial blood volume is sensed as hypovolemia which is the stronger stimulant of ADH secretion than osmolality of plasma. ADH is secreted without an osmotic stimulus if circulation is inadequate. Inpatients with cirrhosis decreased effective circulating volume is a result of arterial vasodilation of the splanchnic circulation, which is due to the increased endothelial release of nitric oxide.

Euvolemic hyponatremia

• Syndrome of inappropriate antidiuresis: The most common cause of hyponatremia (euvolemic) due to either increased level of ADH or gain-of-function mutation of the V2 receptor of ADH. Inappropriate secretion or action of ADH in the absence of osmotic or hemodynamic stimulus is called SIAD ( syndrome of inappropriate diuresis).Nephrogenic SIAD has the same presentation but ADH level is normal.

Diagnostic criteria of SIAD

serum sodium <135 mEq/L Plasma osmolality <275 mOsm/kg , ( NL=275–295 mOsm/kg) urine concentration UOsm >100mOsm/kg , (NL=500-800 mOsm/kg ) Urine sodium >30 (20-40) mmol/L, with normal salt and water intake, (NL=20 mEq/L)† Clinical euvolemia Exclusion of thyroid, adrenal, pituitary and renal insufficiency No recent use of diuretics Supplemental criteria Serum uric acid < 0.24 mmol/l (< 4 mg/dl) Serum urea < 3.6 mmol/l (< 21.6 mg/dl) Failure to correct hyponatremia after 0.9% saline infusion Fractional sodium excretion > 0.5% Fractional urea excretion > 55% Fractional uric acid excretion > 12% Correction of hyponatremia through fluid restriction

† Mmol and Meq are the same for univalent ions like sodium

• Excess water intake: In exercise-associated hyponatremia, increased level of ADH due to hypovolemia and excess free water intake cause hyponatremia. In primary polydipsia, there is an increase in thirst, especially in psychotic patients. The osmotic threshold for thirst is lower than the threshold for ADH release.

• Hormonal: Glucocorticoids have an inhibitory effect on ADH release by the posterior pituitary so cortisol deficiency induces ADH secretion. Thyroid hormone deficiency especially primary hypothyroidism causes hyponatremia and the mechanism is not well-understood. in patients who present with hypothyroidism, there is an elevation to ADH level and reduction in GFR which means decreased ability to excrete diluted urine .Secondary adrenal insufficiency (hypopituitarism), presents with features of SIAD ( euvolemic hyponatremia).

• Medications: Mechanism in which medications can cause hyponatremia are; Interfering with urinary dilution (thiazide diuretics and nonsteroidal anti-inflammatory drugs [NSAIDs]), increasing ADH secretion, persisting ADH effect and reset osmostat.

• Reset osmostat: There is a downward resetting for secretion of ADH regulated by osmoreceptors, therefore, a lower level of plasma sodium concentration is required to completely suppress ADH release and water intake. Pregnancy and drugs are the most common etiologies. In pregnancy, secretion of human chorionic gonadotropin is the main cause of resetting osmostat.

Puedohyponatremia

Considerable elevations of either lipids or proteins in serum causes serum sodium to be measured lower than actual total amount. Plasma osmolality is normal because the total number of solutes are the same e but since the larger portion of plasma is occupied by excess lipids or protein, the measured serum sodium is lower especially with older techniques like flame photometry.

• Blood sampling from a vein that is being infused with hypotonic medications cause serum sodium to be measured lower than actual amount.

• Hyperglycemia can cause hyponatremia, osmotic water movement from cells into the blood, resulting in a relative decrease in serum sodium concentration in the absence of hypo-osmolality.

(for each 100-mg/dL increase in glucose concentration above 100 mg/dL The sodium concentration should be increased by approximately 1.6 to 2 mmol/L )

Hyponatremia represents an excess of water relative to total body sodium, resulting from impaired water excretion by the kidneys or the depletion of sodium in excess of water.

Hypotonic (dilutional) hyponatremia is classified by the extracellular volume status into hypo-, eu- and hyper-volemic hyponatremia.

Term	Definitions
Hyponatremia	Hyponatremia is defined as a serum sodium concentration < 135 mEq/L.
Hypotonic hyponatremia	Hyponatremia with low osmolality (hypotonic hyponatremia) is defined as hyponatremia with a serum osmolality below 280 mOsm/kg.
Hypertonic hyponatremia	Hyponatremia with high osmolality (hypertonic hyponatremia) is defined as hyponatremia with a serum osmolality greater than 295 mOsm/kg.
Isotonic hyponatremia	Hyponatremia with normal osmolality (Isotonic hyponatremia) is defined as hyponatremia with a serum osmolality ranging between 280-295 mOsm/kg.
Hyponatremia based on ECF volume
Hypovolemic hyponatremia	Hyponatremia plus decreased extracellular cellular fluid volume. Usually diagnosed by history and physical examinationshowing water depletion plus spot urine sodium <20 to 30 mmol/L, unless kidney is the source of sodium loss.
Euvolemic hyponatremia	Hyponatremia plus normal extracellular cellular fluid volume. Majority of cases are of this type. Usually diagnosed by spot urine sodium ≥ 20 to 30 mmol/L, unless secondarily sodium depleted.
Hypervolemia hyponatremia	Hyponatremia plus increased extracellular cellular fluid volume. Usually diagnosed by history and physical examinationshowing water retention plus spot urine sodium <20 to 30 mmol/L

Genetics

• Nephrogenic SIAD (syndrome of inappropriate antidiuresis)  : Gain-of-function mutations of the V2 vasopressin receptor gene (AVPR2) causes hyponatremia.
• Pseudohypoaldosteronism
• Aldosterone Biosynthetic Defects
• Gittleman syndrome
• Bartter syndrome

Associated Conditions

• Fanconi syndrome
• Renal tubular acicosis