Medically, Sodium is one of the essential minerals required for homeostasis of body, performing regulation of blood volume, blood pressure, osmotic equilibrium and pH regulation. If anything goes wrong about sodium levels, either decreased as in hyponatremia or increased as in hypernatremia, then it causes a number of disturbances which are usually attributed to a pathology.
In today’s article, we will be discussing about Normal sodium levels, Hyponatremia and Hypernatremia: Definition, Causes, Signs and Symptoms, Diagnostic approach and Treatment.
Normal Value and Physiology
Normal serum sodium concentration ranges between 135-145 mEq/L. Daily sodium requirement is 2 to 3 mEq/kg body weight, though it is generally taken in excess. Major route of loss of sodium through body is via Urinary excretion, which is approximately equal to daily intake of sodium. Other extra-renal losses of sodium include sweating, burns, vomiting or diarrhea.
Sodium is regulated by Renin-Angiotensin mechanism. Whenever there is fluid loss, there is decrease in blood volume, which in turn reduces decrease in renal blood flow. This in turn, activates Renin-Angiotensin mechanism. Renin converts Angiotensinogen into Angiotensin I, which is converted to Angiotensin II by ACE (Angiotensin converting enzyme).
This angiotensin II is reason for Aldosterone release which acts on kidney and thereby, causing sodium and water retention, and hence compensating for the sodium loss. Other causes of Renin-Angiotensin mechanism can be fall in blood pressure, decrease in sodium delivery to macula densa or sympathetic stimulation.
Hyponatremia is defined as plasma sodium going below 135 mEq/L value. Causes of low sodium include sweating, burns, vomiting, diarrhea and administration of diuretics. However, EXCESS OF TOTAL BODY WATER RELATIVELY REDUCES SODIUM LEVELS, causing relative hyponatremia. This is due to syndrome of inappropriate anti-diuretic hormone (SIADH).
Diagnostic approach to Hyponatremia is as follows:
Hyponatremia Clinical Approach
Following table lists the causes of Hyponatremia including Hypovolemic hyponatremia, Normovolemic or Euvolemic hyponatremia and Hypervolemic hyponatremia.
TYPE OF HYPONATREMIA CAUSES
Hypovolemic hyponatremia (Sodium loss in excess of free water) Renal loss: Diuretic use, osmotic diuresis, renal salt-wasting, adrenal insufficiency, pseudo-hypoaldosteronism.
Extra-renal loss: Diarrhea, vomiting, fistula sweat, cerebral salt wasting syndrome, effusions, ascites, drains.
Normovolemic (Euvolemic) hypovolemia (predisposing to SIADH) CNS dieases like meningitis, encephalitis, tumors
Pulmonary diseases like asthma, pneumonia
Drugs use including cyclophosphamide, vincristine
Hypervolemic hyponatremia (excess free water retention) Congestive heart failure (CHF), cirrhosis, renal failure, nephrotic syndrome
Signs and Symptoms
If the sodium concentration falls below 125 mEq/L or within a period of 24 hours, it produces symptoms of hyponatremia. These include, nausea, vomiting, confusion, lethargy and headache. If the low sodium levels continues to persist, it can cause dilated pupils, seizures, coma, arrythmia, central diabetes inspidus or myocardial infarction. Cerebral edema occurs at sodium levels going below 125 mEq/L.
Treatment of hypotension is the first basic, followed by correction of deficit over 48-72 hrs (for chronic) or rapidly (for acute). Fluid restriction is needed for SIADH causing hyponatremia.
Hypernatremia is defined as serum sodium levels going above 150 mEq/L. Major causes of hypernatremia or high sodium levels include reduced body water (either due to loss or reduced intake), excessive intake of sodium or reduced levels of ADH (anti-diuretic hormone) which occurs as in Diabetes inspidus.
Following table lists the causes of hypernatremia, which includes Increased losses of Potassium, Decreased intake or stores of Potassium and Intracellular shift of Potassium from intracellular to extracellular.
Net water loss Pure water loss: Diabetes inspidus, insensible losses, inadequate intake
Renal loss: Polyuria, loop diuretics, osmotic diuretics, post-obstructive
Gastrointestinal loss: Vomiting, nasogastric drainage, diarrhea, lactulose
Hypotonic fluid loss
Hypertonic sodium gain Excess sodium intake: Sodium bicarbonate (HCO3), saline infusion, hypertonic ingestion,ingestion of sodium bicarbonate, hypertonic dialysis
Endocrine sodium gain: Primary hyperaldosteronism, cushing syndrome
Signs and Symptoms
Most important signs of low sodium levels include lethargy or mental status changes, which can develop into seizures or coma. Acute and severe hypernatremia can cause meningeal tearing and intracranial hemorrhage. However, slowly developing hypernatremia is usually tolerated due to body adaptation.
First basic again involves treatment of hypotension regardless of serum sodium levels, followed by correction of deficit over 48-72 hrs (for chronic) or rapidly (for acute). Seizures due to hypernatremia are treated using hypertonic saline. Correction of ongoing fluid losses should be done to correct low sodium levels.