Hyponatremia represents a relative excess of water in relation to sodium. Typically, we diagnose hyponatremia when we see a serum sodium concentration of less than 135mEq/L.
The initial diagnostic approach to the adult patient with hyponatremia consists of a directed history and physical examination as well as selected laboratory tests. On history and physical exam, one can first assess if the patient is hyper-, eu-, or hypo-volemic.
A hypovolemic patient may have a history of vomiting, diarrhea, diuretic therapy and a physical exam significant for decreased skin turgor, a low jugular venous pressure, or orthostatic or persistent hypotension). If they are hypovolemic, the next step is to determine the urine sodium of the patient. If the patient has a urine sodium of over 20mEq/L, the differential etiology is either diuretic therapy or salt-wasting nephropathy. To differentiate between these two diagnoses, it is important to elicit any history of diuretics, especially thiazides, and see if the patient has any evidence of glomerulosclerosis. If the patient has a urine sodium of under 10mEq/L, the differential etiology is either diarrhea, excessive sweating, or third-spacing (i.e. pancreatitis, burns, peritonitis).
A euvolemic patient will not present with any convincing signs or symptoms of hypo- or hyper-volemia. If the euvolemic patient has a urine osmolality of over 100mEq/L, the differential etiology is Syndrome of Inappropriate antidiuretic hormone secretion (SIADH), adrenal insufficiency (i.e. not enough cortisol, resulting in hypersecretion of ADH), or hypothyroidism. SIADH is the most common cause of hyponatremia in euvolemic patients with a high urine osmolality, is diagnosed after other etiologies are excluded. In a euvolemic patient with a urine osmolality of under 100mEq/L, the differential etiology is psychogenic polydipsia (excessive water intake, often seen in patients with psychiatric disorders), or low solute intake (often seen in elderly and/or those who consume a "tea and toast" diet).
In a patient that presents with volume overload, or hypervolemia, the physical exam will often show a high JVP, peripheral edema, and weight gain. In a hypervolemic patient with hyponatremia, if the urine sodium is under 20mEq/L, the differential etiology is congestive heart failure (CHF), cirrhosis, or nephrotic syndrome. The three diagnoses often present similarly, however nephrotic syndrome can be differentiated from CHF and hepatic disease by the presence of proteinuria. If the urine sodium is over 20mEq/L, the differential etiology is acute kidney injury or chronic kidney disease.
In terms of treating hyponatremia, it is important to first be aware of any severe symptoms (coma, seziure, acute respiratory distress syndrome). If the patient is presenting with severe symptoms, the first step is emergent IV 3% NaCl, aiming for P[Na+] rise of about 5mmol/L in the first hour. If the patient does not have any severe symptoms, it is important to be aware of whether the hyponatremia is acute or chronic, chronic being defined as over 48 hours. If the hyponatremia is chronic, then the patient's brain has had time to adapt, so the degree of cerebral edema may be limited and the presentation may include more subtle changes, such as personality changes, nausea, risk of falls, or confusion. In chronic hyponatremia, it is important not to overcorrect the sodium level, because this risks causing osmotic demyelination syndrome (ODS). Risk factors for developing ODS include hyponatremia that is severe, malnutrition, alcohol use, hypokalemia, and cirrhosis. If the hyponatremia is acute, there is a risk of cerebral edema as the water enters the cells of the brain, causing increases in intracranial pressure and resulting in confusion, headache, nausea, vomiting, coma, seizures, and even death. In acute hyponatremia, we are able to correct the sodium more quickly because the brain has not adapted to the state of low sodium. If ODS occurs, it is important to give IV D5W, as well as ADH to stop water diuresis (DDAVP 1-2ug IV).
Treatment of hyponatremia in all patients includes 1) treating the underlying cause and 2) restricting free water intake in SIADH. In acute hyponatremia that is symptomatic, you should correct rapidly with 3% NaCl at 1-2cc/kg/h up to serum Na+ 125-130. In asymptomatic acute hyponatremia, it is important to take general measures to identify and reverse causes of hyponatremia if the serum sodium is over 120mmol/L. If the patient is asymptomatic but has acute hyponatremia with a serum sodium under 120mmol/L, you can treat them as if they are symptomatic.
Treatment of chronic hyponatremia (or unknown timeline), involves water restriction and IV 0.9NS plus furosemide. You may also consider NaCl tablets as a source of sodium.
References:
1. Overview of the treatment of hyponatremia in adults - UptoDate
2. Causes and Management of Hyponatremia - SagePub
3. Pathophysiology and etiology of the syndrome of inappropriate antidiuretic hormone secretion (SIADH)
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