Hyponatremia

It’s 3 AM, and your pager goes off. The call is about a patient seizing; their sodium is 109. Your mind starts racing. What’s your first move? How fast can you correct it? What’s the underlying cause?

Hyponatremia is the most common electrolyte disturbance you’ll encounter in hospitalized patients, and it can be a real diagnostic puzzle. It ranges from an incidental lab finding to a life-threatening emergency. But don't worry—we're here to break it down. Let's walk through a classic case and build a solid framework for your hyponatremia workup and management.

The Clinical Scenario

EMS brings a 70-year-old woman to the Emergency Department after her first-ever seizure. On arrival, she's drowsy and confused, with a GCS of 10. Her family confirms no prior history of seizures. Her medication list includes ramipril, hydrochlorothiazide, and citalopram for hypertension and depression—a classic triad that should immediately get your gears turning.

Minutes after being roomed, she has another tonic-clonic seizure. You administer 10 mg of IV diazepam and the seizure stops. A head CT is ordered, labs are drawn, and she's placed on a monitor.

The arterial blood gas results come back first: Sodium 109 mEq/L.

This is severe, symptomatic hyponatremia. The team administers a rapid IV bolus of 130 mL of 3% sodium chloride (hypertonic saline). They repeat the bolus 20 minutes later. Her sodium rises to 113 mEq/L, she gradually becomes more alert, and the seizures do not recur. She is admitted to the internal medicine service for continued, cautious correction.

First Steps: Is This an Emergency?

Before you dive into a complex diagnostic algorithm, your first question must always be: "Is my patient stable?" In the acute setting, the severity of symptoms dictates the urgency of your intervention. Clinically, this "symptom-based" classification is often more practical than trying to determine if the hyponatremia is acute vs. chronic in those first few frantic minutes.

• Severe Symptoms: The True Emergency. This is hyponatremic encephalopathy until proven otherwise.

  • Seizures

  • Coma (GCS ≤8)

  • Severe confusion or altered mental status

  • Persistent vomiting

• Moderately Severe Symptoms: These are concerning but not immediately life-threatening.

  • Headache

  • Nausea without vomiting

  • General confusion

  • Gait instability

• Mild or No Symptoms: Often discovered as an incidental lab finding.

  • Fatigue or malaise

  • Dizziness

  • Mild concentration issues (mild hyponatremia symptoms)

Our patient presented with seizures, placing her squarely in the "severe" category. This clinical status is precisely why she required immediate, rapid intervention.

The Hyponatremia Workup: Playing Detective

Once the immediate crisis is stabilized, it's time to determine the etiology. A systematic, step-by-step approach is the only way to navigate the hyponatremia maze without getting lost.

Step 1: Check Serum Osmolality

This initial step categorizes the hyponatremia.

• Isotonic Hyponatremia (275–295 mOsm/kg): This is typically pseudohyponatremia—a lab artifact. It occurs when extreme elevations in lipids or proteins interfere with lab investigations. The patient's actual plasma tonicity is normal.

• Hypertonic Hyponatremia (>295 mOsm/kg): This is usually caused by the presence of other effective osmoles in the blood, most commonly glucose. In cases of hyperglycemia and hyponatremia, you must calculate the corrected sodium. The rule of thumb: for every 100 mg/dL increase in glucose above 100, the measured sodium decreases by approximately 1.6–2.4 mEq/L.

• Hypotonic Hyponatremia (<275 mOsm/kg): This is true hyponatremia, representing a relative excess of free water. This is the most common clinical scenario and where the remainder of our workup will focus.

Step 2: Check Urine Osmolality (urine osmolality hyponatremia)

The urine osmolality tells you if the kidneys are responding appropriately by excreting free water.

• Urine Osmolality <100 mOsm/kg: The kidneys are appropriately diluting the urine to get rid of excess water. The problem is simply an intake that exceeds the kidney's excretory capacity. The differential diagnosis includes:

  • Primary Polydipsia: Often referred to as psychogenic polydipsia, frequently seen in patients with psychiatric conditions.

  • Low Solute Intake: The classic "Tea and Toast" diet or Beer Potomania. The kidneys require a certain solute to excrete water, and if the intake is insufficient, they just can't keep up.

Step 3: Check Urine Sodium and Assess Volume Status

If the urine osmolality is ≥100 mOsm/kg, the kidneys are inappropriately concentrating the urine—retaining free water instead of excreting it. This is typically driven by antidiuretic hormone (ADH) activity. At this stage, you must clinically assess the patient’s volume status (looking for signs of hypovolemia such as tachycardia, orthostasis, and dry mucous membranes) versus hypervolemia (edema, JVD).

• Hypovolemic Hyponatremia (UNa <30 mEq/L): The patient is dehydrated, and the kidneys are appropriately conserving sodium and water. Low urine sodium confirms that the kidneys are working hard to retain salt.

  • Causes: Diarrhea, vomiting, burns, or third-spacing of fluids.

• Hypovolemic Hyponatremia (UNa >30 mEq/L): The patient is dehydrated, but the kidneys are “wasting” salt.

  • Causes: Diuretic use (especially thiazides), primary adrenal insufficiency (Addison disease), or cerebral salt wasting.

• Euvolemic Hyponatremia (UNa >30 mEq/L): The patient appears clinically euvolemic, yet a pathologic process is driving water retention. This is a broad category.

  • Syndrome of Inappropriate ADH Secretion (SIADH): The most common cause. SIADH diagnosis is one of exclusion. Often triggered by medications (SSRIs, carbamazepine), CNS disorders, or malignancy.

  • Secondary Adrenal Insufficiency

  • Severe Hypothyroidism (myxedema coma)

• Hypervolemic Hyponatremia (UNa <30 mEq/L): The patient is fluid-overloaded, but effective circulating volume is low. This triggers ADH and aldosterone, prompting the kidneys to aggressively hoard sodium.

  • Causes: Congestive heart failure, cirrhosis, or nephrotic syndrome.

• Hypervolemic Hyponatremia (UNa >30 mEq/L): The patient is fluid-overloaded and losing sodium in the urine.

  • Causes: Acute or chronic renal failure.

Treatment: From Emergency to Maintenance

Emergency Phase: Severe Symptoms

When severe symptoms are present (as seen in our patient), immediate intervention with a bolus of hypertonic saline is mandatory. The goal is not to normalize sodium levels but to raise them just enough (by 4–6 mEq/L) to arrest seizures and prevent brain herniation.

• Dosing: Administer 100–150 mL of 3% NaCl over 10–20 minutes. This bolus can be repeated up to two more times if severe symptoms persist.

• Clinical Pearl: If 3% NaCl isn't immediately available, one 50 mL ampule of 8.4% sodium bicarbonate contains approximately the same sodium load as 100 mL of 3% NaCl.

Correction Phase: The Golden Rule

Once the patient is stabilized, transition to a "slow and steady" correction pace. Why? To avoid the most feared complication: Osmotic Demyelination Syndrome (ODS).

If you correct chronic hyponatremia too quickly, the rapid shift in osmolality can cause water to exit brain cells, leading to demyelination, particularly in the pons (central pontine myelinolysis). This may result in devastating neurological outcomes, including paralysis and locked-in syndrome.

Safe Correction Limits:

• Standard Risk: ≤8-10 mEq/L within the first 24 hours and ≤ 8 mEq/L in any subsequent 24-hour period.

• High Risk for ODS (Na <105, alcoholism, liver failure, malnutrition): Target a stricter limit of ≤6 mEq/L per 24 hours.

  Learn more about hyponatremia overcorrection.

Addressing the Underlying Cause

Long-term management is dictated by your initial diagnostic workup:

• Hypovolemic Hyponatremia: The solution is volume repletion with isotonic fluids, typically 0.9% normal saline.

• Euvolemic Hyponatremia (SIADH):

  • First-line: Fluid restriction (usually <1 L/day) and discontinuation of any offending medications.

  • Second-line: Salt tablets, loop diuretics (to promote free water excretion), or a vasopressin receptor antagonist like tolvaptan.

• Hypervolemic Hyponatremia: The primary goal is to remove excess fluid.

  • Treatment: Fluid and sodium restriction plus diuretic therapy.

High-Yield Takeaways for the Wards

Feeling overwhelmed? Keep these core principles in your pocket:

• Symptoms Drive Urgency: Severe symptoms (seizures, coma) demand an immediate bolus of hypertonic saline.

• The Acute Goal: Raise serum sodium by only 4–6 mEq/L to stabilize the patient, then shift to a maintenance pace.

• Prevent ODS: Correct slowly. Your mantra is "6–8 in 24." For most patients, never exceed an 8 mEq/L rise in 24 hours.

• Your Diagnostic Path: Follow the diagnostic algorithm:

  • Serum Osmolality (Is this true hypotonic hyponatremia?)

  • Urine Osmolality (Are the kidneys diluting or concentrating the urine?)

  • Urine Sodium & Volume Status (Where are the salt and water actually going?)

Mastering hyponatremia takes practice, but with this systematic approach, you can confidently handle that 3 AM page and provide top-tier care. You've got this.