What are hypotonic fluids for a patient with hyponatremia taking Lexapro (escitalopram)?

Hypotonic Fluids: Definition and Clinical Context

Hypotonic fluids are solutions with lower osmolarity than blood plasma that can worsen hyponatremia and should be avoided in patients taking escitalopram (Lexapro), who are already at increased risk for developing low sodium levels. 1

What Are Hypotonic Fluids?

Hypotonic fluids contain fewer dissolved particles (lower osmolarity) than normal blood plasma (275-290 mOsm/kg). 2 Common examples include:

• 0.45% NaCl (half-normal saline): Contains 77 mEq/L sodium with osmolarity ~154 mOsm/L 3
• 0.18% NaCl (quarter-normal saline): Contains ~31 mEq/L sodium 3
• D5W (5% dextrose in water): Provides free water with no sodium 3
• Lactated Ringer's solution: Contains 130 mEq/L sodium with osmolarity 273 mOsm/L, making it slightly hypotonic 4

Critical Warning for Patients on Escitalopram

Patients taking selective serotonin reuptake inhibitors (SSRIs) like escitalopram are at particularly high risk for developing hyponatremia, especially during the first weeks of treatment. 5, 6 The risk is highest in:

• Elderly patients 5
• Patients with lower body weight 5
• Those receiving hypotonic intravenous fluids 2

Why Hypotonic Fluids Are Dangerous in Hyponatremia

The European Society of Cardiology explicitly warns that excessive consumption of hypotonic fluids can cause or worsen hyponatremia. 1 The mechanism involves:

• Impaired free water excretion: SSRIs like escitalopram cause syndrome of inappropriate antidiuretic hormone (SIADH), preventing normal water elimination 5, 6
• Dilutional effect: Hypotonic fluids provide excess free water that dilutes serum sodium further 2
• Risk of hyponatremic encephalopathy: Can lead to confusion, seizures, or death 2

What Fluids Should Be Used Instead?

For patients with hyponatremia or at risk (including those on escitalopram), isotonic fluids are strongly preferred:

• 0.9% normal saline (isotonic): Contains 154 mEq/L sodium with osmolarity 308 mOsm/L 4
• No neurological complications have been reported from using 0.9% NaCl in non-neurosurgical patients 2
• Multiple prospective studies confirm 0.9% NaCl effectively prevents hyponatremia 2

Specific Guidance for Escitalopram Patients

Patients on escitalopram should:

• Avoid excessive consumption of hypotonic fluids (water, tea, other low-sodium beverages) 1
• Have serum sodium monitored closely, especially during the first weeks of treatment 5
• Seek immediate medical attention for symptoms of hyponatremia: malaise, confusion, seizures 5
• Receive isotonic (0.9% NaCl) rather than hypotonic IV fluids if hospitalized 2

Common Clinical Pitfall

Hospital-acquired hyponatremia from hypotonic IV fluids affects 15-30% of hospitalized patients and is entirely preventable by using isotonic maintenance fluids. 4 The practice of administering hypotonic fluids was established over 50 years ago, before recognition that most hospitalized patients have elevated arginine vasopressin (AVP) levels that impair free water excretion. 2