Description

• Sodium cation (Na⁺) is the major extracellular cation and is important osmole in the extracellular space.
• A positive Na⁺ balance is required for growth.
• Serum Na⁺ levels reflect extracellular Na⁺ and are tightly regulated within a narrow range (generally 135 to 145 mEq/L).
• Approximately 2/3 of body mass is Na⁺-containing extracellular fluid space.
• Hyponatremia is defined as serum Na⁺ <135 mEq/L and is graded according to level:
  • Mild hyponatremia (130 to 135 mEq/L)
  • Moderate hyponatremia (125 to 129 mEq/L)
  • Severe hyponatremia (<125 mEq/L)
• Even mild hyponatremia (130 to 135 mEq/L) has measurable effects on cognitive function, and severe hyponatremia can lead to seizures, brain edema, and death.
• Hyponatremia can be factitious in the setting of significantly elevated glucose.

Epidemiology

Hyponatremia is common in the inpatient setting, and incidence for moderate to severe hyponatremia (<130 mEq/L) is ~1% hospitalized pediatric patients and mild hyponatremia (<135 mEq/L) is present in up to 45% of pediatric patients admitted for community-acquired pneumonia.

Risk Factors

• Conditions associated with syndrome of inappropriate antidiuretic hormone secretion (SIADH) including nausea, hypotension, hypovolemia, reduced circulatory volume (congestive heart failure [CHF], nephrosis, and cirrhosis), asthma, pneumonia, mechanical ventilation, fever, stress, pain and specific medications (narcotics, nonsteroidal anti-inflammatory drugs, serotonin reuptake inhibitors, Cytoxan^® and vincristine)
• Conditions associated with increased stool losses (diarrhea, vomiting, loss of gastrointestinal [GI] fluid—surgical or tube drainage) increase the risk of developing hyponatremia.
• Excessive water intake such as in polydipsia or incorrectly mixed formula
• Decreased water excretion such as acute kidney injury

General Prevention

Providing adequate Na⁺ intake and checking serum Na⁺ levels in high-risk patients (GI fluid losses and high risk for SAIDH) is the most important step in prevention of severe hyponatremia.

Pathophysiology

• There are three major mechanisms affecting serum Na⁺ levels: intake, excretion, and total body water. Hyponatremia can develop due to following reasons:
  • Reduced intake of Na⁺
  • Increased losses (GI, renal, or skin)
  • Increased total body water
• Hyponatremic encephalopathy results from hypo-osmolality in the extracellular space and the influx of water into the intracellular space down the concentration gradient, resulting in parenchymal brain swelling and resulting symptoms.

Etiology

• Hyponatremia is usually divided into three classifications: (i) hypovolemic hyponatremia, (ii) euvolemic hyponatremia, and (iii) hypovolemic hyponatremia.
• The most common reason for hyponatremia in hospitalized patients is SIADH.
• The most common cause in the general pediatric population is extrarenal losses including diarrhea and vomiting.