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- 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.
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.
- 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
There are a few causes of hyponatremia that are secondary to monogenic disorders including congenital adrenal hypoplasia, also known as adrenal hypoplasia congenita (AHC).
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.
Due to the high rate of SIADH in hospitalized pediatric patients, isotonic saline solution should be used for maintenance IVF therapy specifically in the postoperative setting and in children with central nervous system (CNS) or pulmonary disease.
- 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.
- 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.