Hypotonic dehydration refers to a condition characterized by the reduction of extracellular fluid accompanied by hyponatremia. Its defining features include a greater loss of sodium (Na+) relative to water, with serum sodium concentration falling below 130 mmol/L and plasma osmolality under 280 mOsm/kg H2O, alongside a reduction in extracellular fluid volume.
Etiology
Significant loss of digestive fluids with only water replenishment is a common cause. Situations such as severe vomiting or prolonged gastrointestinal decompression can lead to substantial losses of sodium-containing digestive fluids, with water or glucose solutions used as replacement fluids.
Fluid accumulation in a third space can also result in hypotonic dehydration. Conditions such as peritonitis or pancreatitis may produce large volumes of ascitic fluid, bowel obstruction may cause intestinal fluid retention within the bowel lumen, and pleuritis may lead to significant pleural effusion.
Prolonged use of sodium-depleting diuretics, such as furosemide, ethacrynic acid, or thiazide diuretics, can lead to sodium loss. Insufficient aldosterone secretion due to adrenal insufficiency and reduced sodium reabsorption in the renal tubules are other contributing factors. Moreover, parenchymal renal diseases or tubular toxicity may increase sodium excretion.
Sodium loss via the skin, such as through excessive sweating or from extensive burns, can lead to significant fluid and sodium depletion. If these losses are replaced by water alone, hypotonic dehydration can result.
Clinical Manifestations
Common symptoms include nausea, vomiting, dizziness, blurred vision, fatigue, and susceptibility to fainting when standing. Thirst is generally absent. When circulating blood volume decreases significantly and renal filtration is reduced, metabolic waste products may accumulate in the body, leading to symptoms such as lethargy, muscle cramps, diminished tendon reflexes, respiratory difficulty, and coma.
Hypotonic dehydration can be classified into three levels based on the degree of sodium deficiency:
- Mild Hyponatremia (serum sodium concentration between 130 mmol/L and <135 mmol/L): Symptoms include fatigue, dizziness, numbness of the extremities, and reduced urinary sodium.
- Moderate Hyponatremia (serum sodium concentration between 120 mmol/L and <130 mmol/L): In addition to the above symptoms, nausea, vomiting, rapid thready pulse, unstable or lowered blood pressure, reduced pulse pressure, collapsed superficial veins, blurred vision, orthostatic fainting, reduced urine output, and an almost complete lack of sodium and chloride in the urine may occur.
- Severe Hyponatremia (serum sodium concentration below 120 mmol/L): Patients may experience confusion, muscle cramping pain, significantly diminished or absent tendon reflexes, stupor, difficulty breathing, or coma. Hypovolemic shock is frequently observed in severe cases.
Diagnosis
A preliminary diagnosis of hypotonic dehydration may be made based on a history of fluid loss and clinical symptoms. Further laboratory tests include:
- Urine analysis: Urine specific gravity often measures below 1.010, with significantly reduced urinary sodium and chloride levels.
- Serum sodium measurement: A serum sodium concentration below 135 mmol/L is indicative, with lower values correlating with greater severity.
- Other laboratory parameters: Red blood cell count, hemoglobin levels, hematocrit, and blood urea nitrogen are often elevated.
Treatment
Treatment focuses on addressing the underlying cause. Given the greater sodium loss compared to water in hypotonic dehydration, intravenous administration of saline or hypertonic saline is used to restore extracellular fluid tonicity and expand blood volume.
Treatment strategies depend on the rate, degree, and symptoms of serum sodium reduction. Sodium supplementation can be calculated using the formula:
Required Sodium (mmol) = [Normal Serum Sodium (mmol/L) - Measured Serum Sodium (mmol/L)] × Body Weight (kg) × 0.6 (or 0.5 for females).
The total calculated amount should be administered in divided doses. Initially, part of the sodium deficit is replenished to relieve acute symptoms, followed by adjustments based on clinical manifestations, serum sodium and chloride concentrations, and arterial blood gas analysis to address the remaining deficit.
In cases of severe hyponatremia with shock, blood volume is first restored to improve microcirculation and perfusion of tissues and organs. When administering hypertonic saline, infusion rates are strictly controlled to ensure no more than 100–150 mL per hour, with subsequent adjustments to the treatment plan based on the patient's condition and changes in serum sodium concentration.