## Distinguishing Hyponatremic from Isotonic Dehydration ### Clinical Presentation Comparison | Feature | Isotonic (Isonatremic) | Hyponatremic | Hypernatremic | |---------|------------------------|--------------|---------------| | **Serum Na⁺** | 130–150 mEq/L | <130 mEq/L | >150 mEq/L | | **Fluid shift** | Proportional (intracellular = extracellular) | Intracellular > extracellular | Intracellular < extracellular | | **Skin turgor** | Loss present | Loss less prominent (puffy appearance) | Marked loss (doughy) | | **Mucous membranes** | Dry | Moist or normal | Very dry | | **Neurological signs** | Absent (unless shock) | **Seizures, altered sensorium, coma** (cerebral edema) | Hyperirritability, high-pitched cry | | **Thirst** | Present | Absent or diminished | Intense | ### Key Point: **Hyponatremic dehydration causes cerebral edema** because water shifts intracellularly due to osmotic gradient. This leads to neurological manifestations (seizures, altered sensorium, coma) that are **disproportionate to the degree of fluid loss**. ### Clinical Pearl: A child with moderate dehydration who develops seizures should raise suspicion for **hyponatremia**, not just hypovolemic shock. The seizure occurs because intracellular fluid accumulates in the brain, raising intracranial pressure. ### High-Yield: **Seizures in a dehydrated child = think hyponatremic dehydration first.** This is the most discriminating and clinically important feature that distinguishes hyponatremic from isotonic dehydration. ### Pathophysiology In hyponatremic dehydration: 1. Low extracellular Na⁺ creates osmotic gradient 2. Water moves INTO cells (including brain cells) 3. Cerebral edema develops → increased ICP 4. Neurological symptoms appear: seizures, confusion, lethargy, coma In isotonic dehydration, fluid loss is proportional from both compartments, so no osmotic gradient exists and no cerebral edema occurs. [cite:Park 26e Ch 6]
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