Hypertonic dehydration, also known as primary dehydration or hypernatremia with reduced extracellular fluid, is characterized by more water loss than sodium loss, a serum sodium concentration of 150 mmol/L, and a plasma osmolality of 310 mOsm/L. When there is more water deficiency than sodium deficiency, the osmolality of extracellular fluid increases, antidiuretic hormone secretion is increased, renal tubular reabsorption of water is increased, and urine output is decreased. Aldosterone secretion increases, and sodium and water reabsorption increases to maintain blood volume. If water deficiency continues, the osmolarity of extracellular fluid increases further, intracellular fluid moves to the extracellular, and eventually the degree of intracellular water deficiency exceeds the degree of extracellular fluid deficiency, which can eventually lead to brain cell dehydration which causes brain dysfunction. Water and sodium are lost at the same time, but the water deficiency is more than sodium deficiency, so the serum sodium is higher than the normal range and the extracellular fluid is hypertonic. In the early stage, the main symptoms are thirst, decreased urine output, weakness, nausea and vomiting, and increased body temperature; signs include water loss. In the late stage, clinical manifestations of brain cell water loss appear, such as irritability, irritability or indifference, sleepiness, convulsions or epileptiform seizures and coma; signs include increased muscle tone and hyperreflexia, etc. Severe cases die as a result.