Most dangerous dehydration is –
Types of Dehydration in diarrhoea

A large amount of water and water-soluble nutritive substances such as electrolytes, metabolites and vitamins are lost from the body during diarrhoea episodes.
Loss of water from the body causes a reduction or shrinkage in the volume of ECF.
Depending on the concentration of sodium, this ECF reduction can be classified into -

Isonatremic dehydration

Sodium loss is parallel to water loss so that sodium concentration in ECF or plasma remains normal (; 140 meq/L).  It occurs in about 50% of diarrheal cases.

Hyponatremic dehydration

Sodium loss is more than ECF loss
It occurs in 40-45% of diarrheal case
Because of low osmolality (due to reduced sodium) water moves from extracellular to intracellular compartment  → signs of dehydration (wrinkled skin due to loss of elasticity).

Hypernatremic dehydration

In about 5% of cases, especially in a child who has been given fluids with more salts, serum level may be elevated (> 150 mEq/L).
Water moves from inside the cells to the extracellular compartment.
This may mask the physical findings of dehydration and the patients may be undertaken as mild dehydration even in the presence of severe dehydration → complications of severe dehydration may occur due to delay in management.
So, hypernatremic dehydration is more dangerous.
Hyponatremic
Hypernatremic
Isonatremic
Non–diarrhoeal cause