The term dehydration tends to be used in a sloppy fashion by many docs, and most medical trainees.
Literally it means lack of water. In practice, though, isolated water loss or lack of water is uncommon and is almost always associated with lack of salt (sodium). So it is the rare patient who lacks only water but has adequate salt (sodium).
The distinction is important from a clinical perspective since the symptoms and treatments are different.
Lack of water leads to elevation in the concentration of sodium in the blood. This leads to changes in brain function and can cause confusion (delirium), coma, and even death. It tends to occur in people who are unable to access water and/or people who can’t tell others they’re thirsty. The classic example would be someone with dementia or with a brain injury such as a stroke.
On the hand, lack of salt (sodium) leads to lower blood/plasma volume (‘hypovolemia’) and thence low blood pressure. The kidneys are particularly susceptible to damage in this situation, but any organ can suffer. The concentration of sodium in the blood can be low, normal, or high depending on whether or not there is associated water deficiency (and depending on whether there has been a separate stimulus to hold on to pure water, i.e. it is ADH mediated).
Treatment of isolated salt (sodium) deficiency is usually by giving ‘normal saline’, an IV solution containing physiologic concentration of sodium. By and large, this is done quickly, over hours. Giving such a solution does not correct the associated water loss (if any).
Treatment of pure water deficiency with IV water (actually glucose and water) must be done slowly, over days. The reason for this is that during the period of water deficiency (when there were high blood concentrations of sodium), there would have been a tendency for water to leave the brain (following the osmotic gradient). To avoid this, the brain generates new osmoles in order to hold on to its water. If you give too much water during such a circumstance it will quickly lower (dilute) the concentration of sodium in the blood. If this happens, the brain (with its new osmoles) will act like a sponge and suck water from the blood. This leads to brain edema (swelling) which can be fatal.
As a rule, hypernatremia%20relative%20to%20electrolyte%20content.) (high blood concentrations of sodium) means lack of water.
Hyponatremia (low blood sodium) indicates an excess of water in the body relative to sodium, and almost always occurs NOT because of excess water intake but because the body is holding on to the normal amount of water it takes in. Indeed, if everything else in the body is working normally, a person needs to ingest about 12 liters of water a day to become hyponatremic. That is a huge amount.