Dehydration is defined as a condition where the water level in the body is insufficient. If the water loss is approximately 1% of the total body weight, it falls under the dehydration. The importance of water in the body is great as all the cells are able to function only if the body has sufficient water. Water also plays the role of excretion of toxins and wastes from the body. The common causes of dehydration are diarrhoea and vomiting. Extreme heat can also cause depletion of the water levels in the body.
What is tonicity and osmosis?
When a solution is able to attract the water molecules from one side of the membrane to the other, it is called as tonicity. If both the solutions have the same properties then it is an isotonic solution. The presence of solutes in the solution will determine its tonicity. The main component responsible for this movement of water in and out of the cells is Sodium (Na)
When water moves from an area of lower tonicity to higher tonicity through a semi permeable membrane, it is called as osmosis. The tonicity is dependant on the concentration of the active substances like sodium within the solution. The exact measurement of the substances that are osmotically active in a given solution is called as Osmolality and its unit of measurement is mOsm/L (milli osmoles per litre)
Normal values of blood plasma osmolality – 285 to 195 mOsm/kg
Types of Dehydration
There are three types of dehydration-hypertonic, isotonic and hypotonic. The classification is done based on the effects on the extracellular fluid that is present between the blood and the cells.
In case of dehydration, the levels of sodium get altered and so the osmolality of the extracellular fluid in the body changes leading to increase or decrease in the fluid retention capacity. Depending on the type of dehydration, the treatment will vary so we shall discuss each type in detail.
(1)Isotonic Dehydration: This type of dehydration is also called as isonatremic or iso molar dehydration. The patient loses water and salt from the body in equal amounts and so the level of sodium in the extracellular fluid remains the same and there is no change in the tonicity. This is the commonest type of dehydration that is seen
Causes: Isotonic dehydration is mostly a result of severe diarrhoea and vomiting where the patient loses a lot of water from the body. Other causes include cholera, excess sweating due to very hot climate and profuse bleeding.
Diagnosis: The diagnosis is done based on history of severe diarrhoea or vomiting. Lab investigations in this case are:
- Blood Tests will show normal osmolality that is 285 to 295 mOsm/kg. The sodium levels will be 130 to 150 mmol/litre which is slightly less than the normal range of 135 to 145 mmol/L
- Urine tests show an increase in the specific gravity as the urine gets concentrated. The total urine volume will be considerably reduced.
(2) Hypertonic Dehydration: The other names for hypertonic dehydration is hypernatremic or hyperosmolar dehydration. It occurs when the amount of water loss from the body is more compared to the sodium loss. As a result, the sodium concentration in the cells and extracellular fluid increases making it hypertonic. This increased tonicity leads to movement of water from the cells to the extracellular fluid. The result is shrinkage in the cell size and consequently the shrinking of brain cells.
Causes of hypertonic dehydration
- Water deprivation
- Hyperventilation
- Profuse sweating
- Diarrhoea in young children especially infants
- Diabetes insipidus
- Heat stroke
- Renal failure
- Accidental drinking of sea water
- Hypertonic solution infusion given by mistake
- Diuretics
- Mistreated diabetes mellitus where the patient has polyuria , osmotic dieresis and diabetic ketoacidosis.
Diagnosis
- Blood tests show osmolality that is greater than 300 mOsm/kg. In hypernatremia, the sodium levels will be greater than 150 mmol/litre and the Blood urea nitrogen will be greatly increased.
- Urine tests show a decrease in the urine output over 24 hours and increase in the urinary specific gravity.
- Reduced glomerular filteration rate (GFR)
(3)Hypotonic Dehydration: Hypotonic dehydration is also called as hyponatremic or hypoosmolar type of dehydration. It happens when the loss of sodium is greater than water. The decrease in sodium leads to reduced tonicity and so the extracellular fluid becomes hypotonic as compared to the fluid within the cells. So water begins to move into the cells from the extracellular spaces; leading to swelling of the cells. The swelling in the brain leads to cerebral edema which is a life threatening condition.
Causes of hypotonic dehydration
- Diarrhoea and vomiting
- Gastric obstructive diseases
- Heat stroke
- Muscle damage
- Burns
- Accidental hypotonic fluid infusion
- Hyponatremic fluid given to children or marathon runners to treat dehydration
- Ketonuria
- Addisons disease
- Malnutrition
- Cystic fibrosis
- Nephropathy
- Renal (kidney) tubular acidosis
- Use of diuretics for a long time.
- High blood pressure is treated with thiazide diuretics along with a low salt diet
- Mannitol
Diagnosis
- Blood tests show osmolality less than 250 mOsm/kg and sodium levels less than 130 meq/litre signifying hyponatremia
- Urine tests show increase in the 24 hour urine output, decrease in the specific gravity and sodium levels of less than 20 meq/litre.
- Increased liver enzymes and pancreatic enzymes
Dehydration complications
Most cases of dehydration are not associated with electrolyte imbalance but if the water loss is more then the following complications may occur depending on the salt that is disbalanced:
- Hyponatremia
- Hypernatremia
- Hyperkalemia
- Hypokalemia
- Hypercalcemia
- Hypermagnesemia
- Hypocalcemia
- Hyperphosphatemia