Mannitol is the generic name for a Food and Drug Administration (FDA) approved drug used as an osmotic diuretic and a mild renal vasodilator. Mannitol is typically administered intravenously, but can also be taken orally, depending on the purpose. Intravenously, it is used to treat excessive intracranial pressure, oliguria, and to expand openings in the blood-brain barrier. Orally, mannitol is used a sweetening agent in confections for people with diabetes and, in higher concentrations, as a laxative for children. When it is called a “baby laxative,” this is usually a slang term referencing mannitol’s use in cutting methamphetamines, heroine, or other drugs used recreationally.
Chemically, mannitol is composed of carbon, hydrogen, and multiple hydroxyl groups (C6H8(OH)6) to make a sugar alcohol. It is prepared in a solution with water at concentrations or five, ten, fifteen, twenty, and twenty-five percent. Mannitol is not permeable, meaning it will not pass through a cell membrane the way many other drugs will. Because mannitol is prone to making a solution acidic, the FDA has approved the adding of sodium bicarbonate to adjust the pH.
As an osmotic diuretic, mannitol raises the osmolarity in the nephron of the kidney so that more water is taken out of the body to be excreted. Nephrons are small units in the kidney that absorbs nutrients, dumps waste, and regulates the balance of water to soluble substances in the body to create urine. Osmolarity measures the amount of solute, for example sodium, to solvent, water. If the body has too little water, the nephron will not allow much water to pass out of the body, but if the body has too much water, the nephron will pass water and sodium out to be excreted with other wastes as urine. Likewise, if there is a high concentration of solute in the nephron, the body will try to achieve a balance by letting water enter through osmosis.
Mannitol passes into the nephron and raises the osmolarity so more water leaves the body. This is how the drug is used in its capacity to decrease intracranial pressure (ICP), or the pressure between the cranium, the tissue of the brain, and the cerebrospinal fluid. Through drawing water out of the body, mannitol lowers the volume of extracellular fluid, thus relieving pressure. This treatment plan, however, has been subject to doubt as to its true effectiveness. Mannitol’s diuretic property also helps open the blood-brain barrier by shriveling the cells that compose the barrier, so there are larger openings for blood to pass through.
When using mannitol, it is important to be aware of the effects on the body’s sodium and water, so that severe dehydration and hypovolemia may be avoided and preexisting conditions such as hyponatremia aren’t aggravated. The patient should have a heart healthy enough to sustain temporary decrease in extracellular fluid and should not receive blood within a short time period of taking mannitol unless absolutely necessary. If it is necessary, sodium chloride should be added to the mannitol solution to prevent pseudoagglutination. For injectable mannitol, the brand Osmitrol® is commonly used.
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