Mannitol must be administered intravenously because it cannot be absorbed from the gastrointestinal tract. This drug may be given as either an intravenous infusion or an intravenous push. For an intravenous infusion, mannitol may be given as a 20% premixed solution (concentration, 200 mg/mL) over 30–60 minutes. For an intravenous push, a 25% solution may be injected over 3–5 minutes. A too-rapid infusion of mannitol may shift intracellular water into the extracellular space, causing cellular dehydration with a high risk of hyponatremia, cardiovascular overload, congestive heart failure, pulmonary edema, and intracranial bleeding.
Urea is unpalatable and thus is used intravenously. Urea has fallen out of favor because of rebound effects (see the earlier section Actions and Uses) and because of its tendency to cause tissue necrosis when it extravasates during administration. However, intravenous administration of urea produces a rapid onset of action, which is usually desirable.
Both mannitol and urea have been associated with subarachnoid hemorrhage attributed to rapid volume overload of the blood vessels and/or rapid shrinkage of the brain with traction of the subarachnoid vessels. This shrinkage is of particular concern in elderly patients, who may already have brain shrinkage from microischemic disease and are therefore at increased risk of bleeding.
These drugs are cleared by the kidneys and produce marked osmotic diuresis that may be troublesome during surgery. Conscious patients should void shortly before surgery, and a urinal or bedpan should be available. If general anesthesia is used, an indwelling urethral catheter may be required to prevent bladder distension.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.