should be cautioned not to drink water or other fluids after administration of the agent because doing so may reduce the osmotic gradient.24

Oral agents

Orally administered hyperosmotic agents are slightly less effective and have a slower onset of action than do the intravenous agents.24 Variable absorption from the gastrointestinal tract makes their effect less predictable. These differences are not great, however, and the oral agents are safer and less likely to produce volumetric overload in patients with borderline cardiac status. Oral agents are not well tolerated by patients with nausea and vomiting.

Glycerol

Glycerol (Glyrol, Osmoglyn), the most commonly prescribed

hyperosmotic agent, is usually administered as a 50% solution in a dose of 1.5–3 ml/kg7,25 (Table 28-1). (Glycerol is also available as a

75% solution.) It begins to lower IOP in 10–30 minutes, reaches

a maximum effect in 45–120 minutes, and has a duration of effect of 4–5 hours.7,25,26 Glycerol has an intense, sweet taste and is more

palatable when given in an iced unsweetened fruit juice or cola base. If necessary, glycerol can be administered repeatedly because it penetrates the eye and other tissues poorly and is confined to the extracellular water.

The major disadvantage of glycerol is the relatively high frequency of associated nausea and vomiting. Glycerol is metabolized in the liver and produces 4.32 kcal/g of energy.The caloric value of glycerol and its metabolites as well as the osmotic dehydration it produces can lead to ketoacidosis and other problems in diabetic patients.27,28

Isosorbide

Isosorbide (Ismotic, Hydronol) is a dihydric alcohol formed by the removal of two molecules of water from sorbitol. It is an effective

oral hyperosmotic agent, administered as a 45% solution in doses of 1.5–4 ml/kg29–31 (see Table 28-1). Its time of onset and duration

of action are similar to those of glycerol.32 Isosorbide is absorbed rapidly from the gastrointestinal tract and is excreted unchanged in the urine. It must be given in somewhat larger doses than glycerol to produce a comparable effect on IOP. Isosorbide is more expensive than glycerol.

Isosorbide is less likely to produce nausea and vomiting but more likely to produce diarrhea than is glycerol.33,34 Because isosorbide is not metabolized and is excreted unchanged in the urine, it does not produce any calories and is thus a better choice for diabetic patients.34 Unfortunately, because of relatively infrequent usage, isosorbide is no longer manufactured in the United States and obtaining it may be difficult or impossible. It is possible to confuse isosorbide with isosorbide dinitrate (Isordil), which is used to treat angina.35

Ethyl alcohol

Ethyl alcohol may be an effective oral hyperosmotic agent when administered as straight spirits or diluted with appropriate mixers to a final dose of about 1.0–1.8 ml/kg of absolute alcohol (about 1–2 ml/kg of a 40–50% solution (80–100 proof  )). Alcohol also induces hypotonic diuresis by inhibiting production of antidiuretic hormone.This prolongs and increases the osmotic gradient.Alcohol enters the eye rapidly, but vitreous penetration is sufficiently delayed to create an osmotic gradient.The effects of alcohol on the CNS as well as on the gastric mucosa limit its chronic use. It is important to know about alcohol’s effect on IOP because a low IOP after a three-martini (or even a one-martini) lunch may not be representative of other afternoon pressures. Like glycerol, ethyl alcohol is metabolized, producing calories that may be a problem for diabetic patients. Its use in this context is limited to emergency situations in which other, more appropriate agents are not available.

Intravenous agents

Intravenously administered hyperosmotic agents produce a more rapid onset of action and a slightly greater effect than do agents