Glycerin is another osmotic solution. Glycerin absorbs water when the 2 are placed in contact with one another and, thus, exerts a hypertonic effect. When glycerin is administered topically to an edematous epithelium, it temporarily clears the edema and its associated corneal haze. The hypertonic effect of glycerin is transient, with its peak effect around 2 minutes. Topical glycerin is painful when applied, so it is advisable to use a topical anesthetic before instillation. Due to its painful nature and its short activity, it is not useful for chronic therapy. However, it is extremely useful to clear epithelial edema to allow visualization when performing gonioscopy or ophthalmoscopy in the office. This is often necessary in patients with acute angle-closure glaucoma. Other than painful stinging, adverse effects to topically applied glycerin are rare.
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