Auditory dehydration testing: glycerol versus urea.

Auditory dehydration testing with oral hyperosmolar substances is commonly used in the evaluation of patients with suspected endolymphatic hydrops. Endolymphatic hydrops is assumed to be temporarily reduced in some cases, resulting in an improvement in hearing. Unfortunately, ingestion of glycerol, the most frequently used oral agent, often causes severe headache, vomiting, or both. Urea has recently been used as an alternative. A study of patients with Meniere's disease was designed to compare the effectiveness of glycerol versus urea in inducing a temporary improvement in hearing thresholds. Three hundred patients received either glycerol or urea. Fifty-seven percent of the glycerol patients had positive hearing results compared with 77% of the urea patients. Temporary hearing improvements following urea ingestion may not be caused by a rise in serum osmolality alone, since this occurs with glycerol but not with urea.