Osmotic diuretics are able to reduce endolymphatic pressure and volume, and hence improve peripheral auditory and vestibular function. After baseline audiometric testing, a glycerol, urea or other osmotic diuretic is administered. Repeat audiometric testing is performed is performed at 3 hours (and sometimes at 1 and 2 hours) post-ingestion. The test is considered positive if: (i) there is a 10 dB or more improvement at 2 or more frequencies (250 to 2,000 Hz), or (ii) there is a 12 % or greater improvement in speech discrimination scores. The test is associated with a number of unpleasant side effects, including headache, nausea, thirst, diarrhea, emesis, diuresis, and dizziness.
Because dehydration tests are relatively specific for endolymphatic hydrops, they may be useful in confirming the presence of disease in patients with atypical presentations. However, because the tests are relatively insensitive, they are not useful to rule out endolymphatic hydrops or as screening tests for the disease.
Although the tests appear relatively specific for endolymphatic hydrops, they are relatively insensitive. Snyder (1974) reported the experience using the glycerol test in 122 patients with a combination of sensorineural hearing loss and tinnitus or vestibular symptoms, in whom endolymphatic hydrops was considered a diagnostic possibility. Fifty percent of patients ultimately found to have endolymphatic hydrops had positive tests. One false-positive was found among the positive tests. In a series of 95 patients with Meniere's disease, Akioka et al (1990) found 47 % to have a positive glycerol dehydration test. Stahle and Klockhoff (1986) reported 60 % of patients with Meniere's disease were found to have positive tests, and that positive tests were only found in ears with Meniere's disease.
Dehydration tests have not been proven to be useful in selecting patients with endolymphatic hydrops who are most likely to respond to surgery. Some authors have suggested that patients with positive glycerol tests are more likely to have beneficial responses to endolymphatic sac decompression, but statistical proof of such a relationship is lacking.
Whether a Meniere's disease patient will have a positive test or not seems to depend in part on the phase of the disease. Tests are more likely negative very early and very late in the course of disease, although the stage of the disease is not predictable from the results of the dehydration testing.
Critics of dehydration testing note that the test is unpleasant, not adequately sensitive, impractical, and subject to significant placebo effects. According to Fagan (1999), dehydration studies are little used these days because they are unpleasant and time consuming. There is only anecdotal evidence that positive responders to dehydration tests may be more likely to respond to endolymphatic sac decompression. Some investigators have found that the results of dehydration testing are highly affected by suggestion to the patients as to what they should expect. These investigators suggest that the use of the dehydration test to select patients for surgery risks induces a bias toward more placebo responders.
In a critical review of diagnostic testing in endolymphatic hydrops, Arts et al (1997) concluded: "At this point, the clinical use of dehydration testing is unclear at best. Despite many legitimate questions with regard to its practicality and sensitivity, there is considerable evidence that a real phenomenon, specific for endolymphatic hydrops, underlies this test. Given this, the test may be helpful in verifying the suspicion of endolymphatic hydrops in patients with atypical presentations. It is unlikely, however, that the choice of therapy will be altered by the results of this test in many instances."
Note: Osmotic diuretics such as urea and isosorbide that can be taken orally have also been used as treatment for endolymphatic hydrops.