**4. Otoacoustic emission**

An otoacoustic emission (OAE) is a low-level sound emitted by the cochlea either spontaneously or evoked by an auditory stimulus. Specifically, OAEs provide information related to the function of the outer hair cells (OHC) [31].

The use of OAEs in the assessment of patients with Meniere's disease has been well documented. According to Van Hufflen et al., patients with Meniere's disease can essentially be divided into four categories. It has been documented that OAEs present in ears with minimal hearing loss and absent in ears where pure tone thresholds exceed 60 dB HL. Both these scenarios are acceptable clinically. Among patients who have hearing thresholds in intermediate range of 30–60 dB HL two scenarios emerge. In the first scenario, OAEs are measurable in large values, while OAEs are not measurable at all. It has been postulated that variable patterns of OAEs in patients with Meniere's disease may be due to more than one precise sites of lesion. It was postulated by Hall that among patients in whom OAEs are present despite a hearing impairment more than 30 dB, the audiometric picture is not representing outer hair cells. He said that in these patients, the outer hair cells may have been spared and data are representative of inner hair cell function [32].

A number of studies have been conducted to determine the utility of OAE and changes in OAE during glycerol test undertaken to diagnose endolymphatic hydrops. In a study, TEOAE and DPOAE were measured before and 3 h after oral administration of glycerol in 22 years of patients with Meniere's disease. The positive result in the glycerol test was observed in 11 of 22 ears. However, of the two OAEs, DPOAE was considered more appropriate than TEOAE for monitoring during the glycerol test because of its high sensitivity in detection of changes in cochlear function. The study concluded that clinical use of OAE, especially DPOAE, as a test complementary to pure tone audiometry during the glycerol test is very useful and will improve the diagnosis of endolymphatic hydrops [33].

The worth of DPOAE to spot minimal inner ear dysfunction which might be due to endolymphatic hydrops and may otherwise go undetected on pure tone testing has been emphasized by a few clinical studies. A study specifically examined a set of patients whose only presenting complaint was fullness of ear/ears. The study used glycerol test with PTA and DPOAE to diagnose subjects at an early stage of Meniere's disease and may have the potential to progress toward the full-blown disease. It was concluded that those patients in whom the only symptom is fullness of ear/ears may potentially be in early stages of the condition [34].

A study was conducted to follow up the dynamics of pure tone threshold and DPOAE amplitude changes induced by glycerol with reference to its activity in inner ear; 38 patients with Meniere's disease and having positive glycerol test were included in the study. It was concluded that audiometry and DPOAE measurements in the glycerol test procedure are the most profitable after the third hour since glycerol administration due to the most significant outcomes of both at this time. Observed changes in pure tone audiometry concern lower frequencies and in DPOAE middle frequencies [25].
