Clinical Electrocochleography in Meniere's Disease: The Value of Low Frequency Stimulus Biasing

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Introduction: The inner ear is a fluid-filled bony cavity divided into 2 parts: the cochlea, which is responsible for hearing, and the vestibular apparatus, which is involved in the maintenance of balance. Meniere’s disease is a clinical syndrome whereby the patient suffers from deafness, tinnitus, ear fullness and vertigo which characteristically fluctuate. It is thought to be due to attacks of increasing fluid in the inner ear (known as endolymphatic hydrops) and currently is diagnosed using clinical history, examination and hearing and balance tests in combination with an electrical test of the cochlea in response to sound known as electrocochleography (ECochG). The changes in the ECochG responses seen in Meniere’s disease are considered to reflect the mechano-electric changes in the inner ear caused by endolymphatic hydrops. There are three main ways that the ECochG can be performed based on the position where the electrode is placed: extratympanic (ET), with the electrode placed external to the ear drum, transtympanic (TT), where the electrode is placed through the ear drum, and round window (RW), which requires a window to be opened in the ear drum for the electrode to be placed, therefore may require general anaesthesia. The closer to the round window that the electrode is placed the more robust the ECochG result is. Transtympanic is the preference of Prof Gibson as it can be done under local anaesthetic but maintains a robust result. The ECochG generates a waveform of which the most useful parts for diagnosing Meniere’s disease are termed the summating potential (SP) and the action potential (AP). There is ongoing debate as how best to use these parts for accurate diagnosis. The SP is thought to reflect the overall change in the electrical gradient in the cochlea created by sound. The basilar membrane is a membrane within the cochlea that is moved by a sound wave and is an important part of sound detection. A theory is that displacement of the basilar membrane is a primary component of generating the SP. If a continuous low frequency sound is transmitted to the ear it causes a shift in the position of the basilar membrane. This is known as low frequency biasing. When the low frequency biasing is transmitted at the same time as a normal ECochG stimulus we would therefore expect to see a change in the SP results, demonstrating that the basilar membrane is mobile. Experiments in guinea pigs using a low frequency bias have confirmed a marked modulation in the SP amplitude. Low frequency biasing studies in humans with endolymphatic hydrops should show less change in the SP because the basilar membrane is already displaced by the disease process and therefore less mobile compared to normal ears. Aim: To determine if the presence of endolymphatic hydrops affects the results of ECochG when low frequency stimulus biasing is introduced. Hypothesis: Low frequency biasing will not affect the basilar membrane as much in those individuals in which endolymphatic hydrops is present, compared to normal ears, because it is already displaced by the disease process present. Potential Significance: This would give the use of ECochG greater diagnostic accuracy for endolymphatic hydrops and Meniere’s disease, making it more useful both as a clinical test and as a method of evaluating the inner ear structural changes occurring in Menie`re’s disease. This may be particularly beneficial for individuals who have unilateral symptoms of Meniere’s where a destructive surgical procedure is contemplated but hydrops in the other ear must be excluded.

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