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Background
Meniere's syndrome/Endolymphatic hydrops
Meniere's disease or Meniere's syndrome is a potentially disabling condition involving varying degrees of fluctuating hearing loss, fluctuating tinnitus, episodic vertigo, and aural fullness (a feeling of fullness, pressure and discomfort in the ear). The syndrome may be idiopathic, in which case it is called Meniere's disease, or secondary to various processes that interfere with the normal resorption of endolymph (e.g., neurosyphilis, viral infections, trauma, congenital anomalies, etc.). The disease appears to strike most commonly persons between 30 and 60 years of age, with men and women affected equally. Incidence of the disease is approximately 250 per million populations. Patients with Meniere's disease have a progressive distention of the endolymphatic space of the inner ear, caused by fluid build-up of the endolymphatic space (endolymphatic hydrops), caused either by overproduction or reduced adsorption. The increased pressure exposes cochlear hair cells responsible for sensing movement and balance to progressive damage and paralysis, resulting in attacks of dizziness, often with nausea and vomiting.
Early in the course of disease, these attacks are usually brief (lasting an hour or so), as the damage to the cochlear hair cells is temporary and the hair cells resume normal function when the hydrops resolves. Chronic repetitive attacks may lead to irreversible damage to the hair cells, and hearing loss can become permanent. The hearing loss and tinnitus are usually unilateral, although up to a quarter of patients may go on to develop a severe bilateral disorder.
Transtympanic electrocochleography (ECOG) can be used to confirm cochlear involvement in hearing loss, and is an objective test for endolymphatic hydrops. ECOG measures the ratio of the summating potential (SP) and the action potential (AP) on the most peripheral portion of the auditory system in response to auditory stimuli. The AP is the summed or averaged activity of the action potentials of the auditory nerve, which are elicited by acoustic stimulation. The SP is generated by the hair cells of the cochlea in response to acoustic stimulation. Surface electrodes, such as those used in auditory brainstem response, cannot record these potentials; electrodes must be placed on or through the tympanic membrane. In ECOG, a fine needle is passed through an anesthetized tympanic membrane and placed in contact with the cochlear hair cells of the inner ear in order to record electrical activity from these cells. The ear is exposed to a train of about 1000 click or tonal stimuli, and action potentials from auditory neurons are recorded for 10 milliseconds after each click. This information is recorded and summated by computer. Patients with endolymphatic hydrops have abnormal waveforms (widening of the waveform with multiple peaks). Endolymphatic hydrops is suggested when the ratio of the summating potential to the action potential is greater than 35 percent.
Electrocochleography allows the diagnosis of Meniere's disease to be confirmed or refuted so that appropriate prognostic advice can be given together with medical or surgical treatments if indicated.
In all patients who have unilateral persistent otological symptoms, a MRI is required to exclude acoustic neuroma, which can mimic the presentation of Ménière's disease. Ménière's is confirmed with an electrocochleogram so that appropriate effective treatments can be applied.
Acute attacks of Meniere's syndrome are treated with antiemetics and sedatives. Long-term treatment is usually medical, including rigid salt restriction and diuretics. Occasionally chemoablation (intratympanic gentamycin) or surgical ablation (labyrinthectomy when hearing is already lost, vestibular nerve section when it is not) is necessary for refractory disease.
Perilymphatic fistula
Electrocochleography has also been used to determine the presence of perilymphatic fistula, based on the SP/AP amplitude ratio. A perilymph fistula (perilymphatic fistula, labyrinthine fistula) is an abnormal communication between the fluid-filled perilymphatic space of the inner ear and the air-filled middle ear cavity, usually through the round or oval windows. This results in sensorineural hearing loss and/or vestibular symptoms.
Most commonly, a tear in the round or oval window leads to loss of perilymph into the middle ear. This may be the result of stapes prosthesis surgery, trauma, barotrauma, bony erosion due to infection or neoplasm, or it may be idiopathic. In children, it is associated with congenital anomalies of the middle or inner ear.
Symptoms of perilymphatic fistula are similar to Meniere's disease, and include sensorineural hearing loss, which may be sudden or fluctuating; aural fullness; and vestibular symptoms (vertigo (with or without head position changes), dysequilibrium, motion intolerance, nausea and vomiting, disorganization of memory and concentration, and perceptual disorganization in complex surroundings (such as crowds or traffic)). Tinnitus occurs in some cases, and can be roaring. In absence of prior surgery or definite traumatic event, it may be difficult to distinguish a perilymph fistula from Meniere's syndrome.
In addition to ECOG, other tests that may be used by otologists for the diagnosis of perilymph fistula include audiograms to detect hearing loss and fistula tests. The subjective fistula test is performed by applying positive and negative pressure to the intact eardrum using a pneumatic otoscope. Positive results include the elicitation of nystagmus or onset of dysequilibrium with the sensation of motion or nausea. Some otologists administer the test with electronystagmography or using a specialized platform. Rigid or flexible endoscopy is performed to look for visible tears or fluid in the middle ear. The final diagnosis is made by direct inspection at the time of surgery, with visualization of perilymph fluid in the middle ear cavity.
Medical therapy is rarely reported. There are some reports of spontaneous healing with bedrest, head elevation to 30 degrees, and avoidance of lifting or middle ear pressure-increasing activities. Surgical treatment is available if conservative therapy fails.
Severe sensorineural deafness
Another clinical application of electrocochleography is identification of wave I of the auditory brainstem response (ABR) during combined electrocochleography-ABR testing, as wave I is frequently difficult to detect in patients with profound hearing loss when electrocochleography is not performed in conjunction with ABR testing. ABR testing involves the measurement of responses along the auditory pathway from cranial nerve VIII to the lateral lemniscus of the auditory brainstem. Five distinct electric waveforms generated in the 8th nerve, brainstem, and other regions in response to acoustic stimulation are examined. Wave I is generated at the distal part of the auditory nerve.
Screening for hearing impairment
According to the U.S. Preventive Services Task Force, electrocochleography is not an appropriate test for routine screening for hearing impairment.
Electrocochleography is available in virtually all otolaryngology departments, takes only 20 minutes or so and requires an otolaryngologist and usually an audiologist.
Perilymphatic pressure measurement
Assessment of perilymphatic pressure has also been used to diagnose Meniere's disease. However, published reports do not support a diagnostic role for this approach. Rosingh and colleagues (1996) did not find any significant differences in perilymphatic pressure measurements between patients with Meniere's disease and young normal hearing subjects. This is in accordance with the findings of Ayache and associates (2000) who concluded that assessment of perilymphatic pressure does not seem to be useful in Meniere's disease. Furthermore, Rosingh and co-workers (2000) reported that perilymphatic pressure measured in the affected ear of patients with Meniere's disease or idiopathic sudden sensorineural hearing loss did not differ significantly from the pressure in the non-affected and normal hearing ear. In a follow-up study by Ayache et al (2002), the authors concluded that perilymphatic pressure measurements by means of the Tympanic Displacement Analyzer are not useful in the evaluation of patients with Meniere’s disease.
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