Electrocochleography (ECochG) is a specialized diagnostic test for the inner ear, providing objective information about its function. This technique records electrical potentials generated within the cochlea and auditory nerve in response to sound stimulation. It is often employed to assess the health of the inner ear and the auditory nerve. The test helps clinicians understand how the ear processes sound and transmits signals to the brain.
Understanding Electrocochleography
Electrocochleography measures distinct electrical potentials produced by the inner ear when stimulated by sound. These potentials include the Cochlear Microphonic (CM), the Summating Potential (SP), and the Action Potential (AP).
The CM is an alternating current voltage that mirrors the waveform of the acoustic stimulus, primarily generated by the outer hair cells within the organ of Corti.
The Summating Potential (SP) is a direct current (DC) potential that arises in response to an alternating current (AC) stimulus. It represents a non-linear response, often reflecting the non-linear vibration of the basilar membrane at higher stimulus intensities.
The Action Potential (AP), also known as the compound action potential, is the summed electrical response of numerous auditory nerve fibers firing synchronously. It is akin to Wave I of the Auditory Brainstem Response (ABR) and mainly originates from nerve fibers in the basal turn of the cochlea that respond to high-frequency sounds.
Conditions Diagnosed by Electrocochleography
Electrocochleography is frequently used to help diagnose and monitor Meniere’s disease, also referred to as endolymphatic hydrops. This condition involves an imbalance of fluid pressure within the inner ear. An abnormal ECochG reading, particularly an elevated Summating Potential (SP) relative to the Action Potential (AP), can indicate this fluid pressure imbalance.
The SP/AP amplitude ratio is the most common measure used in interpreting ECochG results, calculated as the amplitude of the SP divided by the amplitude of the AP. Some research suggests that considering the SP/AP area ratio, which combines both amplitude and duration of the potentials, may improve the diagnostic sensitivity for Meniere’s disease.
Beyond Meniere’s disease, ECochG can assist in diagnosing other conditions. It may be used to identify superior semicircular canal dehiscence (SSCD), a condition where there is a thinning or absence of bone in the inner ear. Additionally, ECochG can help pinpoint the site of lesions in certain types of auditory neuropathy spectrum disorder, categorizing them as presynaptic or postsynaptic. It can also be applied during cochlear implantation to monitor the functional status of the cochlea.
The Electrocochleography Procedure
Before an ECochG test, patients generally do not need to fast or follow strict dietary restrictions. However, it is advisable to inform the healthcare provider about any medications being taken, as some might need to be temporarily adjusted. The patient will typically lie down in a relaxed or sleeping state in a quiet environment for the duration of the test.
During the procedure, electrodes are carefully placed to record the electrical activity. Surface electrodes are positioned on the forehead and earlobes. For more precise measurements, specialized electrodes are used closer to the cochlea. These include extratympanic electrodes, such as TipTrodes or TM-trodes, which are inserted into the ear canal or placed against the eardrum.
Transtympanic electrodes involve a needle electrode passing through the eardrum to rest on the cochlear promontory, which provides the most robust recordings. This method is more invasive and typically requires medical supervision. Auditory stimuli, usually broadband clicks or tone bursts, are presented through insert earphones. Clicks activate a broad frequency range across the cochlea, while tone bursts can provide more frequency-specific information. The test typically takes around 30 minutes to complete.
What the Results Reveal
The data collected during an ECochG test are analyzed to interpret the measured potentials: the Cochlear Microphonic (CM), Summating Potential (SP), and Action Potential (AP).
An elevated SP/AP amplitude ratio is a common indicator of inner ear fluid imbalance, often associated with Meniere’s disease. While a specific cutoff value can vary depending on the electrode type, an SP/AP amplitude ratio greater than 0.45 has been considered a significant diagnostic indicator for Meniere’s disease. The SP/AP area ratio is another measure that considers both the amplitude and duration of the SP and AP components. For instance, an area ratio greater than 1.94 has been considered abnormal in some interpretations.
An abnormal ratio or waveform suggests an issue with the inner ear’s electrical activity or fluid balance. The results are then interpreted by a trained audiologist or ENT specialist, who considers them alongside the patient’s reported symptoms and findings from other diagnostic tests.