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If you feel discomfort in your inner ear, you may experience unpleasant symptoms such as tinnitus, dizziness, and problems with balance and balance. Abnormalities in the vestibular system, which is part of the inner ear, can be a sign of more serious conditions such as endolymphatic hydrops or Meniere’s disease. If your doctor suspects that these symptoms may be the result of a buildup of fluid behind the eardrum, in the inner ear, or in other parts of the ear, then he or she should refer you for an electrocochleography or an ECOG test. This painless procedure evaluates the functions of the inner ear and its electrical potentials. The results help to diagnose the source of clinical symptoms and the further course of treatment.
Electrocochleography (ECoG for short) is a method of recording electrical potentials generated in the inner ear and auditory nerve in response to sound stimulation using an electrode placed in the ear canal or eardrum. The test is performed by a trained otolaryngologist or audiologist and is used to detect increased inner ear pressure (endolymphatic hydrops) or to test and monitor inner ear and auditory nerve function during surgery.
This test is most commonly used to determine if the inner ear (cochlea) has excess fluid pressure. Excessive fluid pressure in the cochlea can cause symptoms such as hearing loss, dizziness, balance problems, and tinnitus. These symptoms are sometimes indicative of some ear pathology, such as Meniere’s disease or endolymphatic hydrops.
The history of the discovery of electrocochleography
A renowned auditory neurophysiologist named Glen Wever discovered ECoG in 1930. He had a great influence on the auditory sciences, including the later work of Hallowell Davis. As you know, Hallowell Davis is a famous physiologist and otolaryngologist, the father of the physiology of hearing and the inner ear. At Princeton, Wever and Bray recorded auditory activity in an anesthetized cat. Using an oscilloscope, amplifiers, and electrodes, they were able to record activity in the cochlea and in the auditory nerve.
According to this story, they substituted the voice from the speaker to the cat during the recording. The words went from the loudspeaker to the cat’s ear, and through electrodes placed on the auditory nerve, they were able to decipher some of what was said through the loudspeaker. In other words, the cat’s ear functioned almost like a microphone, hence the term cochlear microphone potential (MP).
Shortly after the discovery of MT in cats, the work was published in the scientific journal Nauka, one of the most prestigious journals in the scientific arena at the time. People all over the world immediately read this work and began to reproduce their methods already on people. Hallowell Davis, in particular, found that for him it was more than just microphone potential and auditory nerve activity. There was another answer, which he called the summation potential (SP) of the snail. Doctors began to realize that MP and SP were interesting enough concepts, but the biggest discovery recorded in humans was the action potential (AP) of the auditory nerve.
By the late 1950s and early 1960s, audiologist Chuck Berlin and otolaryngologist Bob Reuben at Johns Hopkins University said they could perform electrocochleography in the operating room on children who could not be tested in any other way. They called it an electrophysiological, objective hearing test. Today, unlike our small computers, they had heavy racks of equipment that they brought into the operating room. To place an electrode near the ear, they had to perform a myringotomy. Myringotomy is a procedure in which a small incision is made in the eardrum. For a child who definitely had a hearing loss, it was necessary to make an accurate diagnosis of the severity of the loss.
In the 1960s, doctors began to realize that electrocochleography recording was not that difficult when using a special transtympanic needle, which is a sharp needle placed through the eardrum. The procedure is invasive and the eardrum is immediately restored. If we place the electrode on the promontorium (cape), we can still record these responses without using a myringotomy.
Al Coates was an otolaryngologist at Baylor College of Medicine from the 1960s to the 1980s. He is tasked with assembling the first test battery for electrocochleography (ECoG). He was well versed in vestibular studies, but also did a lot of work on this study. He showed that these responses could be recorded less invasively from the ear canal. Other scientists doctors around the world began to realize that this research technique is of great value in the diagnosis of Meniere’s disease. Many in the late 80’s and early 90’s began to use these procedures very successfully in the operating room. More recently, electrocochleography has been used very widely, including the diagnosis of auditory neuropathy. She became popular and then disappeared. When otoacoustic emissions (OAE) appeared in the early 1970s, electrocochleography was forgotten by most audiologists. But today, she is probably back for a long time, with too many options for diagnosing hearing loss.
Why do we need such a diagnosis?
ECoG is designed to diagnose Meniere’s disease and, in particular, swelling of the inner ear. It may also be abnormal in a perilymphatic fistula. A common feature linking these diseases is an imbalance in pressure between the endolymphatic and perilymphatic spaces of the inner ear.
The study can also be used to show that the cochlea is normal in individuals who are deaf. The microphonic potential of the cochlea may be normal in auditory neuropathy, as well as other disorders in which cochlear function is preserved but the auditory nerve is damaged. Finally, this examination is also used as an indicator of the temporal threshold shift that can be observed in acoustic ear injury.
Indications for conduction
Usually, electrocochleography is performed with sensations of stuffy ears, the presence of noise and ringing, with a noticeable deterioration in hearing. Manipulation is carried out with:
- paroxysmal dizziness, which is repeated with noise and ringing in the ears, decreased hearing function;
- sensorineural hearing loss in one ear;
- periodic dizziness, unsteadiness and unsteady gait;
- feeling of congestion and tightness in the ears;
- diagnosis of endolymphatic hydrops;
- otitis and otosclerosis;
- ear injuries that could lead to a decrease in the function of the sense organ;
- monitoring the success of the treatment of various ear diseases.
The examination is also used in pediatrics to determine the child’s hearing threshold to differentiate diseases such as sensorineural and conductive hearing loss, to assess hearing in children who have an increased excitability of the central nervous system.
Preparation for research
The procedure itself does not require special preparation. Already before the examination, the doctor examines the ear passages, in other words, does an otoscopy, using illumination and a frontal reflector. The frontal reflector is worn in order to reflect the rays into the ear cavity. If necessary, they can clean the ear canals and shells due to the remnants of earwax. The tympanic membrane is then irrigated with isotonic sodium chloride solution.
Also, before coming to the ear examination, you will be asked to remove facial makeup and thoroughly clean your face. It is better not to apply makeup before the procedure.
If we talk about small children, then it is possible to carry out the procedure with general anesthesia. But before that, it is imperative to discuss such issues in advance with the doctor.
Method of conducting
The patient is invited to a special room isolated from various sounds and asked to lie on his side so that the ear to be examined is on top. Then a special electrode is inserted with an otomicroscope, fixed with clamps with a microphone under the strict guidance of a doctor. Another electrode is left in the region of the 7th cervical vertebra. It is intended for grounding. Another electrode with a negative charge is attached at the level of the mastoid process.
Most often, silver-plated cup electrodes filled with an electrically conductive mass are used.
When all the electrodes are attached, the patient will be asked to relax while listening to the doctor’s electrical messages, in the form of, for example, clicking. It is very important that the patient is relaxed for this test, as any stretching or movement of the muscles can slow down the transmission and evaluation of the results. Before the procedure, the patient should be informed that unusual sensations may occur when the electrode comes into contact with the eardrum. This test does not always require a response from the patient. While the patient is listening to the click, the audiologist will measure the electrocochleography response using a computer that uses the necessary information in the form of setting potentials and amplitude graphs.
The audiologist will collect several responses from the ear and look for the presence of a large signal that contains two components: summation potential (SP) and action potential (AP). Both of these waveform components are a direct result of providing sound stimulation to the cochlea. The SP/PD ratio is calculated. The ratio of these two components is the key to deciphering the test results. An increased ratio may indicate excessive fluid pressure in the inner ear.
The study takes about one hour in the normal course and behavior of the patient. The person may be free immediately after the examination, even if local anesthesia was used. Most doctors avoid using anesthesia to perform the test, as anesthesia increases the risk of accidentally injuring the eardrum.
Rehabilitation period
Upon completion of this study, the doctor may schedule a second examination in two weeks. At a follow-up visit, the attending physician will discuss the results of the ECoG and other aspects of the auditory test with the patient. When the doctor receives the report, he or she will compare it with the results of other studies to determine the next step in treatment. If endolymphatic hydrops or Meniere’s disease is suspected, the doctor may ask for additional tests, such as vestibular evoked myogenic potential (VEMP), electroneurography (ENG), or magnetic resonance imaging (MRI), to confirm the diagnosis.
An important role is played by the visualization of the auditory nerve using MRI. Every patient who hears a diagnosis of auditory neuropathy should have an MRI of the auditory nerve. That’s why studies show that one in five patients actually have cochlear nervous system failure, and sometimes no nerves at all. If there is no auditory brain response, but normal otoacoustic emissions and normal summation potential are recorded, it is very important for the patient to undergo an acoustic magnetic resonance imaging to rule out a cochlear nervous system deficiency. If the patient does not have an auditory nerve, a cochlear implant is not the best option. It is known that such patients may also have intracranial problems. It is the detection of auditory neuropathy that often leads to valuable neurological information about the patient.
So, to sum up, only experience and good clinical knowledge is needed to correctly interpret an electrocochleographic study. When interpreting the ECoG, it is important to consider the level of noise, which is usually estimated by obtaining multiple tests. If they are all the same, then the standard deviation should be small and the result is likely to be correct. If they differ greatly, the reliability of the average SP/PD ratio may be questionable. Similarly, if multiple trials are performed and the ratio varies greatly, then the confidence in the correct result should be lower than if the result were not very different from the previous one.
Contraindications for the diagnosis of ear diseases
Usually, electrocochleography is an absolutely safe procedure, without complications and no contraindications. It is carried out regardless of age. Only when the patient has an allergic reaction to local anesthetics used during manipulation, only in this case their use is prohibited or the anesthetic substance is replaced with another one.
During electrocochleography, the patient may feel some unpleasant uncomfortable moments in the sense organs being studied. But there is no need to worry, since these sensations disappear without a trace and without harm to health after the study.
Advantages of this diagnosis of ear diseases
Diagnosis of ear diseases is a valuable test that helps the doctor identify potential problems with the patient’s inner ear. It is electrocochleography that is a painless, highly informative test for examining the hearing organs, both in the early and late stages. It helps the doctor to make the correct diagnosis and decide on a further treatment plan.