An ECoG test, short for electrocochleography, is a diagnostic procedure that measures electrical signals produced by your inner ear in response to sound. It takes about 30 minutes and is most commonly used to help diagnose Ménière’s disease and other inner ear disorders. The test works by picking up tiny electrical potentials generated by the hair cells and auditory nerve inside your cochlea, giving your audiologist or ENT specialist a window into how well your inner ear is functioning.
What the Test Measures
Your inner ear generates several distinct electrical signals when it processes sound, and an ECoG captures three of them. The first is called the cochlear microphonic, an electrical voltage produced by the outer hair cells in your cochlea as they respond to sound waves. The second is the summation potential, which reflects the combined activity of both inner and outer hair cells. The third is the action potential, the electrical firing of your auditory nerve itself.
The most clinically important measurement is the ratio between the summation potential and the action potential, often written as the SP/AP ratio. In a healthy ear, this ratio typically falls below about 0.35. When the ratio is elevated above that threshold, it suggests excess fluid pressure in the inner ear, a condition called endolymphatic hydrops, which is the hallmark of Ménière’s disease.
Why Doctors Order an ECoG
The primary reason for an ECoG is to determine whether endolymphatic hydrops is present. About 90% of clinicians who use the test cite this as their main objective. Ménière’s disease causes a recognizable set of symptoms: fluctuating hearing loss, ringing in the ears (tinnitus), a feeling of pressure or fullness in the ear, and episodes of vertigo. But because other conditions can produce similar symptoms, the ECoG provides an objective measurement to support or rule out the diagnosis.
ECoG is also used in some cases to monitor how well treatment for Ménière’s disease is working over time. If the SP/AP ratio decreases after treatment, it suggests the excess inner ear fluid pressure is improving. The test can additionally play a role during certain ear surgeries, where real-time monitoring of inner ear electrical activity helps protect hearing.
How the Test Is Done
There are two main approaches to performing an ECoG, and which one you experience depends on your clinic. In the extratympanic method, a small foam-tipped electrode is placed in your ear canal, and additional electrodes are placed on your scalp. This version is completely noninvasive and requires no anesthesia. In the transtympanic method, a thin needle electrode is passed through your eardrum and placed on a bony surface called the promontory, closer to the cochlea itself. This version uses local anesthesia to numb the eardrum first.
Research comparing the two methods in the same patients has found no significant difference in the SP/AP ratios they produce, which means the less invasive ear canal approach gives comparable diagnostic information for most people. Once the electrodes are in place, you’ll sit still while a series of clicking sounds are played into your ear through an earphone. The electrodes record how your inner ear responds to each click. The entire process takes roughly 30 minutes, and you can typically go about your day afterward.
How Accurate the Results Are
ECoG is considered a useful diagnostic tool, but it has limitations. One comparative study found that ECoG correctly identified Ménière’s disease in about 77% of confirmed cases. That means roughly one in four people with the condition may show a normal result, particularly if they’re tested between symptomatic episodes when fluid pressure in the inner ear has temporarily normalized.
Because of this, doctors rarely use ECoG as the sole basis for a Ménière’s diagnosis. It works best as one piece of a larger puzzle that includes your symptom history, hearing tests, and sometimes additional vestibular testing. A positive ECoG result in someone with classic Ménière’s symptoms strengthens the diagnosis considerably, but a normal result doesn’t necessarily rule it out.
How ECoG Compares to VEMP Testing
If you’re being evaluated for an inner ear disorder, your doctor may also mention VEMP testing (vestibular evoked myogenic potential). While ECoG measures electrical activity from the cochlea, the hearing part of your inner ear, VEMP assesses the otolith organs: the utricle and saccule, which are responsible for balance and detecting motion. VEMP is noninvasive and involves playing loud sounds while measuring muscle responses in your neck or near your eyes.
In head-to-head comparisons, ECoG has shown a higher detection rate for Ménière’s disease than VEMP, with positive rates of about 77% versus 58%. This difference likely exists because Ménière’s disease can affect the cochlea and the balance organs independently. Some patients have more cochlear involvement, which ECoG catches better, while others have more vestibular involvement. Using both tests together gives a more complete picture of what’s happening across the entire inner ear.
What To Expect From Your Results
Your results will typically be reported as an SP/AP amplitude ratio along with waveform tracings showing each of the three electrical potentials. Normal values vary somewhat between clinics depending on the specific equipment and technique used, but an SP/AP amplitude ratio above roughly 0.35 to 0.40 is generally considered abnormal and suggestive of endolymphatic hydrops. Some clinics also measure the SP/AP area ratio, which uses a slightly different calculation and has its own set of normal ranges.
If your results come back elevated, your doctor will interpret them alongside your symptoms and other test results. If your results are normal but your symptoms strongly suggest Ménière’s disease, you may be asked to repeat the test during or shortly after a symptomatic episode, when abnormal fluid pressure is more likely to be present and detectable.