Electrocochleography (ECoG)

What is Electrocochleography (ECoG)?

Electrocochleography (ECoG) is a diagnostic audiology test that records electrical responses from the cochlea and auditory nerve in reaction to sound stimuli. For that reason, I call it the "ECG of the ear". It is primarily used to assess inner ear function and diagnose conditions such as Ménière’s disease, endolymphatic hydrops, and auditory neuropathy. The test involves placing electrodes near the cochlea to capture these responses.

How is the ECoG test Performed?

ECoG can be conducted using two main techniques:

1. Juxta-tympanic (near the eardrum) electrode 

• A foam electrode or tympanic membrane (TM) electrode is placed on or near the eardrum by the audiologist who performs the test.

• This method is non-invasive and painless, making it suitable for routine use in clinical settings.

• It requires signal averaging (many measurements need to be taken) due to weaker responses compared to the invasive method below.

• Cannot be performed if there is moderate to severe hearing loss (cannot obtain a reliable measurement).

2. Trans-tympanic (through the eardrum) needle electrode

• A fine needle electrode is inserted through the eardrum, resting on the promontory of the middle ear (the bone covering of the cochlea). The needle insertion is usually performed by a neuro-otologist like myself, using an ear speculum through the ear canal (no outside cuts). While I hold the needle in place, the audiologist makes the appropriate measurements. 

• This approach provides high-quality recordings (larger and more precise waveforms) because of its proximity to the cochlea, and allows recording even in the presence of moderate to severe hearing loss.

• However, it is invasive and will cause discomfort. Bear in mind that the needle will have to be inserted twice (once for each ear).

• Thankfully, the recoding only takes 15-20 seconds per ear, so the process is well tolerated by almost all patients. It is important to keep your facial muscles relaxed during the recording despite the discomfort to avoid electrical noise in the recording. 

* The decision to use a juxta-tympanic or trans-tympanic electrode will usually depend on the level of hearing loss. If there is more than 40-45 dB of hearing loss at any frequency (but especially in the low frequencies), the juxta-tympanic electrode will not provide reliable recordings and the needle electrode would be preferred.

General steps for ECoG :

1. The patient lies still in a quiet environment, often in a soundproof booth.

2. The reference electrode is placed on the forehead (sticker electrode, not painful).

3. The ear electrode is selected and inserted (see above).

4. Sound stimuli (clicks or tone bursts) are delivered to the test ear via insert earphones at high intensities

5. Electrodes record electrical responses from the cochlea and auditory nerve. These signals are amplified, filtered, and averaged to produce a clean waveform. Patients are asked to remain still with their eyes closed for accurate results.

6.  The steps are repeated for the other ear (only one ear can be measured at a time).

7. The whole process takes about 30 minutes.

What does the ECoG Measure?

ECoG analyzes three main components:

• Cochlear microphonic (CM): Reflects outer hair cell activity in the cochlea. These hair cells are the cochlea's "amplifier", and produce an electrical current in a healthy ear. This is the noise we record.
  
  
• Summating potential (SP): Represents electrical activity of hair cells or stria vascularis (another part of the inner ear) in response to sound. Thought be a reflection of the inner ear (endolymphatic) pressure.
  
  
• Action potential (AP): Corresponds to the synchronous firing of the hearing nerve fibers. 

How are the ECoG results Interpreted?

The interpretation focuses on specific ratios:

1. SP/AP Amplitude Ratio:

• A higher SP/AP amplitude ratio (>0.4) often indicates conditions like endolymphatic hydrops or Ménière’s disease.
• SP reflects abnormal pressure changes in the inner ear, while AP represents normal nerve firing.

2. SP/AP Area Curve Ratio:

• Instead of just comparing the peak heights (amplitudes) of the SP and AP, this method looks at the total "area under the curve" for each signal, which includes both their size and duration.
• Newer studies suggest that it improves the sensitivity of detecting certain inner ear disorders, particularly Ménière’s disease.
• An area curve ratio > 2.0 is considered abnormal, indicating conditions like endolymphatic hydrops or Ménière’s disease.

What Are The Clinical Applications of ECoG?

ECoG is used for:

• Diagnosing Ménière’s disease and endolymphatic hydrops by identifying elevated SP/AP amplitude ratios (>0.4) or surface ratios (>2.0)

• Evaluating auditory neuropathy by analyzing abnormal AP responses (other tests are also necessary to evaluate for auditory neuropathy).

• Intraoperative monitoring during surgeries involving the auditory system, such as removal of a vestibular schwannoma.

• Enhancing Wave I detection in ABR testing for patients with hearing loss.

Limitations of ECoG

Despite its utility, ECoG has several limitations:

• Invasiveness of the transtympanic methods: Inserting the needle through the eardrum is uncomfortable.

• Technical expertise required: Precise and safe needle electrode insertion and placement requires specialized training. The interpretation also demand skilled professionals. For that reason, the test is not as widely available. Only specialized clinics usually offer it.

• Variability: Results may vary between patients and between ears of the same patient, making interpretation challenging in some cases.

• Limited sensitivity (ability to detect a disease): Studies have shown that ECoG results may not always correlate perfectly with clinical symptoms of Ménière’s disease or other conditions.