Different Tests Used to Detect Hearing Loss
In order to hear normally, the three parts of the earthe outer, middle, and inner earmust work together. The outer ear canal directs sound waves toward the tympanic membrane (eardrum), which conducts them to the three connected bones in the middle ear. These bones then vibrate to conduct sound into the inner ear, where a coiled tube called the cochlea converts sound vibrations into nerve impulses and passes them to the 8th cranial nerve (acoustic nerve) leading to the brain.
Hearing tests are designed not only to detect the presence of hearing loss, but also to determine where in this process a hearing problem originates. Impaired sound transmission through the outer and middle ear is known as conductive hearing loss; dysfunction of the cochlea, acoustic nerve, or higher nerve pathways is termed sensorineural hearing loss; both conditions together are known as mixed hearing loss.
A thorough hearing evaluation usually involves several of the following tests:
Pure tone audiometry, the standard test of hearing level, measures the ability to hear tones at different pitches and volumes—first as the sound is transmitted through air, and then as it travels through vibrations of the bone. By comparing these two types of sound conduction, pure tone audiometry can help to differentiate between conductive, sensorineural, and mixed hearing loss.
The speech recognition test assesses your ability to hear conversational speech by measuring the lowest volume at which you can recognize a series of two-syllable words at least 50% of the time.
The word recognition test evaluates how well you understand single-syllable words that are more difficult to hear (sometimes in the presence of background noise). Poor word recognition results from the sound distortion caused by hearing loss and may be improved by a hearing aid.
Acoustic immittance tests measure the flow of sound energy into the ear and the resistance to that flow, and are valuable for evaluating the eardrum and middle ear for possible conductive hearing loss. In tympanometry, a special probe is used to alter air pressure in the middle ear and measure how sound conduction responds to this change. The acoustic reflex test measures the involuntary contraction of muscles in the middle ear in response to a loud sound (acoustic reflex); this reaction serves to protect the ear from potentially damaging noise levels. Acoustic immittance not only helps to confirm conductive hearing loss, but also may help to diagnose problems affecting the acoustic nerve (which initiates the acoustic reflex).
Otoacoustic emissions (OAEs) are measurable sounds generated by a healthy cochlea in response to sound stimulation; they are produced by the movement of tiny cells, called hair cells, on the cochlear surface. In otoacoustic emissions, a thin probe simultaneously emits a series of clicking noises into the ear and, with a tiny microphone, records the resulting OAEs produced by the cochlea. The absence of OAEs indicates cochlear damage. OAEs are typically measured to determine whether sensorineural hearing loss is due to cochlear dysfunction, or to problems beyond the cochlea affecting the acoustic nerve or brain (known as the retrocochlear area).
The auditory brainstem response (ABR) is the involuntary electrical response that starts in the cochlea, travels through the acoustic nerve and the brainstem, and finally arrives in the hearing centers of the higher brain every time a sound is heard. In auditory brainstem response, clicking sounds or tones are used to stimulate the ABR thousands of times; these responses are measured by electrodes and transmitted to a computer that records and analyzes them. The resulting data help to evaluate the function of your sensorineural hearing tissue and identify the location of any cochlear or retrocochlear lesions.
Purpose of the Hearing Tests
- To reveal the presence, type, and extent of hearing loss
- The acoustic immittance, otoacoustic emissions, and ABR tests may also help to determine the cause of a hearing problem by identifying lesions or dysfunction in particular regions of the ear.
- The speech recognition test assesses hearing sensitivity with or without hearing aids.
- Pure tone audiometry determines the presence or absence of hearing loss. If hearing loss is detected, the audiologist determines the type and degree of hearing loss.
Who Performs Hearing Tests
- These tests are usually done by a doctor who specializes in hearing disorders (audiologist).
- The ABR test may also be performed by a neurologist or neurology technician.
Special Concerns about Hearing Tests
- Pure tone audiometry should be performed at least 16 hours after exposure to loud noises, such as loud music or heavy workplace noise. In addition, the presence of tinnitus (ringing in the ear) may interfere with the results of this test.
- Because disorders of the middle or inner ear may disturb equilibrium as well as hearing, balance tests are sometimes performed in conjunction with hearing tests.
Before the Hearing Tests
- Wash your hair on the night before ABR testing, and do not consume coffee, tea, or caffeinated soft drinks for at least 4 hours before the test.
- Before pure tone audiometry, your ears may be checked for excess wax blockage. If necessary, you may be sent to a physician for wax removal.
What You Experience during Hearing Tests
Pure tone audiometry:
- You will enter a soundproof room and put on earphones.
- A series of tones is played into one ear. You are instructed to give a signal, such as raising a finger or pressing a button, to indicate that you heard the sound. The volume of the tones is lowered progressively until you can no longer hear them, and will then be intensified and lowered several more times. This process is then repeated in the other ear.
- Next, you will remove the earphones and put on a special headband with a small plastic piece that sits behind your ear and conducts sound vibrations through the bone. The series of tones is repeated, and you are again asked to signal when you hear them. This is done in one ear, then in the other.
- In some cases, background noise will be played into one ear to ensure it is not helping the other to hear the tones.
- The test takes about 20 minutes.
Speech and word recognition tests:
- You will wear earphones in a soundproof booth. (Speech recognition testing often follows pure tone audiometry.)
- For speech recognition, you will hear a series of common, two-syllable words in one ear. You will be asked to repeat every word as you hear it. The volume of the words is lowered progressively until you can no longer hear them, and will then be intensified and lowered several more times. This procedure is repeated in the other ear.
- Word recognition testing follows a similar procedure, but uses one-syllable words. In some cases, the words will be presented first in quiet and then in the presence of background noise.
- Each of these tests takes about 10 minutes.
Acoustic immittance tests:
- A special probe is inserted into one ear so that no air can pass in or out of the ear; it feels like an earplug and may cause minor discomfort. (In some cases, silicone putty may be applied around the ear opening to ensure an airtight seal.)
- During tympanometry, the probe will change the pressure in your middle ear. (In rare cases, this may induce transient vertigo; tell the examiner if this happens.)
- For the acoustic reflex test, you will hear a loud tone emitted by the probe, but you do not need to respond. A loud noise may then be presented continuously for 5 to 10 seconds.
- Do not move, speak, or swallow as these tests are being performed. The examiner will warn you when to expect loud tones so you will not be startled.
- These tests take about 2 to 3 minutes each.
- A soft rubber probe is inserted into your outer ear canal and you will hear a series of clicking noises.
- You should sit quietly during the test.
- The test takes about 5 minutes.
Auditory brainstem response:
- You will lie on a bed or reclining chair. The skin on your forehead may be rubbed with a mild abrasive pad or gel to remove excess dry skin, and a special electrode gel is applied to improve electrical conductivity.
- Electrodes are placed on your forehead and on each earlobe, and then the lights are usually dimmed.
- Earphones are placed over your ears, and you will hear a long series of tones and clicking noises in one ear, and then the other.
- The electrodes and paste are removed with warm water and a washcloth.
- Testing usually takes less than 1 hour.
Risks and Complications of Hearing Tests
After the Hearing Tests
- You may return home and resume your normal activities.
- Any electrode paste residue after ABR testing can be removed easily by washing your hair.
Results of Hearing Tests
- Your doctor will review the test results, and consider them along with your symptoms, the visual inspection of your ear, and findings from other tests to reach a diagnosis. In addition to disease or injury affecting ear structures, possible causes for hearing loss include degenerative changes in the ear associated with aging, certain drugs, or repetitive exposure to loud noises. The extent of hearing loss, its treatment, and your prognosis depend on the nature of the problem.
- If your hearing tests, symptoms, and physical exam suggest the presence of a lesion along the hearing pathway, your doctor may recommend an imaging test of the head and neck, such as an MRI or a CT scan, to determine the precise type and location of the lesion.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media