Hearing
Audiology
How is hearing tested?
A standard audiometric test battery consists of:
- Pure tone audiometry
- Speech audiometry
- Immitance audiometry
- includes tympanometry and acoustic reflex testing
The battery is designed to:
- Measure the quietest level the patient can hear pure tones and speech
- Obtain a general idea of the patient’s ability to discriminate speech presented at a clearly audible intensity level
- Provide information on where in the auditory system any hearing loss is coming from
Tests are carried out in a sound-treated booth and the results are recorded on an audiogram.
Follow this link for a good description of how to read an audiogram
Pure tone audiometry is normally carried out at octave frequencies from 250 Hertz (Hz) through to 8000 Hz because this is the frequency range that is most important for hearing speech. For each frequency, tones are varied in intensity until the threshold, the quietest level at which the patient responds 50% of the time, is obtained.
Pure tone testing is usually carried out by both air conduction and bone conduction in order to determine where in the auditory pathway any hearing loss may be coming from. Air conduction testing is usually carried out using earphones and each ear is tested separately. Sometimes, if the patient cannot or will not wear earphones, testing is carried out in the sound field using speakers. Because ears cannot be tested separately in sound field testing the results are considered to represent the hearing of the better ear only.
Bone conduction testing is carried out using a bone oscillator placed on the patient’s mastoid bone. In contrast to air conducted sounds, which travel through the outer and middle ears before reaching the cochlea in the inner ear, bone conducted sounds travel through the skull directly to cochlea. When thresholds obtained by bone conduction are 10 or more decibels better than thresholds obtained by air conduction, the hearing loss is assumed to have a conductive component involving the middle or outer ear. When the air and bone conduction thresholds are within 5 decibels of each other the hearing loss is assumed to be sensorineural, involving the cochlea and/or auditory nerve.
Follow this link for a good description of classification of hearing loss
Speech testing is carried out by air conduction. Speech reception thresholds measure the minimum intensity level a patient can recognize familiar two-syllable words, and word recognition testing, carried out at a level that is clearly audible to the patient, measures the ability to discriminate words. Speech reception thresholds help to verify the accuracy of the pure tone thresholds, and should be within 5 decibels of the average of the pure tone air conduction thresholds at 500, 1000 and 2000 Hz. If there is a significant difference between the speech reception thresholds and the pure tone averages, there may be a non-organic component to the test results, or there may be an error in the testing. Word recognition scores give an idea of how clear speech is for the patient at suprathreshold, clearly audible presentation levels.
Behavioural Audiometric testing of pure tones and speech for young children or patients unable to perform conventional audiometry is often still possible using conditioned play audiometry or visual reinforcement audiometry. In play audiometry children 18 months to about 5 years can be conditioned to perform a play task such as putting a block in a bucket or a peg in a board in response to the test tones and speech signals. Visual reinforcement audiometry uses visual reinforcement in the form of lights or toys in a lighted box to condition infants as young as 6 months to turn their head to speech and tonal stimuli presented through headphones or speakers. For infants under 6 months testing is sometimes carried out using behavioural observation audiometry. In this case testers look for startle or other repeatable behavioural responses to tonal or speech stimuli. While it is often possible to obtain results that are within 5 decibels of the child’s true thresholds with conditioned play or visual reinforcement audiometry, test results obtained with behavioural observation are often unreliable.
Immitance audiometry consists of tympanometry and acoustic reflex testing, and gives information on how the middle ear is functioning.
Tympanometry measures the pressure and mobility of the middle ear system. In this test a probe with a rubber tip is used to seal the ear canal. A probe tone is played into the ear canal while pressure is applied and varied, usually from-200 dPa to +200 dPa. As the pressure varies, the amount of the probe tone reflected back into the probe varies. Values for the mobility of the middle ear system and pressure within the middle ear can be recorded by measuring the amount of the probe tone reflected back. Conditions such as fluid behind the tympanic membrane and ossicular fixation will result in an abnormally stiff system. Disarticulation of the ossicles or a thin monomeric tympanic membrane can result in a hypermobile middle ear system. Eustachian tube dysfunction can result in abnormally high or low pressure in the middle ear with respect to the surrounding atmosphere.
Acoustic reflexes are measured using the same equipment used for tympanometry. The ear canal is sealed and high intensity tones are played into each ear along with the probe tone. If the high intensity tones stimulate an acoustic reflex the stapedial muscle contracts and stiffens the middle ear system. When the middle ear system stiffens more of the probe tone is reflected back into the probe, and the acoustic reflex is considered to be present. The pattern of presence or absence of acoustic reflexes can give information on whether the hearing loss is coming from the middle ear or the inner ear, or from the auditory nerve.
Electrophysiological Testing
Electrophysiological testing is commonly carried out on children who are too young or too sick to undergo behavioural audiometric evaluation, or adults who cannot or will not carry out the tasks required for behavioural testing. Electrophysiological testing does not directly test hearing but does evaluate the function of different components of the auditory system and allows the tester to make accurate estimates of the patient’s hearing abilities. Currently the most common forms of electrophysiologic testing are Otoacoustic Emission (OAE) tests and Auditory Brainstem Response (ABR) tests.
Otoacoustic Emission testing measures the function of the outer hair cells in the cochlea by measuring Otoacoustic Emissions (OAEs), which are very small sounds generated by vibration of the outer hair cells. The presence of these sounds is measured with a small probe placed in the ear canal. There are four types of OAEs: Spontaneous, Transient, Distortion Product and Sustained Frequency OAEs.
Follow this link for a good description of Otoacoustic emissions
Transient and Distortion Product OAEs are most commonly used in clinical testing. Transient OAEs (TOAEs) are elicited by presenting short duration stimuli such as clicks or tone bursts into the ear canal while Distortion Product OAES (DPOAEs) are elicited by presenting two simultaneous tones of different frequencies. The presence of TOAEs or DPOAEs in a particular frequency band suggests that hearing thresholds in that frequency band are 30 decibels or better. Absence of OAEs can be caused by middle ear pathology (eg otitis media) or by cochlear pathology involving the outer hair cells.
The Auditory Brainstem Response (ABR) is an evoked response test which measures the electrical response of the auditory nerve to short stimuli such as clicks or tone bursts, using small electrodes placed on the skull and earlobes. Because this test can be performed while the subject is sleeping it is useful in testing infants or small children who cannot perform the tasks required in behavioural audiometric tests. For this reason it is commonly used in newborn infant screening and in further evaluation of infants or children with suspected hearing loss. However, it is a test of the function of the auditory nerve and not a hearing test, and should therefore be performed in conjunction with behavioural testing wherever possible.
Follow this link for a good description of ABR testing
Description |
Contraindications |
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Conventional Behavioural Audiometric Evaluation
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Carried out in a sound treated booth using headphones, bone oscillator and/or sound field.
Typically evaluates patient's ability to hear pure tones at 250, 500, 1000, 2000, 4000, and 8000 Hz in each ear. Thresholds between 0 and 20 decibels are considered to be in the normal hearing range. Also evaluates speech reception thresholds and word recognition.
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Patient too young or sick to perform the tasks Uncooperative patient |
Other methods of auditory Testing |
Description |
Special Behavioral Audiometric tests for young children or adults unable to perform conventional audiometry |
Behavioural Audiometric testing for young children or patients unable to perform conventional audiometry is often still possible using conditioned play audiometry or visual reinforcement audiometry. In play audiometry children 18 months to approximately 5 years can be conditioned to perform a play task such as putting a block in a bucket or a peg in a board in response to the test tones and speech signals. In visual reinforcement audiometry infants as young as 6 months old can be operantly conditioned to turn their head to speech and tonal stimuli presented through headphones or speakers in the sound field. |
Electrophysiology
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Electrophysiology consists of measurements of:
Electrophysiological methods do not test hearing directly, but they can be used to evaluate the auditory function in children too young or too ill to participate in behavioral assessment2
OAEs and DPOAes reflect the function outer hair cells and are suited to provide reliable information about chances in these structures after cisplatin administration8
Good correlation has been shown between OAEs and behavioural audiometry in Cisplatin treated patients9
DPOAEs could detect hearing loss before it was identified by behavioural methods in a study on pediatric medulloblastoma patients8
Auditory Brainstem Response testing can be carried out while the patient is sleeping and can provide a reliable estimate of frequency specific tone thresholds between 500 and 4000 Hz.
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Relevant Links:
American Speech-Language Hearing Association
