It is much more difficult to test the hearing of non-cooperative patients, such as young children, who should receive assistive listening devices as soon as possible if they are to achieve normal speech development. Objective hearing tests are also important for making expert assessments, which must state fairly and objectively how an accident, working in a noisy environment or other circumstances may have damaged a person’s hearing. This necessitates objective audiometry, which is based on the measurement of reactions in a patient’s ear after stimulation with one or more tones. The results are analysed by an experienced audiologist.
The middle and inner ear are first tested using objective measures. Impedance audiometry is an important tool: this involves testing the eardrum, the ventilation of the middle ear, middle ear effusion (i.e. fluid build-up), and the function of the ossicular chain. Another important tool is otoacoustic emissions measurement, which investigates the function of both the middle and inner ear.
The way in which sound travels as far as the auditory cortex can be tested using electrical reaction audiometry. This can be done using various methods. One problem is (still) unsolved: frequency-specificity, i. e. differentiating at which frequencies there is a hearing loss. However, the methods available make it possible to test where the hearing loss is located and how serious it is. There are some approaches to achieving frequency-specificity, but these (still) require measurement over long durations and deliver less accuracy. This is therefore the subject of international research. One promising approach here is Auditory Steady-State Response (ASSR).
Impedance audiometry measures the acoustic resistance of the middle and inner ear, i. e. whether sound travels through the middle ear uninterrupted or whether there are any obstacles to its passage.
Tympanometry: The eardrum vibrates best if the pressure is equally distributed on both sides of it (in the middle ear and in the outer ear canal). Where there is optimum vibration of the eardrum with the same amount of pressure on each side, most of the sound is passed on, with little reflected back. The amount of sound that is reflected back therefore indicates whether the pressure is the same on both sides of the eardrum, which allows conclusions to be drawn as to whether the patient may have middle ear ventilation problems, fluid in the middle ear or a defect of the eardrum.
Measurement of stapedius reflexes: A stapedius reflex is a form of protection that the ear applies to protect itself from loud noise by reducing the movement of the ossicular chain in the middle ear. The measurement is carried out to examine if, and at which loudness level, the reflexes are elicited.
As pointed out in the description of suprathreshold audiometry, hair cells either amplify or decrease the volume of sound in the inner ear. The sound is not only channelled in the direction of the auditory nerve, but part of it is echoed back to the outside where it can be picked up and recorded by sensitive microphones. The term used for this reflected sound is ‘otoacoustic emissions’. If emissions are present, this reliably indicates the near-normal functioning of the middle and inner ear. However, if no emissions are present, there may be various reasons for this – it does not necessarily mean that an individual’s hearing is impaired.
Although these emissions are always evoked as part of the normal hearing process, for diagnostic purposes they must be elicited in a controlled environment. The principle is that, after presenting a tone stimulus to the ear, measurements are taken to determine whether there is a response. However, if a continuous tone was used, the input and output tones would have the same frequency and could not be differentiated. This is why two different approaches are used.
The eliciting of transient evoked otoacoustic emissions (TEOAE) involves using brief-duration tones and waiting for a response after the tone has died away. Distortion product otoacoustic emissions (DPOAE) exploit the phenomenon that two continuous tones whose frequencies must have a certain ratio generate a third (distorted) tone, which is used to establish whether a response is present or not. As DPOAE can serve only to establish severe hearing loss, TEOAE is usually the preferred option. TEOAE are an integral part of neonatal hearing screening.
Sound travels through the outer, middle and inner ear and is converted into electrical signals in the inner ear. These signals are passed on to the brainstem and the auditory cortex via the auditory nerve. In an evoked response audiometry test, the ear is presented with a series of tones. Electrodes are used to measure when and how the electrical impulses (called potentials) arrive at the auditory brainstem and, subsequently, the auditory cortex. This allows highly reliable conclusions to be drawn as to the location and severity of hearing impairment. It is important that the tested individual lies as motionless as possible throughout the duration of the test.
The standard method used here is the auditory brainstem response (ABR) test, also referred to as the recording of auditory evoked potentials. This procedure is very precise and reliable, although it has the disadvantage that only high frequencies can be tested. ‘Auditory evoked’ means that the responses (potentials) are elicited using acoustic stimuli (tones). In the standard ABR test, these tones are click stimuli. ABR can be applied to an individual who is either awake or asleep (or, alternatively, sedated), which is a great advantage for testing small children.
The largest problem of all objective measurements is that, although only one frequency should be tested at a time, a short tone (such as a click stimulus) contains many different frequencies. A continuous tone contains only one frequency, but does not allow any differentiation between the stimulus and the response. There are various approaches for recording individual frequencies by using other stimulating tones or masking certain frequencies (notched noise ABR or tone pip ABR). A promising approach is chirp ABR, which allows testing high and low frequency spectrums individually.
Cortical evoked response audiometry (CERA) involves recording signals in the auditory cortex (the part of the brain that recognises sound). However, this test has the disadvantage that it can be carried out only if the patient is awake and lying still.
Auditory steady-state response (ASSR) is a test for which an individual is presented with a continuous tone consisting of two frequencies. The potentials are described as ‘steady-state’ because they are continuous and the recording takes place after the tested individual has heard the tone for several seconds’ duration. As with all objective measures using continuous tones, it is difficult to differentiate between the input and output tones. A carrier frequency (the continuous tones that elicit ASSR) is used and a modulation frequency is modulated onto the carrier frequency. For example, if the carrier frequency is 1,000 Hz and the modulation frequency is 80 Hz, this means that the continuous tone presented to the ear has the frequencies 920 Hz, 1,000 Hz and 1,080 Hz. The ‘trick’ with ASSR is that at the brainstem, where the potentials are recorded, potentials are evoked with the frequencies 80 Hz and 1,000 Hz. This means that, if such a tone is presented and a potential of 80 Hz is recorded, it can be concluded that the individual is hearing the carrier frequency of 1,000 Hz.
ASSR works best, and produces the most reliable outcomes, if the tested individual lies as motionless as possible. Ideally the person should be asleep (voluntarily or with sedation). ASSR is the subject of international research because the measuring equipment available today does not as yet provide ideal measurement accuracy.