Otoacoustic emissions (OAE) are sounds produced by motile elements of cochlear outer hair cells. These sounds can be recorded easily. These sounds were first identified and reported by Kemp in 1978.
The main function of OAE tests is to assess the function of cochlear hair cells. These tests could be used:
1. To screen children and neonates for hearing disabilities
2. Estimate hearing sensitivity within a limited range of frequencies
3. To differentiate sensory and neural components in sensorineural hearing loss
4. To rule out malingering (functional hearing loss)
Since the measurment of otoacoustic emissions are recorded from the external auditory canal, the integrity of the middle ear and the cochlear function come into play.
Role played by otoacoustic emissions: As explained earlier otoacoustic emissions are generated by the outer hair cells of cochlea. These otoacoustic emissions play the role of cochlear amplifier. In sensori neural hearing loss the cochlear amplification is lost leading to:
a. Reduction in the hearing level
b. Reduction in the clarity of spoken words
Types of Otoacoustic emissions:
OAE's are classified according to the stimulus employed to elicit them, or by the mechanism that causes them.
Spontaneous otoacoustic emission: These narrow band continuous signals occur without any stimulus. These signals can be detected in a majority of persons with normal pure tone threshold. The clinical value of this signal is limited, as this is not present in all normal ears. The absence of spontaneous otoacoustic emission does not imply cochlear dysfunction. Synchronised otoacoustic emissions are potentials generated by outer hair cells of cochlea which are synchronised to the external stimuli using time averaging techniques. Measurement of these potentials is difficult and highly cumbersome. They do not have any diagnostic / prognosic value as it is not a consistent feature in all normal ears.
Three types of OAE's have been recorded in response to various stimuli. These are also known as evoked oto acoustic emissions.
Stimulus frequency otoacoustic emission:
These potentials are evoked with some kind of acoustic stimulus. The evoking acoustic stimulus is a pure tone one with a low intensity level. Study of these potentials are still in the experimental stage, and hence not widely used.
Transient otoacoustic emission:
(TOAES) These potentials are also known as transient evoked otoacoustic emission. Since it was first described by Kemp it is also known as Kemp echos. The time delay between the stimulus and response allows the examiner to isolate these responses. These echos recorded from normal ears always mirrors the spectrum of the stimulating sound impulse. The probe used to record transient otoacoustic emission has two openings, one for the presentation of a single stimulus like a click, and the other opening which is used to record the transient evoked otoacoustic emission. The second opening is connected to a microphone to enable recording to take place.
Clicks are commonly used as stimuli, sometimes tone burst stimuli can also be used. The stimulus used should be 80 - 85dB sound pressure level. The rate of stimuli should be atleast 60 / minute. When present TOAES occur at frequencies of 500 - 4000 Hz
Distortion Production Otoacoustic emission:
(DPOAEs): These are low intensity signals that occur during stimulation of the ear. A common way to record them is to present the ear with two continuous signals called the primary tones and analyse the spectrum of sounds detected at the external auditory canal. The intensity levels of the signals used are 55 dB and 65 dB respectively.
Prerequsites for obtaining otoacoustic emissions:
1. Unobstructed external auditory canal
2. Perfect seal of external auditory canal with the probe
3. Optimal positioning of the probe
4. Absence of middle ear pathology
5. Functioning cochlear outer hair cells
6. A relatively still patient
7. Quiet recording environment
Interpretation of the recordings made:
Spontaneous otoacoustic emissions: are commonly found in 50% of individuals with normal hearing. It is generally not seen in patients with less than 30 dB hearing level. If spontaneous otoacoustic emissions are elicited in a patient the cochlea could be assumed to be in good health. These emissions are more bilateral than unilateral. They are more commonly recorded in females than in males.
These spontaneous otoacoustic emissions are not associated with tinnitus because the associated cochlear abnormality causes the SOAEs to disappear.
Transient otoacoustic emissions: are usually used to screen neonates for hearing disabilities. These impulses can be recorded only in response to short and transient stimuli. These impulses have a very limited frequency specificity. The presence of these emissions suggest the cochlear sensitivity in the region of 20 - 40 dB or better.
Distortion product otoacoustic emissions: These emissions have greater frequency specificity. These potentials are useful in detection of early detection of cochlear damage i.e. due to noise or drug exposure.
Causes of absent otoacoustic emissions:
1. Poor probe tip placement
2. Standing waves
3. Cerumen occlusion
4. Vernix caseosa in infants
5. Unco operative patient
Outer ear: Stenosis, otitis externa, cysts etc
Tympanic membrane: Perforations. Grommets usually don't complicate recordings
Middle ear: Otosclerosis, ossicular disruption, cholesteatoma, otitis media
Cochlea:Exposure to ototoxic drugs, Noise exposure
Central auditory disorders don't affect otoacoustic emissions.
Otoacoustic emissions play an important role in screening infants for disorders of hearing.