Audiology 5- Objective diagnostic techniques Flashcards
why do you perform objective hearing techniques?
- estimates hearing threshold in difficult test subjects e.g. babies
- differentiate types of peripheral auditory system damage e.g. is the hearing loss conductive or cochlear or due to auditory nerve damage?
- diagnoses middle ear pathologies
what is a tympanogram?
- the output of performing tympanometry
- graph plotted of pressure against middle ear compliance showing a peak at the maximum compliance
what is tympanometry?
- measures both middle ear pressure and the compliance or freedom of movement of the eardrum and ossicular chain
- useful in the diagnosis of middle ear pathologies
- does not give an estimate of hearing threshold.
What is a type A tympanogram?
- a normal tympanometry
What is a type Ad tympanogram?
- disease or head injury eaten away at the ossicles the pump blows and sucks the ear drum in a long way or far out, the peak ends up large due to more movement of the ear drum
- very high compliance (very mobile ear drum)
- hyper flaccid drum
What is a type As tympanogram?
- If someone has otosclerosis the middle ear system is less compliant (stiffer) so when the pump blows, the ear drum does not move.
- a stiffening pathology
- graph is relatively flat with a small peak
- reduced compliance (increased stiffness)
What is a type B tympanogram?
- tympanic membrane is retracted but thick effusion in the middle ear prevents the eardrum from moving and therefore a flat tympanogram.
- if the negative pressure goes on for a long time, the ear drum is very retracted, the negative pressure draws fluid into the middle ear which becomes thick and sticky resulting in e.g. glue ear.
- When the pump blows and sucks, the ear drum doesn’t move. You see a flat line.
What is a type C tympanogram?
- blocked estuation tube, air in the middle ear is absorbed and not replaced due to blockage or dysfunction in the Eustachian tube.
- Results in a negative pressure in the middle ear, the ear drum is retracted. results in a negative middle ear pressure
- results in a negative peak on the tympanogram
what is otoacoustic emissions?
- otoacoustic emissions are sounds produced by the cochlear which travel back through the middle ear and into the ear canal where they can be measured by a mini microphone.
- OAEs can be evoked and then the sounds generated by the cochlear are in response to an auditory stimuli
- OAEs are a by-product of the way a normal cochlea processes sound.
- in the cochlear, at the basilar membrane, the area most tuned to the sound stimulus, outer hair cells contract exaggerating the motion of the basilar membrane and it is the energy imparted by the outer hair cells that produces OAE
how does otoacoustic emissions work?
- a probe containing a miniature loudspeaker and microphone is sealed into the ear canal using a rubber tip
- following the presentation of a broadband click, the response from the cochlea is picked up by the microphone and amplified
- 260 clicks are normally presented and the responses averaged by a computer to improve the signal-to-noise ratio
what do otoacoustic emissions tell us?
- if the cochlea has been damaged, OAEs disappear, so OAEs are a measure of cochlea health
- tells us if the outer hair cells are functioning normally or not
- Evoked OAEs can be recorded in all ears with normal hearing and cover 500-5000Hz
- they are absent when the average hearing threshold exceeds approximately 30dBHL
- they indicate whether threshold is better or worse than 30dB, making it a useful tool for hearing screening
what is an auditory brainstem response (ABR)?
- an auditory evoked potential
- an ABR is the electrical activity from the auditory pathway in the first 10ms following stimulation
- the response represents the activity from the auditory nerve action potential (Wave I) to the inferior colliculus (wave V)
what does auditory brainstem response measure?
- commonly used to measure hearing threshold in babies
-measures the electrical activity in auditory nerve and brainstem following auditory stimulation
- stimulus intensity is decreased and the test repeated until no trace of the response waveform is seen
- wave V is the most robust
- threshold is taken from the lowest stimulus level where wave V was still apparent
what is the auditory brainstem response procedure?
- sound is played into the ear and electrodes are placed on the head to measure the electrical activity of the auditory action potential as it travels up the auditory nerve into brainstem and into the auditory cortex
-up to 1024 clicks are presented to the ear at a constant rate by a transducer
- electrodes pick up the neural response, which is amplified, filtered and then averaged by a computer
- for hearing threshold measurement, the stimulus intensity is decreased until no trace of the waveform is left
what is the cross check principle
- first described by Jerger and Hayes 1976, states results of any single audiometric test cannot be considered valid without verification from another test
what are the comprehensive audiological assessment at different ages?
birth-6 months
- auditory brainstem response
-otoacoustic emissions
-tympanometry
-behavioural observation audiometry
6-24 months
- visual reinforcement audiometry
- otoacoustic emissions
-tympanometry
- auditory brainstem response
24 months- 4 years
- visual reinforcement audiometry/ play audiometry
- otoacoustic emissions
- tympanometry
- auditory brainstem response
what is screening?
- detects hearing impairment in new born babies
- Picks up all those with hearing loss but only those with hearing loss
- must be quick, cheap and easy to administer and sensitive and specific
what are the two methods used in the New born Hearing Screening Programme?
1- Evoked Otoacoustic emissions
- quick, reliable and cheap method of screening for hearing impairment
2- Automated Auditory Brainstem Response
- screening ABR
- only children who fail OAE screen and NICU babies
