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Audiometric tests Air conduction • This test assesses sensitivity when the signal is transmitted through the outer, middle, and inner ear and then to the cortex. Testing may be performed using headphones, insert earphones. Bone conduction • This technique assesses sensitivity when the signal is transmitted through the bones of the skull to the cochlea and then through the auditory pathways of the brain. This type of testing bypasses the outer and middle ear. Masking Masking presents a constant noise to the nontest ear to prevent crossover from the test ear. The purpose of masking is to prevent the nontest ear from detecting the signal , so only the test ear can respond. Pure tone • A pure tone is a single frequency tone with no harmonic content (no overtones). This corresponds to a sine wave. Pure tone Pure tone Pure tone audiometry • An audiometer - electronic device which produces pure tones, the intensity of which can be increased or decreased in 5 db PURE TONE AUDIOMETRY Pure tone Audiometry • In a sound proof room person is seated comfortably. • Ear phones are applied which are color coded. (Red for right ear, Blue for left ear.) • Masking sound is delivered to the non-test ear. • Start with a frequency of 125Hz. & 0 dB. • Gradually increase the dB. till person hears the sound & respond. • Mark the threshold intensity on the audiogram paper. Contd… • Find the threshold of hearing from 125 Hz. to 8000Hz. & mark on the audiogram paper. • Join the points to make air conduction audiogram. • Place the bone vibrator over the mastoid process. • Deliver the sound through the vibrator & find out the threshold of hearing for different frequencies of sound. Contd… • Use different sign to mark the bone conduction audiogram. • Select the other ear and repeat the whole procedure. Audiogram • The audiogram is a chart of hearing sensitivity with frequency charted on the X- axis and intensity on the Y-axis. Intensity is the level of sound power measured in decibels; loudness is the perceptual correlate of intensity. CHL SNHL Speech audiometry • Speech reception threshold Minimum intensity at which 50% of words are correctly repeated Spondee words (baseball, sunlight) Intensity varied in 5db steps Normal – within 10db of pure tone average • Speech discrimination score Measure of ability to understand speech Phonetically balanced words ( pin, sin) at 30 – 40 db above SRT Normal – 90 – 100% Impedance audiometry Acoustic reflex Acoustic Reflex Pathways • Ipsilateral Right ear Left ear • Contralateral Probe right Probe left Uses • • • • • To test hearing in infants and small children To find malingerers To detect cochlear pathology To detect facial n lesions To detect lesions of brainstem Special tests for hearing • Recruitment • Phenomenon of abnormal growth of loudness • Seen in lesions of cochlea Short increment sensitivity index • A continous tone of 20 db above threshold is presented and sustained for 2 minutes • Every 5 seconds tone is increased by 1db and 20 such blips presented • Cochlear deafness – 70 – 100 % • Retrocochlear – 0 – 20 % Tone decay test • Measure of nerve fatigue • Useful in retrocochlear hearing loss • Normally a person can hear a tone continously for 60 seconds. In nerve fatigue he stops hearing earlier • Tone of 4000hz 5db above hearing thresholdfor 60 s • Intensity is increased by 5db till it is heard for 60 s continously • Result is expressed as number of db of decay • >25 db – retrocochlear loss Electrocochleography • Measures electrical potentials in cochlea and VIII n in response to auditory stimuli • Response – • Cochlear microphonics • Summating potential • Action potential • Ratio – SP: AP <30% • Increase in ratio – Meniere’s disease. Auditory brainstem response (ABR) • BERA/ BAER • Auditory brainstem response (ABR) is a neurologic test of auditory brainstem function in response to auditory (click) stimuli. • It is a set of seven positive waves recorded during the first 10 seconds after a click stimuli. They are labeled as I - VII • Auditory brainstem response (ABR) typically uses a click stimulus that generates a response from the hair cells of the cochlea, the signal travels along the auditory pathway from the cochlear nuclear complex to the inferior colliculus in mid brain generates wave I to VIIwave V. ORIGIN OF BAEP Origin of each wave Wave Origin I Cochlear nerve II Dorsal & Ventral cochlear nucleus III Superior olivary complex IV Nucleus of lateral lemniscus V Inferior colliculus VI Medial geniculate body VII Auditory radiation(cortex Intepretation • Latency • Amplitude • Morphology Uses • As a screening procedure for infants • To determine the threshold of hearing in infants • Diagnosis of retrocochlear pathology • Diagnosis of brainstem pathology • To monitor VIIIn intra operatively Oto acoustic emissions • Low intensity sounds produced by outer hair cells of cochlea • They are absent when outer hair cells of cochlea are damaged • OAE is normal in case of retrocochlear hearing loss • • • • • Types Spontaneous Evoked Transient Otoacoustic Emissions (TOAEs) Distortion Product Otoacoustic Emissions (DPOAE) Uses • As a screening test in infants and to test hearing in unco operative individuals • To distinguish cochlear from retrocochlear heaing loss • Useful to diagnose auditory neuropathy THANK YOU