CSD 309: Basic Audiology Week 7 Day 1 PDF

10/3/24 CSD 309: Basic Audiology Week 7 Day 1 Otoacoustic Emissions 1 What is an otoacoustic emission? Low-level sounds generated by the cochlea R...

10/3/24 CSD 309: Basic Audiology Week 7 Day 1 Otoacoustic Emissions 1 What is an otoacoustic emission? Low-level sounds generated by the cochlea Reflect OHC activity (cochlear amplifier) Measured with a sensitive microphone placed in the ear canal Types: Spontaneous Evoked Transient Evoked (TEOAEs) Distortion Product (DPOAEs) + = 3 1  10/3/24 Physiology OAEs are a by-product of the active process in a normal cochlea. Outer hair cells are the most probable origin because OAEs are absent in cases with outer hair cell loss. Any signal presented to the ear has two components: Forward transmission Backward transmission 4 Transmission pathways Forward: The signal proceeds Backward: When the signal through the auditory system toward reaches the cochlea, the active the brain in an efficient manner. process generated by the outer hair cells causes a “ripple” that moves backward along the traveling wave. That “ripple” is transmitted back through the ossicular chain in the middle ear and then converted to an acoustic signal by the tympanic membrane. 5 2  10/3/24 Physiology continued…Outer Hair Cell Motility The acoustic signal (referred to as an acoustic emission) is very small, ranging from 0 to 20 dB SPL, and is directly related to the level of the stimulus. Emissions only occur in healthy cochlea The outer hair cells need to be functioning normally in order to produce emissions 6 Technology The technology used to detect emissions in the ear canal is similar to that of tympanometry. A probe assembly is seated in the ear canal. The probe contains a loudspeaker (tone generator) and a microphone. 7 3  10/3/24 Technology The stimuli are presented through the tone generator into the ear canal. The microphone picks up the low-level acoustic emissions coming back from the cochlea and routes them to a computer for signal averaging. 8 Transient OAEs (TOAEs) These OAEs are elicited using a click stimulus. Clicks stimulate a wide range of frequencies along the basilar membrane. Produce OAEs across a wide range of frequencies. They occur about 4 msec after the onset of the stimulus and they last about 10 msec. Usually present at frequencies between 1000 and 5000 Hz in normal ears. Absent at frequencies with hearing thresholds >30 dB HL. 9 4  10/3/24 Transient Evoked (TEOAE) Tone Generator (dB SPL) Amplitude (dB SPL) Time (ms) Analyzer Microphone (dB SPL) 10 Distortion Product OAEs (DPOAEs) Distortion Product OAEs are evoked by presenting two tones simultaneously. The two tones interact on the basilar membrane to produce a third tone. The third tone can be calculated based on the frequency relationship between the two tones. 2F1-F2 F1 = 4200 Hz F2 = 5000 Hz 2(4200) - 5000 = 8400 - 5000 = 3400 Hz F2 is the higher of the two frequencies (F2>F1). Emissions are present for frequencies between 1000 and 8000 Hz in ears with normal hearing and absent in ears with hearing thresholds >40 dB HL. 11 5  10/3/24 2. Distortion Product (DPOAE) Tone Generator (dB SPL) Amplitude (dB SPL) Time (ms) Analyzer Microphone (dB SPL) 12 OAEs and Hearing Loss OAEs are best used as a screening Can use results to tell if there is or is not a tool and as a part of the diagnostic cochlear insult at specific frequencies. test battery Does not indicate the degree of loss (i.e., Newborn hearing screening threshold). Difficult to test patients Only know that thresholds are better than Functional hearing loss 30 dB HL for TEOAEs or better than 40 Also used to monitor changes in dB HL for DPOAEs for specific frequency cochlear function regions. Ototoxicity Also, OAEs are reduced or absent in Progressive hearing loss cases of significant middle-ear disorders: Noise exposure Negative pressure Middle-ear fluid 13 6