RHS 371 Audiology 1 Lecture 7 PDF

RHS 371 – Audiology I Lecture 7 Clinical Hearing Assessment Pure Tone Audiometry – Part 1 Dr Ibrahim almufarrij Prepared by Ms Haifa Basri  Pure tone Audiometry Definition: The procedure by which sensitivity thresholds for each ear are obtained using pure tone stimuli of...

RHS 371 – Audiology I Lecture 7 Clinical Hearing Assessment Pure Tone Audiometry – Part 1 Dr Ibrahim almufarrij Prepared by Ms Haifa Basri  Pure tone Audiometry Definition: The procedure by which sensitivity thresholds for each ear are obtained using pure tone stimuli of different frequencies (250, 500, 1000, 2000, 4000 & 8000 Hz), 6000 Hz have got strong support to be tested in basic Pure Tone Audiometry.  Threshold The least acoustic intensity which causes a sensation of hearing. The lowest intensity of sound that a person needs to detect its presence. Clinically, it’s the lowest intensity level at which the subject responds to the acoustic stimulus at least 50% of the time.  General information The testing signals are presented by either: 1) Earphone for Air-Conduction (AC) 2) Bone vibrator (oscillator) for Bone-Conduction (BC) 3) Loud-speakers for sound (free)-field The AC followed by BC thresholds are first obtained then compared to each other to distinguish between different types of hearing loss. The test should not exceed 20 minutes for more reliable results if not, patients may benefit from having a short break. 1. Air conduction testing  Procedure of air conduction testing Patient preparation Case history and otoscopic examination should precede the test. Any interfering objects should be removed (Hearing aids, eye glasses…etc) Instructions should be given to the patient before placing the earphone. Why? Instructions Patient should be instructed to sit quietly and to interrupt if s/he feels discomfort. Patients with tinnitus should be instructed to ignore their tinnitus and just focus on the tone, and if they find difficulty at any point they should indicate and this should be written and recorded on the audiogram. The first possible solution is to change the stimulus type from pure tone for example to warble or pulsed tone. Instructions could be given in different wording, as long as it’s comprehensible. Written instructions is a good choice; ̵ It can be emailed to the patient prior to the appointment day. ̵ It could be handed to the patient at the reception upon the patient’s arrival. Example of instructions “I am going to test your hearing by measuring the quietest sounds you can hear. As soon as you hear the sound (tone), press the button, raise your hand or node your head. Keep the button pressed as long as you hear the sound. Release the button whenever you are no longer hearing the tone. No matter what is the sound and how faint it is! just press the button as long as you hear and release it when it’s no longer heard. If you feel uncomfortable or pain at any time, stop me. Any questions?”  Response Strategy Cooperative adults and older children: 1. Handheld switch/button. 2. Hand/finger raising. Asking the pt. to raise his hand/finger on the side corresponding to the ear in which they receives the test signal helps to identify the location of the signal Children or difficult-to-test subjects: 3. Playing a game (take one block and put it in a certain game) like in play audiometry. Transducer Placement Now the earphone should be placed on the patient’s ears (either supra- aural or inserts). Avoid ear collapsing when the super-aural earphones is used. Red=Right, Blue=Left Test order Start at 1000 Hz. Start with the better ear: how do you know and why? ̵ From patient complaints in case history and clinical examination. ̵ Start with the better ear, so the patient gets used to the test, understands the concept, and knows the nature of the stimulus that he will hear. Proceed to 2000, 4000, 8000, then 500, 250 Hz. Fill in the mid-octave frequencies (750, 1500, 3000, 6000) Hz if the threshold difference between 2 consecutive frequencies ≥ 20 dB. 3000, 6000 Hz are useful for tinnitus, HF hearing loss and HA fittings. Retest 1000 Hz just for the first ear. Why? Because it’s more vulnerable to mistakes as it’s the first frequency tested. Retest the other frequencies if the threshold at 1000 Hz change by > 5 dB.  Why start testing at 1000 Hz ? 1. One of the mid-range frequencies to which the human ear is more sensitive to than lower or higher frequencies. 2. Has a more familiar pitch to most listeners. 3. Less affected by background noise and physiological noise than low frequencies. 4. The wavelength in relation to the length of the ear canal makes test-retest reliability better. ❖ Test-retest reliability: Measure the consistency of test results from one trial to the next. Stimulus presentation Start at intensity audible to the patient, 30 dB for normal and 30 dB above the estimated threshold for others. Duration of the tone should vary from 1-3 seconds. Interval between the presentations should be between 1-3 seconds, timing should be unpredictable, why? Testing Method Hughson-Westlake procedure used, down 10 up 5. If no response at the initial presentation, increase by 10 dB until the response emerges. If 80 dB is reached and still no response, increase in 5 dB steps and watch your patient for any discomfort (as the patient might have recruitment, abnormal growth of loudness). Once the positive response is seen, decrease in 10 dB steps until the response disappears. Increase in 5 dB steps until the response re-appears.  Decrease by 10 dB and repeat the process until you get 2/2 or 2-3/4 at the same level. This could be taken as the patient threshold at that frequency. Move on to the next frequency and apply the same steps. Use the audiogram to record the results using the correct symbols for right ear, left ear, AC and BC, masked/unmasked. For unmasked AC threshold, use a circle (O) for the right ear and a cross (X) for the left ear. Audiogram Sample of audiogram with AC symbols 2. Bone conduction testing  Bone conduction testing Bone conduction test bypasses the external and the middle ear and stimulates the cochlea directly. Tone will travel to both cochleae and be perceived by the better one; therefore without masking, it’s not possible to know which ear has detect the signal.  Signal transmission through BC Hearing by bone conduction is resulted from the interaction of 3 events (pathways) : 1. Osseo-tympanic stimulation: Movement of the skull leads to vibration of the air column in the ear canal initiating AC response. 2. Inertial stimulation: Inertia of the Ossicular-chain lag relative to the skull movement causes stapes to move in and out of the oval window into the cochlea similar to AC. 3. Distortional/Compressional stimulation: Direct stimulation of the cochlea. When the skull is set into vibration by TF or BV, it becomes distorted and leads to distortion of cochlear structures, which in turn initiate electrochemical activities relative to that in AC root.  Procedure of bone conduction testing Instructions Patient should be instructed same as in AC. Emphasize the need for response whenever the tone is heard regardless in which ear the tone was heard. Transducer placement Bone vibrator placed either: 1. Over the mastoid of the worse ear (per AC), close to but not touching pinna! and avoid touching the hairline! Not touching pinna! As it might lead to stimulation of AC root and leads to false responses. Avoid touching the hairline! To avoid sliding of the BV from its place and giving inaccurate results and even make the patients uncomfortable during the test time. OR 2. On the middle of the forehead. Mastoid process placement is probably chosen because: 1) BC tones louder at the mastoid process for normal hearing persons, 2) Each mastoid process near to ear being tested, Negative against: The notion that placing the BV on the right mastoid is stimulating the right ear and vice versa is not true as both cochleae are stimulated equally. Advantages of Forehead placement: Artifacts and test-retest differences are less in Forehead placement than mastoid one. More accurate in Ossicular chain abnormality. Easier to fix in its place. Eyeglasses need not to be removed. AC stimulation is more in mastoid placement, due to its near placement to the ear canal. Disadvantages of forehead placement: More intensity is needed to stimulate a normal-hearing person (about 10- 14 dB greater). This site requires a set of correction values, which are available in BS EN ISO 389-3. However, with available disadvantages of mastoid process placement, it’s Test order Test frequencies: from 500 to 2000 Hz. (250 Hz and 4000 and 8000 Hz not tested!) Test order : 1000, 2000, then 500 Hz and no need to retest at 1000 Hz. Which ear to start with is not of importance, Why? Because both enclosed within the same skull and the response will be by the better cochlea, cannot specified which cochlea responding if no masking applied  Stimulus presentation Start the test at AC threshold level for each frequency. Testing method Same as AC method (down 10, up 5). Stop the test and record the result as NR when: For AC, usually at machine limits and no response, or if the patient indicates or feeling discomfort record the result as NR. For BC, The NR should be recorded at lower intensity levels (55 dB at 500 Hz, 70 dB at 1000, 2000 Hz) since at levels greater than these levels, the patient may perceive the tone as vibration (vibrotactile) and not as a sound.  Bone vibrator limitations Testing at frequencies below 500 Hz is not recommended because the subject's threshold might reflect hearing at the second or third harmonic instead of the fundamental frequency. Bone-conduction tests above 2000 Hz are also discouraged due to transducer limitations and calibration issues. However, in certain exceptional cases, testing at these lower or higher frequencies may be necessary depending on the specific investigation. At higher frequencies (3000 and 4000 Hz), airborne sound is generated by the bone vibrator, this may lead to a false air-bone gap (improve the BC threshold). Using ear plug may help to prevent this problem. Ear plug shall not be used at lower frequencies ≤ 2000 Hz as the occlusion effect may reduce the thresholds at these frequencies and become better.  Occlusion effect (OE) Covering the ear by any means (earmold, earphone, earplug) during BC testing, a stronger signal will reach the cochlea, this boost is known as OE. It usually happens at 1000 Hz or less. So occluded BC thresholds are better than unoccluded ones. OE happens only when the cartilaginous part of the ear is covered not the bony portion only. OE is absent in CHL cases. Only can be experienced by normal hearing subjects or with SNHL. OE uses : ̵ Used clinically to help in diagnosing CHL as in Bing test. ̵ Used also to determine the amount of masking needed in BC testing.  OE values The amount of OE can simply be found by calculating the difference between unoccluded and occluded BC thresholds. At 500 Hz: OE = 20 dB (supra-aural headphone) and 10 dB (insert earphone) At 1000 Hz: OE = 10 dB (supra-aural headphone) and 0 dB (insert earphone) At 2000 and 4000 Hz: 0 dB  OE implications for test procedure At low frequencies ≤ 1000 Hz, when masking required keep the non-tested ear (NTE) covered with the ear phone and do not close the tested ear by any mean At 3000 & 4000 Hz, ear plug should be used to block the TE while the ear phone covering the NTE (to deliver the noise), why? (to prevent BC thresholds enhancement (decreased) at these frequencies, fault ABG resulted)  Some tips about Bone Conduction People with normal AC thresholds have normal BC thresholds. In other words, BC thresholds are always the same or better than AC thresholds. Sometimes, BC thresholds are 10 dB poorer than AC thresholds and this may be seen because of allowable variability. But BC thresholds should not be significantly poorer than AC thresholds. In case of greater differences, re-testing with careful attention to bone vibrator placement, if still a significant difference is there reinstruct the patient, and if the problem persists check the device calibration. BC thresholds are always normally tested unless AC thresholds are 0 dB HL or better.  Final step After both AC and BC thresholds are obtained, they should be recorded on the audiogram using the correct symbols. The audiogram will be used to diagnose the possible type and degree of hearing loss the patient has. The diagnosis is done by: 1. Comparing AC and BC thresholds. 2. Calculating the Pure Tone Average, sometimes might be enough to show the degree of hearing loss impact on the patient’s communication. Audiometric symbols Questions?