Why do we test hearing ?Is the hearing normal?What is the degree of hearing loss?What type of hearing loss is it?
Principles of hearingUnits for measuring sound exposure Sound Pressure Level (SPL)• expressed in μPa or Pa• range from 20 μPa (hearing threshold) till 20 Pa (painthreshold), Decibel dB of sound pressure level (dB SPL)• defined as: 20 log10 p1/p0 where p1 is actually measuredsound pressure level of a given sound, and p0 is a referencevalue of 20μPa, which corresponds to the lowest hearingthreshold of the young, healthy ear.• In the logarithmic scale the range of human ear’s audiblesounds is from 0 dB SPL (hearing threshold) to 120-140 dBSPL (pain threshold)
Principles of hearingTable 1: Examples of sound pressure levels in relation tohearing threshold and pain threshold (in dB SPL)The rangeof human ear’s audible sounds goes from 0 dB SPL (hearingthreshold) to 120-140 dB SPL (pain threshold)Source / observingsituationTypical sound pressurelevel (dB SPL)Hearing threshold 0 dBLeaves fluttering 20 dBWhisper in an ear 30 dBNormal speech conversationfor a participant60 dBCars/vehicles for a closeobserver60-100 dBAirplane taking-off for aclose observer120 dBPain threshold 120-140 dB20dB is 10 times 0dB, 40dB is 10 times20dB, 60dB is 1000 times louder than 0dBSource: SCENIHR, Pohealth risks of exposure to nopersonal music players andphones including a music pfunction (2008) , Section 3.3.3
Principles of hearingBone conductionAir conduction
Types of hearing loss Conductive hearing loss Sensorineural Hearing Loss
Types of hearing loss Mixed Hearing Loss Central Hearing Loss
Severity of hearing lossDegree of hearing loss Hearing loss range (dB HL)Normal –10 to 15Slight 16 to 25Mild 26 to 40Moderate 41 to 55Moderately severe 56 to 70Severe 71 to 90Profound 91+Source: Clark, J. G. (1981). Uses and abuses of hearing loss classification. Asha, 23,493–500.The table below shows one of the more commonly used classification systems.The numbers are representative of the patients hearing loss range in decibels (dB HL).
Severity of hearing loss The American Academy of Ophthalmology andOtolaryngology (AAOO) guidelines (Revised in1979) states:“the ability to understand normal everydayspeech at a distance of about 5 feet does notnoticeably deteriorate as long as the hearing lossdoes not exceed an average value of 25 dB at 500,1000 and 2000 Hz”
Hearing Loss Causes Age- The tiny hairs get damaged and are less able to respond to soundwaves. Hearing loss can progress over the course of several years. Loud noise-Exposure to loud noises damage the hair cells in the cochlea.. Infections-During an ear infection, fluid can build up in the middle ear. Perforated eardrum-Depending on the size of the perforation, theremay be a mild or moderate hearing loss. Tumors- eg acoustic neuroma (vestibular schwannoma) and meningioma. Trauma-Injuries such as a skull fracture or a punctured eardrum cancause severe hearing loss. Medications- the aminoglycoside class of antibiotics (streptomycin,neomycin, kanamycin), large quantities of aspirin, chemotherapy drugs(cisplatin, carboplatin), Vicodin (in large quantities), macrolide antibiotics(erythromycin) can cause hearing loss. Genes- Genetic hearing loss often begins with hearing loss diagnosed atbirth
Initial Otoscopic examination Tympanometry BERA EChocG OAE (OtoacousticEmission)Assessment of hearing Speech test Loud Whisper Tuning fork test Weber Rinne Schwabach Audiometry Speech audiometry Pure Tone Audiometry
Initial Otoscopic examination Performed with a hand held otoscope Ear canal and tympanic membraneare observed. Tympanic membrane is seen for:• Light reflection• Differentiation of its part• Mobility
Speech test Simplest of all Involves testing ability to hear words withoutusing any visual information. Patient should repeat 5 words spoken loudly ata distance of approx. 5 metre. The whispered voice test involves the testerblocking one of patients ears and testing hearingby whispering words at varying volumes.
Tuning fork test Used to differentiate between conductive andsensorineural hearing loss. Larger forks vibrate at slower frequency. Tuning forks with frequency 256 or 512 hz are used
Principle of Tuning fork testCHL (OE or ME Disorder) Sounds delivered to the ear via AC will beattenuated If the sound is delivered to the ear via BC,bypassing the OE & ME, then the sound will beheard normally assuming there is no disorderSNHL (OE & ME Are Free From Disorders) Sounds delivered to the ear via BC will also beattenuated
Weber’s Test Vibrating tuning fork is placed on the patientsforehead (or in the middle line). The vibrations are tyransmitted by boneconduction to cochlea. The pt. should state if the tone is heardin the left ear, right ear, or both ears. If the sound lateralizes, the patient may haveeither an ipsilateral conductive hearing loss or acontralateral sensorineural hearing loss
Rinne’s Test Compares the level of air and bone conductionof the same ear. base of a tuning fork is placed to the mastoidarea (bone con.), and then after the sound is nolonger appreciated, the vibrating top is placednear the external ear canal (air con.)
Schwabach’s Test compares the patients bone conduction to that ofthe examiners If the patient stops hearing before the examiner,this suggests a sensorineural loss If the patient hears it longer than the examiner,this suggests a conductive lossThis test is contingent on the examiner having normal hearing…..
Audiometry Audiometry (from the Latin audīre, "to hear", andmetria, “to measure) is a branch of Audiology, andis the science of measuring hearing acuity forvariations in sound intensity and frequency. An audiometer is the device used to produce soundof varying intensity and frequency.Ear Phone to testAir conduction thresholdResponse switchBone oscillator to testBone conduction threshold
Pure Tone Audiometry A pure tone is a tone having a single specificfrequency. The frequency of the tone is determinedby the rate or speed at which the sound sourcevibrates Pure tone audiometry is the use of pure tones toassess an individual’s hearing. The results of this testing are plotted on theaudiogram. Pure tones are generated by an audiometer andpresented to the patient via headphones or, insome cases, through loudspeakers. Can be air conduction audiometry or boneconduction audiometry
Pure Tone Air Audiometry procedure The audiologist present pure tones of one frequency tothe patient, initially at an intensity level that it isassumed they can hear quite well. The intensity (loudness) of the tone is decreased in 10 to15 dB steps. This is continued, with tones being presented for one totwo seconds, until the patient no longer responds. The intensity is then raised in 5 dB steps until thepatient responds, decreased again and increased againin 5 dB steps until the patient responds. This lowest audible intensity is defined as the patient’sthreshold for the particular frequency and is marked assuch on the audiogram
Pure Tone Air Audiometry procedureInteraural attenuation and masking At certain intensity levels, the signal presentedto the test ear will cross over and be heard in thenon-test ear. The inter-aural attenuation rate, orthe intensity difference at which a sound will beheard in the non-test ear, is approximately 40dB for air conduction signals presented throughcircumaural earphones. the masking noise used for pure toneaudiometry is narrow-band noise;generally thesame frequency as the pure tone beingpresented to the test ear.
Pure Tone Air Audiometry audiogramSensorineural impairment Conductive disease
Tympanometry Tympanometry is an examination used to testthe condition of the middle ear and mobility ofthe eardrum (tympanic membrane) and theconduction bones by creating variations of airpressure in the ear canal. In evaluating hearing loss, tympanometrypermits a distinction between sensorineural andconductive hearing loss, when evaluation is notapparent via Weber and Rinne testing
Principles of Tympanometry Introduces a pure tone into ear canal through 3-function probe tip• Manometer (pump) varies air pressure against TM(controls mobility)• Speaker introduces 220Hz probe tone• Microphone measures loudness in ear canal
Normal tympanogram (Type A)• Peak at 0 daPa• Best movement of drum when no extra pressure on either side of TM
Other Type A tympanogramsPeak at 0daPa, butunusually high amplitude? Ossicular disruptionPeak at 0daPa, butunusually low amplitude? Stapes fixation
Flat tympanogram (Type B)When tymp is flat,usually means 1 of 3things:1. Artefact2. Fluid in ME3. PerforationLook at EAM vol.If large = perforationIf normal = fluid
Negative tympanogram (Type C)Can be further dividedinto:C1 – peak between 0and -200 daPaC2 – peak less than -200daPa
Otoacoustic Emission They are low intensity sounds produced byouter hair cells of a normal cochlea Can be elicited by a very sensitive microphoneplaced in EAC (External Ear Canal) Absent when OHC are damaged Thus serve to test cochlear functioning
USES of OAE’s As a screening test for neonates Distinguish cochlear from retrocochlear HL To test hearing in mentally challanged anduncooperative individuals after sedation(Note- sedation doesn’t interferes with OAE’s)
ElectroCochleoGraphy (EChocG) It measures electrical potentials arising in thecochlea and VIII nerve in response to auditorystimuli within first 5 millisec Response is in the form of• Cochlear microphonics (CM)• Summation potential and (SM)• Action potentials (AP) Two methods are widely used:• Transtympanic• Extratympanic
ElectroCochleoGraphy(EChocG)Transtympanic membraneelectrodes: These are needleelectrodes, which are placed over thepromotory by penetrating the eardrumIntrameatal electrodes: Theseelectrodes are placed in the externalauditory canal.
ElectroCochleoGraphy (EChocG) Normal electrocochleogram from the tympanic membrane to clickspresented in alternating polarity at 80 dB HL. The amplitudes ofthe Summating Potential (SP) and Action Potential (AP) can bemeasured from peak-to-trough (left panel), or with reference to abaseline value (right panel). mean SP/AP amplitude ratio to click stimuli for normal subjects isapproximately 0.25 + 0.10 standard deviations (SD). An SP/APamplitude ratio greater than 45 % (2 SDs above the norm) isconsidered to be enlarged.
ElectroCochleoGraphy (EChocG) usage Objective identification and monitoring of meniere’sdisease and endolymphatic hydrops (EH): Enlarged SPreflects pathologic condition of EH. SP-AP amplituderatio is more consistent with the presence of meniere’sdisease. On average, SP-AP ratio of 0.45 or greater isconsidered abnormal. Intraoperative monitoring: Surgical operation onbrainstem or cerebellum may bear risk of damage to theauditory system. ECochG recording during surgeryassess the functional integrity of the peripheral andbrainstem pathway directly during operation.
BERA To be dealt in subsequent seminar by Dr. Saif
Final Thought“Tests are not infallible, they areonly as good as those taking,administering and interpretingthem…”