Audiometry Dr. Vishal Sharma
Pure Tone Audiometry • 5 up, 10 down technique used with single frequency tones to find hearing threshold. • 2 correct responses out of 3 is acceptable. • Air conduction measured for 1K, 2K, 4K, 8K, 500, 250 & 125 Hz via head phone. • Bone conduction measured for 1K, 2K, 4K, 500 & 250 Hz via bone vibrator. Masking of other ear. • Normal hearing for AC & BC is at 0 dB.
Pure Tone Average Calculated by taking arithmetic mean of air conduction thresholds at 500, 1000 & 2000 Hz (speech frequencies)
Low frequency conductive HL Otitis media with effusion
High frequency SNHL Presbyacusis, ototoxicity, acoustic neuroma
Uses of pure tone audiogram 1. To find type of hearing loss 2. To find degree of hearing loss 3. For prescription of hearing aid 4. Predict hearing improvement after ear surgery 5. To predict speech reception threshold 6.A record for future medico-legal reference
Speech Audiometry Speech Reception Threshold (S.R.T.):Minimum intensity at which 50% of spondee (disyllable with equal stress) words are correctly identified. S.R.T. is normally within 10 dB of Pure Tone Average. Speech Discrimination Score (S.D.S.):Percentageof phonetically balanced (single syllable) words correctly identified at 40 dB above S.R.T.
Speech Audiometry PB max Score:Maximum SDS at any intensity. Uses of Speech Audiometry • Differ b/w cochlear & retro-cochlear lesions. • Volume of hearing aid fixed at PB max score • In functional deafness: SRT > + 10 dB of pure tone average.
Tests for Recruitment Recruitment is abnormal growth in perception of sound intensity. Tests of recruitment are done to diagnose a cochlear pathology. Tests used are: 1. Short Increment Sensitivity Index (SISI) Test 2. Alternate Binaural Loudness Balance (ABLB) Test
S.I.S.I. Test (Jerger, 1959) • Continuous tone given 20 dB above hearing threshold & sustained for 2 min. • Every 5 sec, tone intensity increased by 1 db and 20 such blips are given. • SISI score = % of blips heard. • 70-100 % in cochlear deafness • 0-20 % in conductive & nerve deafness
A.B.L.B. Test (Fowler, 1936) Pure tone is presented alternately to deaf & normal ear. Intensity heard in normal ear is adjusted to match with deaf ear. Test started 20 dB above threshold in normal ear & repeated with 10 dB raises till loudness is matched in both ears. Initial difference is maintained, decreased & increased in conductive, cochlear & retro- cochlear lesions respectively.
Threshold Tone Decay Test • Olsen & Noffsinger (1974) • Detects abnormal auditory adaptation due to nerve fatigue caused by a retro-cochlear lesion. • Pure tone presented 20 dB above hearing threshold, continuously for 1 min. If pt stops hearing earlier, intensity ed by 5 dB & restart. • Test continued till pt hears tone continuously for 1 min or intensity increment (decay) > 25 dB
Impedance Audiometer Probe A = oscillator (220 Hz). B = air pump C = microphone to pick up reflected sound
Impedance Audiometry 1. Tympanometry 2. Acoustic reflex (Stapedial reflex) Principles of Tympanometry a. Less compliant T.M. reflects more sound. b. Maximum compliance of T.M. denotes equal pressure in E.A.C. & middle ear.
Type B (fluid in middle ear) EAC volume = 1.8 ml
Type B (T.M. perforation, grommet) EAC volume = 3.2 ml
Type B (E.A.C. obstruction) EAC volume = 0.4 ml
Acoustic Reflex Loud sound > 70 dB above hearing threshold, causes B/L contraction of stapedius muscles, detected by tympanometry as se in compliance.
Uses of Acoustic Reflex 1. Objective hearing test in infants & malingerers 2. Presence of reflex at <60 dB above threshold is seen in cochlear lesion due to recruitment 3. Reflex amplitude decay of > 50 % within 10 sec is seen in retro-cochlear lesion 4. Absence of reflex seen in facial nerve lesion proximal to stapedius nv & in severe deafness 5. I/L reflex present, C/L absent in brainstem lesion
Electro-cochleography Measures auditory stimulus related cochlear potentials by placing an electrode within external auditory canal / on tympanic membrane / trans- tympanic placement on round window. 3 major components: a. Cochlear microphonics:from outer hair cells b. Summating potential:from inner hair cells c. Compound Action potential:from auditory nerve