Tests of hearing:-
- Finger friction test
- Watch test
- Speech test; whisper and conversational speech
- Tuning fork tests
- Pure-tone audiometry
- Impedance audiometry / tympanometry
- Brain-stem evoked response audiometry
Finger friction test, watch test, clinical speech test and tuning fork tests are often used for hearing screening or bed side testing.
Tuning-forks test :–
256, 512 and 1024 Hz frequency, vibrating tuning forks are placed in front of ear (to test air conduction), behind the ear and over the forehead (to test bone conduction; here sound is directly transmitted to the cochlea) to get an idea of hearing, and degree & type of hearing impairment if any.
The tuning fork should be struck against elbow or knee, and not against a table or desk to avoid undesirable harmonics.
Result of tuning fork test:
Rinne test described as being positive (normal/conductive deafess) or negative (sensorineural deafness),
Weber test described as being central (normal) or lateralized to one ear (same side conductive loss or other side sensorineural loss; interpreted along with Rinne test)
Schwabach’s test where patients bone conduction is compared with that of examiner’s..
Audiometer is an electronic device which produces pure-tones.
Person has to put on headphones and then listen for some different tones from low pitch to high pitch. Person has to push a button or raise hand when he hears each tone.
Hearing is measured at different frequencies from (250 Hz to 8,000 Hz). Results in decibels are charted in the form of graph called audiogram.
This procedure is called Pure-tone Audiometry.
It is a subjective test of hearing and cooperation and understanding of patient is of utmost importance.
Audiometry is required: –
1. To know degree and type of hearing loss.
2. To keep record and for medico-legal purposes.
3. Before prescribing a hearing-aid.
Figure 1: An audiogram of an ear with normal hearing
Result interpretation; Reading Audiogram-
Normal hearing: less than 15-25 decibels
Minimal loss: 16 to 25 decibel loss
Mild loss: 26-40 decibel loss
Moderate loss: 41-55 decibel loss
Moderately severe Loss – 56 to 70 decibel loss
Severe loss: 71 to 90 decibel loss
Profound: 91 decibel loss and above
To know the type of hearing loss bone conduction is tested, If hearing is better by bone conduction a conductive loss is present.
Patient’s ability to hear and understand speech is measured.
Certain number of words presented via head phones or free field loud speakers, and person asked to repeat the words.
Speech reception threshold is minimum intensity at which a person is able to repeat at least 50% of words correctly. Here a set of two syllable words with equal stress on each syllable (spondee words) are used.
Speech discrimination score is percentage of words heard correctly when phonetically balanced words at 30 to 40 decibel above the person’s hearing threshold are presented.
A score of 90 – 100% is considered excellent, 80 – 89% is good, 70 – 79% is fair, 60 – 69% is poor, <50% is considered very poor.
Tympanometry or Impedance audiometry –
Objective test of hearing; particularly useful to test hearing in infant and child, to know patency of Eustachian tube, and to know condition of middle ear like presence of fluid or fixation (as in otosclerosis) or dislocation of ossicles.
Ossicles are 3 small bones present in middle ear namely malleus, incus and stapes.
Equipment consists of a probe which snugly fits into external ear canal.
Tymanometry is the dynamic measurement of middle ear pressure through measuring mobility of the tympanic membrane. The resultant changes in air pressure variation are plotted on a graph called tympanogram.
Acoustic reflex testing consists of response contraction of subjecting the ear to a loud sound a rough method of evaluating hearing.
Interpretation of tympanogram:-
Fig-2: Different types of tympanograms (pic taken from http://ivertigo.net/graphics/v14.gif)
The shape of the tympanogram suggests how the eardrum is functioning.
1. Normal eardrum movement is shows a well-formed peak, where height of the peak indicates the amount of eardrum compliance, (type A).
2. A flat line indicates little or no eardrum movement. This type of tympanogram (type B)is commonly seen when fluid is present behind the eardrum or when there is a hole or perforation in the eardrum .
3. A peak to the left of the normal pressure range (type c) means adequate eardrum movement with negative middle ear pressure, seen in persistent cold or initial phase of fluid accumulation or recovery phase of fluid accumulation.
Reduced mobility with normal middle ear pressure (type As) is seen in otosclerosis.
Normal middle ear pressure with hyper-mobility (type Ad) is seen in disruption of ossicular chain.
Brain stem evoked response audiometry (BERA): –
Auditory brain stem response (ABR) audiometry, Brainstem auditory evoked response audiometry (BAER)
It is a test of neurological activity of auditory nerve and brain stem in response to auditory stimuli.
Electrodes are placed on scalp and ear lobe and patient is asked to relax and remain still .
Clinical uses of BERA
1. It is an effective screening tool for evaluating cases of deafness due to retro cochlear pathology i.e. (Acoustic schwannoma).
2. Used in screening newborns for deafness
3. Used for intra-operative monitoring of central and peripheral nervous system
4. Monitoring patients in intensive care units
5. Diagnosing suspected demyelinated disorders (multiple sclerosis)