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INTEGRATED AUDIOMETRIC TESTSFOR COCHLEAR AND ETROCOCHLEARPATHOLOGIES
Jyotsna Nadkarni
Audiologist and Speech Pathologist, Bombay Hospital Medical Research Centre, Mumbai 400 020.
In the earlier years differential diagnosis for audiometric test results were limited. The first audiometric test battery came into existence with the inclusion of speech tests in 1924. “Supra-threshold” audiometric tests were reported in 1930’s.With the pure tone Audiogram, comparison of air and bone conduction results assisted in the diagnosis of conductive, sensorineural and mixed hearing impairment. Speech tests alone were of little diagnostic value. So the special audiometric test to differentiate cochlear pathology vs retro-cochlear pathology came into existence.
Indications for the Special Audiometric Test are sensori-neural hearing loss associated with the symptoms like giddiness, tinnitus, vomiting, nausea, positional vertigo, imbalance etc.
SPECIAL AUDIOMETRIC TESTS
I. Subjective (Behavioral) Tests
Pure Tone Audiometry Tests
i) TDT : (Tone Decay Test) : TDT is the most commonly used tests because the test can be reliably carried out on any pure tone audiometer and helps in diagnosing neural lesions like acoustic neuroma.
In TDT a sustained air conducted ear tone is presented while assessment of resulting changes in auditory threshold is made.
There are two commonly used methods:
1. Carhart’s TD Test
2. Rosenberg’s TD Test
Results of TDT
1. Normal 0.5 dB in 60 seconds.
2. Mild 10-15 dB in 60 seconds.
3. Moderate 20-25 dB in 60 seconds.
Marked TD Indicates Retro-Cochlear Pathology
II. SISI : (Short Increment Sensitivity Index)
The SISI test determines the capacity of a patient to detect a brief 1 dB increment at a 20 dB supra-threshold tone (called carrier tone) in various frequencies (preferably at 1000 Hz and 4000 Hz). This increase in the intensity of the carrier tone may be varied from 6 dB to 1 dB, Twenty such 1 dB increments are presented in the test ear and patient is asked to count how many of these 1 dB increments he could correctly identify. This multiplied by 5, gives the percentages SISI score.
Interpretation of SISI Test
Jerger’s classification for the test is done at 1000 Hz and above: - If SISI score is above 70% (+ve SISI) - Cochlear pathology. If SISI score is < 30% (-ve SISI) - Disorder elsewhere than inner ear.
Though SISI test is useful in distinguishing between cochlear and retro-cochlear lesions, it is not entirely foolproof and has its own limitations. So it should be used as a part of audiological test battery and the results must be interpreted in the light of other audiological findings.
III. Other Audiological Tests for Recruitment
Recruitment : A patient with a loudness recruitment responds to increased stimulus intensity with an abnormally rapid rise in the loudness. Often occurs in cochlear pathology. This phenomenon is often related to dysfunction of sense organ and may occur in cochlea with an alteration in endolymph as well as in those with structural changes in organ of corti.
Recruitment Tests
a. Loudness discomfort level (LDL) : In the normal ear, the LD threshold is between 90 dB and 105 dB. Narrow dynamic range indicate abnormal loudness growth and recruitment which is observed in the cochlear pathology.
b. ABLB : (Alternate Binaural Loudness Balance) : It is a standard behavioral test for recruitment. A pure tone test is done when there is a threshold difference of > 20 dB between the ears at the test frequency (the better ear is relatively normal).
The purpose of ABLB test is to compare the growth of loudness in the impaired ear with the growth of loudness in the opposite (normal) ear to demonstrate the degree of recruitment.
This test is mainly used for unilateral hearing loss cases where the two different loudness levels of a tone is alternately given in a test ear and normal ear. Patient is asked to find out whether they heard equal in loudness or one is softer or louder than the other and then ladder gram is plotted to find out whether there is a recruitment in the test ear.
Limitation
This test can be administered mainly in unilateral hearing loss.
IV. Glycerol Test
A pure tone audiometry test is done and then the patient is given about 70 ml (1.5/kg body weight) of glycerol orally, preferably in the fasting state. The PTA is done at intervals of 30 to 40 minutes for about 3 hrs. In presence of Meniere’s disease, there is usually an improvement in the hearing is at least of 5 to 10 dB in 3 adjacent octaves or 10 to 15 dB in two adjacent octaves. In advanced cases of Meniere’s disease when permanent cochlear damage is present, the test will not be +ve.
V. Bekesy Audiometry
Though at present this test is superfluous, it was a part of the site-of-lesion test battery.
Jerger’s classification of Bekesy audiograms is as follows:
1. Type I - Normal hearing or conductive loss.
2. Type II - -Cochlear hearing loss.
3. Type III - -8th nerve lesion.
4. Type IV - More closely resembling type IIthan type III.
2. SPEECH AUDIOMETRY
1. SRT (Speech reception threshold) : SRT is the intensity level at which a person is capable of repeating 50% of spondee words (Bisyllabic equally stressed words). SRT is carried out to find out the sensitivity of speech or hearing loss for speech. If it doesn’t correlate with average pure tone threshold then you can suspect retro-cochlear pathology.
2. SDS (Speech Discrimination Score) : It is a procedure of establishing the percentage of correctly perceived phonetically balanced monosyllabic words or consonant vowel (CV) combinations presented at a comfortable suprathreshold level (Generally 40 dB SL)
SD is reduced in sensori neural loss cases and much more in cases with retro-cochlear lesions (SD score is < 30%).
II. OBJECTIVE TEST BATTERY FOR LOCALISING SITE OF LESION
1. Impedance Audiometry
Impedance Audiometry has been one of the major advancements in the field of audiology and otoneurology in recent times. It is becoming increasingly popular because of the wide range of otological and neurological information it provides about the nature and anatomical site of lesion.
i. Metz Recruitment Test : In a normal ear the acoustic reflex threshold is between 70 - 105 dB above the pure tone hearing threshold level. Metz in 1952 demonstrated that in presence of cochlear lesions the gap between the acoustic reflex threshold and the pure tone audiometry hearing threshold level (called dynamic range) is considerably reduced (i.e. < 60 dB). The phenomenon may be attributed to the abnormally rapid growth of loudness that occurs in loudness recruitment which is a characteristic of cochlear pathology.
ii. Acoustic Reflex Decay Test : The test is usually done at 500 and 1000 Hz (Since some decay may occur in normal ears at 2000 and above). The sound stimulus is presented at an intensity of 10 dB above acoustic reflex threshold for the particular frequency for a duration of 10 seconds. The reflex amplitude is measured in the meter of the impedance audiometer. If the amplitude falls to less than 50% within 5 seconds, then it is interpreted that there is an abnormal decay of acoustic reflex. This abnormal decay is indicative of a neural or retro-cochlear pathology.
2. BERA
BERA is an electro-physiological test procedure which studies the electrical potential generated at the various levels of the auditory system starting from cochlea to cortex.
Fig.2 Normal BERA wave pattern Typical wave pattern : Origin of peak
1. Auditory nerve
2. Cochlear nucleus
3. Superior olivary complex
4. Lateral lemniscus
5. Inferior colliculus
6 and 7. Medical geniculate body
The BERA test helps us not only in identifying lesions in the 8th cranial nerve, but also the lesions in the brainstem region which affect the auditory pathway. The BERA response obtained in a particular case will depend upon the nature, location, and size of the lesion; e.g. if there is a small unilateral intracanalicular acoustic neuroma the BERA response will be of a particular type (increase in wave I and wave III interpeak latency). If there is a large acoustic neuroma in the CP angle region pressing upon the brainstem there is an increase in wave III and V latency on the contra lateral side, etc.
Fig.3 : ABR shows a poor morphology and reduced amplitude wave v. It also shows an abnormal i-v interwave latency and extended interaural latency difference for the ear.
Limitation
1. All waves are absent in severe hearing loss as well as in a large acoustic neuroma.
2. A normal BERA response virtually rules out an acoustic neuroma; but doesn’t at all rule out intrinsic brainstem lesion or even non-acoustic tumour of the CP angle e.g. Meningioma.
CONCLUSION
There are myriads of test available, some behavioral and some electrophysiological, for localizing the site of the pathology in sensori-neural lesion; but none of these tests are foolproof. A detailed history, thorough clinical examination with the battery of audiometric tests available, help a clinician to come to a correct diagnosis.
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