SPECIAL TESTS OF DIAGNOSTIC AUDIOLOGY

INTRODUCTION

All types of Audiometry are diagnostic since it contributes in some way to the ultimate localization of auditory disorder.

For ex. The relationship between AC and BC thresholds is one of the principle tests for differentiating conductive hearing loss from Sensorineural hearing loss.

The auditory disorders are broadly classified into:

1.      Peripheral Auditory System

2.      Central Auditory System

On these lines, Audiological tests can be divided into 2 categories:

1.      Audiological tests suited to the evaluation of peripheral auditory system

2.      Audiological tests more appropriate to the evaluation of central auditory system.

Auditory disorders are broadly classified as central and peripheral on the basis of location of cochlear nuclei i.e. the lesion distal to cochlear nucleus may be said to affect Peripheral Auditory system while lesions proximal to the boundary at the first synapse may be said to affect Central auditory system.

Common characteristics of peripheral disorders are:

-          Loss of hearing sensitivity

-          Fair to poor speech discrimination score

-          Distortion in auditory system

-          Abnormal adaptation

-          Ipsilateral symptoms (symptoms originating from same side of defect)

As in pure tone signals there is a spectral concentration of energy, hence the test using pure tone signals play a significantly more important role in the evaluation of peripheral disorders.

Common characteristics of central disorders are:

-          Virtually normal hearing sensitivity

-          Occasionally high frequency hearing loss or artificial configuration of hearing loss

-          Poor or very poor discrimination score irrespective of normal hearing sensitivity

-          Tinnitus associated with giddiness

-          Blackouts in front of eyes.

-          Contra lateral symptoms (occurring in other sides of defect)

However a characteristic feature which is common to both the categories is the impaired ability to transmit complex signals like speech, hence test using speech signals are significantly important in the evaluation of both peripheral and central disorders.

Special hearing test may be categorized in number of ways, such as:

1          (a) Special test using pure tones

            (b) Special test using speech

2          (a) Special tests administered at threshold level

(b) Special test administered at suprathreshold level

3          (a) Special test requiring voluntary behavior responses from patient

            (b) Special test doesn’t requiring voluntary behavior responses from patient

4          (a) Special tests designed for assessment of Cochlear pathology.

            (b) Special test designed for assessment of Retro Cochlear pathology

The test for cochlear evaluation using pure tone as stimulus includes:

1.      Differential Limen of Intensity                       (DLI)

2.      Short Increment Sensitivity Index                   (SISI)

3.      Alternate Binaural Loudness Balance  Test    (ABLB)

4.      Monaural Loudness Balance Test                   (MLB)

While, test using speech as stimulus, includes SRT, WRS, MCL, UCL (LDL-Loudness Discomfort Level), and Dynamic range.

Test for Retro cochlear evaluation using pure tone stimulus includes Tone Decay Test administered at threshold and supra threshold level including Supra Threshold Adaptation Test (STAT). While, the using speech stimulus is used WRS, PIPB functions, Roleover Effect Index and test for Central Auditory Processing Disorder (CAPD)

Besides these tests, the other tests which were to be used earlier for differential diagnosis of Cochlear pathology and Retro Cochlear Pathology, includes:

-          Beckhesy Audiometry

-          Brief tone Audiometry

-          Diplacusis Test

However, with the advancement of science and technology also with the availability of more sophisticated objective tests, e.g. CT SCAN, MRI, MRA scan, PET scan, etc. the efficiency of most of these tests have been questioned. The clinical value of the given diagnostic tests can be expressed by sensitivity and specificity and thus their predictive values were found or detected for differential diagnosis of Cochlear Pathology and Retro Cochlear Pathology.

According to Cohm-et-al, 1986, the sensitivity can be defined as,

“A portion of population with a particular disease which was correctly diagnosed by using tests”

And specificity is defined as,

“The proportion of population who doesn’t have the disease and give a negative response.”

Among all the special Audiological tests, the most commonly or popularly used site of lesion test are Tone Decay Test (TDT), Beckhessy Audiometry and SISI test. Johnson-et-al (1977) reported that TDT and BAT have a sensitivity of less than 50% for diagnosing cochlear and Retro Cochlear Pathology, whereas, Thomson and other reported a higher sensitivity of about 70% for TDT for identifying RCP. The performance of test is measured in terms of % of correct detection (HIT) and the % of false referral is referring to as false alarm of MISS.

RECOMMENDED PROTOCOL FOR SPECIAL TESTS:

There is no universally accepted recommended protocol of special test for site of lesion but it all depends upon feasibility in terms of equipment, time availability, available manpower, case load, experience and skill of the clinician.

The recommended test battery procedure is:

1.      Case history with highlighted symptoms and signs

2.      Otoscopy

3.      Pure tone Audiometry (PTA)

4.      Speech Audiometry with SRT, WRS, MCL and UCL

5.      TDT- For screening purpose (Olsem & Offsinger) and for more appropriate diagnosis (Carhart with Lings)

6.      SISI- Conventional (by Jerger & Jerger) and Modified (by Thomson-et-al)

7.      Test for recruitment- ABLB (for unilateral hearing loss) and MLB (for bilateral hearing loss)

8.      Protocol for using Speech special test

(a)   For CP use MCL and UCL

(b)   For RCP use WRS, PIPB functions and Roll Over Index (ROI)

TONE DECAY TESTS (TDT)

In clinical practice, Tone Decay is either measured at or near threshold or well above threshold, thus Tone Decay can be classified into 2 groups:

1.      Threshold Tone Decay

2.      Supra Threshold Tone Decay

Threshold Tone Decay (TTD) is defined as reduction in the sensitivity resulting from the presence of a barely audible tone while Supra Threshold Tone Decay refers to loss of audibility as a result of stimulation which is presented at higher presentation level.

HOOD’S TONE DECAY TEST

Hood (1956) reported a more elaborate procedure of Tone Decay. He elaborate the procedure in 5 steps:

1.      Patient’s threshold is obtained at 3 frequencies.

2.      Patient is instructed to raise his hand or a finger as soon as he hears the tone and lower it down as the signal fades.

3.      Tone is presented at 5 dB above the Patients’s threshold.

4.      If the patient hears the tone completely for 1 minute then the tone is switched off and the test is reported negative for same frequency and same procedure is carried out for other frequencies. But if the patient signals that the tone is no more audible, then to is turned off and patient is allowed to have rest of 60 sec.

5.      Intensity of tone is increased by 10 dB and procedure is repeated until an intensity level is reached which permits a sensation of the tone. The difference between the presentation level after the test and final level at which the tone becomes audible is considered as the amount of Tone Decay.

CARHART TONE DECAY TEST

Carhart (1957), reported a procedure for measuring Tone decay which was developed by him at North Western University. Purpose of the test was to develop such technique which could be time saving so that it can be conducted very rapidly in any clinical situation with an standard audiometer.

1.      Patient’s threshold is obtained at 3 frequencies.

2.      Patient is instructed to raise his hand or a finger as soon as he hears the tone and lower it down as the signal fades.

3.      The test is administered 5 dB below the subject’s established threshold and then ascended in 5 dB steps without interruption until the subject responds. As soon as the subject responds, timing is started by switching on stop watch. If the tone is heard for a full one minute then the test is terminated.

4.      But if the subject indicates that he no longer hears the tone before the minute criteria is reached, then the intensity of tone is increased by 5 dB without interrupting the tone, but setting the stop watch back to zero and starting timing for a minute again.

5.      A record is kept for the numbers of second the tone is audible.

6.      The tone is continued to be raised in 5 dB steps until a intensity is reached that allows the subject to perceive the tone for full minute. The amount of decay occurring at each level suggests / indicates the amount of decay the subject is showing.

7.      For saving the time, Carhart suggested that the test should be terminated when the subject fails to respond 30 dB above threshold depending on the amount of decay shown by the subject for different levels with respect to time.

8.      If the amount of decay is occurring abnormally with respect to time then the test should be terminated instead of counting further till tone becomes audible for 60 sec or till the audiometric limit is reached.

OLSEM AND OFFSINGER

Olsem and Offsinger 1974 gave the modified version of Carhart TDT, which is one of the most popular administered tests for screening purpose. They suggested beginning the test at 20 dBSL at 3 frequencies. In this, studies of 20 patients with surgically confirmed 8th nerve humor the standard Carhart test and 20 dBSL technique were equally sensitive in identifying recession. Their presentation at 20 dBSL makes the patient’s task much easier, resulting in more reliable finding of threshold. However, because of poor validity it has been recommended to support this procedure, such as Rosenberg 1 minute modification of Carhart TDT andGreen’s modification of TDT.

ROSENBERG 1 MINUTE MODIFICATION OF CARHART TDT

Finding Carhart’s method as combursome and time consuming, Rosenberg 1958 proposed a short term modified version of Carhart test. He proposed that no records need to be kept of number of seconds the tone is audible, at each intensity level, as restoration for any given tone is limited to a level of 60 sec, but he continued the tone without interruption of timing for 60 sec.

In this procedure from 1 to 4th step are similar tot hat of the Carhart procedure, but in the 5th step, if the patient indicates that he no longer hears the tone before 1 minute is completed, then intensity of the tone is raised by 5 dB without interrupting the stimulus and without stopping the watch.

In the 6th step, the tone is continued to be raised in 5 dB steps, each time when the tone becomes inaudible. At the end of 60 sec, the tone is turned off and amount of tone decay is computed in dB.

GREEN’S MODIFIED TDT

In 1960, Greens noticed that some patients with Retro Cochlear Pathology (RCP), experience loss of tonality before loss of audibility on Carhart TDT. Hence, he modified the instructions of Carhart procedure.

1.      The patient is seated in an arm a chair and he is asked to maintain his/her elbow in contact with the arm of the chair while he signals. He/she is instructed to raise arm perpendicular to the arm of the chair, if the tone is perceived, and to lower his/her arm to the rest position if the sound becomes completely inaudible and at 45 degree if he loses tonality but the tone is still audible.

2.      The patient is cautioned against adjusting earphones or chewing while the test is ON.

3.      The subjective change in the quality of pure tone to noise lacking tonality has been called Tone Perversion.

OWEN’S TDT

In 1964, Owen proposed a modified version of Hood’s technique. He incorporated a rest period of 10 sec instead of 60 sec.

However, he gave the interpretation of Td, both in terms of amount of TD upto 20 dBSL (in dB) and the pattern of TD (in sec at succeeding levels). On this finding, he proposed 3 categories:

1)      Normal

2)      Cochlear

3)      Retro Cochlear types

JERGER & JERGER SUPRATHRESHOLD ADAPTATION TEST (STAT)

On the basis of Vanhover hypothesis, symptoms of abnormal TD first appear significantly only at highest testable sound intensity. Jerger & Jerger, 1975, proposed a simplified suprathreshold TDT. Test is administered at 3 frequencies: 500 Hz, 1 KHz and 2 KHz.

Procedure:

1.      Patient is instructed to signal as long as he hears the sound in test ear. At the same time non-test ear is masked with white noise at 90 dBSPL level.

2.      A continuous 500 Hz test tone at 110 dBSPL is presented until the patient says that he no longer hears the tone or until 60 sec have lapsed, whichever comes first.

3.      If the patient has responded for complete 60 sec for test frequency then the test is code Negative for that frequency and the test is code Positive if the patient fails to respond for complete 60 sec.

4.      To ensure that the patient has grasped the essential measures of listening task, the test tone is pulsed for 60 sec. If the patient indicates that he heard the pulse and not the tone for 60 sec, then he is likely that he is responding to the test in proper manner.

5.      Test is then carried out at 1 KHz and 2 KHz.

INTERPRETATION OF TDT:

TD is a powerful diagnostic procedure for RCP. However, It is only one of the tests of battery that has been considered sensitive for differential diagnosis between Cochlear Pathology and Retro Cochlear Pathology.

According to Rosenberg, 1958:

0          -           5 dB Decay                  Normal or Conductive

10        -           15 dB Decay                Mild

20        -           25 dB Decay                Moderate

30        -           >35 dB Decay              Marked Decay

Marked tone decay almost always indicates RCP.

-          Glaslow, 1968 stated that positive TD is one where there is at least 30 dB decay.

-          Tillman, 1969 agreed that patients with RCP, typically have TD exceeding 30 dB.

However, at the same time it would be dangerous to assume that anyone with 30 dB decay, has RCP. While everyone with less than this amount, doesn’t have.

A more predictive way of looking at TD is that each dB of decay above 15 dB, should raise the suspicion that RCP lesion may exist.

The greater the TD and the number of frequencies involved, particularly the low frequencies, and then there is greater possibility of serious pathology.

The index of suspicion should also be raised if the rate of decay doesn’t diminished with increased stimulus intensity. Patients with acoustic tumor, frequently exhibit extreme an often complete TD. However, tumor size appears to be related to the severity of symptoms. Partial or complete TD was found in 60% of tumors classified as large, while, 40% of tumor is classified as small.

OWEN’S INTERPRETATION OF TD:

LEVEL ABOVE THRESHOLD (dB)	PARTS OF DECAY (TIME TAKEN)
	TYPE I	TYPE II					TYPE III
		A	B	C	D	E
5	60	25	17	15	12	5	14
10	-	60	34	26	23	14	12
15	-	-	60	40	30	18	12
20	-	-	-	60	39	21	10

TYPE I            No decay in 60 seconds. Should be present in Normal / Conductive hearing loss and in some patients with lesions of auditory portion of inner ear.

TYPE II          Progressively slower decay as the level is raised in 5 dB steps. Strongly suggestive of inner ear pathology.

TYPE III         In this, even with increase in intensity upto 20 dB from presentation level, the patient is unable to sustain the tone for increased period of time strongly suggestive of lesion of auditory nerve.

RECRUITMENT: Abnormal growth of loudness with increase in intensity.

Fowler noted that equal loudness between the recruiting impaired ear with normal ear can be achieved only with larger sensation levels (SLs) to the normal ear. Eg. A tone at SL of 60 dB in normal ear and 30 dB in impaired ear may sound equally loud.

This result suggests that the growth of loudness requiring an intensity increase of 60 dB in normal ear is achieved with an intensity increase of 30 dB in impaired ear. This indicates that recruitment for loudness growth must be occurring much more in impaired ear. This is due to abnormality in cochlea such as hypersensitivity of haircells due to damage.

            Recruitment is a landmark feature of SNHL of cochlear origin. Reverse Recruitment / Decruitment is a hallmark feature of SNHL of Retro Cochlear region. When recruitment is found to be associated with presence of cochlear pathology then the recruitment is known as complete recruitment. When the recruitment is associated with cochlea then the concept is known asPartial Recruitment.

LOUDNESS BALANCE TEST (LBT)

LBT was developed and popularised by Edmond Fowler 1986. The term recruitment was coined by Fowler in 1928 which defines abnormal growth of loudness at suprathreshold level.

The test for detection of recruitment in unilateral cases was developed by Fowler and is known as ABLB. Scott Reger 1936 is credited with the development of loudness balance procedure which is applicable to bilateral symmetrical hearing losses when there is normal hearing at some frequencies.

ALTERNATE BINAURAL LOUDNESS BALANCE TEST (ABLB)

This technique was initially described by Fowler, 1986, but corresponding modifications were proposed by Hood & Jerger in new course. Currently Fowler’s ABLB is the most preferred and commonly used procedure with Hood & Jerger modifications whereas, the technique or MLB was described by Rager 1936, which is helpful in detecting the recruitment in clients with bilateral hearing loss. ABLB compares loudness growth between 2 different frequencies in the same ear. ABLB is a pure tone test which is done when there is a threshold difference of more than 20 dB between ears at the test frequencies and the better ear is relatively normal.

            The purpose of ABLB test is to compare the growth of loudness in an impaired ear with the normal growth of loudness in the opposite / normal ear.

PROCEDURE:

For the administration of ABLB test, there is a need of 2 or dual channel audiometer with capability of alternatively pulsing 2 tones of identical frequency from one ear to other. The intensity of each tone must be individually controlled from a separate hearing level dial.

            The intensity is held constant in one ear while it is varied in other ear until the listener judges both signals to be of equal loudness. The ear with constant or fixed intensity is termed as Reference ear and the ear with variable intensity is termed asVariable ear. The intensity levels in the variable ear are matched with that of loudness levels of reference ear.

The client is instructed to state whether the variable tone is “softer than”, “louder than” or “equal” in loudness to the reference ear. The client is instructed that he/she will hear 2 tones, one constant in loudness and other variable.

            Judgement of loudness is to be made only from the variable tone to the reference tone; also the client should be cautioned to pay attention only to the loudness changes and to ignore other differences. This is required because in case of ABLB and MLB, where the same tone is presented in each ear, may sound different because of the phenomenon called Diplacusis.

Carber, 1978, stated 4 variables which need to be considered while administering ABLB technique:

I)                    Psychophysical method: Method limits v/s Method of adjustment: The clinician controls both the reference and variable ear intensity settings whereas the clinician sets the level in reference ear while client control the variable tone intensity. Hood 1969, supports methods of limit whereas Jerger 1962, recommends method of adjustment. According to him, when the control of stimulus is in the client’s hand then it has advantage of reducing the potential biases by the clinician.

II)                  Reference v/c variable ear: this is the most argued option amongst clinician. Cols & others, 1974, respected Jerger’s suggestion of using poorer ear as the reference ear, whereas, Hood, 1961, followed Fowler’s suggestion of using the good or the better ear as the reference ear. According to Hood, using the better ear as reference ear would result in less variability in responses from the client as the poor being recruiting ear will be more sensitive to changes In intensity. Jerger, 1962, stated that if the clinician is interested in detection of presence or absence of recruitment in poorer ear, then a few fixed settings in the poorer ear are sufficient enough to achieve the desirable result, but if the clinician interest is in plotting of loudness growth function as well as the determination of presence or absence of recruitment in the poorer ear then the Fowler’s procedure is more appropriate.

III)                Number of Loudness Balance Levels: to some degree the number of loudness balance level are related to the following:

o   Which ear is the reference ear

o   What is the basic rationale of the test i.e. whether the aim of the search for recruitment or the determination of loudness growth function is the desired goal.

According to Jerger, when poorer ear is used as reference ear then minimum 2 and maximum 3 levels are sufficient to find out recruitment, this will save time of the clinician.

According to Jerger, the use of good ear as reference ear would not only require more levels of determination of the presence or absence of recruitment but would also result in time consumption. The number of sound levels required, are more in number when good ear is the reference ear.

According to Fowler, more number of levels permits diagnostic judgment as well as determination of accurate loudness growth function.

IV)               Duty cycle and Alternation rate: Jerger, 1962, recommended a tone alternation rate of 1 per second with a duty cycle of 500msec ON and OFF per ear (i.e. the tone is ON half the time in one ear and half the time in other ear)

Majority of diagnostic audiometers works on these parameters though maximum clinicians are satisfied with such settings but Hood 1969, stated that this may lead to adaptation, hence he suggested that a duty cycle of 600msec as a better option. However, this is not possible for the manufacturer to provide such alteration; hence maximum clinical audiometer has a duty cycle of 500msec per ear.

PLOTTING OF RESULTS AND INTERPRETATION: 2 primary methods have been practiced in plotting loudness balance results:

1.      LADDER GRAM: Plotting on pure tone audiogram and concept is given by Jerger. In ladder gram, equal loudness judgment between 2 cases is connected by a straight line, the presence or absence of recruitment is judged by inspection of the line pattern.

2.      GRAPHICAL FORMANT: this method is commonly known as Fowler plot. Here the graphs are plotted of the same data as the ladder gram. The good ear is plotted on X-axis and poorer ear is plotted on Y-axis. The 45 degree diagonal represents idealized results for loudness balance judgment between 2 normal ears. Comparison of lines connecting the equal loudness points to the idealized function determines presence or absence of recruitment.

Jerger 1962 suggested 4 results are possible:

1.      COMPLETE RECRUITMENT: is present when reference and variable ears are judged equally loud at equal HL + 10 dB.

2.      If the equal loudness judgments are made at equal SL + 10 dB, then NO RECRUITMENT is present.

3.      PARTIAL RECRUITMENT: is seen if equal loudness judgment falls off between those of complete or no recruitment. At the highest intensity level, equal loudness is just at 85 dBHL in good ear and 100 dBHL in poor ear.

4.      The phenomenon of DECRUITMENT is seen less frequently than other patterns. In this case, the poorer ear needs an ever increasing amount of intensity for a signal to sound equally loud to the good ear. One should suspect decruitment when loudness judgment shows a SL difference of 50 dB or more in the poorer ear than in the better ear, which is seen in case of RCP.

DIFFERENTIAL LIMEN OF INTENSITY (DLI)

DLI of recruitment are based on the measurement of rate of change of loudness as the intensity increases. The rate of change in loudness with increase in intensity is greater in recruiting ears than non-recruiting ears. The change in intensity in dB which results in a just barely noticeable loudness change is known as Intensity Diffrential Limen for Loudness.

            Leushior & Zwislocki (1948), gave recruitment test which was first designed IDL test for detection of recruitment in unilateral ear. The test involves an amplitude modulated tone as the criteria for the detection of the presence of recruitment in the ear. The size of amplitude modulation which results in perception of loudness change is called as Critical Percentage Modulation (CPM), if it is smaller than that in normal hearing ears then recruitment is present, while if it is the same size as that in normal recruiting ears then recruitment is absent. The test is done at 40 dB SL since the IDL in normal hearing ears at this level is independent of frequencies. Persons having CPM less than 8% but considered as recruitment.

            Neuberger, 1950, reported that all of his 14 patients with unilateral recruitment (as evidenced by the results of ABLB) had reduced IDL using the Leushior, Zwislocki (1948), however the test has following drawbacks:

1.      The CPM could exceed 8% in recruiting ears if the HL was mild i.e. >40 dBHL or if the audiometric configuration was not steeply sloping.

2.      The normal hearing listeners could obtain IDL less than 8% with practice.

3.      Individual variability is very high.

DENNIS & NAUNTON TEST:

Dennis & Naunton, 1950, develop another recruitment test based on IDL for loudness. This test is often referred to as memory method for measuring the IDL. Their technique involves the sequential presentation of 2 tones of the same frequencies for 2 ears i.e. frequency are presented to an ear and the intensity of second tone is that it was just noticeably louder than first tone. Then the intensity of the second tone was again varied until the patient reported that it was just noticeably softer than the first tone. The test is done at 4 & 44 dBSL if the IDL is reduced with increasing SL levels then the ear was suppose to have decruitment or non-recruitment. While if the IDL increase or remains unchanged with increasing sensational levels then the ear was based on a relative rather than absolute measure of IDL for loudness.

As compared to L&Z test, D&N test gains more popularity. The only limitation of D&N test is that it is time consuming although it is difficult to present the tone at 4 & 44 dBSL with available clinical audiometer.

Drawbacks-

-          The critical percentage modulation could exceed 8% in recruiting ears if the hearing loss was mild i.e. less than 40dB or if the audiometric configuration was not steeply sloping.

-          The normal hearing listener could obtain IDL less than 8% with practice.

-          Individual variability is very high.

-          According to Harsh-et-al, 1954, the IDL was considered as majority of earlier researchers as a measure of recruitment is not direct measure of recruitment rather than indirect one.

PERFORMANCE INTENSITY PHONETICALLY BALANCE (PIPB)

The study of relative intelligibility of spondees at different intensity levels is termed as articulation curve. Similarly the study of intelligibility of PB word list at different intensity level is termed as PIPB function or PIPB curve. PIPB curve is a graph showing the correct % of speech discrimination material as a function of intensity. The graph usually shows the discrimination score on the Y-axis and the sensation level on the X-axis. The study of PIPB function helps in determining the differential diagnosis of cochlea and RCPs. Jerger and Jerger 1971 describe the use of PIPB function as the method of screening for disorders of central auditory nervous system. They observed that patients with normal hearing sensitivity shown the differences in the score as the intensity increased with shapes of PIPB curve goes on increasing with the increase in intensity level upto such level where plateau is reached where there is no improvement in score with further increase in intensity level.

This similar curve is also obtained in patients with conductive hearing losses. However in cases with Cochlear SNHL the shape of the curve is much similar to normal and conductive hearing loss but they may take more energy for reaching the maximum scores and the scores is never 100% in such cases. However it has been seen that in cases of RCPS as the intensity is increased the score reduces especially after reaching the plateau. This phenomenon is referred to as roll-over effect. ROE suggest a lesion on the side of the brain opposite the better ear whenever the ratio is less than 0.4. However the poorer ratio is obtained on the same side if the lesion is present on the same side. Jerger & Jerger further observed a significant difference in score between two ears though both ears have equal sensitivity for pure tones. When it is usually suggestive of a central lesion on that side of the brain when you obtained at poorer score.

While obtaining PIPB curve or ROE, the PB words are usually presented 10, 20, 30, etc. i.e. in 10 dB steps above the PTA & SRT level up to the maximum level of audiometer and MCL whichever is greater. Some authors have recommended the presentation of materials in 20dB steps above SRT. PIPB function can be calculated by obtaining the level at which maximum score is obtained and the level at which min. score is obtained.

A RO Ratio can be calculated by the following formula-

                  Roll over ratio= PBmax. - PBmin.

                                               PBmax.

Where,

PBmax. Is the highest speech discrimination score

PBmin. Is the minimum score, the level at which further increases in intensity not causing any change. A ROR of 0.4 is suggestive of cochlear region while that of 0.5 or greater is suggestive of 8th nerve lesion.

Limitations of PIPB

-          The procedure is not standardized and equally helpful for all test materials for WRS i.e. ratios vary from test to test.

-          For greater accuracy and precision of results more no. of levels should be tested.                                                                                     

DIAGNOSTIC SIGNIFICANCE OF ROLL OVER ON PERFORMANCE INTENSITY FUNCTION

Jerger & Jerger 1962 obtained a performance intensity / articulation function using half list of PB 50th in subjects with cochlear hearing impairment, brainstem pathology and 8th nerve pathology.

They found that cochlear hearing impaired subjects had poor PB maximum scores in the hearing impaired ears and that subjects with 8th nerve pathology had even more poorer scores in the pathological ear. However, subjects with brainstem lesions had reduced PB maximum scores in the ear contra lateral to the lesion.