SHORT INCRIMENT SENSITIVITY INDEX

Introduction:

Jerger et al (1959) noted that hyper sensitivity to small changes in intensity at low SL may be unrelated to the intensity DL.

The DLI issue was refocused by Jerger, Shedd, and Harford (1959), who pointed out that the ability to detect small intensity differences is useful in the dif-ferential diagnosis no matter what the DLI-recruitment relation may or may not be. They also modified the nature of the intensity discrimination test, making the task easier for the patient to take, and easier for the clinician to administer and interpret. Instead of directly measuring the size of the DLI, their short increment sensitivity index (SISI) presents the patient with increments having a predetermined size of (usually) 1 dB, and simply asks her to indicate when she hears them.

The basic structure of the SISI is depicted , which shows that the stimulus in the SISI has two aspects. The first component is an ongoing tone that stays on for the entire duration of the test. This carrier tone is presented at 20 dB SL. The second element is a 200-millisecond increment of 1 dB that is superimposed on top of the carrier tone. Twenty 1 dB pulses are superimposed on the carrier tone, spaced at 5-second intervals; and the patient simply listens to the carrier tone and indicates whenever these brief pulses are heard.

Jerger et al (1959) also developed the SISI test, guided by the PRINCIPLE that persons with cochlear pathology might demonstrate to small intensity as increments super imposed on a sustained rather than an interrupted tone. 

The fundamental principle of the SISI test is that many of the pulses should be heard if the ear being tested has a cochlear disorder. On the other hand, fewer increments should be detected if the test ear has a retrocochlear disorder, a conductive loss, or normal hearing. 

Figure : the signal used to perform SISI

The stimuli had rise fall time of 50 ms and total duration of 300 ms. Each increment was 1 dB. The test presentation level was 20 dB SL. The patient was instructed to indicate whenever he or she found certain little jump in loudness and not to respond when uncertain.

A test turn consisted of 8 increments. The first 5 increments were 5 dB in size, to familiarize the patients with the task, and then 20, 1 dB increments were given. To prevent false positive and false negative responses, 3 increments were inserted between the 5^th, 6^th, 10^th, 11^th and 16^th increments. The size of these 3 depended upon the nature of previous responses. If the patient responded to 2 or fewer of the previous 5^th increments, the 6^th increment was increased to 5 dB. If they responded to 3 or more of the previous 5^th, the 6^th increment was decreased to 0 dB.

The SISI was derived from the responses to the 20, 1dB increments and was expressed as a percentage, with each response equivalent to a score of 5%. Jerger (1961) considered scores between 60%-100% as positive, which is indicative of cochlear pathology, scores between 20 and 50% as questionable and 0-15% consistent with conductive or RCP.

SISI test variants:

There are 5 different variants of SISI tests.

1. one dB increments at 20dB sound level (classical SISI) - High scores suggest a cochlear lesion.

2. Two to 5 dB increments at 20 dB sound level - Low scores suggest a retrocochlear lesion.

3. One dB increments at high sound levels (75dB) - Low scores suggest a retrocochlear lesion.

4. Increment sizes varied from 1 - 5 dB at 20 dB sound level - poorer scores in one ear than the other (when their thresholds are approximately equal) suggests a central lesion opposite the ear with the lower score.

5. One dB increments at sound levels ranging from 20 dB to high levels (about 75dB) in 10 dB steps for both ears. Difference in the rate at which scores increase suggests a retrocochlear lesion. The disorder is located on the same side as the ear which has not shown normal increases in intensity.

The SISI score is calculated by multiplying the number of the 1 dB increments that are correctly detected by 5, giving a percentage of correct responses. Scores between 0 and 70% are negative for cochlear pathological conditions; scores of 75% and above are positive for cochlear pathological conditions. Modifications to the classical procedure included reducing the initial five 5 dB practice increments in 1 dB steps (i.e., 4, 3, and 2 dB) to eliminate the sudden reduction of the signal to 1 dB and reducing the number of 1 dB increments by half, from 20 to 10.

MODIFICATIONS

High level SISI

Thompson (1963) suggested administering the SISI at 75dB HL. He reasoned that at this level, normal hearing, cochlear impaired and conductive impaired subjects would obtain positive scores while only retro cochlear impaired subjects would obtain negative scores.

Cooper and Owen (1976) recommended a minimum presentation level of 90 dB SL. In case where the hearing loss was greater than or equal to 70 dB, they suggested 20 dB SL presentations level, lower if 20 dB SL exceeded the audiometric limits.

Young and Harbert (1967) recommended administration at 70 dB SL or more if required for audibility. They concluded that positive score at this level indicates that ear is responding like a normal hearing ear, a negative score indicates the presence of abnormal adaptation.

Increment size:

Herbert et al (1969) suggested using an increment of 1.5 dB based on the finding that the minimum increment detected by normal and recruiting ears i.e. between 0.5- 1.5 dB and the minimum increment that could be detected in abnormally adapting ears exceeded 1.5 dB.

Contra lateral Masking:

Blegvad (1969) evaluated effects of masking with BBN at 80 dB SL on SISI scores of the affected ears for 32 patients with cochlear hearing loss (unilateral). It resulted in a dramatic improvement in SISI scores for some subjects.

Perhaps the increase in SISI scores is related to the release from masking phenomenon. That is, internal noise such as flow of blood may sometimes mask out the increment. Consequently, when noise is added to the contra lateral is then perceived as spatially separate from the increment, which can be heard. Burns et al (1982) suggested using contra lateral masking only when the level of crossover in the non test ear exceeds 30 dB SL, provided that the bone conduction threshold is below the crossover threshold of the SISI carrier tone. They suggested the following formula for the minimum effective masking level for the SISI test.