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Short increment sensitivity index test (SISI)
Dr. T. Balasubramanian M.S. D.L.O.
Introduction: The SISI test is still widely used to determine whether the patient is having cochlear pathology. This test is based on a phenomenon known as recruitment (abnormal loudness growth). It was Dix and Hallpike who related th presence of loudness recrutiment to cochlear pathology.
Difference limen for intensity (DLI): is the smallest change in the intensity of a pure tone which can just be detected. It is usual for patients with normal hearing to have difficulty in detecting small changes in intensity close to threshold. Patients with cochlear pathology will be able to appreciate the change in intensity better because of the phenomenon of recuritment. DIL could safely be assumed to be an indirect indicator of the phenomenon of recruitment. It was Luscher and Zwislocki who developed a DIL test which remained popular for quire sometime.
SISI test proper was introduced by Jerger and Shedd in 1959 as a test for the phenomenon of recruitment. In this test a pure tone was presented to the patient at a sound level of 20dB. A small increase in intensity is superimposed upon the steady state tone at periodic intervals. The size of the increment varied from 5 - 1 dB. Jerger clearly demonstrated that the ability to detect the 1 dB increments was restricted to patients with cochlear pathology. This ability was absent in patients with normal hearing or with a conductive hearing loss. This test is very simple to perform and is less confusing than the DIL test. The SISI test differs from the classical DLI tests in that the patient's precise DLI is not explored. It just tests the cochlea's ability to repond to a transient signal of small amplitude.
Diagram showing the signal used to perform SISI
As shown in the fig. a carrier tone is introduced into the patient's ear at a Sound level of 20 dB. Every 5 seconds a short increment is superimposed, starting with 5 dB increments. The signal has an on-off time of 50 msec and 5 seconds elapse between increments. The patient is instructed to indicate when he hears a brief jump in the loudness of the tone. After 5 such jumps (to condition the patient) the size of the increment is lowered to 1 dB marking the begining of the SISI test. Twenty 1 dB increments are introduced and the subject is required to indicate when the increment is heard each time. If a number of consecutive increments heard exceeds 5 then the examiner should delete several increments to ensure that the subject is reponding to the change in intensity and not the time interval. If the patient fails to respond to several increments in a row the increment size can be increased for retraining the patient before proceeding with the test proper. These steps will avoid false negative and false positive results.
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.
This test helps in determining the site of lesion in the auditory system by determining whether a disorder is cochlear or noncochlear. The diagnostic accuracy of this test depends upon the amount of hearing loss a patient may have.