Introduction: Short increment sensitivity index test . A hearing test used to determine differential sensitivity to changes in loudness.
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.
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.
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
The "negative" SISI-test : The SISI-test has gained increased recognition during the past few years and can now be considered as the most important recruitment hearing test. In addition to the positive, negative and not determinable result there are frequent cases of incorrect negative interpretation. A "negative" SISI-test can be the result of insufficient or excessive hearing loss, missing adaptation, pathological threshold shift, aggravation, psychogenic deafness, cerebral sclerosis, imbecility etc. A negative result always requires additional audiometric differential-diagnosis.
Uses of SISI test: The SISI test is still widely used to determine whether the patient is having cochlear pathology.
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.
The SISI test differentiates between cochlear and retrocochlear disorders. A patient with a
cochlear disorder will be able to perceive the increments of 1 dB, a patient with a retrocochlear
disorder will not (Stach 1998).
