What is the Short Increment Sensitivity Index (SISI) test?
The Short Increment Sensitivity Index (SISI) test is a hearing assessment that evaluates the function of the inner ear and auditory nerve. It measures the ability of the inner ear to detect small changes in sound intensity that patients with normal hearing wouldn’t be able to perceive.
The ability to recognise and respond to such changes is called hyperacusis - an over-sensitivity to sound that is usually caused through damage to the cochlea (cochlear origin) or damage to the ascending auditory pathway (retro cochlear origin).
This can regularly be used in a test battery when assessing a patients’ suitability for a cochlear implant, as it compares two different lesions.
What’s involved in a SISI test?
During a SISI test, a patient listens to a series of tones presented at 20dB above their pure tone threshold for the test frequency. Then short increments of intensity are applied in bursts (1dB increments louder than the continuous tone).1
The patient will need to respond to these increments to indicate they can perceive the change. A normal listener shouldn’t be able to perceive an increment this small. In total, 20 increment bursts will happen and the test result is calculated as a percentage.
Why perform a SISI test?
The SISI test is commonly used to help further diagnose and identify the lesion of sensorineural hearing loss, which is caused by damage to the inner ear or auditory nerve. Evaluating the sensitivity of the inner ear towards small changes in volume provides important information about the function of these structures and guides appropriate treatment.
Overall, the SISI test is a valuable tool for assessing hearing function and can help healthcare providers diagnose and treat a variety of conditions that affect the inner ear and auditory nerve. It can also be used to differentiate between cochlear and retro cochlear disorders because a patient with a cochlear disorder can perceive the increments of 1dB, whereas a patient with a retro cochlear disorder can’t.
How to perform a SISI test
Here are some basic steps for performing a SISI test:
- At the start of the test, make sure the patient is comfortable and understands the procedure
- Provide the patient with headphones or insert earphones
- Play a tone at a specific frequency and supra-threshold loudness. This will serve as the reference tone and will be continuous. The frequency and loudness should vary depending on the individual's hearing level as well as specific testing protocol
- Present a second tone as a short-duration increment, this is the "stimulus", at the same frequency as the reference tone but at a slightly higher intensity (1dB higher)
- Ask the patient to flag if they perceive a change in loudness between the reference tone and the stimulus tone by pressing a button, raising a hand, or giving a verbal response
- This stimulation should be repeated 20 times to provide a percentage score of how many correct increments the patient has identified
- Calculate the SISI score by subtracting the number of correct responses from the total number of trials. The result should be expressed as a percentage. A score of 0-20% is considered normal, while a score of 20-65% is inconclusive, and a score above 75% is positive.
It is essential to note that the SISI test should be performed by a licensed audiologist or trained healthcare professional. The interpretation of results requires expertise and should be done in the context of the patient's overall hearing evaluation.
SISI test results
The SISI test should be conducted at 20dB SL for all frequencies. If the patient does not manage to get a high score on the SISI test, this could be indicative of retro cochlear damage. The final calculation of the percentage score indicates whether the test has shown a positive or negative indication of recruitment.
Result SISI test conclusion recruitment
If the patient scores above 70%, this is a positive indication and is indicative of the loss being due to a cochlear lesion. Results lower than 30% are perceived to be negative and indicative of a normal listener or a patient with a central lesion (as a cause of their hearing impairment). Results within the range of 30% to 70% are indifferent and a lesion can’t be determined.
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1J. Jerger (1962) The SISI Test, International Audiology, 1:2, 246-247