SISI Test (Short Increment Sensitivity Index)

Definition and History

The SISI Test (Short Increment Sensitivity Index) is a suprathreshold hearing test that evaluates the function of the auditory system—especially the inner ear (cochlea) and the auditory nerve. It measures a person’s ability to detect very small changes in sound intensity. While individuals with normal hearing typically cannot detect such minute increases, those with cochlear damage (sensorineural hearing loss) may sometimes perceive them. The principle underlying SISI is the phenomenon called recruitment. Recruitment is characterized by an abnormally rapid growth in perceived loudness with increasing sound level in cochlear hearing losses—meaning the dynamic range between the hearing threshold and uncomfortable loudness is narrowed. Therefore, SISI measures the increased sensitivity to small intensity fluctuations that arises when the cochlea is damaged.

The test was developed in the late 1950s. In 1959, James Jerger and colleagues (Shedd and Harford) described the test to distinguish cochlear-origin hearing loss from retrocochlear (auditory nerve or central) loss. Jerger’s work in the 1950s showed that patients with cochlear pathology could detect smaller intensity changes than normal, forming the basis of the test. Its emergence closely followed Carhart’s tone decay test; both aimed to localize the site of lesion (cochlea vs. nerve), providing audiologists with crucial tools for diagnosing conditions such as acoustic neuroma at that time. In short, as the 1960s began, the SISI test attracted significant attention in diagnostic audiology and became a groundbreaking method for detecting cochlear damage.

Test Administration (Method and Procedure)

Administering the SISI requires a clinical audiometer capable of special settings. Each ear is tested separately, typically at fundamental frequencies such as 500, 1000, 2000, and 4000 Hz (with a focus on frequencies where hearing loss is present). Before the test, clear instructions are given: “You will hear a continuous tone. From time to time the loudness of this tone will increase very slightly and suddenly. Please raise your hand (or press the button) whenever you notice this change.” This ensures the listener knows what to listen for.

A typical procedure is as follows: a continuous reference tone is presented at the test frequency at 20 dB SL (20 dB above the listener’s threshold). While the tone is sustained, approximately every 5 seconds a very brief (~0.2 s) and small increment is superimposed. This increment is typically +1 dB (i.e., the tone’s level is increased by 1 dB for an instant). The patient signals whenever they detect this tiny “jump.” In total, 20 increments of 1 dB are presented at 5-second intervals. Whether the listener responds to each increment is recorded. The test lasts about 1–2 minutes and requires sustained concentration. To maintain attention and reduce response bias, some protocols include an occasional larger increment (e.g., +5 dB) after every five small ones. Brief practice runs with 2–5 dB increments before the test can also help familiarize the listener with the sensation of a 1 dB change and reduce false positives/negatives.

After all presentations, the number of correctly detected increments is tallied. For example, detecting 10 out of 20 increments yields a 50% score (correct detections × 5). The SISI score is reported as this percentage. If needed, contralateral masking noise is used to isolate the true response of the test ear (especially when the test ear has significant loss and the opposite ear hears well). The SISI must be administered by an audiologist or trained professional and interpreted by a specialist, because proper execution and interpretation require specific expertise.

Role and Function in Clinical Diagnosis

The most important clinical use of SISI is to help determine whether a sensorineural hearing loss is cochlear (inner ear) or retrocochlear (auditory nerve or central pathway) in origin. This distinction is critical for treatment and rehabilitation planning. The logic is: if the loss is cochlear (e.g., damage to outer or inner hair cells), the person may begin to detect very small intensity changes such as 1 dB above a certain level (because recruitment is present). In contrast, if the loss stems from post-cochlear structures (e.g., an acoustic neuroma), the person typically does not detect such small increments, as the problem lies not in “loudness growth” but in neural transmission—so recruitment is absent. Thus, a high SISI percentage (detecting many 1 dB increments) suggests a cochlear pathology, whereas a low score (failing to detect most small increments) points toward a retrocochlear problem. For instance, a result of 80% may indicate damage to the inner ear’s sensory cells, while a result of 10% could suggest an issue with the auditory nerve.

In the 1960s and 1970s, SISI and similar suprathreshold tests were widely used for diagnosis. At that time, advanced tests such as ABR (auditory brainstem response) or MRI were not routine, so SISI and the tone decay test were crucial for differentiating cochlear lesions from eighth-nerve lesions. Together with Carhart’s tone decay, SISI guided early detection of auditory nerve tumors (vestibular schwannoma). For example, in a patient with unilateral hearing loss, if SISI is 0% in the affected ear (no detection of 1 dB changes) while the contralateral ear is normal, this would raise concern for a retrocochlear lesion and prompt further imaging.

With technological advances, the routine use of behavioral tests like SISI has diminished. Objective measures (e.g., acoustic reflexes, ABR, and MRI) now play leading roles in diagnosis. Nevertheless, SISI has not been abandoned entirely; some clinics still use it to confirm cochlear pathology or to assess the inner ear’s capacity to detect small changes. For instance, in evaluating candidates for cochlear implantation, SISI may be part of a test battery to judge whether the patient benefits from hearing aids or to confirm the lesion is confined to the cochlea. SISI can also aid in evaluating hyperacusis (excess sensitivity to sound). For example, if someone with normal thresholds reports an unusually high SISI score (detecting many 1 dB changes), this may indicate heightened central auditory processing sensitivity—even in the absence of cochlear recruitment. Distinguishing hyperacusis from phonophobia (fear of sound) is important, and SISI can contribute to that clinical decision-making.

In summary, SISI’s core clinical role is to refine the cause of hearing loss and inform appropriate treatment/device planning. If cochlear damage is identified, hearing rehabilitation (e.g., hearing aids or implants) takes center stage; if a retrocochlear condition is suspected, neurological evaluation and management (e.g., tumor workup) become priorities. SISI provides a valuable data point for this differential.

Role and Contribution in Hearing Aid Fitting

SISI results can also guide hearing aid fitting. In patients with positive SISI (high percentage, indicating recruitment), cochlear damage implies a reduced dynamic range—the gap between audibility and discomfort is narrowed. When fitting a hearing aid, low-level sounds must be sufficiently amplified while high-level sounds must be restricted to avoid discomfort. Modern digital hearing aids with multichannel compression are designed precisely for this: they automatically attenuate loud inputs and enhance soft inputs so the user hears comfortably across levels. If SISI shows that a person detects 1 dB changes, it suggests low tolerance for loud sounds and the need to carefully limit gain. This information helps the audiologist set target gains and MPO (maximum power output) during programming.

For example, if a cochlear-loss patient has an SISI of 80%, the clinician knows the patient is highly sensitive to small loudness increases. A fitting that rapidly compresses high-level inputs to prevent discomfort will be critical. Otherwise, a linear (non-compressive) device could cause sudden, uncomfortable loudness jumps. Frequency-specific SISI results are also useful: because SISI can be obtained per frequency, it can reveal recruitment confined to certain bands. If, say, only 1000 Hz yields 80% and other frequencies are negative, recruitment may be limited to a specific region; gain in those bands should be adjusted more conservatively.

Conversely, in a negative SISI case (failure to detect 1 dB changes) with sensorineural loss, the site of lesion may be retrocochlear. Expected benefit from amplification differs in such cases; retrocochlear pathologies often feature distortion and speech intelligibility issues. If SISI and other tests suggest a retrocochlear condition, the audiologist sets hearing aid expectations accordingly and may emphasize supplemental strategies such as speechreading and aural rehabilitation. While SISI’s primary role is to detect cochlear pathology, a negative SISI contributes indirectly by signaling “this may not be a typical cochlear loss.”

Finally, SISI and related suprathreshold tests are also used to gauge loudness tolerance in hearing aid users. In patients with hyperacusis or sound tolerance problems, SISI can help anticipate how much amplification is tolerable. If someone with normal thresholds still shows an unusually high SISI, they may be more sensitive to environmental sounds than expected when using a device. This would argue for softer compression and tailored noise-management strategies. In short, SISI provides clues about inner-ear sensitivity that support individualized hearing aid settings.

Advantages and Limitations

Advantages. SISI is relatively quick and noninvasive. It is easy to administer in a standard audiometric booth. It is particularly sensitive for detecting cochlear pathology—where conventional audiometry only measures threshold, SISI examines the inner ear’s response to intensity changes, adding a diagnostic dimension. For example, conditions like Ménière’s disease often yield positive SISI, supporting the diagnosis, whereas otosclerosis (a conductive loss) or retrocochlear losses tend to be negative, aiding differential diagnosis. Historically, SISI played a significant role in alerting audiologists to acoustic tumors, making it a classic in audiology. Even today, some protocols combine SISI with acoustic reflex testing to illuminate the nature of sensorineural loss. Its ability to demonstrate recruitment objectively also provides valuable information for hearing rehabilitation planning (e.g., cochlear implant candidacy or advanced digital fitting). SISI can help distinguish hyperacusis from phonophobia and can be used to anticipate a patient’s uncomfortable loudness levels.

Limitations. SISI has constraints and caveats. First, it requires a certain degree of hearing loss to be meaningful. Generally, at frequencies where thresholds are better than ~40 dB HL, SISI is unlikely to yield positive results at 20 dB SL, since in normal or mildly impaired ears without cochlear recruitment, 1 dB increments are not perceived—so the test will naturally be negative. Thus, SISI is best applied in at least moderate (~≥40 dB HL) sensorineural losses. Likewise, if hearing loss is very severe (e.g., thresholds >80 dB HL), accuracy may decline. In such cases, 20 dB SL can push absolute intensities to around 100 dB, where detection of 1 dB steps may occur for reasons unrelated to true recruitment. Indeed, sources caution against administering SISI at absolute levels above ~60 dB HL, as very high intensities can artificially facilitate detection of small changes and confound results.

Another limitation is that SISI depends entirely on subjective responses. Cooperation and attention directly affect outcomes. Young children, individuals with cognitive impairment, or those with concentration difficulties may not provide reliable responses, limiting use in these populations. Interpretation can also be challenging with intermediate scores. For example, around 40% may be ambiguous—does it indicate mild cochlear involvement or hint at a retrocochlear issue? Values in the 20–70% range are often considered “questionable” or “inconclusive,” necessitating correlation with other tests (e.g., ABR, MRI, or additional audiologic measures). Specificity is not absolute either: rarely, conditions like hyperacusis may yield sensitivity to 1 dB increments without cochlear damage. Therefore, SISI alone is not diagnostic; it must be interpreted within a comprehensive assessment.

Although routine use has declined with technological progress, the SISI test still provides unique information. Today, audiologists may prefer objective measures (e.g., brainstem responses, otoacoustic emissions, or stapedius reflexes) that can indirectly address the cochlear vs. retrocochlear question SISI targets. Even so, SISI’s advantage is that it probes this distinction by directly observing the inner ear’s behavior—its ability to detect a 1 dB difference. Consequently, when used in the right patient and for the right indication, SISI remains a valuable, albeit carefully applied and interpreted, audiologic test. The data it yields can guide both diagnosis and rehabilitation planning for ENT physicians and audiologists.

Sources

Compiled from a multilingual literature review (English, Spanish, German, and French) and rendered into Turkish in the original article. Each source supported different aspects of the SISI test (history, procedure, clinical use, device fitting, advantages, and limitations). This synthesis aims to offer a perspective that is both academically sound and accessible to readers unfamiliar with the topic.