The Short Increment Sensitivity Index, or SISI, asks a simple question: can the patient hear very small changes in loudness? The answer turns out to separate cochlear hearing loss from the alternatives, because the underlying mechanism — recruitment — is a cochlear phenomenon.

Recruitment, briefly

In a healthy cochlea the outer hair cells provide an active amplifier that boosts quiet sounds far more than loud ones. When outer hair cells are damaged, that compression is lost: above threshold, loudness then grows abnormally steeply. A small increase in physical intensity produces a large jump in perceived loudness. This abnormally rapid growth of loudness is recruitment, and it is a hallmark of cochlear pathology.

Trainee Recruitment explains why a person with a cochlear loss may say a sound is inaudible one moment and uncomfortably loud the next, with little room in between. The same steep loudness growth is what makes them unusually good at detecting tiny intensity increments — which is exactly what the SISI measures.

The test

The SISI presents a steady carrier tone at 20 dB above the patient’s threshold for that frequency — that is, at 20 dB sensation level. Riding on that steady tone, the audiometer introduces twenty brief 1 dB increments, and the patient signals each time a small jump in loudness is detected.³

Increments detected: 14 / 20

SISI score: 70%POSITIVE

High sensitivity to 1 dB increments — favours a cochlear site of lesion (abnormal loudness growth / recruitment).

Figure 1. The SISI carrier tone and its 1 dB increments. A steady carrier tone carries twenty brief 1 dB increments. Use the controls to step through the increments and to set how many are detected; the score and interpretation update from the verified scoring rule.

Scoring

The score is the number of increments the patient detected, multiplied by five, giving a percentage out of twenty possible increments. The interpretation follows three broad bands:

70–100% — positive. The patient detects most of the tiny increments. This favours a cochlear site of lesion, where recruitment makes small loudness changes easy to hear.
0–30% — negative. The patient detects few increments. This favours a retrocochlear lesion, a conductive loss, or a normally hearing ear, none of which produce recruitment.
31–69% — questionable. An intermediate result that cannot be interpreted confidently on its own.

Trainee The exact band boundaries vary slightly between sources — some place the cochlear cut-off at 70%, others a little higher — so the SISI is best read as part of a battery rather than in isolation. A high-level variant presents the carrier tone at a high intensity instead of a fixed 20 dB SL; at high levels even normal and cochlear-impaired ears score high, so a low high-level score becomes a more specific pointer to retrocochlear pathology.⁸

ClinicianThe SISI was developed before modern imaging and electrophysiology, and as a stand-alone site-of-lesion test it has been largely superseded for detecting retrocochlear disease. Its enduring teaching value is conceptual: it makes recruitment measurable and concrete, and it illustrates the central diagnostic move of audiometry — using a behavioural response to localise a lesion within the auditory system. A positive SISI remains useful confirmatory evidence of a cochlear process in conditions such as Ménière’s disease and noise-induced hearing loss.