Where the SISI probes how loudness grows, the tone-decay test probes whether a tone can be sustained. A nerve that fatigues abnormally fast — pathological adaptation — is a strong pointer toward retrocochlear disease.

Adaptation and abnormal adaptation

Even a normal ear shows a little adaptation: hold a faint continuous tone and it may seem to fade slightly. A diseased auditory nerve does this to a pathological degree. The tone simply disappears for the listener within seconds, even though it is still being presented at the same level. Measuring how much extra intensity is needed to keep the tone audible quantifies that abnormal adaptation.

The Carhart threshold tone-decay test

The classic procedure, described by Carhart, is straightforward.¹A continuous tone is presented at 5 dB above the patient’s threshold — 5 dB sensation level. The patient signals for as long as the tone is heard. If it fades before 60 seconds, the level is raised by 5 dB and timing starts again. This repeats until the tone is heard for a full minute, or until the audiometer runs out of output. The total rise in dB is the tone decay.

Total decay: 15 dB

MILD decay

Decay of 15 dB (mild). Adaptation is present but below the threshold of strong retrocochlear suspicion (30 dB).

Figure 1. The tone-decay staircase. Each time the tone fades within 60 seconds the level rises 5 dB. The total rise is the decay. Use the slider to set the decay and see how the staircase and the grade change.

Grading the result

The amount of decay is graded into broad bands, following the scheme attributed to Rosenberg:⁵

0–5 dB — normal; also the expected finding in a purely conductive loss.
10–15 dB — mild decay.
20–25 dB — moderate decay.
30 dB or more — marked decay, which strongly raises the suspicion of a retrocochlear lesion.

Trainee The 30 dB figure is a guide, not a verdict. A more cautious way to read the test is that every decibel of decay above about 15 dB should raise concern incrementally — marked decay makes a retrocochlear lesion likely, not certain, and modest decay does not exclude one. Cochlear losses, particularly severe ones, can show some decay too. The test is a pointer that earns its place in a battery, not a stand-alone diagnosis.

Clinician SISI and tone decay make a natural pair, and together they sketch the cochlear-versus- retrocochlear distinction. A cochlear lesion classically gives a positive SISI (recruitment present) and little tone decay. A retrocochlear lesion classically gives a negative SISI (no recruitment) and marked tone decay. Like the SISI, threshold tone decay has been largely overtaken by auditory brainstem response testing and MRI for actually detecting tumours — but as a teaching tool it makes pathological adaptation tangible, and the cochlear/retrocochlear contrast it draws is exactly the one the disease modules build on.

A note of caution

The chapter would be misleading if it left the impression that a number from a behavioural test settles a diagnosis. It does not. Tone decay, the SISI and the audiogram together narrow a differential and direct further investigation. Marked tone decay should prompt imaging and specialist assessment, not a conclusion.