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Special Hearing Tests

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Special Hearing Tests

All types of Auditory Disorders can be broadly classified into 2 categories, namely Peripheral Auditory Disorders and Central Auditory Disorders. The impaired ability to transmit signals, however, is a characteristic feature common to both the disorders.

With pure tone audiogram, special audiometric tests came into existence. Such special tests can further be characterized in the following number of ways:

  • Special tests using pure tone
  • Special tests using threshold level
  • Special tests for assessing cochlear and retro-cochlear pathology
  • Special tests requiring behavioural response

Special Audiometric Hearing Tests

Tone Decay Test

Tone Decay Test (TDT) is a subjective behavioural test that is used to detect and measure auditory fatigue and neural lesions such as acoustic neuroma. It’s an essentially powerful procedure for auditory nerve damage

Tone decay is measured either at or near threshold or well above it, and therefore is classified into 2 groups

  • Threshold Tone Decay: Reduction in sensitivity arising from the presence of hardly audible tone
  • Supra Threshold Tone Decay: Loss of audibility through stimulation at higher presentation level

In the case of patients with normal hearing, a tone slightly above their hearing threshold can be heard straight for 60 seconds. This test produces a measure of decibels of decay, which is the numbers of decibels above the patient’s hearing threshold.

Clinical Procedure for Tone Decay Test

Once the patient is relaxed, a tone with a frequency of 4000 Hz is passed through the patient’s ears at an intensity of 5 dB above their threshold of hearing. In case the patient stops hearing before 60 seconds, the tone’s intensity is increased by another 5 decibels.

The process is repeated until the decibel level reaches a point where the patient is able to hear the tone for straight 60 seconds, and finally, the resultant measure is produced as decibels of decay.

A decay of 0.5 dB indicates a normal hearing level, while that of 10-15 dB indicates a mild hearing loss. Any decay above 15 dB and up till 20 db suggests a cochlear hearing loss and that above 25 db suggests a vestibulocochlear nerve damage.

Short Increment Sensitivity Index Test

The Short Increment Sensitivity Index (SISI) Test is widely used to determine if a patient is having a cochlear pathology or not and is based on a phenomenon called recruitment, i.e. abnormal loudness growth.

This hearing test determines a patient’s capacity to detect a 1 decibel increment at a 20 decibel threshold tone, i.e. the carrier tone, in frequencies between 1000 and 4000 Hertz.


Clinical Procedure for SISI Test

Once the patient is relaxed, a pure tone at 20 dB is passed through their ears. At periodic intervals, a meager escalation in its intensity is superimposed (varying from 1dB to 5 dB).

Patients with normal hearing or conductive hearing loss do not identify the increments, while those with cochlear pathology do.

There are, however, certain variants to the SISI auditory test. Some of them are:

  • 1 decibel increment at 20 decibel sound level: high scores implying a cochlear lesion
  • 2-5 decibel increment: low score suggesting a retro-cochlear lesion


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