The tone decay test (also known as the threshold tone decay test or TTDT) is used in audiology to detect and measure auditory fatigue. It was developed by Raymond Carhart in 1957. In people with normal hearing, a tone whose intensity is only slightly above their absolute threshold of hearing can be heard continuously for 60 seconds. The tone decay test produces a measure of the "decibels of decay", i.e. the number of decibels above the patient's absolute threshold of hearing that are required for the tone to be heard for 60 seconds. A decay of between 15 and 20 decibels is indicative of cochlear hearing loss. A decay of more than 25 decibels is indicative of damage to the vestibulocochlear nerve.

PROCEDURE

A tone at the frequency of 4000 Hz is presented for 60 seconds at an intensity of 5 decibels above the patient's absolute threshold of hearing. If the patient stops hearing the tone before 60 seconds, the intensity level is increased by another 5 decibels with the procedure repeated until the tone can be heard for the full 60 seconds or until no decibel level can be found where the tone can be heard for the full 60 seconds. The resultant measure is given as the decibels of decay.

INTERPRETATION OF TDT

0-5 dB Decay - Normal or Conductive

10-15 dB Decay - Mild

20-25 dB Decay - Moderate

30->35 dB Decay - Marked Decay

ADVANTAGE

• Low cost and general accessibility

DISADVANTAGE

• Pathophysiologic essence of tone decay is not very well known. The actual value of any tone decay procedure in accurately identifying 8 cranial nerve pathology has not been extensively investigated