TDT Hearing Test، TD is a powerful diagnostic procedure for RetroCochlearPathology (damage to the auditory nerve). However, It is only one of the tests of battery that has been considered sensitive for differential diagnosis between Cochlear Pathology and Retro Cochlear Pathology. According to Rosenberg, 1958:
0-5 dB Decay – Normal or Conductive
10-15 dB Decay – Mild
20-25 dB Decay – Moderate
30->35 dB Decay – Marked Decay
decay frequency vs time
Marked tone decay almost always indicates RCP. Glaslow, 1968 stated that positive TD is one where there is at least 30 dB decay. Tillman, 1969 agreed that patients with RCP, typically have TD exceeding 30 dB. However, at the same time it would be dangerous to assume that anyone with 30 dB decay, has RCP. While everyone with less than this amount, does not have. A more predictive way of looking at TD is that each dB of decay above 15 dB, should raise the suspicion that RCP lesion may exist. The greater the TD and the number of frequencies involved, particularly the low frequencies, and then there is greater possibility of serious pathology. The index of suspicion should also be raised if the rate of decay does not diminish with increased stimulus intensity. Patients with acoustic tumor, frequently exhibit extreme an often complete TD. However, tumor size appears to be related to the severity of symptoms. Partial or complete TD was found in 60% of tumors classified as large, while, 40% of tumor is classified as small..
Fowler noted that equal loudness between the recruiting impaired ear with normal ear can be achieved only with larger sensation levels (SLs) to the normal ear. E.g. A tone at SL of 60 dB in normal ear and 30 dB in impaired ear may sound equally loud. This result suggests that the growth of loudness requiring an intensity increase of 60 dB in normal ear is achieved with an intensity increase of 30 dB in impaired ear. This indicates that recruitment for loudness growth must be occurring much more in impaired ear. This is due to abnormality in cochlea such as hypersensitivity of haircells due to damage. Recruitment is a landmark feature of SNHL of cochlear origin. Reverse Recruitment / Decruitment is a hallmark feature of SNHL of Retro Cochlear region. When recruitment is found to be associated with presence of cochlear pathology then the recruitment is known as complete recruitment. When the recruitment is associated with cochlea then the concept is known as Partial Recruitment.