Tests to identify cochlear and retro cochlear pathology

A common thread among many of the traditional behavioral tests for distinguishing cochlear and retrocochlear disorders is the perception of intensity and how it is affected by pathology. We shall see, however, that the ability of these kinds of tests to confidently separate cochlear and retrocochlear disorders has actually been rather disappointing, and their use has decreased over the years (Martin, Champlin, & Chambers 1998).

In spite of this, we will cover these tests in some detail not just because one will find the need to use them from time to time, but also because this knowledge provides the future clinician with (a) insight into the nature of hearing impairment, (b) familiarity with

various approaches used to assess auditory skills, and (c) the all-important foundation needed for understanding the literature in the field.

In clinical practice, Tone Decay is either measured at or near threshold or well above threshold, thus Tone Decay can be classified into 2 groups:

1.      Threshold Tone Decay

2.      Supra Threshold Tone Decay

Threshold Tone Decay (TTD) is defined as reduction in the sensitivity resulting from the presence of a barely audible tone while Supra Threshold Tone Decay refers to loss of audibility as a result of stimulation which is presented at higher presentation level.

Threshold Tone Decay Tests

A continuous tone sounds less loud after it has been on for some time compared with when it was first turned on, or it may fade away altogether. The decrease in the tone’s loudness over time is usually called loudness adaptation, and the situation in which it dies out completely is called threshold adaptation or tone decay. Adaptation is due to the reduction of the neural response to continuous stimulation over time, and is common to all sensory systems (Marks 1974). Adaptation per se is a normal phenomenon, but excessive amounts of adaptation reflect the possibility of certain pathologies. It is for this reason that adaptation tests are often used as

clinical site-of-lesion tests. Most clinical adaptation procedures are threshold tone decay tests, which measure adaptation in terms of whether a continuous tone fades away completely within a certain amount of time, usually 60 seconds. The patient’s task is easily understood in terms of these typical instructions: “You will hear a continuous tone for a period of time, which might

be a few seconds or a few minutes. Raise your finger (or hand) as soon as the tone starts and keep it up as long as the tone is audible. Put your finger down whenever the tone fades away. Pick it up again if the tone comes back, and hold it up as long as you can still hear it. It is very important that you do not say anything or make any sounds during this test because that would interrupt the tone. Remember, don’t make any sounds, keep your finger raised whenever you hear the tone, and down whenever you don’t hear it.” Because the patient may be holding

his hand or finger up for some time, it is a good idea to have him support his elbow on the arm of his chair. Many audiologists have the patient press and release a response signal button instead of holding up and lowering his finger or hand. Carhart (1957) suggested that tone decay tests (TDTs) be administered to each ear at 500, 1000, 2000, and 4000 Hz, but most audiologists select the frequencies to be tested on a patient-by-patient basis. Both ears should be tested at each frequency selected because this permits the clinician to compare the two ears as well as to determine whether abnormal tone decay is present bilaterally. Of course, each ear is tested separately.

Carhart Tone Decay Test

In the Carhart threshold tone decay test (1957), a test tone is presented to the patient at threshold (0 dB SL) for 60 seconds. If the patient hears the tone for a full minute at the initial level, then the test is over. However, if the patient lowers his finger, indicating that the tone faded away before 60 seconds have passed, then the audiologist (1) increases the level by 5 dB without interrupting the tone, and (2) begins timing a new 60-second period as soon as the patient raises his hand. If the tone is heard for a full minute at 5 dB SL, then the test is over. However, if the tone fades away before 60 seconds are up, then the level is again raised 5 dB and a new minute is begun. This procedure continues until the patient is able to hear the tone for 60 seconds, or until the maximum limits of the audiometer are reached. Tone decay test results are expressed as the amount of tone decay, which is simply the sensation level at which the tone was heard for 60 seconds. For example, if the tone was heard for 1 minute at threshold, then there would be 0 dB of tone decay; and if the tone was heard for 60 seconds at 5 dB SL, then there was 5 dB of tone decay. Similarly, if the tone could not be heard for a full minute until the

level was raised to 45 dB SL, then there would be 45 dB of tone decay. Normal individuals and those with conductive abnormalities are expected to have little or no threshold adaptation. Cochlear losses may come with varying degrees of tone decay, which may range up to perhaps 30 dB, but excessive tone decay of 35 dB or more is associated with retrocochlear pathologies (Carhart 1957; Tillman 1969; Morales-Garcia & Hood 1972; Olsen & Noffsinger 1974; Sanders, Josey, & Glasscock 1974; Olsen & Kurdziel 1976). Thus, if the TDT is viewed as a test for retrocochlear involvement, then ≤ 30 dB of tone decay is usually interpreted as “negative,” and > 30 dB of tone decay is “positive.” Tone decay test outcomes should be documented separately for each ear in terms of the number of decibels of tone decay at each frequency tested, to which one might add an interpretation (such as “positive” or “negative”). One should never record “positive” or “negative” without the actual results. You can always figure out whether a result was positive or negative from the amount of tone decay, but you could never go back and deduce the actual amount of tone decay from a record that says only “positive” or “negative.” These points apply to all diagnostic procedures.

 Olsen-Noffsinger Tone Decay Test

The Olsen-Noffsinger tone decay test (1974) is identical to the Carhart TDT except that the test tone is initially presented at 20 dB SL instead of at threshold. Beginning at 20 dB SL is desirable for several reasons. It makes the test simpler for the patient to take because a 20 dB SL test tone is much easier to detect than one given at threshold. It is also easier to distinguish it from any tinnitus that the patient may have. In addition, starting the test at 20 dB SL can shorten the test time by as much as 4 minutes for every frequency tested. Reducing the test time makes the experience less fatiguing for the patient and saves clinician time, which is always at a premium mium. The Olsen-Noffsinger modification relies on the premise that amounts of tone decay up to 20 dB on the Carhart TDT are interpreted as negative. Thus, omitting the test trials that would have been given at 0 to +15 dB SL should not change any diagnostic decisions. It has been found that the Carhart and Olsen- Noffsinger procedures yield similar results in terms of when the results are positive versus negative (Olsen & Noffsinger 1974; Olsen & Kurdziel 1976). The outcome of the Olsen-Noffsinger TDT is recorded as follows: If the patient hears the initial (20 dB SL) test tone for a full minute, then one records the results as “≤ 20 dB tone decay.” Greater amountsof tone decay are recorded in the same way as for theCarhart TDT. The Olsen-Noffsinger TDT Is sometimes misconstruedas a tone decay “screening” test because mostpatients are able to hear the initial test tone for the full 60 seconds. It should be stressed that the reason why many patients do not have to be tested beyond the 20 dB SL starting level is simply that they do not have more than 20 dB of tone decay. One should remember that the Olsen-Noffsinger is a full-fledged TDT that yields the actual amount of significant tone decay > 20 dB, just like the Carhart procedure.

Other Modifications of the Carhart Tone Decay Test

There are several other modifications of the Carhart TDT of which the student should be aware. The Yantis (1959) modification begins testing at 5 dB SL insteadof at threshold. This modification is so commonly used that it is not distinguished from the Carhart by

most clinicians. Sorensen’s (1960, 1962) modification requires the patient to hear the test tone for 90 seconds instead of 60 seconds, and is performed only at 2000 Hz. This procedure is rarely used. The Rosenberg (1958,1969) modified tone decay test begins like the Carhart test but lasts only 60 seconds from start to finish. If the patient hears the tone for 60 seconds at threshold, then the test is over and there is 0 dB of tone decay. If the tone fades away before the end of one minute, then the clinician does the following: As with the Carhart TDT, she increases the intensity in 5 dB steps without interrupting the tone until the patient raises his hand. Every time the patient lowers his hand, the audiologist again raises the tone in 5 dB steps until the patient hears the tone again, and so on. However, unlike the Carhart TDT, she does not begin timing a new minute with every level increment. Instead, the clock keeps running until a total of 60 seconds has elapsed since the tone was originally turned on. The amount of tone decay is the sensation level reached at the end of 60 sec onds. For example, if the threshold was 35 dB HL, the tone starts at this level and one begins timing for 60 seconds. If the attenuator has been raised by a total of 25 dB to 60 dB HL at the end of one minute, then there has been 25 dB of tone decay. Notice that the Rosenberg test ignores how long the tone was actually heard at any given level. Green’s (1963) modified tone decay test involves administering the Rosenberg 1-minute test with a

significant change in the instructions. The patient is told to lower his hand completely if the tone fades away and to lower his hand partially if the tone loses its tonal quality (even though it might still be audible). The modified instructions are based on the observation that some patients with retrocochlear pathologies hear a change in the character of the tone in which it loses it tonal quality, becoming noise-like, before its audibility is completely lost (Pestalozza & Cioce 1962; Sorensen 1962; Green 1963). This phenomenon is called atonality or tone perversion (Parker & Decker 1971).

Owens Tone Decay Test

Owens (1964a) introduced a modification of a tone decay procedure originated by Hood (1955). Unlike the Carhart test and its modifications, which concentrate on the amount of adaptation, the Owens tone decay test focuses upon the pattern of tone decay. The test begins by presenting a continuous test tone at 5 dB SL. As with the Carhart TDT, the Owens test ends if the patient hears the tone for 60 seconds at this initial level. However, if the tone fades away before 60 seconds, the tone is turned off for a 20-second rest (recovery) period. After the 20-second rest, the tone is reintroduced at 10 dB SL (i.e., 5 dB higher), and a new 60-second period begins. If the tone is heard for a full minute at 10 dB SL, then the test is over. However, if the tone fades away before a full minute, then the tone is turned off for another 20-second rest period, after which it is given again at 15 dB SL. The same procedure is followed for the 15 dB SL tone. If necessary, the tone is presented for

another 1-minute period at 20 dB SL, but this is the last level tested regardless of whether the tone is heard for 60 seconds or less. The audiologist records how many seconds the tone was heard at each of the levels presented, and the test is interpreted in terms of the pattern of how many seconds the tone was heard at each of the four test levels. Fig.1 shows the various patterns (types) of tone decay described by Owens (1964a). The type I pattern involves being able to hear the initial (5 dB SL) tone for a full minute, and is associated with normal ears and those with cochlear impairments.

Overall Assessment of Threshold Tone Decay Tests

Tone decay appears to be the only classical site-oflesion technique that is still routinely used by a majority of audiologists (Martin et al 1998). Several studies have compared the accuracy of threshold adaptation tests as indicators of retrocochlear pathology (e.g.,

Parker & Decker 1971; Olsen & Noffsinger 1974; Sanders, Josey, & Glasscock 1974). Overall, they have shown the Carhart-type TDTs are most sensitive procedures. This appears to hold true whether the test begins at threshold, 5 dB SL (Yantis 1959), or 20 dB SL (Olsen & Noffsinger 1974). This kind of TDT is thus the one of choice, with the Olsen-Noffsinger modification being the most efficient. The Owens TDT is particularly valuable when the severity of a hearing loss makes it impossible to determine the amount of tone decay using the Carhart or similar procedures (Silman et al 1981). In contrast, the Rosenberg 1-minute TDT is not as effective at identifying retrocochlear lesions as the Carhart, Olsen-Noffsinger, or Owens procedure (Parker & Decker 1971; Olsen & Noffsinger 1974) and is not recommended. Green’s modification of the Rosenberg TDT has not been compared with other tests that do not use the atonality criterion. It is not clear whether atonality per se should be used as a criterion for tone decay testing because little if any research actually addresses this issue. More tone decay can be obtained when the patient responds to either atonality or inaudibility compared with inaudibility alone. However, the experience of the author and his colleagues (e.g., Silman & Silverman 1991) has been that using the atonality criterion increases the number of false-positive TDT results, and this is especially problematic when testing elderly patients.

Suprathreshold Adaptation Test

Jerger and Jerger (1975a) suggested a tone decay test performed at high levels instead of beginning at threshold, called the suprathreshold adaptation test (STAT). Here, a continuous test tone lasting a total of 60 seconds is presented at 110 dB SPL.2 (This corresponds to ~ 105 dB HL when the test is done at 1000 Hz, and to 100 dB HL when testing at 500 Hz or 2000 Hz.) As with threshold tone decay tests, the patient is told to keep her hand raised as long as she hears the tone, and to lower her hand if it fades away completely. If the high-intensity tone is heard for the full minute, then the test is over and the result is negative. If the tone fades away before 60 seconds are up, then the patient is retested with a pulsing tone for confirmatory purposes. If the patient keeps her hand up for the full 60 seconds in response to the pulsing tone, then her failure to keep responding to the continuous tone is attributed to abnormal adaptation. The test is thus confirmed to be positive, suggesting a retrocochlear disorder. However, if she fails to respond to the pulsing tone for 1 minute, then the test result is not considered to be valid because tone decay should not occur with a pulsed tone. The correct identification rates for cochlear and retrocochlear cases, respectively, are 100% and 45% when the STAT is done at 500 and 1000 Hz, 95% and 54% at 500 to 2000 Hz, and 13% and 70% at 500 to 4000 Hz (Jerger & Jerger 1975a; Turner et al 1984).