Acoustic Reflect Tests:  Caution using high intensity stimulus

Acoustic reflex tests are valuable assessment tools used to help determine whether hearing loss is due to middle ear, cochlear, or retrocochlear pathology.  Acoustic reflex test procedures for those individuals who may have retrocochlear pathology requires presenting stimuli at very high intensities.  In addition, at these high intensity levels, acoustic reflex responses are often absent with retrocochlear pathologies.  The purpose of this article is to inform Audiologists of the inherent dangers of testing at high intensities and what must be done to prevent causing permanent threshold shifts due to unsafe testing procedures.  

Published reports have made several recommendations regarding which levels of testing are within safe levels.  Wilson & Margolis (1999) state that testing should not exceed 110 dB SPL.  Jacobson & Northern (1991) say that testing should not exceed 110 dB HL (see decimal scales).  If this is the case, why then are commercially available instruments allowing for testing at levels in excess of 120 dB HL or higher?  There have been documented examples of clients with SNHL who have suffered permanent threshold shift due to high stimulus intensity testing (Hunter et al. 1999). 

Individuals have a wide range of susceptibility to noise induced PTS, and there is not a reliable method of predicting who is more susceptible than others. 

As we know, pure tone stimuli focuses a lot of sound energy on a small area of the basilar membrane and according from data obtained by Ward 1991, PTS has not been evident with signal levels below 120 dB SPL.  Audiologists need to know however, that 120 dB SPL will vary in dB SPL, depending on the frequency of the stimulus, the earphone type and the size of the ear canal.  With this in mind, Hunter et al. 1999 recommend an upper limit of 120 dB SPL for acoustic reflex testing.  It is important to note however, that smaller ear canal volumes will result in an increased SPL over larger ear canals (thus extra care must be taken when assessing young children and infants).  It is the Audiologist's responsibility to determine the SPL of the reflex activating stimulus through calibration procedures to determine whether or not the signal intensity is within safe limits for testing.   

The increase in ART as a function of hearing level also bears a direct relationship to the percent of ears that can be tested at safe levels with ARD tests, because ARD is performed 10 dB above the ART level. 

According to the Hunter et al. 1999 study, all those individuals who produce measurable acoustic reflex responses above 105 dB SPL will not be testable for reflex decay due to safety reason.  That is because reflex decay is measured at 10 dB above the level of the reflex decay, or in this case, above 115 dB SPL. 

The figure of 115 dB SPL is not a standardized value for the upper limits of ARD testing.  In fact, ANSI S3.39 (1987) provides specifications for aural acoustic immittance instruments, but does not specify reference values for reflex activator stimuli.  I am merely bringing it to the attention of Audiologists and related hearing professionals that there is documented evidence that cases of permanent threshold shift have occurred with stimulus levels above 115 dB SPL.

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