Historical Vignettes in Audiology #1 “The (SAL) Revisited”

Jeffrey Weihing, Ph.D.

Frank Musiek, Ph.D.

Recently several students have presented audiology cases of masking dilemmas related to major conductive hearing losses. Our response has often been “why not use the SAL (sensorineural acuity level) test?”. This was the impetus for us to address the present topic. The procedure remains useful in certain cases and yet is not known by many audiologists. It is most useful in cases where a masking dilemma is noted due to bilateral conductive hearing loss.

This procedure was introduced by Rainville in 1959 as an alternative to standard air- and bone-conduction testing options for measuring sensorineural hearing level (e.g., cochlear sensitivity) {{1}} [[1]] Rainville M. A new method of masking for the determination of bone conduction curves, Beltone Translations, number 11, 1959.[[1]]. In many audiology circles this procedure was referred to as the Rainville technique at the time. About a year later Jerger and Tillman published a modification of the Rainville technique supporting it’s usefulness {{2}} [[2]] Jerger J., Tillman, T. A new method for the clinical determination of sensorineural acuity level (SAL), Archives of Otolarngology, 1960; 71, 948-953.[[2]].

In a general sense, the procedure takes advantage of the fact that masking introduced by a bone oscillator will cause a greater shift in cochlear sensitivity for patients with conductive hearing loss than for those with sensorineural hearing loss.  A patient with conductive hearing loss who has an air-conduction threshold beyond normal limits will still have 0 dB HL cochlear sensitivity, while a patient with sensorineural hearing loss will have cochlear sensitivity that is generally equal to the air-conduction threshold.  As a result, introduction of masking noise via the bone oscillator will shift the air-conduction threshold of patients with a conductive hearing loss because of their normal cochlear sensitivity, but will have little or no effect on the patient with sensorineural hearing loss.

Arguably the most useful aspect of the SAL test is that it allows for measurement of sensorineural hearing level in cases of bilateral conductive hearing loss.  To perform the SAL test one does the following: thresholds to tonal or speech stimuli are obtained in quiet via air conduction, and then a second time while masking is introduced through the bone oscillator.  Masking is generally presented at equipment limits for the stimulus used and the bone oscillator is placed on the forehead. Sensorineural hearing level can be calculated by subtracting the difference of the masked threshold and the degree of effective masking used from the threshold in quiet. Put another way, Sensorineural Hearing Level = Threshold in Quiet – (Masked Threshold – Effective Masking Level) {{3}} [[3]] Simon, G.  The sensorineural acuity level (SAL) technique with narrow band noise J. Aud. Res., 1964; 3 , 191 -194.[[3]].

As an example, take the case where a bilateral conductive hearing loss is present yielding air-conduction thresholds of 50 dB HL in both ears.  Introduction of 60 dB of effective masking via bone conduction (e.g., 60 dB dial reading on most audiometers) would shift the air-conduction threshold to 110 in both ears.   To compute the actual sensorineural hearing level, the computation in the previous paragraph is made: 50 – (110 – 60), or 0 dB HL.

In contrast, consider a patient with a mild, bilateral sensorineural hearing loss. As with the previous example, this patient will also have air-conduction thresholds of 50 dB HL bilaterally, though in this case it is because of cochlear dysfunction.  Introduction of 60 dB of effective masking shifts the threshold only slightly to 60 dB HL.  Again, to obtain the sensorineural threshold we compute: 50 – (60 – 60), or 50 dB HL.  This result indicates that the hearing loss is sensorineural.

Research has investigated the relative value of the SAL technique in the clinic.  Due to the unique bone oscillator placement and manner of masking, normative values for bone oscillator force level have been established {{4}} [[4]] Beattie R., Daily, L. Bone conduction force values for sensorineural acuity level (SAL) test.  Journal of Auditory Research, 1982; 22, 51-60.[[4]].  It is good practice, however, to establish these values on a clinic specific basis.  Variability in SAL responses is generally similar to other audiometric tests, such as air-conduction pure-tone audiometry {{5}} [[5]] Jerger, J. Threshold shift variability in SAL technique.  Archives of Otolaryngology, 1965; 81, 641-642.[[5]].  It has also been shown to be a valid indicator of conductive pathology in true clinical cases {{6}} [[6]] Jerger, J., Verville, J., Shea, J., Kinchen. Comparison of SAL and bone conduction audiometry in the prediction of gain from stapes surgery. Archives of Otolaryngology, 1965; 60, 467-478.[[6]], although an occlusion effect can be introduced in the low frequencies {{7}} [[7]] Goetzinger, C., Porter. T. Study of the sensorineural acuity test.  Journal of the American Auditory Society, 1976; 1, 135-144.[[7]].  Finally, alternative SAL methodologies have also been suggested for threshold auditory brainstem response testing, showing some success {{8}} [[8]] Gayle, E., Hicks, M. Auditory brainstem response sensory assessment by bone conduction masking. Archives of Otolaryngology, 1980; 106, 392-395.[[8]]; {{9}} [[9]] Webb, K. C., & Greenberg, H. J. Bone-conduction masking for threshold assessment in auditory brain stem response testing. Ear and Hearing, 1963; 4, 261–266.[[9]].