Asymmetrical Low Frequency Hearing Loss and Acoustic Neuroma

Question

Answer

While an asymmetric hearing loss, most often in the higher frequencies, is typically considered a red flag for a retrocochlear pathology, one couldn't rule out the possibility of an asymmetric low frequency hearing loss resulting from the presence of an acoustic neuroma. There are reported incidences of tumor patients with only low-frequency hearing losses (Johnson, 1977).

An acoustic neuroma (also known as a vestibular schwannoma) is an abnormal growth of tissue impinging on the 8th cranial nerve. It should be noted that the 8th cranial nerve actually has several branches that course through the internal auditory canal along with the facial nerve. Those branches include the auditory branch, the superior vestibular branch, and the inferior vestibular branch. The anatomy needs to be considered, as the nerve fibers affected will depend on the location of the tumor. High frequency fibers from the basal turns of the cochlea are located inferiorly and superiorly. Low frequency fibers from the apical turns of the cochlea are located in the medial portion of the auditory nerve adjacent to the interior vestibular branch.

Standard click ABR has been used for years as a neurodiagnostic test to screen for the presence of an 8th nerve tumor impinging on the auditory branch. However, standard latency measures used to characterize abnormal ABR findings may not detect a tumor affecting low frequency nerve fibers because it is the activity and synchronous firing of the higher frequency nerve fibers which determine ABR latency. While clicks do stimulate a wide frequency region of the cochlea, time delays along the basilar membrane cause phase cancellation of lower frequency responses, so the resultant ABR is based upon the activity of those nerve fibers associated with higher frequencies. If a tumor does not affect enough of these fibers, as is the case for small acoustic neuromas, or acoustic neuromas affecting lower frequency fibers, the ABR will miss them.

A new method to called "Derived Band" or "Stacked" ABR has been developed by Dr. Manuel Don and his colleagues in an effort to detect smaller tumors (Don and Kwon, 2002). If you have a patient who you suspect as having an acoustic neuroma, the gold standard test is still the standard click ABR. However, should the standard ABR results come back within normal limits, a Stacked ABR can then be performed.

The Stacked ABR method essentially measures activity from the cochlea using broadband stimuli and dividing it up into 5 octave bands. These bands are derived by presenting click stimuli in the presence of a high pass masking noise. The high pass cutoff frequency is progressively lowered masking more and more basal regions of the cochlea. The waveform derived by the subtraction of a given waveform from the previous waveform produces a response which represents activity from the frequency region in between the two high pass cutoff frequencies. Due to the nature of the traveling wave along the basilar membrane, as the derived waveforms represent activity from more apical regions, the wave V latency is prolonged. To compensate for latencies shifts, the wave V component for each of the derived waveforms is aligned (or "stacked"), and then the waveforms are added together. The amplitude of the summated wave V is then measured. Should the amplitude of the summated wave V be smaller than the normative value, it would suggest that activity from one or more of the octave regions from each of the derived octave bands has been affected by the presence of a tumor.

While the 'Stacked ABR' has been demonstrated to show good sensitivity and specificity to small tumor detection, it is not available at all clinics. Currently, you can obtain the Stacked ABR test for your clinic through Bio-Logic Systems Corp. If Stacked ABR is not available to you, and you have a patient with an asymmetric hearing loss that passes a standard ABR test, it would not necessarily be a bad decision to suggest an MRI to rule out the presence of a small tumor.

References:

Johnson, E.W. (1977). Auditory test results in 500 cases of acoustic neuroma. Arch Otolaryngol., 103 (3), 152-158.

Don, M. and Kwong, B. (2002). Auditory brainstem response: Differential diagnosis. In J. Katz (Ed. )Handbook of Clinical Audiology, (5th ed). (pp 274-297). Baltimore: Lippincott, Williams & Wilkins.

For further information on Stacked ABR see the following references:

Don M., Masuda A., Nelson R., & Brackmann D. (1997). Successful detection of small acoustic tumors using the stacked derived-band auditory brain stem response amplitude. American Journal of Otology, 18(5), 608-621.

Don, M. (2002). Auditory brainstem response testing in acoustic neuroma diagnosis. Current Opinion in Otolaryngology & Head and Neck Surgery, 10, 376-381.

Don M., Kwong B., Tanaka C., Brackmann D. & Nelson, R. (2005). The stacked ABR: a sensitive and specific screening tool for detecting small acoustic tumors. Audiol Neurootol., 10(5), 274-290.

Dr. Tina Prout is an Assistant Professor at the University of Northern Colorado. She specializes in auditory electrophysiology. Her research interests include auditory evoked potentials, auditory processing disorders, and cochlear implants.