A recently-released Danish study shows the auditory brainstem evoked response (ABR) test only detects 77-80% of vestibular schwannomas (VS; acoustic neuroma),1 and a recent Mass Eye & Ear study seems to explain why by detailing a newly-discovered second way a VS causes hearing loss due to ototoxic secretions causing sensorineural hearing loss.2 This article will explain the linkage of these two findings; and more importantly, why both the speech rollover test and diagnostic ABR should no longer be depended upon when an asymmetrical hearing loss is detected, to rule out a potentially lethal schwannoma.
The study at Massachusetts Eye and Ear showed that in some cases of vestibular schwannoma, a sometimes-lethal tumor often associated with neurofibromatosis Type 2 (NF2), secretions from the tumor contain toxic molecules that damage the inner ear. The findings, published online in Scientific Reports, explain why some vestibular schwannomas cause sensorineural hearing loss even though they are not large enough to compress nearby structures such as the auditory nerve which control hearing, hence possibly not showing up on a diagnostic ABR.
The vestibular schwannoma, also known as an acoustic neuroma, is a benign tumor which attaches itself to the auditory nerve bundle. Often associated with neurofibramatosis type II (NF2),4 in fact these can occur at random when NF2 is not present. However, in many cases VS-induced asymmetrical hearing loss is the first symptom of NF2, which is why it is crucial for the clinician to radiologically investigate all asymmetrical hearing losses.
“What’s written in textbooks is that these tumors cause hearing loss by growing to the point of compressing the auditory nerve,” said senior author and Mass Eye & Ear otologic surgeon Konstantina M Stankovic, MD, PhD. “We knew that it couldn’t be as simple as that, because there are large tumors that do not cause hearing loss and little ones that do.”
Let’s unpack these last three paragraphs: Traditionally, the hallmark of a VS (either NF2 or random) pressing on the auditory nerve causes neural firing dys-synchrony (a member of the ANSD spectrum), triggering abnormal results in the speech rollover test, i.e. meaning any hearing professional bothering to perform this basic test would catch a VS, as the louder presentation level would uncover the distortion the patient is hearing via the lower score.
However, what this study shows is that there is a second mechanism a VS can cause hearing damage that will not show up on the speech rollover test; and as we will discuss the Danish study1 in a moment, neither on a diagnostic ABR. Apparently, a percentage of VS’ which can be too small to exert enough pressure on the auditory nerve to cause loss of synchrony at the dorsal cochlear nucleus can sometimes emit the ototoxic compound TNFa, which has been implicated in other forms of sensorineural hearing loss… And although this damage mechanism would not reveal itself on the rollover or ABR, it would show up on the pure tone audiogram. This is why we admonish all hearing care professionals to refer patients who have either an asymmetrical SNHL component .OR. fail the speech rollover test to a competent otologist or neurologist for radiologic followup.
Wrapping a bow around the Boston study, we have a Danish study comparing the effectiveness of the diagnostic ABR to the MRI. Interestingly, this was a cost-benefit study, the cost for an ABR was €326 vs just €319 for the MRI (about $350). From the study’s conclusion,
In the past, ABR was a valid and cost-effective tool when screening for VS, but considering the improved availability and reduced cost of MRI, we conclude that contemporary ABR is medically and financially obsolete in VS screening. Contemporary ABR is not valid as a screening tool when considering the low sensitivity, the low specificity and the low positive predictive value.
- For asymmetrical thresholds, ABR and speech rollover test will miss VS’ which emit toxic secretions but are too small to press against the auditory nerve;
- For symmetrical thresholds the ABR & speech rollover will detect the non-toxin secreting VS if it is large enough to press against the AN due to the different damage mechanism;
- MRI followup by an otologist is mandatory for either asymmetrical hearing thresholds .OR. failed speech rollover test (with or without confirming ABR).
- Auditory brainstem response: A valid and cost-effective screening tool for vestibular schwannoma? Rafique I1, Wennervaldt K1, Melchiors J1,2, Caye-Thomasen P1,2. Acta Otolaryngol. 2016 Jul;136(7):660-2. doi: 10.3109/00016489.2016.1157726. Epub 2016 Apr 4;
- Secreted Factors from Human Vestibular Schwannomas Can Cause Cochlear Damage. Sonam Dilwali, Lukas D. Landegger, Vitor Y. R. Soares, Daniel G. Deschler, & Konstantina M. Stankovic, Scientific Reports, December 22, 2015;
- Toxic secretions from intracranial tumor damage the inner ear. Mass Eye & Ear press release by Suzanne Day, December 22, 2015;
- Neurofibramatosis Type II. National Library of Medicine, Genetics Home Reference. Retrieved September 5, 2016;
Hat tip: Brainstem Auditory Evoked Responses (BAER or ABR): Timothy Hain MD, Dizziness-and-balance.com