Frank Musiek, Ph.D.
Peripheral hearing loss is not commonly associated with multiple sclerosis (MS). Rather, MS is often associated with central auditory dysfunction not usually portrayed by pure tone hearing loss. However, sensorineural hearing loss (SNHL) associated with MS, reflected by pure tone threshold deficits does occur. Though not always the case, loss of hearing sensitivity in MS can be attributable to damage to the auditory nerve. There is logic to this. That is, MS is a disease that affects myelin integrity, and the auditory nerve is wrapped in a myelin sheath. It follows then, if MS damages myelin in the auditory nerve, SNHL would likely evolve. So let’s take a quick look at myelin and the auditory nerve.
There is no question that by far, most myelin in the auditory system is not in the auditory nerve but rather the central auditory system. This is why auditory dysfunction associated with MS is most often related to the central auditory system and noted by poor performance on behavioral, electrophysiologic, and electroacoustic central auditory tests. However, that avenue of discussion is not our aim here.
The auditory nerve is about 22 – 26 mm long, coursing from the terminal buttons of the hair cells to the root entry zone1 of the cochlear nucleus in the pons. Type I auditory nerve fibers connect to inner hair cells (IHCs). They are essentially unmyelinated as they leave the IHCs but become myelinated along their pathway through the internal auditory meatus (IAM). Type II auditory nerve fibers are much fewer in number (5- 10% of total). They are thin and mostly unmyelinated throughout their length.
The myelin covering the auditory nerve is considered Schwann myelin (meaning that it originates from Schwann cells). The greater the amount of myelin on a given nerve fiber, the faster the electrical impulses travel down the axon of the nerve cell. For reference, the compound action potential recorded from the auditory nerve has a latency of about 2 msec. for a moderate intensity click stimulus. This potential is also recognized as wave I of the ABR waveform (see Musiek & Baran, 2020; Gonzalez-Gonzalez & Cazevieille, 2019).
Hearing Loss and MS
Hearing loss, as reflected on the pure tone audiogram, has been associated with MS and probable involvement of the auditory nerve via imaging studies (Furman et al., 1989). In these kinds of cases, the demyelination in MS results in impeded impulse transmission along the auditory nerve, which results in poorer audiometric thresholds (Gonzalez-Gonzalez & Cazevieille, 2019).
Related to this reduced hearing sensitivity is that fewer healthy neurons are available to carry the auditory stimulus and some fibers cannot fire at their normal firing rates, which also contributes to depressed pure tone thresholds. This pathologic mechanism can also produce tinnitus. In addition, MS plaques form on sites of damage, which also impedes impulse transmission. In periods of exacerbations of the disease, swelling around these plaques can occur, resulting in sudden loss of hearing. When the exacerbation quiets, the swelling subsides and better hearing returns (Musiek et al., 2121). This action can occur across the hearing and balance systems. Therefore, simultaneously occurring sudden hearing loss, tinnitus, and vestibular symptoms related to MS could be (and have been) confused with Meniere’s disease.
Hearing loss (either gradual or sudden), tinnitus, and vestibular symptoms including vertigo can be related to MS. These symptoms become worse with exacerbations of the disease and improve with recovery from these periods of exacerbation. Trends in regard to the configuration of pure tone hearing loss are difficult to surmise due to the variability observed in these cases. If there is a trend, high frequency hearing loss would be the most logical predicted configuration, as is the case with many other auditory disorders. Likewise, there are no predictable trends regarding degree of hearing loss related to MS and auditory nerve involvement (see Leite et al., 2014 for review).
There is evidence to show that auditory nerve involvement could occur in MS without effects on the pure tone audiogram. In an interesting study by Verma and Lynn (1985), 9 out of 40 definitively diagnosed MS patients with normal audiograms showed an abnormal ABR wave I with either latency or morphology deviancies. This of course implicates auditory nerve dysfunction. This early study by Verma and Lynn also has striking relationships to the now renowned study by Kujawa and Liberman (2009) which showed normal detection thresholds but abnormal ABR wave I in animals incurring temporary threshold shift from high intensity sound exposure.
One of the most interesting and instructive case studies was reported by Furman et al. (1989). This patient was a young woman with MS who rapidly developed a profound, high frequency, sensorineural hearing loss, very poor speech recognition, and absent ipsilateral and contralateral acoustic reflexes for the affected ear. Interestingly, this patient’s hearing loss essentially resolved over the course of a little more than a week. An ABR at the time of presentation was clearly abnormal (Wave I present, remaining waves absent), and a repeat ABR when the patient’s hearing returned was still abnormal but slightly changed. An MRI conducted when the patient presented at the clinic was indeed interesting. This imaging showed a lesioned area at the auditory nerve root entry zonea and the cochlear nucleus of the brainstem. This case reflects the pattern of symptoms and audiological findings discussed in this commentary.
In closing this commentary, a few key points seem worth reiterating. First, MS will commonly manifest with symptoms and audiological findings consistent with central auditory involvement. However, MS can less commonly affect the peripheral auditory nerve. When this happens, chances increase that loss of hearing sensitivity will manifest.
However, this may not always be the case, as MS lesions in the caudal brainstem have also been linked to pure tone hearing loss (Musiek, et al., 1994). Sudden hearing loss can be, and has been, associated with MS, along with other symptoms of special interest to audiologists, including vertigo and tinnitus. Finally, the understanding of myelin anatomy in the auditory nerve is a key issue in appreciating the auditory clinical impact of demyelinating diseases such as MS.
1. Root entry zone – the area where auditory nerve fibers penetrate into the cochlear nucleus, specifically between the anterior and posterior ventral cochlear nucleus.
References:
- Furman, J. M., Durrant, J. D., & Hirsch, W. L. (1989). Eighth nerve signs in a case of multiple sclerosis. American Journal of Otolaryngology, 10(6), 376-381.
- Leite, H. F. B., Leite, J. D. C. B., de Araújo Melo, M. H., Vasconcelos, C. C. F., & de Papais Alvarenga, R. M. (2014). Deafness in patients with multiple sclerosis. Audiology and Neurotology, 19(4), 261-266.
- Musiek, F. E., Shinn, J. B., Baran, J. A., & Jones, R. O. (2020). Disorders of the auditory system (2nd ed.). Plural Publishing.
- Musiek, F. E., & Baran, J. A. (2018). The auditory system: Anatomy, physiology, and clinical correlates (2nd ed.). Plural Publishing.
- Musiek, F.E., Baran, J. & Pinheiro, M. (1994). Neuroaudiology: Case studies. Singular Publishing Group.
- Verma, N. P., & Lynn, G. E. (1985). Auditory evoked responses in multiple sclerosis: Wave I abnormality. Archives of Otolaryngology, 111(1), 22-24.
- Gonzalez-Gonzalez, S., & Cazevieille, C. (2019). Myelination of the auditory nerve: Functions and pathology. Scientific Journal of Research and Reviews,1(3). https://doi.org/10.33552/SJRR.2019.01.000513
