Hearing Aids & the Ear Canal

Resolving a Root Cause of Failure to Fit: Re-examining the Ear Canal

By Max Stanley Chartrand and Glenys Anne Chartrand

Frequent complaints of earmold discomfort, non-acoustic occlusion, sensations of fullness, and chronic itching while wearing hearing aids still continue to drive at least some of the failure-to-fit rate for dispensing professionals.

We need to apply our science to both acoustic and non-acoustic issues that influence customer satisfaction

Despite steady progress in the development of hearing aid and assistive technology, the industry continues to lose ground in terms of penetrating an otherwise rapidly growing hearing-impaired population.1 We note that returns for credit (RFC), factory remakes, and large numbers of in-office modifications—often totally unrelated to acoustical issues—constitute a large portion of the reasons why the industry fails to improve market penetration.

Frequent complaints of earmold discomfort, non-acoustic occlusion, sensations of fullness, and/or chronic itching while wearing hearing aids still continue to be responsible for at least some of the failure-to-fit rate in the industry. Consequently, every RFC, factory remake, and in-office modification contributes to the cost of the next hearing instruments sold in the marketplace.2,3

A great amount of research and product development has been devoted to fighting occlusion, feedback, and other acoustic-related problems such as hearing in noise. Likewise, throughout the history of hearing care, manufacturers have focused on reducing the size of the hearing aid in order to minimize perceived stigma issues. While these efforts are certainly important in helping increase customer satisfaction with hearing instruments, very little attention seems to focus on both the short-term and chronic physical comfort issues related to the actual wearing of a hearing aid and its physical presence within the ear canal. Additionally, discounting cursory anatomical diagrams of the ear canal and perhaps some mention of the thin epithelial layer of skin covering the bony portion of the ear canal, surprisingly little education centers on the dynamic ear canal—where about 70% of hearing aids reside and through which 100% of all hearing aid sound is eventually transmitted.

It appears that, rather than research and address the root causes of these non-acoustic complaints, the hearing industry continues to search for amplification strategies that bypass entirely issues related to the ear canal. For example, truly ingenious and useful products like retro-auricular devices, middle-ear implants, open-ear configurations, and bone-anchored hearing aids have been developed.

While each of these amplification strategies hold promise for small segments of the hearing-impaired market, a much larger portion of the market is resigning itself to another, far worse option: Avoiding hearing aids in the first place. Whether by pre-empting their search for help, giving up during fitting trials, or consigning their hearing aids to dresser drawers (and not ear canals), consumer perceptions and experiences relative to the physical fit and comfort of a hearing aid represent formidable barriers to the future of hearing health care.

More than an Ear Impression Issue
Prizanski and Berge4 point out the need to utilize dynamic ear canal principles when making ear impressions. They estimate that approximately 50% of poor fittings result from poor impression techniques, and that only 43% of professionals participating in a recent international hearing health conference actually utilize dynamic impression techniques.

Oliviera et al5 have long suggested that the problem may not be just the ear impression, but instead a lack of understanding of the dynamic physiology of the human ear canal. They find that, in over half (51%) of hearing aid users, the shape of the outer region of the EAC changes by more than 10% when the jaw is opened. The remaining 49% of hearing aid users exhibited changes of less than 10%. Arguably, that is a lot of dimensional variation during the frequent movements of speaking, chewing, smiling, etc. Additionally, most hearing aid wearers experience a volume change in the ear canal that is asymmetric in terms of the magnitude and direction of the moving ear canal. These movements potentially contribute to complaints of acoustic and resonant distortion (eg, feedback), and feelings of fullness and/or discomfort (occlusion) in some hearing aid users.

FIGURE 1. Relationship between keratin status and vagus over-sensitivity during otoblock insertion.

From a study of 27 hearing aid users, it is suggested that the status of the keratin (corneum stratum) layer of the external canal lumen has a direct relationship to the sensitivity of EAC neuroreflexes.6 By extension, the absence of keratin could magnify the problems explored by Oliviera et al5 and further complicate adaptation to hearing aids and earmolds. In the Chartrand study,6 37% of hearing aid users exhibited a significant Vagus or Arnold’s Reflex (coughing, gagging, eyes watering, etc) when a foreign object—such as an otoblock, otolight, or earmold—was inserted into the EAC. The correlation between keratin status (eg, absent or peeling, thin, or thick keratin levels) and a demonstrable Vagus reflex was found to suggest a strong (r=0.735) correlation using the Pearson Product Moment line of regression (Figure 1). In other words, the chances for inciting a cough, gag, or other vagus response via the Arnold’s branch increases in relative proportion to the lack of keratin in the EAC.

Primer on EAC Keratin
Keratin, or stratum corneum, constitutes the inorganic outermost layer of protective tissue in the human ear canal. It is the primary structure for maintaining homeostasis, hydration, and pH flora in the entire EAC. Without a good, healthy layer of keratin, ceruminous and sebaceous secretions fail to mix into the beneficial compound known as “earwax.” Hence, fungi, yeasts, and bacteria—and deeper yet, pseudomonas—may flourish, causing itching, infection, and in some cases, chronic inflammation while wearing hearing aids or earmolds.7

FIGURE 2. Example of the visible desquamation lines along the ear canal when healthy keratin is present. Spaced appropriately, these lines reveal keratin that can help maintain homeostasis and keep the ear canal clear of dead skin cells, excessive cerumen, and potentially septic debris.

The desquamation process of EAC keratin is such that it steadily carries dead skin cells, debris and aged cerumen toward of the aperture of EAC at the rate of approximately 1 mm per day (or about 1/2” per month). Through video otoscopy, it can be seen that a good, healthy layer of keratin presents visible “desquamation lines” as shown in Figure 2.8-10

FIGURE 3. Example of mechanically disturbed keratin peeling off the ear canal, leaving the epithelium open to itching, infection, and/or unshielded neuroreflexes that can complicate adaptation to hearing aids.

In our experience, we find that most cases of impacted earwax (and keratosis obturans) result from disturbances in the keratin layer. Keratin that is mechanically removed using cotton swabs, chemically removed with boric acid or hydrogen peroxide, or systemically or metabolically reduced due to acidosis or diabetes mellitus II may cause itching of the ears and/or chronic external otitis (Figure 3). The first two of these causes (mechanical and chemical) are arguably preventable, while the third (systemic or metabolic disease) may be managed or addressed only by controlling the underlying condition or disease. Hence, it is arguable that the majority of problems arising during the fitting process arise from subclinical issues that are outside the FDA Red Flag conditions for referral.11

Potential Pitfalls to Fitting Success
Relative to hearing aid adaptation, probably the most important role for keratin is its ability (or, if absent, its inability) to shield the EAC neuroreflexes from over-sensitivity while wearing hearing aids, earmolds, and other prosthetic applications. Without a good, healthy layer of keratin, the new hearing aid user might not adapt to the hearing aids and may give up too quickly, returning the instrument to the dispensing professional.

These neuroreflexes are innervated via various mechanoreceptors, particularly the EAC’s hair follicles at the surface of the skin, Meissner corpuscles situated immediately below the surface of the skin, and Pacinian corpuscles located deeper within the tissues.12-16 Designed by nature to respond to temperature changes, tactile pressure, and movement of any object or matter entering the ear canal, they transmit neural information (afferent and efferent) to various points along the EAC and tympanic membrane (TM) to prepare for threats to homeostasis (eg, the dynamic equilibrium maintained by the body). These threats to homeostasis include things like insects, cold air, water, debris, foreign objects, and developing infection.6,16,17

Such defensive mechanisms involve reflex arcs that can evoke hyper-vascularization,14 coughing or gagging,18,19 tissue swelling,20 and tension on the TM3 arising along the EAC to the TM. Some of these arcs involve both sympathetic and parasympathetic reactions, meaning the neuroreceptors cease synaptic firing after a time, while others exhibit only sympathetic responses with little or no parasympathetic (adaptation) control.

EAC neuroreflexes most involved with hearing aid and earmold adaptation appear to be:

Lymphatic Reflex (LR). Innervated by epithelial mechanoreceptors (hair follicles, Meissner, and Pacinian corpuscles), LR presents varying degrees of tissue swelling reciprocal to the degree of pressure upon insertion of hearing aid or earmold. Under most conditions, this reflex exhibits an excellent parasympathetic response. So, like wearing a wristwatch or piece of jewelry, it usually calms after a period of adjustment.9,19,21

Trigeminal Reflex (TR). Involving various branches of the trigeminal, facial, and subsidiary nerves, TR causes hypervascularization (blood and lymph fluid) specific to the TM upon insertion of a otoscopic speculum, impression otoblock, hearing aid, or earmold. In cases of hearing aid/earmold use, this may artificially create the need for more gain and output in some individuals that do not exhibit a parasympathetic (adaptation) reaction over time.6,19

Vagus Reflex (VR). Involving Arnold’s branch, glossopharyngeal, and various interneural connections, can cause cough, gag, effortful phonation, and nausea in some individuals upon insertion of an otoscopic speculum, ear impression otoblock, hearing aid, or earmold into the EAC. VR usually shows excellent parasympathetic (adaptation) response over time.19,22,23

Industry-Wide Effort Needed
After studying the nature and mechanisms of these neuroreflexes for more than two decades, and while currently conducting research on the interaction between these phenomena and hearing aid fitting success, we find that this effort should be of immense importance for the advancement of all sectors of the hearing care field. Preliminary conclusions suggest that, when intelligently considered within an auditory rehabilitative framework, many would-be failures to fit can be resolved.

Current practice, on the other hand, tends to ignore these natural structural and neurological processes. Instead, consensus appears to accept a static physiology model, which explains most cases of failure to fit in either psychosocial or idiopathic terms, as pointed out by some researchers.5,24,25

For that reason, we strongly encourage an intense industry-wide research effort to be launched so that methodologies, materials, and counseling strategies may be developed to help dispensing professionals to resolve and/or avoid the all-to-frequent pitfalls of current practice. In so doing, a good starting place would be to routinely observe and document each prospective hearing aid user’s keratin status via video otoscopy. Furthermore, taking careful and thorough health histories can reveal potential underlying chronic disease, pharmaceutical side-effects, and even personal ear-health practices that can negatively impact fitting success.

From there, we can develop better protocols for detecting neuroreflex activity affecting the wearing of hearing aids, and better impression-taking and remedial earmold modification methods. Certainly, called into play would be the search for less toxic shell and earmold materials, as well as improved ways to work with the dynamic aspects of hearing correction.

In so doing, hearing aid manufacturers may produce improved auditory prostheses, while dramatically reducing returns for credit and costly remakes. Dispensing professionals will be able to make more ergonomically correct ear impressions, reduce user complaints, and put more focus where it is needed the most: in better outcomes and more effective auditory rehabilitation.26 Likewise, ENT physicians will receive more timely referrals and better communications from the dispensing community, and researchers will be aided in the search for better diagnostic and treatment approaches.

The large and growing market of hearing-impaired individuals not presently being served will feel more confident about raising their hands for help with problems that, when left untreated, cause considerable communicative, cognitive, and social stress in their lives. Finally, the consumer’s experience in accepting and utilizing today’s solutions for auditory enhancement will be a happier, more successful event for everyone.

Max Stanley Chartrand and Glenys Anne Chartrand serve as director of research and director of rehabilitation, respectively, for DigiCare Hearing Research & Rehabilitation, Colorado City, Colo, and they serve as consultants to the hearing industry.

 

Correspondence can be addressed to HR or Max Stanley Chartrand, DigiCare Hearing Research & Rehabilitation, PO Box 706, Rye, CO 81069; email: chartrandmax@aol.com.

References
1. Chartrand MS, Chartrand GA. If a tree fell in the forest: What’s really holding back the market? The Hearing Review. 2004;10(10):21-23.
2. Barber S. Hearing Aid “Sticker Shock” and Things to Consider When Purchasing. Available at: http://www.healthyhearing.com/library/article_content.asp?article_id=710; Accessed: retrieved on January 24, 2006; Posted May 16, 2005.
3. Chartrand MS. Hearing Instrument Counseling: Practical Strategies for Counselling the Hearing Impaired. 2nd edition. Livonia, MI: International Institute for Hearing Instruments Studies (IIHIS); 1999.
4. Pirzanski C, Berge B. An ear impression technique that works. The Hearing Review. 2002;9(12):24-27,53.
5. Oliviera R, Babcock M, Venem M, Hoeker G, Parish B, Vasant K. The dynamic ear canal and its implications: The problem may be the ear, and not the impression. The Hearing Review. 2005;12(2):18-19, 82.
6. Chartrand MS. Identifying “neuroreflexes” of the external ear canal. Available at: http://www.audiologyonline.com/articles/arc_disp.asp?article_id=1374;    Accessed: November 29, 2005; Posted May 2005.
7. Chartrand MS. The importance of the keration layer in successful fitting and treatment. Hearing Professional. 2004; July-Aug:14-18.
8. Persaud RAP, Hajioff D, Thevasagayam MS, Wareing MJ, Wright A. Keratosis obturans and external ear canal cholesteatoma: How and why we should distinguish between these conditions. Clin Otolaryngol & Allied Sciences. 2004;29(6):577-581.
9. Vrabic J, Underbrink M. Grand Rounds Presentation: Infections of the External Ear. University of Texas, Dept of Otolaryngology-Head and Neck Surgery; March 21, 2001.
10. Naiberg J, Berger G, Hawke M. The pathologic features of keratosis obturans and cholesteatoma of the external auditory canal. Arch Otolaryngol-Head & Neck Surg. 1984;110(10):751-756.
11. Chartrand MS. Video otoscopy observation and referral: The FDA Red Flags. Hearing Professional. 2003; Jan-Feb:9-14.
12. Guclu B, Bolanowski SJ, Pawson L. End-to-end linkage (EEL) clustering algorithm: A study on the distributions of meissner corpuscles in the skin. J Computational Neuroscience. 2003;15(1):19-27.
13. Pare M, Elde R, Mazurkiewics JE, Smith AM, Rice FL. The meissner corpuscle revised: A multiafferented mechanoreceptor with nociceptor immunochemical properties. J Neuroscience. 2001;21(18):7236-7246.
14. Smoliar E, Smoliar A, Belkin VS. Innervation of human trigeminal nerve blood vessels. Cells, Tissues, Organs. 1999;165(1):40-44.
15. Kryzanski W. Analysis of a model of membrane potential for a skin receptor nerve. Buffalo, NY: State University of New York at Buffalo; AAT 9801317; 1997.
16. Bensmaia SJ, The Vibrations of Texture. Chapel Hill, NC: Univ of North Carolina at Chapel Hill AAT; 2003.
17. Checkosky CM. Temporal summation in the Pacinian channel: Implications for the neural code for detecting vibrotactile stimuli. Rochester, NY: University of Rochester; AAT 9226009; 1991.
18. Gupta D, Verma S, Vishwakarma SK. Anatomic basis of Arnold’s ear-cough reflex. Surgical Radiological Anat. 1986;8(4):217-220.
19. Reid CC. Lymphatics of the Eye, Ear, Nose and Throat. In: Millard FP ed. Applied Anatomy of the Lymphatics. Available at: http://www.meridianinstitute.com/eamt/files/millard/millcont.html;   Accessed: February 13, 2005; 1922.
20. Johnson A, Hawke M. The nonauditory physiology of the external ear. In: Santos-Sacchi J, ed. Physiology of the Ear. New York: Raven Press; 1981:41-58.
21. Nafe JP, Wagoner KS. The nature of pressure adaptation. J Gen Psychol. 1941;25:323-351.
22. Amin MR, Koufman JA, July-August). Vagal Neuropathy after Upper Respiratory Infection: A Viral Etiology? Amer J Otolaryngol. 2001;22(4):251-256.
23. Hadjikoutis S, Wiles CM, Eccles R. Cough in motor neuron disease: A review of Mechanisms. Quarterly Jour Med. 1999;92:487-494.
24. Resnick SB. Breakdown in the fitting process. An idiopathic approach to Fitting: A Matter of Perspective. Available at: http://www.vard.org/mono/ear/resnick.htm; Accessed: November 9, 2005; 1999.
25. Rehabilitative Engineering Research Center on Technology Transfer. Stakeholder Forum on Hearing Enhancement, Earmolds: Problem Statement. Available at: http://cosmos.buffalo.edu/t2rerc/pubs/ forums/hearing/forum/earmolds/ problem_statement; Accessed: December 23, 2005.
26. Wayner DS. Aural Rehabilitation adds value, lifts satisfaction, cuts returns. Hear Jour. 2005;58(12):30-38.