Audiology Online

Identifying “Neuro-reflexes” of the External Ear Canal.

Max Stanley Chartrand

Abstract:

This study addresses measures and methods for identifying three under-recognized neuroreflexes of the external ear canal.  These three neuro-reflexes are related to hearing instrument dispensing practice, as they often complicate and may cause failure to fit hearing instruments. By using an External Ear Neuro-reflex Checklist (EENC) clinical tasks such as video-otoscopy, cerumen management, otoblock insertion, ear-impression taking, and hearing aid adaptation can be more effectively managed. The EENC is a significant measurement tool and helps expose potential fitting problems during the evaluative process. Increased awareness of little-known ear dynamics can help reduce negative incidences and unnecessary remakes, modification and returns for credit.  

Introduction:

            The physiology of the human external ear canal poses significant challenges with respect to wearing and fitting hearing aids, wearing and fitting ear protection devices, using stethoscopes, and the application of various inserts for hearing and ear measurement techniques and many other functions.  Modern ear and hearing devices require cooperation and accommodation of “pre-modern” natural defensive mechanisms designed to protect the external ear from foreign objects, infection, injury, invasion, and environmental exposure (Durrant and Lovrinic, 1984).  Disregard of these same defensive mechanisms can cause discomfort or rejection of an otherwise beneficial or necessary auditory task.

            The percentage of people who experience negative or perhaps unsatisfactory events or experiences during hearing loss evaluation and hearing aid exploration is significant. The Hearing Industries Association (HIA) statistics revealed a high rate of “return for credit” for hearing aids, hovering around 20% (Ross, 2002). Further evidence of dissatisfaction is apparent in the number of hearing aids “in the dresser drawer” (Kochkin, 2000). Additionally, shell and earmold remakes at hearing aid factories and earmold laboratories and countless in-office modifications made every day, indicate the presence of an underlying problem (Chartrand, 1999).  

            As a result of extensive clinical observation and a review of the current literature, three neuro-reflex mechanisms have been identified. These three neuro-reflexes affect hearing health practice procedures and hearing instrument success (Chartrand, 2004a). For the purposes of this paper, these neuro-reflexes are referred to as:

·    The Vagus Reflex- This reflex may be evoked during insertion of the otoblock, cerumen removal, and rarely, with hearing aid use. This reflex is evidenced by cough, gag, and/or watering eyes during any of these activities.

·    The Trigeminal Reflex is sometimes called the “red reflex,” can cause excessive vascularization and thickening of the tympanic membrane (TM) during otoscopy, otoblock insertion, and during hearing aid wearing.

·    The Lymphatic Reflex is a slower reflex which may result from over-wearing of hearing aids during the early adaptation period. This reflex is evidenced by excessive swelling of tissues and soreness while wearing a custom fitted earmold or hearing aid. Sometimes, even a perfectly fitted device (earmold or hearing aid) causes this reflex. The lymphatic response may appear to be due to an allergic reaction, or the result of a poor fit. Physical modification (buffing, drilling, grinding etc.) during the lymphatic response period potentially results in loose fittings, acoustic feedback and a compromise of the integrity of the amplification system -- acoustically and physically.

Physiologically what we find is an elaborate, interconnected reflex system. The reflex system consists of somatic afferent sensory fibers and sympathetic and parasympathetic efferent motor fibers, which respond to external stimuli on the mechanoreceptors (Pacinian Corpuscles and, to a lesser extent, Meissner’s Corpuscles) and hair follicle receptors (Grenness, 1999; Kress and Zeilhofer, 1999; Spray, 1986). When movement or occupation occurs in the external ear canal, it is perceived as a “threat” and the mission of these mechanoreceptors is to maintain chemical, thermal, bacteriological, and ionic homeostasis within the external ear, thus -- eliminating the threat.

            Neurologically, cranial, cervical and auxiliary nerves are implicated in the neurocomplex which innervates the external canal region.  Although most of these have nothing to do with hearing, they impact the ear and surrounding tissue. The tympanic branch of the glossopharyngeal nerve (CN IX), the mandibular and maxillary branches of the trigeminal nerve (CN V), the mandibular, submandibular, chorda tympani, greater petrosal and tympanic plexus of the facial nerve (CN VII), Arnold’s branch of the vagus nerve (CN X), and others, interconnect in a complex and difficult to predict (with regard to their impact on hearing prosthetics) manner (UC-Davis, 2005).

            One perplexing challenge is adaptation to tactile pressure in the external ear canal. Elsewhere in the human body, adaptation to tactile pressure on a cutaneous sensory organ occurs only a short while after movement ceases, such as wearing a wristwatch or jewelry (Nafe and Wagoner, 1941).  However, regarding prosthetics in the external ear canal, movement never ceases because of mandibular, facial, and other dynamics of the head and neck region. In fact, at the aperture of the ear, dimensional changes of 10% (or more) occur with the simple opening and closing of the mouth (Oliviera et al., 2005; Oliviera et al., 1992).  Therefore, it is almost impossible for sensory receptors in the external ear to stop firing while a rigidly fixed object (i.e., hearing aid or earmold) is placed in an excessively movable ear canal (Kolpe and Oliviera, 2003).  Aging ears can be even more adversely affected by these factors (Willott, 1981). 

            The degree of sensitivity varies widely in individuals, often depending upon the thickness of epithelium and the corneum stratum (keratin layer) over the surface of the external ear canal and tympanic membrane (Chartrand, 2004b; Kolpe and Oliviera, 2003; Naiberg, Proops, and Hawke, 1984).  The most important factor regarding whether an individual will adapt comfortably to aural prosthetics may hinge on whether the keratin layer of their external ear tissue—which shields otherwise sensitive neuroreflexes as well as maintain physical homeostasis—is healthy and intact (Chartrand, 2004a).

To help organize these autonomic mechanisms into practical models that can be observed, understood, and accommodated (where possible) in the course of hearing health practice, each neuro-reflex has been defined in behavioral terms in Table 1 (below).

Table 1. Description of external ear neuro-reflexes affecting hearing instrument dispensing procedures and activities.

Neuroreflex         Causal/Action                Neural Attribution           Symptoms

Method:

            Participants- Twenty-seven hearing impaired individuals (18 male, 9 female) between the ages of 49 and 87, with a mean age of 68.3 years, were studied during the course of the hearing aid fitting process. No attempt was made to randomize participants. Participants were recruited by informed consent as they presented at three dispensing offices in southern Colorado and Northern New Mexico over a two-month period.

            In many respects this subject population (n = 27) was representative of that found in the typical hearing health practice. For instance, there is a much higher concentration of serious hearing impairment in the older population, and the incidence of hearing impairment in males over sixty years of age is roughly twice that of females in the same age-group when comparing age and degree of hearing impairment (Staab, 1990).

Measures/Apparatus: Equipment and materials used to make our observations consisted of the following:

                American Electromedic AE-105 Tympanometer

                                                                Qualitone CD-3 Acoustic Appraiser

                                                                MedRx Video Otoscope

                                                                Welch-Allyn Hand-held Fiber-optic Otoscope

                                                                Fiber Ear Vision Ear Light Set

                                                                Westone Labs open-cell foam otoblocks

                                                                Dreve impression syringe

                                                                Dreve Oto-Form A/k silicone impression material

                                                                Cerumen management implements and solutions

                                                                Miracell Botanical Solution

                A standardized checklist, the External Ear Neuro-reflex Checklist (EENC) with rating scale, was used to make qualitative measurements and observations of each task and their resulting reflexes (the EENC can be found in Appendix A). 

            Procedure: Clinical tasks relative to assessing and evaluating hearing, and the hearing aid fitting process were performed. Careful observations (EENC) were recorded during; Video otoscopy, cerumen management, Oto-block insertion, impression material insertion & removal, earmold insertion, and post-fitting adaptation.

Subject verbal responses and experimenter observations were considered in the rating scale for degree of effect during each task. Table 2 (below) summarizes reflex indications relative to a rating scale of 0-3 (i.e., 0 = non-existent, 1 = weak, 2 = moderate, and 3 = strong):

Table 2.  EENC rating scale key.

\

Neuroreflex                  Non-existent        Weak                         Moderate                      Strong

    The Vagus Reflex (involving mostly Arnold’s Branch) was rated according to patient reports and involuntary coughing, gagging, and/or eyes watering during any of the targeted clinical tasks. Trigeminal Reflex observations were made during and after certain tasks with a hand-held fiber-optic otoscope. Most of the responses for the Lymphatic Reflex were elicited during adaptation of new hearing aids or earmolds. 

            Data Analysis:  Two sets of data were calculated from this study:

a-The Pearson Product Moment Correlation co-efficient between

keratin status and an elicited Vagus Reflex during otoblock insertion

b- Quantification of how often any of the targeted reflexes occur

at any point in the process.

To determine an association between “keratin status” and vagus sensitivity during otoblock insertion, we designated keratin status by number.  Normal appearance was designated “0,” thin or granulated appearance was assigned “1,”  a “peeling” or “peeled” appearance was assigned a value of “2’” and an appearance that was absent/swabbed was assigned a value of “3.” Vagus sensitivity was quantified in reverse order (0 = non-existent, 1 = weak, 2 = moderate and 3 = strong).

Table 3. Ordered arrangement of subjects (n=27) re keratin status vs vagus reflex sensitivity

     Subject                Keratin            Vagus Reflex

        (N)                      Status               Sensitivity    

Exhibit B shows calculations for inferential and descriptive statistics from the above data. For a visual correlational representation of the data, a scatterplot with line of regression (r = .735) is shown in Figure 1:

Figure 1. Relationship between keratin status and vagus oversensitivity during otoblock insertion.

The second set of data concerned how often any of the three reflexes occurred in the subject (n = 27) population at any point in the evaluation, fitting, and post-fitting process.  An “X” represents reflexes were observed (moderate or strong only), an “0” represents no significant reflex was observed.

Pass/fail decisions were based on the following criteria: Vagus if a cough reflex was evoked in at least two procedures, Trigeminal if a visible and rapid vascular dilation occurred during two or more procedures, and Lymphatic upon discomfort or soreness during the adaptation period.  Data is tabulated in Table 4 as follows:

Table 4. Incidence of significant neuroreflex activity within the subject (n = 27) population.

     Subject                 Vagus                   Trigeminal             Lymphatic

     (N = 27)                 Reflex                       Reflex                    Reflex