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This article originally appeared in The Hearing Professional May—June 2005

Working with Neuroreflexes of the External Ear Canal

By Max S. Chartrand, BC-HIS

Mrs. Brown coughs and her eyes water as you insert an otoblock into her ear to take an ear impression. Mrs. Ortega complains that the longer she wears her new hearing aids the tighter they feel in her ears.

The above-described reflexes are present in normal ear physiology. In the study, it was found that the the keratin layer of their external ear tissue—which can shield otherwise sensitive neuroreflexes as well as maintain physical homeostasis—is healthy and intact (Chartrand, 2004a).

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

Ask yourself, what do all these patients have in common? First, each is complaining of an external ear neuroreflex that is evoked during the course of dispensing activity. Second, conventional wisdom simply does not have an answer to directly address their problems.

For each task involved we keep bumping into the natural defense mechanisms designed to protect the external ear from foreign objects, infection, injury/invasion and environmental exposure (Chartrand, 1999; Durrant and Lovrinic, 1984). For sure, the disregard of these mechanisms too often causes discomfort, delay and even rejection in the fitting and acclimatization process of a new hearing aid.

Ross (2002) and Kochkin (2000) report continued high levels of credit returns and/or “dresser drawer hearing aids,” which continue to plague the hearing industry. Another concern is the countless remakes and shell modifications that are still the mainstay of everyday dispensing practice.

In an effort to identify, label and understand three of these neuroreflexes, the following have been identified (Chartrand, 2005; Chartrand, 2004b):

The Vagus Reflex

Evoked in about 41% of cases during insertion of the otoblock, the vagus reflex is sometimes called the “cough reflex” innervated in Arnold’s branch as it meanders in and around the external ear, terminating at the tympanic plexus of the eardrum. It also can cause gagging, eye watering and other referred reflexes. During the acclimatization process of a new hearing aid fitting it has also been known to cause non-acoustic occlusion or a complaint of feeling “like I have a head cold,” while wearing the hearing aid.

The Trigeminal Reflex

Sometimes called the “red reflex,” the trigeminal reflex can be seen in up to 70% of patients during otoscopy. In extreme cases, the trigeminal reflex appears to be early otitis media, although it is merely hypersensitive vascularization. In most cases, however, it evokes a mild increase in blood and lymphatic fluid across the tissue above the eardrum and coursing down the handle of malleus toward the manubrium. Since there is no parasympathetic action to counter or limit this reflex, simple acclimatization is not usually a solution. Hence, an overly tight earmold can cause the need for more gain and output than otherwise necessary. These are the mystery “power mongers.”

The Lymphatic Reflex

The lymphatic reflex is evidenced by excessive tissue swelling and complaints of soreness during the early stages of wearing a new earmold or hearing aid. It reportedly affects about 27% of new hearing aid users, especially CIC and deep-fitting patients. Fortunately, parasympathetic mechanisms allow eventual acclimatization if a carefully designed wearing schedule is followed.

Neuroreflexes can be described as an elaborate, interconnected system consisting 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). The mission of mechanoreceptors is to maintain chemical, temperature, bacteriological and ionic homeostasis in the external ear. Hence, when a movement happens in the external ear canal, it is perceived as a threat to the homeostasis by these mechanoreceptors (Carlson, 2004).

Cranial, cervical and auxiliary nerves involved in this neurocomplex innervate the external ear canal region. These have essentially nothing whatsoever to do with the sense of hearing, and involve the tympanic branch of glossopharyngeal (CN IX), mandibular and maxillary branch of trigeminal (CN V), mandibular and submandibular, chorda tympani, greater petrosal, and tympanic plexus of facial (CN VII), Arnold’s branch of vagus (CN X) and others, such as a branch of IV cervical, ciliary ganglion and general sensory nerves, interconnecting yet to other nerves and neural pathways (UC–Davis, 2005).

One of the most perplexing neurological challenges for achieving comfort in hearing aids is adaptation to tactile pressure in the external ear canal. Elsewhere in the human body, adaptation to tactile movement 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, in the case of wearing a device in the external ear canal, the sense of movement never ceases because of mandibular, facial and other dynamics of the head and neck region. In fact, most ear canals at the aperture and beyond experience dimensional changes of 10%–2% or more with the simple opening and closing of one’s mouth (Oliviera et al., 2005; Oliviera et al., 1992). Therefore, it is almost impossible for sensory receptors in the external ear to stop firing when a rigidly fixed object (hearing aid) is placed in an excessively movable ear canal (Kolpe and Oliviera, 2003).

Presumably, the aging ear experiences these phenomena even more adversely than in younger age groups (Willott, 1981).

The above-described reflexes are present in normal ear physiology. Their degree of sensitivity varies widely in individuals, depending upon the thickness of the epithelium and keratin layer (corneum stratum) in the external canal (Chartrand, 2004b; Kolpe and Oliviera, 2003; Naiberg, Proops, and Hawke, 1984). In the study, it was found that the single most important factor in whether an individual user adapted comfortably to wearing hearing aids generally hinges on whether the keratin layer of their external ear tissue—which can shield otherwise sensitive neuroreflexes as well as maintain physical homeostasis—is healthy and intact (Chartrand, 2004a).

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

For instance, the correlation between keratin status (as observed via video otoscopy) and incidence of cough reflex (vagus) during otoblock insertion was a strong 71%.

Conversely, when desquamation lines in the ear canal and adequate keratin formation over the outer layer of tissue were observed, there was considerably less tendency for evoking a cough, gag or watering eyes reflex during otoblock insertion and other fitting tasks (figure 2).


Figure 2. Keratin status was found to correlate strongly with the degree of external ear neuroreflex sensitivity. The left photo is of an ear canal that has had a portion of its keratin removed with a cotton swab, causing the ears to itch and to become more sensitive to wearing a hearing aid. The right photo shows a healthy layer of keratin and an ear comfortable while wearing a hearing aid.

Therefore, the first line of defense for resolving complications due to external ear reflex oversensitivity is to refrain from using cotton swabs, foreign objects of any kind and chemicals in the ear canal such as boric acid and hydrogen peroxide. Left alone, healthy ear canals are self-cleaning, and usually cover the epithelium quite well with an adequate keratin layer (Chartrand, 2004a).

To help better understand the autonomic mechanisms of these reflexes and how they affect the success or failure of a hearing aid fitting, they’ve been organized into behavioral terms in Table 1.

Neuroreflex

Causal/Action

Neural Attribution

Symptoms

Vagus Reflex (also known as the “cough reflex”) Sympathetic motor reflex caused by light touch upon the superior/inferior and anterior metal wall of the ear canal. 1) Pacinian corpuscles; 2) Arnold’s branch, vagus; 3) Internal acoustic meatus, facial; 4) Glossopharyngeal (no parasympathetic activity in this reflex). Cough, gag, and/or eyes watering upon insertion of otoscopy, otoblock and impression-taking. Also causes non-acoustic occlusion while wearing hearing aids in some cases.
Lymphatic Reflex Sympathetic response to pressure on the surface of the skin, as well as deeper structures.

Facial, interneural cross-connections, involving mechanoreceptors, Pacinian corpuscles (fast action), Meissner’s corpuscles (slow action), and hair follicles.

Swelling, and soreness after wearing device for period of time. Later causes increased feedback and acoustic detriment when earmold modifications are made during swelling.
Trigeminal Reflex (also known as “red refleax”) Sympathetic and parasympathetic vascularization at TM, when pressure is applied in outer third of ear canal. 1) Mechanoreceptors, Pacinian and Meissner’s corpuscles and hair follicles; 2) Facial sensory and motor neurons for sympathetic and parasympathetic response to stimuli. Vasodilation at the TM during otoscopy or hearing aid wear. Can cause need for more gain/output to overcome TM impedance.

First Line of Defense: A Healthy Keratin Layer

Obviously, the first line of defense in resolving complaints due to an oversensitive ear canal’s neuroreflexes is to encourage a healthy layer of keratin over the epithelium of the ear canal. This can easily be monitored by video otoscopy, letting the patient observe their current status firsthand. This will give them further reason to avoid practices and substances that undermine good ear canal health and understand its relationship to their fitting success.

Next, it is suggested that hearing instrument dispensers utilize a dynamic ear impression technique:

During earmold modification, to relieve complaints of non-acoustic occlusion, tissue swelling and/or overstimulation of the red reflex, taper (do not shorten) the length of the canal of the hearing aid shell or earmold. In cases of mild losses, you may be able to go to an appropriate open-configuration instrument, which essentially avoids any neuroreflex involvement. But, in most cases, the patient will need the acoustics of the instrument in a standard configuration.

Finally, you’ll need to customize your patient’s wearing schedule to accommodate their neuroreflex sensitivity as much as for any other consideration. For most of your new hearing aid patients, a good rule-of-thumb for a wearing schedule:

1st day—one hour in/one hour out during waking hours

2nd day—one hour in/one hour out

3rd day—two hours in/one hour out

4th day—two hours in/one hour out

5th day—three hours in/one hour out

6th day—three hours in/one hour out

7th day—four hours in/one hour out

8th day—four hours in/one hour out

9th day—wear all day with one mid-day break for an hour

10th day—wear all day with a break if needed

Of course, there are other considerations for acclimatization, such as neurological and psychological factors. A good auditory rehabilitation procedure to accompany the wearing schedule is skillfully outlined by Dingler, Hanson and Morris (1995).

However, no matter how a program is designed, frequent and timely post-fitting visits are required to monitor user progress and to head off problems before they discourage the patient. Even experienced users that are being fitted with new instruments or earmolds require a carefully planned wearing schedule.

If the above considerations are made, far fewer shells and earmolds would need to be remade. Needless credit returns and failed trials will be curtailed. Certainly, the time a hearing instrument dispenser spends modifying earmolds will be significantly reduced, leaving him or her free to perform the important work of counseling and motivating a patient to success. THP

External Ear Neuroreflex Checklist

The following checklist may be utilized by any hearing instrument dispenser in their practice to detect and resolve fitting problems.

Scoring Instructions: During execution of each of the following clinical tasks and post-fitting adaptation experiences, rate the incidence, observation or report of any of the noted reflexes and rate each reflex by circling the corresponding degree of sensitivity. (See reflex indications key at bottom of page.)

1. Clinical Task: Video Otoscopy

Rating scale:
Nonexistent
Weak
Moderate
Strong
a. Vagus Reflex
b. Trigeminal
c. Lymphatic

Keratin Status: (check one) M Absent   Swabbed  Peeled   Granulated   Thin   Normal

 

2. Clinical Task: Cerumen Management

Rating scale:
Nonexistent
Weak
Moderate
Strong
a. Vagus Reflex
b. Trigeminal
c. Lymphatic

Method used: (check one)  N/A   Loop   Syringe Suction   Other

 

3. Clinical Task: Otoblock Insertion

Rating scale:
Nonexistent
Weak
Moderate
Strong
a. Vagus Reflex
b. Trigeminal
c. Lymphatic

Type of otoblock used: (check one) Cotton   Foam Latex

 

4. Clinical Task: Impression Material Insertion

Rating scale:
Nonexistent
Weak
Moderate
Strong
a. Vagus Reflex
b. Trigeminal
c. Lymphatic

Type of material: (check one)  Oil/Powder Silicone: Low Viscosity   Med. Viscosity   High Viscosity

 

5. Clinical Task: Insertion of Earmold

Rating scale:
Nonexistent
Weak
Moderate
Strong
a. Vagus Reflex
b. Trigeminal
c. Lymphatic

Depth of earmold in the canal: (check one)  shallow moderate   deep fitting

 

6. Post-fitting Adaptation Experience: After Three Day’s Hearing Aid Wear

Rating scale:
Nonexistent
Weak
Moderate
Strong
a. Vagus Reflex
b. Trigeminal
c. Lymphatic

Observation Notes: _____________________________________

REFLEX INDICATIONS KEY: Vagus: Cough, Gag, Non-acoustic Occlusion;
Trigeminal: Red Reflex, Increased Impedance; Lymphatic: Discomfort, Tissue Swelling, Soreness

Bibliography

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