BAER Testing in a Dog with Bilateral External Ear Canal Atresia

Introduction

The brainstem auditory evoked response (BAER) test is a valuable tool for assessing hearing loss in animals because behavioral hearing testing used in humans is difficult to perform. The BAER test employs repeated auditory stimuli that evoke electric waveforms in the auditory system. The BAER consists of up to seven distinct waves, which develop within 10 msec after the stimulus. Although it is not entirely understood what anatomic structures produce each waveform, studies suggest that waves 1–3 are produced by the acoustic nerve, cochlear nucleus, and superior olivary complex. The origin of waves 4–7 are thought to be from one or more of the following: the nucleus of the lateral lemniscus, the inferior colliculus, or the medial geniculate nucleus.^1,2 Scalp electrodes are used to record this peripheral nerve and brainstem activity. The BAER test can be used to evaluate whether an auditory stimulus is appropriately received, and help determine whether an animal, infant, or nonresponsive adult human can hear.¹

Hearing impairment may be due to either conductive or sensorineural causes. Conductive hearing impairment occurs due to abnormalities within the external ear canal or middle ear which diminish the ability of sound waves to reach the cochlea. Sensorineural hearing impairment occurs due to disease of the cochlea, eighth cranial nerve, or auditory pathway. Two different forms of stimuli to the cochlea can be used to differentiate conductive from sensorineural impairment: air- and bone-conducted stimuli. Air-conducted stimuli use a series of repetitive clicks produced by earphones, whereas bone-conducted stimuli use a vibrating transducer to directly stimulate the hair cells of the cochlea, thereby bypassing the external and middle ear.^1–3 A conductive hearing impairment should have an abnormal air-conducted BAER and an intact bone-conducted BAER. With sensorineural impairment, both the air- and bone-conducted BAERs will be abnormal.^1,3 In the following case, air-conducted and bone-conducted BAER data were used to diagnose conductive hearing impairment and to provide prognostic information regarding a patient's likelihood of hearing following ear canal reconstruction.

Case Report

A 3 yr old, 43 kg, castrated male Labrador retriever presented to the Matthew J. Ryan Veterinary Hospital of the University of Pennsylvania for evaluation and treatment of deafness. According to the owners, the dog would only respond to high-pitched voices and to sounds made close to his head.

On physical examination, the dog did not behaviorally respond to the sound of people talking normally within the room, but he would respond to loud voices. The right pinna was absent and the left pinna was of normal shape and size. No opening of the external acoustic meatus on either side could be found. Instead, a shallow invagination of epithelium was present (Figures 1A, B). The tubular auricular cartilage could be palpated below the epithelium covering the shallow invagination. The remainder of the physical and neurologic examination was within normal limits. Bilateral external ear canal atresia was diagnosed. The total cause for hearing impairment could not be immediately identified, but could have been due to a lack of sound conduction in the external ear canal or middle ear, disease of the cochlea or eighth cranial nerve, or a combination of both abnormalities. The owners requested information as to whether hearing could be restored following surgery on the external ears.

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The dog was anesthetized with isofluorane^a for computed tomography (CT) of the head and BAER testing to further characterize the ear disease. The dog was placed in sternal recumbency and a CT of the head (5 mm slice thickness and interval, bone and soft tissue algorithm) with and without IV contrast (90 mL of Omnipaque 350^b) revealed bilateral distension of the external ear canals with a lack of communication between the proximal vertical canals and the external auditory meatus. The horizontal and vertical canals of both ears were tortuous, but patent, along their course, with each vertical canal ending subcutaneously at what would be the external acoustic meatus in a normal dog. Both external ear canals were filled with two types of noncontrast-enhancing material: smaller fractions of high-density fluid or low-density soft-tissue material (approximately 25 Hounsfield units [HU]) and larger fractions of less dense material (approximately -50 HU). This larger fraction was compared with densities of subcutaneous fat (approximately -100 HU) and cerebrospinal fluid (approximately 5 HU) in this imaging study as a reference for comparative determination of the ear content. The larger fraction of less dense material corresponded to a midpoint density between fat and cerebrospinal fluid. The left canal was sharply demarcated by a bulging tympanic membrane from a mostly air-filled tympanic bulla with a small amount of gravity-dependent material. The right middle ear was filled with similar material as well as a small amount of air dorsally. The bony structures of the middle and inner ear appeared normal bilaterally (Figure 2). The imaging diagnosis was bilateral closed ear canals filled with mixed-density material, which the authors suspected was a combination of cerumen and inspissated fluid.

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BAER testing was performed on a DANTEC Cantata^c using 12 mm, 29 gauge subdermal needle recording electrodes. The active electrode was placed over the osseous bulla of the stimulated ear, the reference electrode over the vertex of the skull, and the ground electrode over the contralateral osseous bulla. For the air-conducted stimulation, rectangular, rarefaction clicks (0.1 msec duration) were delivered to the stimulated ear at 10 Hz via a headphone using a 24 cm plastic tube connected to a soft foam earpiece. The earpiece was placed within the shallow depression of the external ear where the opening would be expected. The intensity of the stimulus was varied from a maximum of 132 decibels (dB) sound pressure level (SPL) to 66 dB SPL. A white masking noise was applied to the contralateral ear at 40 dB SPL below the stimulus intensity delivered to the stimulated ear. The filter settings of the amplifier were 5 kHz and 100 Hz. Five hundred evoked responses were averaged for each tracing obtained. Responses were recorded at an amplifier calibration of 2 μV/cm for 10 msec. Response threshold was defined as the sound intensity at which an evoked waveform was first visible as the sound intensity was gradually increased. The attenuator was decreased in 3 dB steps, and a signal averaged response was sought at each step. If an evoked response was observed, the attenuator was then decreased by another 3 dB and the BAER response again observed. This continued until a sound level was reached at which an averaged evoked response could be identified.

Bone-conducted stimulus was performed in similar fashion substituting the earpiece stimulus for a bone vibrator^d held directly against the osseous bulla of the ipsilateral ear. The intensity of the bone vibrator was set at the maximum intensity (132 dB), and rarefaction and condensation stimuli were averaged.

Results of the air-conducted stimulus BAER test revealed an elevated response threshold at 102 dB in both the left and right ears, and waves that appeared subjectively small at 132 dB. Bone-conducted testing evoked waveforms that were a well-developed BAER at the single intensity tested (Figure 3). Based on the results of the BAER testing, air-conductive hearing impairment was diagnosed, and surgery was elected to open the external ear canals.

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A lateral ear canal resection (the Zepp procedure) was performed bilaterally 1 mo later to open the ear canals and improve conductive hearing.^4,5 Both ear canals were found to be tortuous. The right tympanic membrane was ruptured, but the left was intact. Large amounts of brown ceruminous material were removed from both ear canals and submitted for culture. Escherichia coli cultured from the right ear was sensitive to all antimicrobials on the sensitivity panel, and the dog was subsequently treated with amoxicillin trihydrate/clavulanate potassium^e (17.4 mg/kg per os q 12 hr) for 10 days.

Auditory function was reassessed 2 days postsurgically with BAER using the same procedure as described above. The dog was sedated with hydromorphone^f (0.1 mg/kg IV) and acepromazine^g (0.01 mg/kg IV). Results showed that air-conducted stimulus thresholds were approaching normal, at 81 dB on the right and 66 dB on the left (Figure 4). Bone-conducted stimuli at 132 dB evoked responses similar to the preoperative waveforms. The owners also noted apparent improvement in hearing ability at home.

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Discussion

The acoustic signal used for the BAER test is typically an air-conducted stimulus. A normal response depends on the integrity of the ear canal, tympanic membrane, middle ear, and auditory pathway. A reduction in amplitude of the evoked waveforms (and the elevation in response threshold) may occur with damage to any of these structures. In contrast, the bone-conducted stimulus bypasses the external and middle ear and stimulates the cochlea directly. A reduction in bone-conducted BAER is only expected with sensorineural disease.^1,3

Previous reports of ear canal atresia in dogs and one cat describe presenting signs of ear discomfort, drainage, or nasal discharge, requiring surgical correction.^6–13 In contrast, the dog in this case report showed no clinical signs associated with external ear inflammation, did not appear painful, and presented solely for decreased hearing acuity. The owners were intent on determining whether surgery would correct the obvious hearing impairment before proceeding. Although CT clearly identified obstruction of the external and middle ear that could have caused a conductive hearing impairment, it could not be determined whether the cochlea and eighth cranial nerve were functional and whether surgery to correct the external ear disease would result in improved hearing. Bone-conducted BAER confirmed that the cochlea and eighth cranial nerve were functioning, that conductive deafness was the likely cause for the hearing impairment, and that surgery should successfully improve hearing. Improvement in auditory response was confirmed using postsurgical BAER testing. The presence of E. coli suggested that, similar to previous cases of atresia of the external acoustic meatus, pain or draining tracts may have developed and surgical intervention would have been warranted even without the possibility of increasing hearing acuity.

There are few reported examples in the veterinary literature where bone-conducted stimuli were used to diagnose conductive hearing loss.^14,15 The use of bone-conducted stimuli was not described in other case reports of atresia of the external acoustic meatus; however, it was used to diagnose conductive deafness in cases of proliferative otitis externa where sensorineural damage was also considered.^6–15 The lowest dB air-conducted stimulus that would still produce BAER waveforms was elevated in dogs with proliferative otitis externa compared with normal dogs, whereas bone-conducted response thresholds were normal in some of the diseased dogs.^14,15 Elevated bone-conducted response thresholds in these studies were considered to have a component of sensorineural deafness.^14,15

Finally, the CT revealed some interesting features in this dog. Middle and external ear debris is usually consistent with thickened fluid based on density (CT) and intensity (MRI) in cases of otitis media, otitis externa, or eustachian tube dysfunction.¹⁶ This was only the case for a small fraction of the ear canal content in this dog. There was a well-separated, less dense fraction that showed characteristics of a lipid-containing fluid, such as cerumen, with a density between fluid and fat. Similar imaging characteristics were also displayed in two previous case reports of external ear canal atresia, but the authors did not specifically commenton it.^10,12 These findings may be unique to external ear canal atresia and potentially aid in differentiation between otitis and ear canal atresia. More CT studies of external ear canal atresia need to be reviewed to validate this finding.

Conclusion

This case report demonstrates that bone-conducted BAER testing can be used to differentiate sensorineural deafness from conductive deafness. BAER testing can also provide useful prognostic information regarding whether surgical correction of conduction abnormalities will improve hearing.