Brainstem Auditory Evoked Response (BAER) in Canine Otitis Externa

Sense of hearing is very important in animals for communication and to safeguard themselves from impending hazards. Peripheral deafness may be  conductive or sensorineural. The blockage of transmission of sound waves from the environment to the cochlea causes conductive deafness while sensorineural deafness occurs due to affections of cochlear structures, auditory nerve and auditory components of brain stem and central nervous system (Scheifele et al., 2012).
       
Inflammatory conditions of the ear can lead to conductive and sensorineural type of deafness. Otitis interna has the potential to produce sensorineural deafness (Strain, 2012), while otitis externa and otitis media can cause conductive deafness due to attenuation of the auditory stimulus by the of presence of exudates, ear canal stenosis, thickening and perforation of the tympanic membrane or damage to the auditory ossicular chain (Eger and Lindsay, 1997).
       
Diagnosis of deafness by behavioural means is unreliable, because it is difficult to identify unilateral deafness or partial hearing loss by this technique. Dogs with normal hearing ability may not respond because of situational anxiety, or may discontinue responding upon getting accustomed to the stimulus. Additionally, partially hearing impaired and deaf dogs may respond by detecting the stimulus through other senses, from visual cues, vibration or even air movement (Oishi et al., 2012; Strain, 2012).
       
Brainstem auditory evoked response (BAER) first described in veterinary research applications in 1970 (Jewett, 1970) is a time - locked wave pattern with 5 - 7 peaks, measuring the electrical activity arising from cochlea and auditory pathway within first 10 millisecond (ms) of providing acoustic stimulation (Strain, 1996). Determining the type and extent of deafness is informative in understanding the impact it has on the animal’s life, the prognosis of the condition, further treatment options and breeding implications. Conductive deafness is due to blockage of sounds by damage to middle or external ear and it can be often corrected medically or by corrective surgical procedures (Béraud, 2012; Mason et al., 2013; Strain, 2015). With the above background, this study was conducted to determine the effect of otitis externa on hearing with BAER technique.

Animals
 
Twenty five dogs presented to the Teaching Veterinary Clinical Complex of Madras Veterinary College were selected for the study. The study was conducted from September 2018 to July 2019. In all cases, written owner consent was obtained before admission of the dog for investigation. Complete physical and otoscopic examination of the dogs was performed along with routine blood tests and skull radiography. Each ear of the dogs was assigned a grade of severity of otitis depending on the clinical and otoscopic evaluation. Grade 1 ears had no history or clinical signs of inflammation while grade 2 ears had mild otitis with inflammed ear canals exhibiting slight ear discharge. Lastly grade 3 encompassed ears which revealed severe stenosis of and excessive discharge in ear canal.
 
Neurodiagnostic testing
 
The test was performed using BAER apparatus (RMS Medulla 201, Recorders and Medicare Systems (P) Ltd., Panchkula, Haryana, India). Dogs were prepared by clipping the hair at the site of insertion of electrodes on the head region and cleaning it thoroughly with surgical spirit. The dogs were anaesthetized and maintained in ventral recumbency position. A stimulus of 90 decibel sound pressure level (dB SPL) was delivered to each test ear via over the head phones and non - test ear was provided a mask stimulus. A total of 1000 click stimuli were delivered at the rate of 33 clicks per second. The filters were adjusted to have high pass filter at 100 hertz (Hz) and low pass filter at 3 kilohertz (kHz). Three readings were obtained for each ear to determine the repeatability and natural response of waves. The subdermal needle electrodes (Inomed Medizintechnik GmbH, Emmendingen, Germany) were inserted as per 10 - 20 protocol with positive electrode at vertex and negative electrode at test ear mastoid region and ground electrode at mastoid of non - test ear. The signals were amplified with an absolute gain setting of 0.5 microvolt (µV). The V^th wave was recognized by the deep trough following its peak. The absolute latencies of all five waves, inter peak latencies of I - III, III - V and I - V and wave I to V amplitude ratio were measured.
 
Statistical analysis
 
The data thus obtained was analysed statistically using one way ANOVA test in IBM^® SPSS Software version 20 to compare the significant difference among the three grades of ears with respect to the various latencies and amplitude ratios.

Distribution of ears according to severity of disease showed 26 normal ears (Grade 1), 10 with OE of Grade 2 and 14  with Grade 3 OE. The 46/50 ears showed the 5 peaked waveform with distinct wave V followed by deep trough. The normal BAER wave pattern is shown in Fig 1. The mean absolute latencies of grade 1 ears, were within the range noted in other canine BAER studies (Besalti et al., 2008; Kemper et al., 2013; Plonek et al., 2017). The wave I began within 1.0 to 1.5 ms after the stimulus with each succeeding wave occurring within 1ms intervals which was in accordance with previously established research findings (Sims, 1988).

Two dogs did not show any response bilaterally upon BAER test. Among these two, one had a ruptured tympanic membrane while the other had severe bilateral ear canal stenosis. Hence they were declared severely hearing impaired and excluded from further analysis. The absence of BAER response due to these causes has been reported previously also (Eger and Lindsay, 1997; Perry et al., 2017; Strain, 1999).
 

       
The BAER wave pattern of a dog with both ears of grade 3 is shown in Fig 2. The mean ± SE (standard error) values of the parameters of BAER tests are given in the Table 1. There was highly significant difference (p<0.01) between latency of wave I in group 1 and group 3. However, there was no statistically significant difference in the latency between grade 1 and 2 ears as well as between grade 2 and 3 ears. The latencies of other waves did not show any statistically significant difference between grade 1 and 2 ears while the grade 3 ears were significantly (p<0.05) different from the other two. There was no statistically significant difference in inter - peak latencies between all three groups.
 

 

       
The prolongation of latencies of waves in the grade 3 ears may be attributed to the conductive lesions in outer ear which delay onset of wave I primarily and other waves as well. Since, the cause of delay was caudal to the generators of the waves, inter - peak latencies were not affected (Hall, 1992; Sims, 1988).
       
Studies in the past have recorded that conductive deafness due to otitis resulted in prolonged latencies of wave I and V, only wave I or all five waves (Béraud, 2012; Besalti et al., 2008; Eger and Lindsay, 1997). 
       
Our study results were in accordance with previous reports, that chronic otitis externa largely did not result in complete deafness in dogs but rather lead to conductive type of hearing impairment. This is the result of stenosis or absolute obliteration of vertical ear canal and occlusion with discharge (Eger and Lindsay, 1997; Strain, 1999).
       
Presbycussis or age related hearing impairment, occurs in dogs from 8 to 10 years of age (Ter Haar et al., 2008). In our clinical cases, two dogs were 8 years and one was 8.5 years old. These three dogs had unilateral OE. The affected ears showed increased latency values as compare to other normal ears. This ruled out effect of presbycussis on the overall hearing of the dogs.
       
The wave I: wave V amplitude ratios were observed to be significantly  lower for grade 3 ears as compare to that in  grade 1 and grade 2 ears. However, no statistically significant difference was found  in these ratios between grade 1 and 2 ears. Thus, there was significant reduction in wave I / V amplitude ratio as the severity of otitis increased, similar to previous reports (Eger and Lindsay, 1997, Plonek et al., 2017). Both the reports indicate that the amplitude of wave V increased while that of wave I reduced as the severity of hearing impairment increased.
       
The absence of significant difference BAER parameters between mildly affected dogs (grade 2) and healthy ears (grade 1) could be due to the infection level being mild without severe lesions of the ear canal and tympanic membrane (Steiss et al., 1994).
       
Hence, the BAER findings signify hearing impairment, correlated with the clinical and diagnostic findings of severe OE. As such BAER may be used as a promising diagnostic modality to determine hearing impairment in dogs with OE. This can help to plan further therapeutic protocol, effectively ameliorate clinical signs and prevent progression to complete deafness.