Comparison of auditory brainstem response peak measures using ear lobe, mastoid, and custom ear canal reference electrodes.

Auditory brainstem response (ABR) peak measures were compared for a custom ear canal electrode and traditional mastoid and ear lobe reference electrode sites with the assumption that the ear canal electrode would yield larger Wave I amplitudes. In this study, Waves I, III, and V latencies and amplitudes were measured and compared in twenty participants between the ages of 18 and 50, with particular interest in a potential Wave I advantage using the custom ear canal reference electrode. The statistical analysis suggested that all reference electrode sites yielded comparable results with no statistical differences in peak latency or peak-to-trough amplitude for Waves I, III, and V. Although the custom ear canal electrode did not produce larger Wave I amplitudes over the other two references, a deeper placement of the ear canal electrode might have yielded different results.

Introduction

A prolongation of the Wave I-V inter-peak latency of the auditory brainstem response (ABR) has been regarded as one of several useful indices of a neurological lesion of cranial nerve VIII.[1,2] However, Wave I is sometimes obscured because of poor signal-to-noise ratio in the resultant average, peripheral hearing loss, and/ or the location and placement of electrodes on the head and ears.[3] The inability to visualize Wave I or V can negatively influence the clinical sensitivity and specificity of the ABR, which are generally high.[4] Previous studies in the literature have demonstrated that a reference (inverting) electrode placed superficially within the ear canal (i.e., extratympanic) may enhance the amplitude of Wave I.[1,5-8]

Harder and Arlinger[5] assessed a custom ear canal electrode involving a silver-coated acrylic sheet placed as near the tympanic annulus in the conduction paste-coated ear canal with microsurgical forceps. They reported that Wave I amplitude for the ear canal electrode was on average twice the size of Wave I recorded from the mastoid. Wave V between the ear canal and mastoid references was non-significant. Yanz and Dodds[6] assessed a custom ear canal electrode involving a conductive paste-coated foam earplug threaded with silver wire and a small transducer passed through the core of the foam. They also reported significantly larger Wave I amplitudes with the ear canal electrode compared to mastoid, but unlike Harder and Arlinger,[5] they found the opposite effect with Wave V being significantly larger for the mastoid electrode. Ruth et al.[1] essentially replicated the findings of Yanz and Dodds[6] with a, then, newly commercially made ear canal electrode (i.e., Enhancer 1 electrode system). The Enhancer 1 involved a head worn device with built-in electrodes and a transducer/electrode system for the ear canal with a disposable foam eartip impregnated with conductive gel. The Enhancer 1 electrode system is no longer being manufactured, but a viable alternative using a commercially-available gold-foiled ear canal electrode (i.e., TIPtrode™) has been available for the last couple of decades and works with tubal insert earphones that often come with evoked potential systems. Using the TIPtrode, Bauch and Olsen[7] also reported Wave I enhancement compared to the mastoid, but they reported no differences for Wave V between the two electrodes. Additionally, with the TIPtrode they reported that Wave I was identified more frequently in patients with sensorineural hearing loss compared to mastoid electrode. Finally, Gaddam and Ferraro[8] modified the TIPtrode by shortening it for pediatric patients. They reported results similar to Bauch and Olsen[7] with larger Wave I amplitudes for the ear canal electrode, but no statistical difference between ear canal and mastoid electrodes for Wave V. Taken together, all of these studies indicated Wave I with ear canal electrode, regardless of design. However, the report of differences and/or the magnitude of differences across studies between the ear canal and mastoid electrodes varied depending on the depth of insertion and the type of transducer used (i.e., headphone or earphone). Unfortunately, none of these studies reported the effect of using an earlobe reference electrode.

The principle reason for Wave I enhancement is attributed to the shorter distance between the ear canal electrode and the generator of interest, which is cranial nerve VIII.[6,9] Past studies have compared Wave I amplitudes between the ear canal and mastoid electrodes, leaving a paucity of information about how either compare with an earlobe electrode. Additionally, there have been anecdoctal statements by some clinicians that an ear canal electrode is advantageous over the other two placements, but this has never been verified under clinically-similar test conditions. In this study, we test a custom ear canal electrode, similar to the TIPtrode™, and compare with traditional ear lobe and mastoid electrode placements. The primary purpose of this study was to compare Wave I, III, and V latency and peak-to-trough amplitude measures of fixed-intensity level ABR recordings in a sample of normal hearing adults through the use of earlobe, mastoid, and ear canal reference electrodes. A secondary, but lesser purpose of this study was to evaluate the custom-made, ear canal electrode using easily accessible materials for clinical use.

Materials and Methods

Participants

Twenty young adult participants (10 males, 10 females) between the ages of 18 and 50 years were recruited for the study. Participants had thresholds of 25 dB HL or better at octave frequencies from 250 to 8000 Hz, as well as, Type A tympanograms to qualify for this study.

Custom ear canal electrode

Custom ear canal electrodes were made using 10 mm ER3-14B disposable foam ear tips and a 4 in. length of 32-gauge silver wire. First, a small hole was formed along the outside of the plastic tubing of the foam ear tip, so that the wire could be threaded from the lateral to medial ends. After passing the wire through, the medial end of the wire was then curled around the foam and tucked back into the lateral end of the foam (again, on the outside of the plastic tubing) to ensure that the end of the silver wire would not scratch the ear canal surface upon insertion. On the foam surface, a short segment of the silver wire is visible, which will later come in contact with the posterior wall of the ear canal at the first bend. The lateral end of the wire was left exposed to be used with an alligator clip electrode during recordings and could be trimmed to size. These custom electrodes are assembled using easily accessible materials, and they appear to be similar in concept and design as other commercial and non-commercial ear canal electrodes. Figure 1 illustrates the parts and final construction of the custom ear canal electrode.

Figure 1

Custom ear canal electrode. The basic parts include a 32-gauge piece of silver wire and 10 mm ER3-14B ear tip (A). A small hole is punctured through the foam next to the plastic straw. The silver wire is passed through, looped around to the lateral end of the straw, and tucked back into the hole. The lateral end of the wire is then pulled until the wire is snug on the surface of the foam (B). An alligator clip connected to the electrode box is attached, and the final product serves both as an electrode and sound port.

Equipment setup and participant preparation

For the ABR, participants were then instructed to lie in a reclining chair in a non-acoustically-shielded clinical test room. The Bio-Logic Navigator Pro (Version 6.2.0) was used as the evoked potential system with proprietary tubal insert earphone transducers. Broadband clicks (0.1 ms duration) were presented to each subject at a rate of 17.7/s with stimulus intensity level set at 115.5 dB ppeSPL (or 80 dB nHL). Rarefaction clicks was chosen based on data by Ruth et al.[10] that demonstrated that Wave I is enhanced with this polarity. All waveforms were collected using a window size of 10.66 ms with 256 points, an amplifier gain of 100,000, a bandpass filter of 100 and 3000 Hz, and with an artifact rejection level of ±23.75 μV.

A montage of four commercially available silver-silver-chloride (Ag-AgCl) disc electrodes was attached to each participant: Fz, Fpz, A2 (right earlobe), and M2 (right mastoid). Prior to electrode placement (including ear canal electrode), skin areas where the electrode would be applied were prepared with an alcohol wipe and a mildly abrasive electrode prep gel using a cotton swab or gauze pad to ensure adhesion and conductivity. The region of the first ear canal bend was abraded using a cotton swab applicator. All disc electrodes were filled with Ten-20 conduction paste. Preliminary otoscopic exam was conducted prior to insertion of the custom ear canal electrode. The ear canal electrode was coated in ultrasonic gel (Burdick ElectroShock, Milton, WI) with the exposed silver wire positioned on the first bend of the right posterior ear canal wall. This location was used to ensure consistency of placement from participant to participant. Impedances were kept below 5000 Ohms with interelectrode impedances kept below 2000 Ohms.

Experimental conditions

During the study, three different test conditions were implemented using a two-channel setup to collect data. Because we were most interested in comparing the three electrodes around the ear, the three test conditions were as follows: ear canal vs. earlobe, ear canal vs. mastoid, and earlobe vs. mastoid. Because only 2-channels could be recorded at a time with the Bio-Logic system, one of the three ear electrodes was unplugged during each condition. By nature of each condition, all reference electrode placements were repeated once. All data was collected and labeled by two of the authors who were trained on all procedures. The first author served as a secondary independent judge for the labeling of all waves without regard to which reference was used and any differences in judgment were resolved until there was 100% agreement.

Waveform and data analysis

For data reduction purposes, Waves I, III, and V latencies and peak-to-trough amplitudes were averaged between the repeated trials. The mean latencies and peak-to-trough amplitudes along with their relative standard deviations were obtained for all Wave-Electrode combinations. For Wave V, we selected the maximal peak in the vicinity of Wave IV and V. Three separate multivariate analysis of variance (MANOVA) models at each wave (i.e., Wave I, III, V) were used to compare mean differences in electrodes in terms of their relative latencies and peak-to-trough amplitudes. A significance level of .05 was used for all analyses. Analyses were conducted using SAS 9.2 (SAS Institute Inc., Cary, NC, USA).

Results

Three separate multivariate analysis of variance (MANOVA) models at each wave (i.e., Wave I, III, V) were used to compare mean differences in electrodes in terms of their relative latencies and peak-to-trough amplitudes. There were no statistical differences among the three electrode types at Wave I (Wilk's Lambda=0.97, F(4,112)=0.42, P=0.7939), Wave III (Wilk's Lambda=0.85, F(4,112)=2.43, P=0.0518), or Wave V (Wilk's Lambda=0.93, F(4,112)=1.03, P=0.3975). Peak latency means and standard deviations for Waves I, III, and V are shown in Table 1 associated with each of the reference electrodes. Peak-to-trough amplitude means and standard deviations for Waves I, III, and V are shown in Table 2 associated with each of the reference electrodes.

Table 1

Latency descriptive statistics for Waves I, III, and V.

Gender	EC-I	EL-I	MS-I	EC-III	EL-III	MS-III	EC-V	EL-V	MS-V
F	1.32	1.57	1.74	3.66	3.82	3.66	5.28	5.32	5.40
F	1.32	1.66	1.62	3.66	3.78	3.61	5.32	5.45	5.36
M	1.37	1.66	1.49	3.66	3.78	3.49	5.53	5.57	5.45
M	1.37	1.41	1.49	3.57	3.82	3.57	5.49	5.20	5.4
M	1.53	1.37	1.49	3.66	3.70	3.57	5.28	5.20	5.45
F	1.57	1.49	1.49	3.66	3.74	3.61	5.28	5.28	5.45
F	1.49	1.41	1.49	3.74	3.78	3.74	5.32	5.32	5.49
M	1.49	1.41	1.49	3.74	3.74	3.74	5.15	5.36	5.40
F	1.57	1.45	1.53	3.66	3.82	3.49	5.49	5.45	5.40
F	1.53	1.49	1.49	3.57	3.78	3.57	5.53	5.45	5.45
M	1.53	1.53	1.55	3.61	3.61	3.57	5.36	5.36	5.36
M	1.51	1.47	1.49	3.86	3.82	3.82	5.45	5.59	5.32
M	1.49	1.45	1.47	3.61	3.53	3.53	5.62	5.65	5.64
M	1.78	1.83	1.78	3.87	3.89	3.78	5.80	5.76	5.78
F	1.51	1.51	1.47	3.72	3.66	3.64	5.16	4.95	5.45
F	1.41	1.24	1.28	3.74	3.70	3.78	5.61	5.49	5.64
F	1.45	1.45	1.51	3.66	3.68	3.70	5.09	5.62	5.36
M	1.64	1.62	1.43	3.82	3.60	3.80	5.49	5.60	5.57
M	1.57	1.45	1.53	3.66	3.82	3.49	5.49	5.45	5.40
F	1.49	1.41	1.49	3.61	3.61	3.57	5.15	5.36	5.40
Mean	1.50	1.49	1.52	3.69	3.73	3.64	5.39	5.42	5.46
SD	0.11	0.13	0.10	0.09	0.09	0.11	0.19	0.19	0.12

EC, ear canal; EL, earlobe; MS, mastoid.

Table 2

Peak-to-trough amplitude descriptive statistics for Waves I, III, and V.

Gender	EC-I	EL-I	MS-I	EC-III	EL-III	MS-III	EC-V	EL-V	MS-V
F	0.51	0.58	0.69	0.24	0.22	0.3	0.66	0.49	0.53
F	0.46	0.43	0.43	0.34	0.34	0.33	0.78	0.46	0.64
M	0.40	0.40	0.37	0.36	0.23	0.30	0.75	0.66	0.51
M	0.52	0.54	0.54	0.66	0.60	0.60	0.42	0.46	0.45
M	0.45	0.29	0.27	0.21	0.20	0.30	0.48	0.41	0.46
F	0.53	0.37	0.34	0.21	0.45	0.08	0.37	0.21	0.16
F	0.36	0.28	0.41	0.56	0.43	0.50	0.67	0.33	0.63
M	0.43	0.45	0.61	0.28	0.26	0.34	0.76	0.70	0.52
F	0.16	0.18	0.14	0.35	0.32	0.32	0.29	0.28	0.18
F	0.33	0.27	0.48	0.33	0.24	0.21	0.40	0.31	0.57
M	0.68	0.60	0.45	0.47	0.38	0.56	0.73	0.75	0.81
M	0.79	0.71	0.45	0.41	0.43	0.47	0.69	0.69	0.85
M	0.66	0.53	0.55	0.32	0.52	0.45	0.62	0.52	0.61
M	0.34	0.23	0.17	0.31	0.31	0.32	0.35	0.35	0.41
F	0.56	0.41	0.63	0.40	0.40	0.41	0.49	0.57	0.41
F	0.53	0.58	0.55	0.53	0.53	0.44	0.53	0.45	0.32
F	0.56	0.58	0.55	0.56	0.41	0.53	0.29	0.14	0.35
M	0.31	0.27	0.28	0.23	0.24	0.28	0.57	0.47	0.44
M	0.33	0.27	0.48	0.33	0.24	0.21	0.40	0.31	0.57
F	0.68	0.60	0.45	0.47	0.38	0.45	0.62	0.52	0.61
Mean	0.47	0.42	0.43	0.37	0.35	0.36	0.53	0.44	0.49
SD	0.15	0.15	0.15	0.13	0.12	0.13	0.16	0.17	0.18

EC, ear canal; EL, earlobe; MS, mastoid.

Discussion

The results indicated no statistical advantage of the ear canal reference electrode for Wave I enhancement when compared to more conventional ear lobe or mastoid reference electrode placements. However, two other finds are potentially relevant to clinicians. First, in spite of lack of statistically significant differences between reference electrodes, the data would suggest that the custom ear canal electrode works just as good as, if not better, than the other two electrodes as references. Additionally, the ear canal electrode did not require tape to be held in place, nor did it involve much cleanup. Although not directly related to the question, there was a statistically significant advantage for the ear canal electrode for Wave V over the ear canal electrode, which might be useful for ABR threshold procedures as demonstrated previously both by Ruth et al.[1] and more recently by Gaddam and Ferraro[8] when compared to a mastoid reference electrode. However, the true advantage of the ear canal reference over mastoid and earlobe reference for enhancement of Wave V with decreasing intensity cannot be determined from this study. Wave I discrepancies between the results of this study and with others[1,5-7] are likely due to a number of different factors including: participant makeup (gender, age, and physical size), small sample size, transducer type, stimulus type and parameters, materials used, and depth of the ear canal electrode. Although a small number of participants did have a noticeably larger Wave I with the ear canal reference, we believe a deeper insertion past the first bend of the ear canal might have yielded a different statistical result. The first bend of the ear canal was chosen as a visual place marker to ensure consistent depth of insertion across participants. A deeper insertion of the ear canal would require silver wire of longer length and will need to be insulated except at its very tip. Telfon-coated silver wire is available, or nontoxic clear fingernail polish should also work. Although the custom ear canal used in this study could also be a factor in the present results, it is conceptually not unlike other ear canal electrodes described in the literature or manufactured. Future studies should aim to explore several of these factors at once before firm conclusions can be drawn regarding Wave I enhancement with ear canal electrodes. A comparison between the custom ear canal electrode and the TIPtrode™ is also warranted. We are certain that a deeper insertion of the ear canal electrode (past the first bend, but not past the second) would have enhanced Wave I much more consistently across participants, as is suggested from past studies. However, to maintain consistency in the control of the ear canal electrode placement for this study, we did not place the ear canal electrode past the first bend in order to achieve good contact with the posterior wall of the ear canal. Placement of an ear canal electrode ultimately depends on the skill and comfort level of the clinician.