2.1 Study participants
The study was designed as a prospective comparative experimental study at a tertiary referral center. Based on the primary outcome measures, the study sample size was estimated to 25 participants. Healthy adult test subjects between 18 and 40 years were included in the study. Screening audiograms were conducted in all subjects. Exclusion criteria were otologic or neurotologic disorders as well as an air-bone gap ≥ 10 dB at 500 Hz.
The study protocol was reviewed and approved by the ethics committee of the Medical Faculty of Martin Luther University Halle-Wittenberg and the University Hospital Halle (approval number: 2018-76). The study was performed in accordance with the ethical standards of the Declaration of Helsinki. Written informed consent was obtained from all patients before inclusion to this study. For publication of the image in Figure 1 in an online open access publication, written informed consent was obtained by the pictured test person.
2.2 Experimental set-up
The Eclipse EP25 system (Interacoustics A/S, Middelfart, Denmark) was used for stimulation and recording. Electromyographic (EMG) signals were recorded in a -20 to 80 ms window relative to the onset of the stimulus and bandpass filtered between 10-1000 Hz. The artifact rejection level was set to 400 mV. Responses to at least 200 stimuli were averaged. Self-adhesive Neuroline 720 surface electrodes (Ambu A/S, Ballerup, Denmark) were used to record the EMG signals after the skin was prepared to provide impedances of less than 5 kW. The electrodes were placed over the middle of the sternocleidomastoid muscle ipsilateral to the stimulated side and referenced to an electrode over the sternum for cVEMP testing. For oVEMP recordings, the electrodes were placed as bipolar montage on the infra-orbital ridge 1 cm below the lower eyelid contralateral to the stimulated side and about 2 cm inferior to the first electrode.
For BCV stimulation, the B81 and the Mini-Shaker 4810 were used. The B81 was fitted with the standard P333 steel spring headband (Radioear, New Eagle, USA) to provide coupling forces of 5.4 ± 0.5 N. The Mini-Shaker was fitted with an experimental apparatus to control the coupling force (see Fig 1). The apparatus consisted of a chuck holding an aluminum rod in a frame fixed to the shaker itself, and a preload force adjustment. The latter consisted of a compression spring with a spring constant of 0.49 N/mm. A static force of approximately 5 N was adjusted by screwing the preload force adjustment compressing the spring by 10 mm (10 mm x 0,5 N/mm = 5 N). The aluminum rod ended with a plane circular tip surface of approximately 1.75 mm2 which served as an adapter to be coupled to the hairline at the test person’s head. The whole system was fixed to a flexible mounting arm. The Mini-Shaker was used with the power amplifier 2718 (Brüel & Kjær Sound & Vibration Measurement A/S, Denmark). For ACS stimulation, ER-3A insert earphones (3M, St. Paul, MS, USA) were used.
The stimuli were short 500 Hz tone bursts (0-1-0, i.e.: 0 cycles rise/fall time, 1 cycle plateau) for each measurement. The BCV stimuli were calibrated on an artificial mastoid 4930 (Brüel & Kjær Sound & Vibration Measurement A/S, Denmark). An Infinii Vision 2000 X-Series oscilloscope (Keysight Technologies, Santa Rosa, CL, USA) was used to read the output peak-to-peak equivalent vibratory force levels (peVFL). The maximum stimulus intensity was 142 dB peVFL for the B81 and 144 dB peVFL for the Mini-Shaker. For ACS, the standard clinical units and calibration in dB nHL was used. The maximum stimulus intensity was 100 dB nHL.
2.3 Procedures
Ocular and cervical VEMPs were measured on the right and left side of all subjects.
2.3.1 cVEMPs
For cVEMP recordings, the patients were sitting on a chair and were asked to turn their head to the shoulder contralateral to the stimulated side 22. VEMP responses were recorded from the ipsilateral sternocleidomastoid muscle. The EMG signal was monitored and feedback was provided by the examiner to ensure sufficient tonic muscle contraction (50 - 200 mV). To calculate the background EMG, the root mean square of the EMG signal was averaged over the pre-stimulus window and for each recording frame. EMG scaling was performed by normalizing the cVEMP p13-n23 peak-to-peak amplitude to the background EMG in order to reduce the impact of muscle contraction on cVEMP amplitudes.
Cervical VEMPs were measured to BCV using the B81 positioned on the mastoid and to ACS using the ER-3A insert earphones. The Mini-Shaker was not used for cVEMP measurements because the experimental apparatus required a fixed head position and could not be applied to the head during cVEMP measurements (turning the head to the contralateral shoulder was necessary).
2.3.2 oVEMPs
For oVEMP testing, the patients were asked to lie in supine position and look up, maintaining an upward gaze of 30°. VEMP responses were recorded from the ocular muscles contralateral to the stimulated side.
Ocular VEMPs were measured to BCV using the B81 on the mastoid and on the forehead as well as using the Mini-Shaker on the forehead. During the measurements with the Mini-Shaker the test person’s head was positioned on a fixation pillow (mediPlac, Borchen, Germany) to prevent head tilt and therefore prevent a change of coupling force adjusted with the experimental apparatus described earlier. Ocular VEMPs to ACS were measured using the ER-3A insert earphones.
2.4 Data analysis
The VEMP responses were analyzed in OtoAccess software (Interacoustics A/S, Middelfart, Denmark). A response was judged to be present when it was clearly larger than the pre-stimulus waveforms, i.e., the background noise. The amplitudes of all myogenic potentials were measured from peak to peak (p13-n23 for cVEMPs, n10-p15 for oVEMPs). The asymmetry ratio (AR) in % was calculated by dividing the difference between the larger and smaller amplitude by the sum of the amplitudes multiplied by 100.
Descriptive statistics were used to report latency, amplitude, threshold and asymmetry data. Quantitative data were presented as mean, standard deviation (±), and range (minimum and maximum).
Response rates were calculated for each stimulation mode (cVEMP: B81 at mastoid, AC; oVEMP: B81 at mastoid, B81 at forehead, Mini-Shaker at forehead, AC) for the right and left sides and were compared using the Chi-square test. For confirmed associations (p < 0.05), post-hoc analysis was performed by pairwise Chi-square tests. To consider multiple comparisons, the p value was reduced according to the Bonferroni correction (cVEMPs: 4 stimulation modes with six pairwise comparisons, p = 0.05/6 = 0.008). Qualitative data were presented as graphs, if appropriate. SPSS 25 for Windows software (IBM, Armonk, NY, USA) was used for all statistical analyses. Graphs were created in GraphPad Prism 8 (Graphpad Software, San Diego, CA, USA).