Study subjects
For this retrospective study, the medical records of consecutive patients with VM or MD, who had received VEMP testing at the German Center of Vertigo and Balance Disorders or the Department of Neurology at the University Hospital of the Ludwig-Maximilians-Universität, Munich, Germany, in 2012–2013 and 2017–2019, were screened. One hundred (100) patients were included in the MD and the VM group, respectively. The MD group comprised 85 patients with definite MD (dMD) and 15 patients with probable MD (pMD), while the VM group consisted of 35 patients with definite VM (dVM) and 65 patients with probable VM (pVM). In addition to those patients, four MD and five VM patients, who had received serial VEMP measurements over time, were identified and analyzed separately.
All subjects underwent a thorough neurotological workup, i.e., history-taking, clinical examination, and additional vestibular diagnostics, including caloric irrigation of the horizontal canals, video head impulse testing for the semicircular canal function in the high-frequency range, examination of the subjective visual vertical and fundus photography with a scanning laser ophthalmoscope for static graviceptive function. In addition, c- and oVEMPs, posturography, gait analysis, and pure tone audiometry were performed if necessary.
Patients were included in the study if they had received VEMP testing and fulfilled the diagnostic criteria of the Bárány Society for either vestibular migraine [
8] or Menière’s disease [
5]. All patient records were reviewed by a neurotology specialist for the presence of the diagnostic criteria defining the definite and probable forms of the disorders before inclusion in the study. Patients were excluded from the study if they had reported overlapping symptoms of MD and VM or if they fulfilled the diagnostic criteria for both disorders based on their history.
Moreover, the following groups of patients were excluded: bilateral MD, MD with a history of migraine, patients with further vestibular or neurological disorders (e.g., benign paroxysmal positional vertigo, vestibular paroxysmia, inner and outer labyrinthine fistula, vestibular neuritis, vestibular schwannoma, cerebellar ataxia, extrapyramidal motor disorders, dementia, multiple sclerosis, stroke), middle ear disease (e.g., cholesteatoma, otosclerosis, chronic otitis media, tympanic effusion), or an air–bone gap in pure tone audiometry on the day of the VEMP recording. Furthermore, patients with a history of ear surgery (including but not limited to tympanoplasty, stapes surgery, endolymphatic sac surgery, intratympanic gentamicin, and labyrinthectomy), brain surgery, or concussion were excluded.
VEMP recordings
VEMPs were recorded as described previously [
1] with one of the three VEMP platforms routinely used in the German Center for Vertigo and Balance Disorders and the Department of Neurology, i.e., the Nicolet on Viking EDX evoked potential system (Natus, Pleasanton, CA, USA), the Eclipse platform (Interacoustics, Middelfart, Denmark), or the Neuropack M1 platform (Nihon Kohden, Tokyo, Japan). Only those VEMP responses that were clearly discernible from background noise were included in the analysis. To avoid bias due to different recording platforms and examiners, only asymmetry ratios of VEMP amplitudes and latencies were analyzed in detail (see “
Outcome parameters”).
oVEMPs
For oVEMPs, the surface recording electrode was placed on the infraorbital rim, the reference electrode 1–2 cm below, and the ground electrode was fixed around the wrist. Patients lay supine and looked up during the recording to increase the oVEMP n10 amplitude [
2]. A BCV stimulus was delivered to the midline of the forehead at the hairline (Fz) by a powerful bone-conduction device (minishaker 4810, Bruel and Kjaer, Naerum, Denmark) connected to an amplifier (type 2718, Bruel and Kjaer, Naerum, Denmark). A custom-made Matlab program (MathWorks, Natick, MA, USA) was used to produce 500 Hz tone burst BCV stimuli (rise/fall time: 0 ms, plateau: 2 ms, driving voltage: 5 V, stimulus repetition rate: 3 pulses per second (pps)). Responses below the right and left eyes were recorded simultaneously; the analysis window was 20 ms from stimulus onset. The EMG signal was amplified and bandpass filtered (10 Hz – 1.5 kHz). Twenty unrectified traces were averaged per recording, and at least two trials were run in one subject to ensure reliability and reproducibility of the VEMP signal. Amplitudes and latencies from reproducible recordings were averaged for the right and left ear, respectively (see “
Outcome parameters”).
cVEMPs
For cVEMPs, the recording electrode was placed over the mid third of the sternocleidomastoid muscle (SCM), and the reference electrode over the sternoclavicular junction. The ground electrode was fixed around the wrist. During the recording, the subject lifted the head in the midline from a semi-recumbent position to maintain sufficient symmetric muscular activity in the SCM of either side for recording the inhibitory cVEMP response [
2]. 500 Hz ACS tone burst stimuli (130 dB peak sound pressure level, rise/fall time: 1 ms, plateau: 5 ms, 3 pps) were applied consecutively to either ear by TDH-39P headphones (Telephonics, Framingdale, NY, USA). The recording window was 50 ms from stimulus onset. The EMG signal was amplified and bandpass filtered (10 Hz – 2 kHz), and 50 unrectified traces were averaged per recording. As described for oVEMPs above, amplitudes and latencies were calculated as average from at least two reproducible recordings.
Outcome parameters
For oVEMPs, amplitudes and latencies of the first negative (n10) and the first positive (p15) peak were determined as a measure of contralateral (mainly) utricular function, while the first positive (p13) and the first negative peak (n23) of the cVEMP response were used as parameters for ipsilateral (mainly) saccular function [
2].
To rule out any systematic differences between the three different sets of recording equipment and different examiners, we did not compare absolute amplitudes and latencies between subjects, but only ARs between the right and left sides within each subject. Amplitude ARs were calculated as described before [
28]:
$${\text{AR}}\, = \left| {{\text{ right amplitude }}{-}{\text{ left amplitude }}} \right|{\text{ }}{ / }{\text{ }}\left| {{\text{ right amplitude}}\, + \,{\text{left amplitude }}} \right|.$$
AR values range between 0 (symmetric response on both sides) and 1 (absent response on one side). Asymmetry ratios were determined for oVEMP n10p15 amplitudes and cVEMP p13n23 amplitudes. ARs > 0.3 (oVEMPs) and ARs > 0.4 (cVEMPs) were considered to indicate asymmetry between the two sides, respectively, based on the normative values of our laboratory as well as data from the literature [
2,
28].
Latency ARs were calculated accordingly for n10 and p15 latencies (oVEMPs) and p13 and n23 latencies (cVEMPs) in addition to n10p15 and p13n23 inter-peak intervals. All values are presented in mean ± standard deviation (SD).
Data analysis
Data were entered into an Excel 2013 spreadsheet (Microsoft, Redmond, WA, USA) and analyzed with GraphPad Prism 8.3.1 software (GraphPad, San Diego, CA, USA). For all statistical tests, an (adjusted) p value < 0.05 was considered to indicate a statistically significant difference. Age between the MD and VM groups was compared using the two-sided unpaired Student’s t test, gender distribution between the two groups was analyzed with the two-sided Fisher’s exact test. Simple linear regression was employed to determine a possible correlation between VEMP ARs and age, and a p value < 0.05 was considered to indicate a slope significantly different from zero. For the comparison of amplitude ARs between different groups, we used the Brown-Forsythe and Welch ANOVA test, as SDs were significantly different for the individual groups. ANOVA was followed by Dunnett’s T3 test to correct for multiple comparisons. Finally, latency ARs between the MD and the VM groups were analyzed by two-sided unpaired t tests corrected for multiple comparisons.