The occlusion effect in bone conducted cVEMP

1 Introduction

Vestibular evoked myogenic potentials (VEMP) are transient alteration in tonic activity of muscles elicited by auditory stimulation of the otolithic organs of the ear. The cervical VEMP (cVEMP) is generated by stimulating the saccule and recording the relaxation of the sternocleidomastoid (SCM) muscle. The auditory stimulation can by mediated by air or bone conduction [5]. As VEMPs are the main tool to study the function of the otolithic organs and given the great impact of dysfunction of these organs on the well-being of individuals, VEMPs have become an important tool in the evaluation of dizzy patients. In addition to measuring otolithic organs activity VEMPs can also be used in the evaluation other inner ear disorders such as the presence of an active inner ear third window or the effects of surgery.

Occluding the meatus of the external auditory canal (EAC) lowers the thresholds for bone conducted (BC) auditory cochlear stimulation as compared to the non-occluded condition. The perceived loudness of an auditory stimulation is increased [16]. This is known as the occlusion effect and was described 80 years ago [7, 17]. It is most prominent with stimulation by low frequencies. The mechanism causing the occlusion effect may be attributed to the vibration of the sound waves in the air trapped in the external auditory canal. For cochlear stimulation the occlusion effect has been studied extensively [15]. To the best of our knowledge the occlusion effect on BC VEMP responses has not been previously described. We hypothesized that occluding the external meatus would lower the sound intensity thresholds required to generate VEMPS and will increase the supra-thresholds amplitude of the response.

2 Material and methods

The study was approved by the University of Ariel ethics committee. Twenty volunteers with normal hearing were recruited. Their age ranged from 20 to 36 years. Each volunteer signed an informed consent form.

The right ear was tested in all subjects. Behavioral audiograms without and with ear plugs were performed. The extent of hearing loss caused by the plugging was established. The procedure for VEMP testing is next described. The subject was seated and asked to turn his head to contract the SCM. The appropriate extent of contraction of the SCM was verified via a propriety patient monitor provided by the Interacoustic Eclipse (Interacoustics, Middelfart Denmark) system utilized. It provides the patient with a visual reference of the level of muscle contraction as compared to the level appropriate for the test procedure. The range of the visual reference feedback is between 20 and 200 uV RMS. The subjects were encouraged to keep the contraction to a middle of the visual reference corresponding with 100 uV, but the setting allowed for VEMP recording within a wide range of 50–150 μV RMS. VEMPs were measured using four electrodes: foreheads as a ground, both SCM were prepared for testing although only the right was used for the study, and a forth electrode was attached to the area of the clavicular joint). Bone was stimulated (Eclipse, Interacoustics, Middelfart Denmark) over the mastoid with a tone burst, 500 Hz, 55 dB nHL stimulation at a rate of 5.1/second, rarefaction and repeated 200 times. The Interacoustics default of 2 cycles of rise-time, 2 cycles of fall-time and 2 plateaus were used, yielding a 12 ms stimulation time at 500 Hz. The thresholds were established by reducing the stimulation intensity in 5 dB steps. The maximal raw amplitude of the response was the span between the p1 and n1 waves. Measurements were made with an open and plugged (occluded) ear.

The thresholds and amplitudes between these two conditions were compared by paired t-tests with SPSS software.

3 Results

The effect of pugging the external auditory meatus was measured in the change in air conducted hearing thresholds, BC-VEMP thresholds and BC-VEMP amplitudes.

3.2 BC-VEMP Thresholds

The non-occluded thresholds were generated at average of 38.25 dB HL (SD 5.91, range 30–50 dB). The occluded thresholds were generated at an average of 36.75 dB HL (SD = 4.37, range 30–50 dB). The difference between the non-occluded and occluded condition did not reach statistical significance (Fig. 1A and 1B).

OPEN IN VIEWER

Fig. 1.

Recording of cervical VEMP (vestibular evoke myogenic potentials) to bone conducted 500 Hz stimulus in the same subject with an open external auditory canal (Fig. 1A) and with its meatus occluded (Fig. 1B). A noticeable increase in the amplitude of the responses is seen (P1-N1). The thresholds for triggering the responses are similar.

4 Discussion

VEMPs are the most direct and practical method to measure the function of the otolith organs. The otolith organs sense linear acceleration, including gravity, hence crucial to our normal orientation in space and for maintaining normal posture. Establishing the functional status of the otolith organs can help the diagnosis of and direct treatment in a variety of clinical settings. In a patient suspected of having Meniere’s syndrome, VEMP thresholds may be elevated or non-recordable in the diseased side or sides [10]. In presence of an active third inner ear thresholds are typically reduced as compared to a non-active window [8]. VEMP response can help evaluate the effect of surgeries on the inner ear (i.e. implantation of the cochlea [9] or stapedotomy).

Vestibular evoked myogenic potentials (VEMP) record transient alteration in tonic activity of muscles elicited by an auditory stimulation of otolith organs. The triggering sound can be a click, a tone or the more recently described a chirp [12]. In addition, VEMP responses can be recorded to an electrical stimulation [13]. VEMPs can be elicited by either air or bone conducted auditory stimulation [18]. Cervical VEMPs cVEMP are dominated by saccular stimulation and sternocleidomastoid relaxation, and ocular VEMP (oVEMP) by utricular stimulation and extraocular muscle activity [4]. This project has focused on cVEMP testing.

It has been long known that occluding the EAC increase the loudness perception of bone conducted sound [7]. The occlusion effect is thought to result from sound waves transmitted through the bone being captured in the EAC, resulting in an increased sound pressure on the tympanic membrane. In the non-occluded condition sound escapes through the external meatus and the EAC acts as a high pass filter as less of the low tone sound pressure is lost. Hence, the occlusion effect is most prominent in the low tones, being 10–20 dB at 250 and 500 Hz and barely measurable above 1000 Hz [1, 14]. The effect is present in externally generated sounds as well as the listeners own voice. It is of significance that VEMP are mostly triggered by low tones stimulation, overlapping with those influenced by the occlusion effect.

BC may trigger VEMP by the same pathways leading to sound perception mediated by the cochlea. For example, vibrations of the skull trigger a movement in the perilymph and endolymph exerting pressure on the basilar membrane. Similar mechanisms are probably involved in the creation of BC VEMP response. Hence BC VEMP may be influenced by the occlusion effect. To the best of our knowledge the occlusion effect on BC-generated VEMPs is presented here for the first time.

During VEMP recording a number of factors are measured: The thresholds for eliciting the response, the maximal amplitude achieved by supra-threshold stimulation and latency from stimulus to response. The VEMP response can be stimulated by sounds of a number of frequencies, creating a “VEMPogram” depicted in each of the tested frequencies. The amplitude of supra-threshold responses is dependent on the amount of energy above thresholds reaching the stimulated organ. In this study a comparison between the thresholds for VEMP responses did not differ significantly between testing the non-occluded and occluded ears. However, the amplitude of responses did differ between these groups. This finding is most probably the result of the elevated sound intensities reaching the saccule in the occluded condition. Hence the occlusion effect is present in BC VEMP.

It has been shown that increase in the intensity of stimulation of VEMP results in larger amplitudes [6]. The occlusion effect increase sound intensities measured by objective tools in the external auditory canal [14]. Together these findings explain the increased amplitudes observed in response to the occluded vs. non occluded EAC.

Although there was a trend towards lower thresholds in the occluded vs. non-occluded VEMP it did not reach statistical significance. The group tested was small. There is an age dependent reduction of VEMP amplitudes [2]. The stimulation was set to 500 Hz, the lowest attainable by the measuring equipment. According to Aazh et al. the maximal occlusion effect on BC auditory thresholds was measured at 250 Hz [1]. Thresholds increased (and occlusion effect decreased) as stimulated frequencies increased. Repeating the currently reported study with a larger group of young subjects and at 250 Hz may result in decrease in thresholds of the occluded BC stimulated ears.

The degree of the effect of occlusion was smaller in the VEMP responses as compared to those seen in cochlear responses to auditory stimulation. This may reflect the fact that BC saccular excitation is more purely a direct bone conducted effect and therefore little altered by changes in air-conducted sound pressures induced by occlusion (unlike the cochlea). BC is relatively more effective for exciting the otoliths than the cochlea when compared to AC sound. Alternatively, and less plausible, is that the lower stimulation level above the thresholds for the saccular VEMP responses as compared to the cochlear, particularly when testing normal subjects as was the case in this study. If this is indeed the explanation utilizing a stronger bone conductor could create a larger occlusion effect on VEMP responses.

This finding has a number of clinically relevant implications. In the surgically altered ear the occlusion effect may differ between ears. In the patient who underwent a canal wall down mastoidectomy in one ear a conductive hearing loss is almost always present in the operated ear, likely resulting in asymmetrical air-conducted hearing and VEMPs thresholds. Hence, VEMPs are better studied with BC rather than AC. Since the space between the occluded meatus and drum is of a larger volume it is reasonable to assume the occlusion effect will be even larger than that measured in the current study. Since we have documented the effect of occlusion on VEMPs hearing aid fitting may also be influenced. The effect of occlusion on hearing aid use has long been recognized [11]. Potential solutions and the conflict with feedback were explored as well [3]. However, no data is available to describe the possible effect on the function of the otolith organ. Conceivably, especially in single side hearing users an effect on balance can be created. Users of bone conducting hearing implants could benefit, at least in theory, by the occlusion effect to improve hearing perception through the device. Exploring this possibility must take into account the probable effects on the otolithic organs.

In this study measurements were made on a small group of healthy volunteers. The main limitation of the study stems from the lack of precise quantification of the SCM contraction level. As a consequence, the findings regarding the increase in amplitude cannot be regarded as conclusive. It should be repeated in more subjects and in additional frequencies. In addition, increasing in intensity of the BC stimulation may reveal a change in VEMP thresholds between the occluded and non-occluded conditions in addition to that found in the supra-thresholds amplitude. Future studies should also be planned to better separate the effects of occlusion vs. conductive hearing loss that were inseparable in the current study.

In conclusion, BC VEMP are influenced by the occlusion effects similarly to auditory responses perceived by the cochlea.