New approaches to VEMP measurement

Transcription

1 New approaches to VEMP measurement Steve Bell, Hearing and Balance Centre, ISVR, University of Southampton Collaborators: Neil Todd, Jennifer Parker, Mike Griffin

2 Motivation It is desirable to have good clinical test methods for balance disorders, but the vestibular system is difficult to stimulate/test Calorics are widely used, but only test the lateral canals at low frequency. There are several new methods Head Impulse Testing, VEMPs and vibration induced nystagmus that have potential to test more of the vestibular system

3 Background: What is a VEMP? Colebatch and Halmagyi (1992) first described activity in the sternocleidomastoid muscle in response to loud clicks As early as 1935 von Békésy had reported that loud sounds evoked head movements and proposed that this was due to vestibular stimulation. Bickford et al. (1964) reported an inion response to loud sounds that was present in deaf patients with intact vestibular function More recently activity has been recorded in other muscles in particular the occular muscles, the so called o-vemp

4 Types of VEMP: The cervical VEMP cvemp from the SCM muscle is proportional to the tension in the muscle as it is an inhibitory response. The subject needs to have their neck in tension. Response is primarily ipsilateral. The ovemp can also be stimulated by loud sounds. Responses are seen in the (inferior) extraoccular muscles. The response is primarily contralateral. VEMPS can be elicited using a number of stimuli: Sound, bc, electrical, tendon hammers and vibration There is still some debate over pathways Location 1 Location 2 Location 3 Location 4 For a review of VEMPs see : Rosengren, Welgampola and Colbatch (2010)

5 Vibration induced VEMPS ovemp responses to vibration applied to the head with a minishaker were first documented by Todd et al. (2008). The minishaker causes translation of the head so the response may arise from the linear VOR Responses are seen in the eye muscles (ovemp) It is thought that linear translation activates the utricle, not the saccule. Bilateral occular responses are seen The vestibular system appears to have surprising sensitivity to vibration better than hearing at 100 Hz

6 Vibration induced responses appear to be a way to selectively test the utricle However coupling 100 Hz vibration to the skull through the skin may not be optimal Questions: Can coupling of the vibration to the skull be improved? Is the o-vemp response the same as the VOR?

7 Experiment 1: Stimulating ovemps via a bite bar Can ovemp measurement be improved by applying vibration to the jaw rather than with a mini-shaker on the head? Griffin et al measured head acceleration using a bite-bar, which was better than measurement on the skin as it avoid the problem of tissue compliance. (Also see Moore and Popelka, 2013 dental hearing aid) What is the relationship between stimulus duration and ovemp measurement?

8 Design 10 subjects (6F, 4M) mean age 27 O-VEMP measurements from inferior eye muscles Stimulus generation from Cambridge Electronic Design micro1401. Responses recorded via CED 1902 amplifier Hz bandwidth. Gain Stimuli presented at 5 Hz. 0.4 g Minishaker at Fz, Cz or inion. Bite bar with x, y or z stimulation. 1:4:1 stimuli used for effect of position 100 Hz tone bursts of increasing duration (1:2:1, 1:4:1, 1:8:1 etc.) Fz and sideways bitebar Effect of gaze elevation explored Pilot work showed a complex relationship between head movement and applied acceleration at some shaker positions

9 Effect of stimulation type and position on the ovemp response Repeated measures analysis of variance (RMANOVA) identified that type of stimulation had a significant effect on the amplitude of the response (p<0.001 onetailed).

10 Effect of stimulus duration on responses

11 Summary of findings from the bite bar study The vibration induced ovemp is enhanced by applying vibration to the teeth, which might have application to clinical measurement The response duration follows stimulus duration this is difference from the effect of stimulus duration on the cvemp. It appears that the ovemp response opposes or mirrors the applied acceleration.. So is the vibration induced ovemp a high frequency manifestation of the VOR?

12 Experiment 2: The relationship between the VOR and the ovemp Six healthy male subjects with no history of vestibular or neurological impairment Whole body vibration was applied from 0.5 to 64 Hz Subjects lay on their back on a 1m hydraulic vibrator O-VEMPS and EOG were measured using CED 1902 amplifiers Eye targets at 20⁰ eye elevation were either fixed to the moving platform, or fixed to the ground Movement is x axis (front-back). 0.1 g acceleration Eyes will verge on a fixed target

13 Whole body vibration 0.5 Hz

14 Whole body vibration 2 Hz

15 At low frequency EOG shows how eyes track the target

16 .. but the ovemp activity appears to track acceleration 0.5 Hz 1 Hz 2 Hz

17 At higher frequency a more typical vibration induced ovemp is seen (although platform ringing is an issue)

18 At low frequency, platform movement is large. Eye tracking is dominant. Large eye movement effects are seen when the target moves relative to the platform the subject lies on At high frequency the response increases. This appears to be continuous with the ovemp at higher frequency tuning to 100 Hz At mid-frequency tracking breaks down We expect to see the (linear) VOR

19 Main findings of whole body vibration study At high frequency there is an WBV ovemp response that appears similar to ovemp responses obtained with minishakers At low frequency visual tracking is dominant At mid-frequency the response amplitude increases which is consistent with a linear VOR. Previous studies suggest that linear VOR has near unity gain at around 10 Hz The mid-frequency response appears to be continuous with an increasing ovemp at higher frequency possibly peaking around 100 Hz

20 Summary Vibration induced ovemps may be a way to test utricular function Using a bite bar appears to improve coupling of vibration to the head The vibration ovemp is a compensatory reflex this is fundamentally different from the cvemp We think that there is a continuum from the VOR at mid frequency to the vibration induced ovemp at high frequency There is potential to measure the linear VOR in more detail

21 References Griffin,M.J. (1975) Vertical vibration of seated subjects: effects of posture, vibration level and frequency Aviation, Space and Environmental Medicine, 46,(3), Moore, B.C.J. and Popelka, G.R. Preliminary comparison and bone-anchored hearing instruments and a dental device as treatments for unilateral hearing loss IJA; 52(10), Rosengren, S.M., Welgampola, M.S. Colebatch J.G. (2010) Vestibular evoked myogenic potentials: Past, present and future Clinical neurophysiology, 121(5): Todd,N.P., Bell,S.L., Paillard,A.C., Griffin,M.J. (2012) Contributions of ocular vestibular evoked myogenic potentials (OVEMPs) and the electro-oculogram (EOG) to periocular potentials produced by whole-body vibration. Journal of Applied Physiology 113 (10): Todd, N.P.M, Rosengren, S.M., Colebatch, J.G. (2008) Ocular vestibular evoked myogenic potentials (OVEMPs) produced by impulsive transmastoid accelerations Clinical Neurophysiology 119;