Single motor unit responses underlying cervical vestibular evoked myogenic potentials produced by bone-conducted stimuli

- Bone-conducted (BC) stimulation is a useful cVEMP stimulus in patients with conductive hearing loss as it bypasses the middle ear.
- Our single motor unit data show that the mainly inhibitory cVEMP may change polarity with different directions of BC stimulation to become an excitatory reflex.
- In some conditions the BC cVEMP is likely to receive contributions from end organs in addition to the saccule, such as the utricle.

ObjectiveCervical vestibular evoked myogenic potentials (cVEMPs) are muscle reflexes recorded from the sternocleidomastoid (SCM) neck muscles following vestibular activation with air- or bone-conducted (BC) stimulation. We investigated the effect of different forms of BC stimulation on the single motor unit response underlying the cVEMP.MethodsWe tested 8 healthy human subjects with 5 different stimuli. Motor units were recorded with thin concentric needle electrodes; surface potentials were recorded simultaneously.ResultsThe polarity of the initial change (at approx. 15 ms) in single motor unit activity reflected the polarity of the surface cVEMPs: a short-latency decrease in activity (inhibition) was seen with the four stimuli that produced a positive surface potential (p13), while an initial increase in activity (excitation) was seen with the stimulus that produced a negative surface potential.ConclusionsBC stimulation with common clinical stimuli usually produces an inhibition in single motor unit activity in the ipsilateral SCM muscle. However the projections activated by BC stimulation are not exclusively inhibitory in nature and depend upon the shape and direction of the stimulus.SignificanceThe utricle is likely to contribute to some BC cVEMPs, as some stimuli clearly evoke an excitation that is not likely to be saccular in origin.