Parameters of click and tone burst evoked potentials

For both the click evoked masseter vestibular-evoked myogenic potentials and 500 Hz tone burst evoked masseter vestibular-evoked myogenic potentials, no ear effect was observed for p11 and n21 latencies for both ipsilateral and contralateral stimulation. Previous studies on click evoked masseter vestibular-evoked myogenic potentials have also reported no ear effect on p11 and n21 latencies for ipsilateral and contralateral stimulation.^{Reference de Natale, Ginatempo, Mercante, Manca, Magnano and Ortu8,Reference Loi, Manca, Ginatempo and Deriu10,Reference Vignesh, Singh and Rajalakshmi11} We found prolonged p11-n21 latencies for contralateral stimulation for both the click and tone burst stimulus in the present study. In addition, the amplitude of the p11-n21 contralateral peaks was higher than the ipsilateral p11-n21 peaks for both the click and the 500 Hz tone burst stimuli.

The larger amplitude of p11-n21 peaks for the contralateral stimulation could be because of differences between the ipsilateral and contralateral vestibulomasseteric reflex pathways. In an animal study, it was confirmed that the activity of the trigeminal motoneurons are affected by stimulating or lesioning the vestibular receptors.^{Reference Tolu, Pugliatti, Lacana, Chessa, Caria and Simula22} During the caloric stimulation, the spontaneous firing of the masseter motor units increases tonically.^{Reference Tolu, Caria, Chessa, Melis, Simula and Podda23} This response also gets modulated by otolith stimulation. During the electrical stimulation, the masseter motoneurons have excitatory effects.^{Reference Tolu, Caria, Chessa, Melis, Simula and Podda23} Furthermore, the ipsilateral responses occur earlier than the contralateral responses.^{Reference Deriu, Podda, Chessa and Tolu24} The latency and duration of the vestibular-evoked trigeminal responses suggest polysynaptic pathways between the vestibular system and the motor trigeminal nuclei, with the contralateral pathway being stronger than the ipsilateral pathway.^{Reference Deriu, Giaconi, Rothwell and Tolu25} As the contralateral pathways are stronger than the ipsilateral pathways, the amplitude of the masseter vestibular-evoked myogenic potential peaks could be higher for the contralateral stimulation.

The present study also showed latency prolongation of masseter vestibular-evoked myogenic potential peaks for contralateral stimulations. However, previous studies have shown no differences between the latency and amplitude of masseter vestibular-evoked myogenic potentials recorded for ipsilateral and contralateral stimulations using click and tone burst stimuli.^{Reference Deriu, Tolu and Rothwell1,Reference de Natale, Ginatempo, Mercante, Manca, Magnano and Ortu8,Reference Loi, Manca, Ginatempo and Deriu10,Reference Vignesh, Singh and Rajalakshmi11,Reference Deriu, Podda, Milia, Chessa, Sau and Pastorino15} The recent studies on masseter vestibular-evoked myogenic potentials recording using electrical stimulation on humans^{Reference Deriu, Tolu and Rothwell1} do not match the data obtained from animals.^{Reference Tolu, Pugliatti, Lacana, Chessa, Caria and Simula22–Reference Deriu, Podda, Chessa and Tolu24,Reference Tolu and Pugliatti26} This could be because of the anatomical differences in vestibulomasseteric reflex pathways between humans and animals. The vestibulomasseteric pathways in humans represent the monosynaptic pathways’ activity, whereas the asymmetric excitatory responses could be because of the activity of the multisynaptic pathways.^{Reference Deriu, Giaconi, Rothwell and Tolu25} Further data in clinical populations would be required to understand the contribution of the ipsilateral and contralateral pathways to the vestibulomasseteric reflex system. However, the presence or absence of the ipsilateral and contralateral masseter vestibular-evoked myogenic potential responses can help differentiate peripheral and central pathology. For example, lesions confined to the inner ear and affecting the otolith organs would result in abnormality of both the ipsilateral and contralateral responses. However, if the ipsilateral masseter vestibular-evoked myogenic potential responses are present but the contralateral responses are absent, it would indicate a pathology at the neural level.

Comparison of p11 and n21 parameters

The results of the present study also suggest no significant difference in latency of masseter vestibular-evoked myogenic potential peaks between click and tone burst for ipsilateral and contralateral stimulations. To the best of our knowledge, this is the first study that has compared the latency and amplitude of masseter vestibular-evoked myogenic potentials between click and tone burst stimuli. Earlier studies have utilised either a click or a 500 Hz tone burst stimuli to compare the latency and amplitude of masseter vestibular-evoked myogenic potentials.^{Reference Deriu, Tolu and Rothwell1,Reference Deriu, Tolu and Rothwell2,Reference de Natale, Ginatempo, Mercante, Manca, Magnano and Ortu8,Reference Vignesh, Singh and Rajalakshmi11,Reference Thirusangu and Sinha12,Reference Deriu, Ortu, Capobianco, Giaconi, Melis and Aiello14} Vignesh et al.^{Reference Vignesh, Singh and Rajalakshmi11} recorded masseter vestibular-evoked myogenic potentials using a 500 Hz tone burst stimuli and compared the latency of masseter vestibular-evoked myogenic potentials with one of the previous studies.^{Reference de Natale, Ginatempo, Mercante, Manca, Magnano and Ortu8} Vignesh et al.^{Reference Vignesh, Singh and Rajalakshmi11} reported prolonged latency of masseter vestibular-evoked myogenic potential peaks recorded with 500 Hz tone burst stimulus compared with the click stimulus. This may not be an ideal way to compare the latency of masseter vestibular-evoked myogenic potential between the two stimuli. The difference in latency between the two studies could be because of several factors, such as patient variables and the different equipment used in various clinical setups.

However, previous studies have reported prolonged p13 and n23 peaks for 500 Hz tone burst evoked cervical vestibular-evoked myogenic potential compared with click evoked cervical vestibular-evoked myogenic potential.^{Reference Kumar, Sinha, Bharti and Barman16,Reference Viciana and Lopez-Escamez17,Reference Wu, Shiao, Yang and Lee19} In the literature, only one study reported a larger amplitude of cervical vestibular-evoked myogenic potential recorded with click stimulus than that recorded with 500 Hz tone burst stimulus.^{Reference Cheng, Huang and Young27} The present study shows no significant difference in latency between the two stimuli for the ipsilateral and contralateral stimulations. We hypothesise that because of excitation of both the monosynaptic and multisynaptic pathways during the ipsilateral and contralateral stimulations, latency difference may not exist for the masseter vestibular-evoked myogenic potential peaks for the two stimuli.

However, when we used bilateral stimulations, we could find a significant difference in latency between the two stimuli. The 500 Hz tone burst latency evoked masseter vestibular-evoked myogenic potential to bilateral stimulations was greater than the click stimulus. Some reports suggest double or triple firing of the vestibular nerve fibres to 500 Hz tone burst stimulus. The 500 Hz short tone burst stimulus responses might be because of the second or third action potential spike.^{Reference Cheng, Huang and Young27}

In the present study, we found a larger amplitude for the 500 Hz tone burst stimulus for ipsilateral, contralateral and bilateral stimulations. Vignesh et al.^{Reference Vignesh, Singh and Rajalakshmi11} also reported a higher amplitude of 500 Hz tone burst evoked masseter vestibular-evoked myogenic potential compared with that in the study by de Natale et al.^{Reference de Natale, Ginatempo, Mercante, Manca, Magnano and Ortu8} Although Vignesh et al.^{Reference Vignesh, Singh and Rajalakshmi11} used a lower intensity level to evoke 500 Hz tone burst masseter vestibular-evoked myogenic potential, the amplitude was higher than the click evoked masseter vestibular-evoked myogenic potential. The acoustic energy delivered to the inner ear, the transmission of the sound through the middle ear and the resonance of otolith organs determine the amplitude of the vestibular-evoked myogenic potential.^{Reference Viciana and Lopez-Escamez17} Furthermore, when matched for equal sound pressure levels, the 500 Hz tone burst has higher acoustic energy than the click stimulus. In response to both the click and the 500 Hz tone burst stimuli, an action potential will be generated in vestibular neurons. The acoustic energy of click stimulus below 1 kHz is lesser than for the high frequencies.^{Reference Todd, Rosengren, Govender and Colebatch28} The difference in acoustic energy at various frequencies for click stimulus happens because the sound pressure level reflects the overall contribution for both the low and the high frequencies. However, the 500 Hz tone burst stimulus overall energy is centred within the otolithic organs’ resonance frequency.^{Reference Todd, Rosengren, Govender and Colebatch28}

In the present study, we also found that bilateral responses were larger than ipsilateral responses for both the click and the 500 Hz tone burst stimuli. Previous studies^{Reference Deriu, Tolu and Rothwell2,Reference de Natale, Ginatempo, Mercante, Manca, Magnano and Ortu8,Reference Loi, Manca, Ginatempo and Deriu10} have also reported a larger response of masseter vestibular-evoked myogenic potential to bilateral stimulations than ipsilateral ones. The difference in amplitude between the ipsilateral and bilateral stimulations suggests different motor units for activating these pathways.^{Reference Loi, Manca, Ginatempo and Deriu10} The clinician and researchers must evaluate both ipsilateral and bilateral pathways in individuals with various vestibular disorders. The masseter vestibular-evoked myogenic potential amplitude difference between ipsilateral and bilateral stimulations can clear up questionable test results.^{Reference Loi, Manca, Ginatempo and Deriu10} The bilateral stimulation will also increase masseter vestibular-evoked myogenic potential detection rates because of higher amplitudes.^{Reference de Natale, Ginatempo, Mercante, Manca, Magnano and Ortu8} Bilateral responses may also help differentiate between the lesions of the peripheral and central vestibular disorders.

Prospective clinical applications

Masseter vestibular-evoked myogenic potentials can be used as an additional tool to assess the otolith function in individuals with various peripheral and central vestibular pathologies. The results of the various studies suggest that masseter vestibular-evoked myogenic potential is a better tool than cervical vestibular-evoked myogenic potential and ocular vestibular-evoked myogenic potential in identifying brainstem lesions in multiple pathologies.^{Reference Magnano, Pes, Pilurzi, Cabboi, Ginatempo and Giaconi3,Reference de Natale, Ginatempo, Paulus, Manca, Mercante and Pes5,Reference de Natale, Ginatempo, Paulus, Pes, Manca and Tolu6,Reference Puligheddu, Figorilli, Serra, Laccu, Congiu and Tamburrino9,Reference Vignesh, Bhoopathy, Singh and Rajalakshmi13} Multiple sclerosis is one of the neurodegenerative diseases that affect the brainstem severely. Brainstem dysfunction in patients with multiple sclerosis can result in severe disability. The brainstem lesions in patients with multiple sclerosis are often undetected by conventional tests.^{Reference Magnano, Pes, Cabboi, Pilurzi, Ginatempo and Achene4} Magnano et al.^{Reference Magnano, Pes, Cabboi, Pilurzi, Ginatempo and Achene4} reported that masseter vestibular-evoked myogenic potential could detect brainstem lesions in patients with multiple sclerosis with no clinical symptoms and normal magnetic resonance imaging results. Magnano et al.^{Reference Magnano, Pes, Pilurzi, Cabboi, Ginatempo and Giaconi3} also reported that the lesions of the medulla result in an abnormality of the vestibulomasseteric reflexes. However, limited studies have reported the effectiveness of masseter vestibular-evoked myogenic potentials in detecting brainstem dysfunction in various pathologies except for multiple sclerosis, Parkinson's disease and idiopathic sleep disorders. The vestibular system stabilises the masseter muscle during sudden head tilt upward or downward.^{Reference Deriu, Tolu and Rothwell2} It would be interesting to see the effect of various vestibular pathologies on vestibular influences on motor output to the masseter muscles. Future studies should focus on the usefulness of masseter vestibular-evoked myogenic potential in various peripheral and central vestibular disorders.