The Medial Canthus Reference Electrode Is not Electrically Indifferent to the Ocular Vestibular Evoked Myogenic Potential.

OBJECTIVE: To determine whether a reference electrode placed over the contralateral medical canthus is electrically indifferent to the ocular vestibular evoked myogenic potential (oVEMP). STUDY DESIGN: Prospective observational study. SETTING: Tertiary academic center. PARTICIPANTS: Nineteen otologically normal subjects. METHODS: oVEMPs were recorded using a 500 Hz tone burst at 125 dB pSPL using both a conventional dual infra-orbital derivation and contralateral infraorbital electrode referenced to a medial canthus electrode known as the "belly-tendon" derivation. RESULTS: The belly-tendon derivation produced significantly larger oVEMP amplitudes. This appears to be due to a polarity inversion of the first negative peak that occurs when the reference electrode is placed over the medial canthus. Thus, the belly-tendon derivation was associated with reference contamination, but in this instance the reference contamination produced an augmented oVEMP amplitude. CONCLUSIONS: Neither the dual infra-orbital nor belly-tendon electrode montages are electrically neutral. Reference contamination is the source of both a decrease in amplitude using conventional recording techniques and increased amplitude using the belly-tendon montage. The results support the contention that the belly-tendon montage generates a reversed polarity response, i.e., algebraically subtracted (i.e., added in the differential amplifier) from the response recorded at the infraorbital midline enhancing the oVEMP amplitude. It should be noted that the amplitude gained when using the belly-tendon montage may also be due to electromyographic (EMG) generated by extraocular muscles other than the inferior oblique generator, resulting in a loss of specificity. The authors recommend the belly-tendon montage be used in clinical situations where the oVEMP is absent or low in amplitude.

Anterior Canal benign paroxysmal positional Vertigo following surgical Management of superior canal dehiscence.

OBJECTIVE: Benign paroxysmal positional vertigo (BPPV) is a common post-surgical finding in patients managed for superior semicircular canal dehiscence (SSCD). The posterior semicircular canal has been reported as the involved canal in the majority of cases of post-surgical BPPV, with only two cases reported of lateral canal involvement. The objective of this report is to present a case in which an anterior semicircular canal BPPV response was identified in a patient following surgical management for SSCD. METHOD: This case report presents an adult with residual dizziness following surgical management of SSCD and vestibular rehabilitation therapy (VRT). During subsequent evaluation of vestibular function, a transient and torsional, down-beating nystagmus was provoked along with vertigo during Dix-Hallpike positioning to the right. This was consistent with BPPV affecting the left superior (anterior) semicircular canal. RESULTS: The patient was treated with a repositioning maneuver to manage anterior semicircular canal BPPV and no nystagmus response was recorded with post-repositioning Dix-Hallpike test. Review of radiographic images, obtained prior to vestibular function testing, showed a hyperintensity in the area of the left anterior semicircular canal ampulla. It was felt this was likely a bone chip from the SSCD repair that was pushing against the ampulla with further mobile debris within the canal. CONCLUSION: It is reported that BPPV is a common complication in patients surgically managed for SSCD. Posterior semicircular canal BPPV is reported most often, with a couple of cases of lateral semicircular canal BPPV also reported. As far as we are aware, the current case represents the first report of anterior semicircular canal BPPV in this type of patient.

Ocular and Cervical Vestibular Evoked Myogenic Potentials in Patients With Vestibular Migraine.

OBJECTIVE: To evaluate the relationship between normal and abnormal ocular vestibular evoked myogenic potentials (oVEMP) and cervical vestibular evoked myogenic potentials (cVEMP) in patients with and without vestibular migraine (VM). STUDY DESIGN: Retrospective review of oVEMP and cVEMP results in patients with vestibular disorders who were assessed clinically and completed vestibular function studies. Data were extracted from a deidentified RedCap Repository. SETTING: Tertiary care multispecialty medical center. PATIENTS: Subjects were 212 consecutive adults meeting prespecified inclusion criteria who were evaluated in the Balance Disorders Clinic at Vanderbilt University Medical Center between 2011 and 2017. Patients with bilaterally absent VEMPs were excluded from the study. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Proportions of subjects with or without VM in one of the following four test outcomes: normal cVEMP/normal oVEMP, abnormal cVEMP/abnormal oVEMP, abnormal cVEMP/normal oVEMP, and normal cVEMP/abnormal oVEMP. RESULTS: There was a significant relationship between VM and cVEMP and oVEMP test outcomes. CONCLUSION: Patients with VM are more likely than subjects with vestibular disorders other than migraine to exhibit normal cVEMP responses in the presence of unilaterally abnormal oVEMP responses. Such a VEMP pattern may be a biomarker of VM and further supports a possible pathophysiologic relationship between the utriculo-ocular reflex and VM.