Activation of Guinea Pig Irregular Semicircular Canal Afferents by 100 Hz Vibration: Clinical Implications for Vibration-induced Nystagmus and Vestibular-evoked Myogenic Potentials

[The "difficult" patient-pearls and pitfalls of vestibular diagnostic tests: Part 2 : Difficult aspects of vestibular laboratory testing]

Patients with the cardinal symptoms "vertigo" or "dizziness" may be a real challenge for the treating otorhinolaryngologist. While the first part of this educational series was focused on history taking and bedside neurotological examination, the present paper is devoted to difficult aspects of vestibular laboratory testing, including getting the indication right, what to do if my patient is not able to fully cooperate during the tests, how to choose the adequate diagnostic procedure depending on the patient's comorbidities, how to interpret discordant results of various tests. Finally the paper addresses which conclusions can be drawn (and cannot be drawn) from normal findings in vestibular testing and how to communicate this result to the dizzy patient.

Vestibular Testing-New Physiological Results for the Optimization of Clinical VEMP Stimuli

Both auditory and vestibular primary afferent neurons can be activated by sound and vibration. This review relates the differences between them to the different receptor/synaptic mechanisms of the two systems, as shown by indicators of peripheral function-cochlear and vestibular compound action potentials (cCAPs and vCAPs)-to click stimulation as recorded in animal studies. Sound- and vibration-sensitive type 1 receptors at the striola of the utricular macula are enveloped by the unique calyx afferent ending, which has three modes of synaptic transmission. Glutamate is the transmitter for both cochlear and vestibular primary afferents; however, blocking glutamate transmission has very little effect on vCAPs but greatly reduces cCAPs. We suggest that the ultrafast non-quantal synaptic mechanism called resistive coupling is the cause of the short latency vestibular afferent responses and related results-failure of transmitter blockade, masking, and temporal precision. This "ultrafast" non-quantal transmission is effectively electrical coupling that is dependent on the membrane potentials of the calyx and the type 1 receptor. The major clinical implication is that decreasing stimulus rise time increases vCAP response, corresponding to the increased VEMP response in human subjects. Short rise times are optimal in human clinical VEMP testing, whereas long rise times are mandatory for audiometric threshold testing.

A Review of Neural Data and Modelling to Explain How a Semicircular Canal Dehiscence (SCD) Causes Enhanced VEMPs, Skull Vibration Induced Nystagmus (SVIN), and the Tullio Phenomenon

Angular acceleration stimulation of a semicircular canal causes an increased firing rate in primary canal afferent neurons that result in nystagmus in healthy adult animals. However, increased firing rate in canal afferent neurons can also be caused by sound or vibration in patients after a semicircular canal dehiscence, and so these unusual stimuli will also cause nystagmus. The recent data and model by Iversen and Rabbitt show that sound or vibration may increase firing rate either by neural activation locked to the individual cycles of the stimulus or by slow changes in firing rate due to fluid pumping ("acoustic streaming"), which causes cupula deflection. Both mechanisms will act to increase the primary afferent firing rate and so trigger nystagmus. The primary afferent data in guinea pigs indicate that in some situations, these two mechanisms may oppose each other. This review has shown how these three clinical phenomena-skull vibration-induced nystagmus, enhanced vestibular evoked myogenic potentials, and the Tullio phenomenon-have a common tie: they are caused by the new response of semicircular canal afferent neurons to sound and vibration after a semicircular canal dehiscence.

Fifty Years of Development of the Skull Vibration-Induced Nystagmus Test

This review enumerates most of the studies on the Skull Vibration-Induced Nystagmus Test (SVINT) in the past 50 years from different research groups around the world. It is an attempt to demonstrate the evolution of this test and its increased interest around the globe. It explores clinical studies and animal studies, both permitting a better understanding of the importance of SVINT and its pathophysiology.

The Neural Basis of Skull Vibration Induced Nystagmus (SVIN)

I list a summary of the major clinical observations of SVIN in patients with total unilateral vestibular loss (TUVL) and show how basic results from neurophysiology can explain these clinical observations. The account integrates results from single neuron recordings of identified semicircular canal and otolith afferent neurons in guinea pigs in response to low frequency skull vibration with evidence of the eye movement response in cats to selective semicircular canal stimulation (both individual and combined) and a simple model of nystagmus generation to show how these results explain most of the major characteristics of SVIN.

Skull Vibration-Induced Nystagmus Test in a Human Model of Horizontal Canal Plugging

Background/Aim: the aim of this study was to assess the skull vibration-induced nystagmus test (SVINT) results and vestibular residual function after horizontal semicircular canal (HSCC) plugging. Methods: In this retrospective chart review performed in a tertiary referral center, 11 patients who underwent unilateral horizontal semicircular canal plugging (uHSCCP) for disabling Menière’s disease (MD) were included. The skull vibration-induced nystagmus (SVIN) slow-phase velocity (SPV) was compared with the results of the caloric test (CaT), video head impulse test (VHIT), and cervical vestibular-evoked myogenic potentials (cVEMP) performed on the same day. Results: Overall, 10 patients had a strong SVIN beating toward the intact side (Horizontal SVIN-SPV: 8.8°/s ± 5.6°/s), 10 had a significant or severe ipsilateral CaT hypofunction, 10 had an ipsilateral horizontal VHIT gain impairment, and 3 had altered cVEMP on the operated side. Five had sensorineural hearing worsening. SVIN-positive results were correlated with CaT and horizontal VHIT (HVHIT) results (p < 0.05) but not with cVEMP. SVIN-SPV was correlated with CaT hypofunction in % (p < 0.05). Comparison of pre- and postoperative CaT % hypofunction showed a significant worsening (p = 0.028). Conclusion: SVINT results in a human model of horizontal canal plugging are well correlated with vestibular tests exploring horizontal canal function, but not with cVEMP. SVINT always showed a strong lesional nystagmus beating away from the lesion side. SVIN acts as a good marker of HSCC function. This surgical technique showed invasiveness regarding horizontal canal vestibular function.

Rare Disorders of the Vestibular Labyrinth: of Zebras, Chameleons and Wolves in Sheep's Clothing

The differential diagnosis of vertigo syndromes is a challenging issue, as many - and in particular - rare disorders of the vestibular labyrinth can hide behind the very common symptoms of "vertigo" and "dizziness". The following article presents an overview of those rare disorders of the balance organ that are of special interest for the otorhinolaryngologist dealing with vertigo disorders. For a better orientation, these disorders are categorized as acute (AVS), episodic (EVS) and chronic vestibular syndromes (CVS) according to their clinical presentation. The main focus lies on EVS sorted by their duration and the presence/absence of triggering factors (seconds, no triggers: vestibular paroxysmia, Tumarkin attacks; seconds, sound and pressure induced: "third window" syndromes; seconds to minutes, positional: rare variants and differential diagnoses of benign paroxysmal positional vertigo; hours to days, spontaneous: intralabyrinthine schwannomas, endolymphatic sac tumors, autoimmune disorders of the inner ear). Furthermore, rare causes of AVS (inferior vestibular neuritis, otolith organ specific dysfunction, vascular labyrinthine disorders, acute bilateral vestibulopathy) and CVS (chronic bilateral vestibulopathy) are covered. In each case, special emphasis is laid on the decisive diagnostic test for the identification of the rare disease and "red flags" for potentially dangerous disorders (e. g. labyrinthine infarction/hemorrhage). Thus, this chapter may serve as a clinical companion for the otorhinolaryngologist aiding in the efficient diagnosis and treatment of rare disorders of the vestibular labyrinth.

How to do and why perform the skull vibration-induced nystagmus test

The skull vibration-induced nystagmus test (SVINT) is a global vestibular test stimulating otoliths and semicircular canals at a frequency of 100Hz, not modified by vestibular compensation, which may reveal vibration-induced nystagmus (VIN). Bone-conducted vibration applied to the mastoid processes and the vertex instantaneously induces predominantly low-velocity (∼10°/s) horizontal nystagmus, with rapid phases beating away from the affected side in patients with unilateral vestibular loss (UVL). VIN starts and stops immediately with stimulation, is continuous, reproducible, beats in the same direction irrespective of which mastoid process is stimulated, with no or little habituation. The SVINT acts like a vestibular Weber test. In peripheral UVL, the SVINT is a good marker of vestibular asymmetry and demonstrates pathological nystagmus beating towards the healthy side in 90% of cases of vestibular neuritis, 71% of cases of Menière's diseases and 44 to 78% of vestibular schwannomas. In superior semicircular canal dehiscence, VIN usually beats towards the affected side due to facilitation of bone conduction related to the presence of a third window. Stimulation of the vertex is more effective than in UVL patients, with sensitivity extending to higher frequencies, up to 700Hz. Observation of vibration-induced nystagmus then reveals equally represented vertical, torsional, and horizontal components beating towards the affected ear, suggesting dominant, but not exclusive, stimulation of the dehiscent superior semicircular canal.

Bone-Conducted oVEMP Latency Delays Assist in the Differential Diagnosis of Large Air-Conducted oVEMP Amplitudes

Background: A sensitive test for Superior Semicircular Canal Dehiscence (SCD) is the air-conducted, ocular vestibular evoked myogenic potential (AC oVEMP). However, not all patients with large AC oVEMPs have SCD. This retrospective study sought to identify alternate diagnoses also producing enlarged AC oVEMPs and investigated bone-conducted (BC) oVEMP outcome measures that would help differentiate between these, and cases of SCD.

Methods: We reviewed the clinical records and BC oVEMP results of 65 patients (86 ears) presenting with dizziness or balance problems who underwent CT imaging to investigate enlarged 105 dB nHL click AC oVEMP amplitudes. All patients were tested with BC oVEMPs using two different stimuli (1 ms square-wave pulse and 8 ms 125 Hz sine wave). Logistic regression and odds ratios were used to determine the efficacy of BC oVEMP amplitudes and latencies in differentiating between enlarged AC oVEMP amplitudes due to dehiscence from those with an alternate diagnosis.

Results: Fifty-three ears (61.6%) with enlarged AC oVEMP amplitudes were identified as having frank dehiscence on imaging; 33 (38.4%) had alternate diagnoses that included thinning of the bone covering (near dehiscence, n = 13), vestibular migraine (n = 12 ears of 10 patients), enlarged vestibular aqueduct syndrome (n = 2) and other causes of recurrent episodic vertigo (n = 6). BC oVEMP amplitudes of dehiscent and non-dehiscent ears were not significantly different (p > 0.05); distributions of both groups overlapped with the range of healthy controls. There were significant differences in BC oVEMP latencies between dehiscent and non-dehiscent ears for both stimuli (p < 0.001). A prolonged n1 125 Hz latency (>11.5 ms) was the best predictor of dehiscence (odd ratio = 27.8; 95% CI:7.0-111.4); abnormal n1 latencies were identified in 79.2% of ears with dehiscence compared with 9.1% of ears without dehiscence.

Conclusions: A two-step protocol of click AC oVEMP amplitudes and 125 Hz BC oVEMP latency measures optimizes the specificity of VEMP testing in SCD.

[Effective diagnostics for vertigo, dizziness and equilibrium disorders]

Vertigo, dizziness and equilibrium disorders are symptoms with a variety of causes. First, four cardinal questions (type and duration of the vertigo, triggering factors, accompanying symptoms) must be answered. After that, the search for a spontaneous nystagmus (differentiation of peripheral and central disorder using the HINTS[head impulse, nystagmus, test of skew]-test ) and, as part of a positioning examination, the search for a benign paroxysmal positional vertigo (BPPV) are necessary. If the result is negative an instrument-based receptor-specific examination is carried out. The caloric examination (low-frequency stimulus) tests the horizontal semicircular canal and the superior vestibular nerve, whereas the 3‑D video head impulse test (vHIT, high-frequency stimulus) is used to analyze all three semicircular canals as well as the superior and inferior vestibular nerves. Analysis of the cervical vestibular evoked myogenic potential (cVEMP) checks the function of the sacculus and that of the ocular VEMP (oVEMP) checks the function of the utriculus. The final overall analysis usually gives a definitive diagnosis or at least provides a suspected diagnosis, which then determines the further diagnostic procedure (e.g. targeted radiological diagnostics if vestibular paroxysmia, superior canal dehiscence or a vestibular schwannoma are suspected).