Superior Semicircular Canal Dehiscence Syndrome.

Superior canal dehiscence syndrome (SCDS) is a vestibular disorder caused by a pathologic third window into the labyrinth that can present with autophony, sound- or pressure-induced vertigo, and chronic disequilibrium among other vestibulocochlear symptoms. Careful history taking and examination in conjunction with appropriate diagnostic testing can accurately diagnose the syndrome. Key examination techniques include fixation-suppressed ocular motor examination investigating for sound- or pressure-induced eye movements in the plane of the semicircular canal. Audiometry, vestibular evoked myogenic potentials, and computed tomography confirm the diagnosis. Corrective surgical techniques can be curative, but many patients find their symptoms are not severe enough to undergo surgery. Although a primarily peripheral vestibular disorder, as first-line consultants for most dizziness complaints, neurologists will serve their patients well by understanding SCDS and its role in the differential diagnosis of vestibular disorders.

Neural correlates of sensory substitution in vestibular pathways following complete vestibular loss.

Sensory substitution is the term typically used in reference to sensory prosthetic devices designed to replace input from one defective modality with input from another modality. Such devices allow an alternative encoding of sensory information that is no longer directly provided by the defective modality in a purposeful and goal-directed manner. The behavioral recovery that follows complete vestibular loss is impressive and has long been thought to take advantage of a natural form of sensory substitution in which head motion information is no longer provided by vestibular inputs, but instead by extravestibular inputs such as proprioceptive and motor efference copy signals. Here we examined the neuronal correlates of this behavioral recovery after complete vestibular loss in alert behaving monkeys (Macaca mulatta). We show for the first time that extravestibular inputs substitute for the vestibular inputs to stabilize gaze at the level of single neurons in the vestibulo-ocular reflex premotor circuitry. The summed weighting of neck proprioceptive and efference copy information was sufficient to explain simultaneously observed behavioral improvements in gaze stability. Furthermore, by altering correspondence between intended and actual head movement we revealed a fourfold increase in the weight of neck motor efference copy signals consistent with the enhanced behavioral recovery observed when head movements are voluntary versus unexpected. Thus, together our results provide direct evidence that the substitution by extravestibular inputs in vestibular pathways provides a neural correlate for the improvements in gaze stability that are observed following the total loss of vestibular inputs.

Neural correlates of motor learning in the vestibulo-ocular reflex: dynamic regulation of multimodal integration in the macaque vestibular system.

Motor learning is required for the reacquisition of skills that have been compromised as a result of brain lesion or disease, as well as for the acquisition of new skills. Behaviors with well characterized anatomy and physiology are required to yield significant insight into changes that occur in the brain during motor learning. The vestibulo-ocular reflex (VOR) is well suited to establish connections between neurons, neural circuits, and motor performance during learning. Here, we examined the linkage between neuronal and behavioral VOR responses in alert behaving monkeys (Macaca mulatta) during the impressive recovery that occurs after unilateral vestibular loss. We show, for the first time, that motor learning is characterized by the dynamic reweighting of inputs from different modalities (i.e., vestibular vs extravestibular) at the level of the single neurons that constitute the first central stage of vestibular processing. Specifically, two types of information, which did not influence neuronal responses before the lesion, had an important role during compensation. First, unmasked neck proprioceptive inputs played a critical role in the early stages of this process demonstrated by faster and more substantial recovery of vestibular responses in proprioceptive sensitive neurons. Second, neuronal and VOR responses were significantly enhanced during active relative to passive head motion later in the compensation process (>3 weeks). Together, our findings provide evidence linking the dynamic regulation of multimodal integration at the level of single neurons and behavioral recovery, suggesting a role for homeostatic mechanisms in VOR motor learning.

Multimodal integration after unilateral labyrinthine lesion: single vestibular nuclei neuron responses and implications for postural compensation.

Plasticity in neuronal responses is necessary for compensation following brain lesions and adaptation to new conditions and motor learning. In a previous study, we showed that compensatory changes in the vestibuloocular reflex (VOR) following unilateral vestibular loss were characterized by dynamic reweighting of inputs from vestibular and extravestibular modalities at the level of single neurons that constitute the first central stage of VOR signal processing. Here, we studied another class of neurons, i.e., the vestibular-only neurons, in the vestibular nuclei that mediate vestibulospinal reflexes and provide information for higher brain areas. We investigated changes in the relative contribution of vestibular, neck proprioceptive, and efference copy signals in the response of these neurons during compensation after contralateral vestibular loss in Macaca mulata monkeys. We show that the time course of recovery of vestibular sensitivity of neurons corresponds with that of lower extremity muscle and tendon reflexes reported in previous studies. More important, we found that information from neck proprioceptors, which did not influence neuronal responses before the lesion, were unmasked after lesion. Such inputs influenced the early stages of the compensation process evidenced by faster and more substantial recovery of the resting discharge in proprioceptive-sensitive neurons. Interestingly, unlike our previous study of VOR interneurons, the improvement in the sensitivity of the two groups of neurons did not show any difference in the early or late stages after lesion. Finally, neuronal responses during active head movements were not different before and after lesion and were attenuated relative to passive movements over the course of recovery, similar to that observed in control conditions. Comparison of compensatory changes observed in the vestibuloocular and vestibulospinal pathways provides evidence for similarities and differences between the two classes of neurons that mediate these pathways at the functional and cellular levels.

Canal dehiscence.

PURPOSE OF REVIEW: The aim is to review canal dehiscence involving the superior, lateral, and posterior semicircular canals. The main focus will be on superior semicircular canal dehiscence. RECENT FINDINGS: Canal dehiscence involving the superior, lateral, and posterior semicircular canal can have different etiologies, including developmental abnormality, congenital defect, chronic otitis media with cholesteatoma, and high-riding jugular bulb. However, their clinical presentation can be very similar, with patients complaining of vertigo, oscillopsia, and sometimes hearing loss. Canal dehiscence causes an abnormal communication between the inner ear and the surrounding structures. This creates a third mobile window within the inner ear, disrupting its normal mechanics and causing symptoms. SUMMARY: Superior semicircular canal dehiscence is now a well-established entity in the medical literature. Surgical repair is effective at relieving patients' vestibular symptoms. Lateral semicircular canal dehiscence is usually associated with chronic otitis media. Posterior semicircular canal dehiscence is a rare entity, with similar clinical presentations and treatment options as the other canal dehiscences.

Variation in response dynamics of regular and irregular vestibular-nerve afferents during sinusoidal head rotations and currents in the chinchilla.

In mammals, vestibular-nerve afferents that innervate only type I hair cells (calyx-only afferents) respond nearly in phase with head acceleration for high-frequency motion, whereas afferents that innervate both type I and type II (dimorphic) or only type II (bouton-only) hair cells respond more in phase with head velocity. Afferents that exhibit irregular background discharge rates have a larger phase lead re-head velocity than those that fire more regularly. The goal of this study was to investigate the cause of the variation in phase lead between regular and irregular afferents at high-frequency head rotations. Under the assumption that externally applied galvanic currents act directly on the nerve, we derived a transfer function describing the dynamics of a semicircular canal and its hair cells through comparison of responses to sinusoidally modulated head velocity and currents. Responses of all afferents were fit well with a transfer function with one zero (lead term). Best-fit lead terms describing responses to current for each group of afferents were similar to the lead term describing responses to head velocity for regular afferents (0.006 s + 1). This finding indicated that the pre-synaptic and synaptic inputs to regular afferents were likely to be pure velocity transducers. However, the variation in phase lead between regular and irregular afferents could not be explained solely by the ratio of type I to II hair cells (Baird et al 1988), suggesting that the variation was caused by a combination of pre- (type of hair cell) and post-synaptic properties.

Unidirectional rotations produce asymmetric changes in horizontal VOR gain before and after unilateral labyrinthectomy in macaques.

Unilateral vestibular lesions cause marked asymmetry in the horizontal vestibulo-ocular reflex (VOR) during rapid head rotations, with VOR gain being lower for head rotations toward the lesion than for rotations in the opposite direction. Reducing this gain asymmetry by enhancing ipsilesional responses would be an important step toward improving gaze stability following vestibular lesions. To that end, there were two goals in this study. First, we wanted to determine whether we could selectively increase VOR gain in only one rotational direction in normal monkeys by exposing them to a training session comprised of a 3-h series of rotations in only one direction (1,000°/s² acceleration to a plateau of 150°/s for 1 s) while they wore 1.7 × magnifying spectacles. Second, in monkeys with unilateral vestibular lesions, we designed a paradigm intended to reduce the gain asymmetry by rotating the monkeys toward the side of the lesion in the same way as above but without spectacles. There were three main findings (1) unidirectional rotations with magnifying spectacles result in gain asymmetry in normal monkeys, (2) gain asymmetry is reduced when animals are rotated towards the side of the labyrinthectomy via the ipsilesional rotation paradigm, and (3) repeated training causes lasting reduction in VOR gain asymmetry.

Complementary gain modifications of the cervico-ocular (COR) and angular vestibulo-ocular (aVOR) reflexes after canal plugging.

To determine whether the COR compensates for the loss of aVOR gain, independent of species, we studied cynomolgus and rhesus monkeys in which all six semicircular canals were plugged. Gains and phases of the aVOR and COR were determined at frequencies ranging from 0.02 to 6 Hz and fit with model-based transfer functions. Following canal plugging in a rhesus monkey, the acute stage aVOR gain was small and there were absent responses to thrusts of yaw rotation. In the chronic state, aVOR behavior was characterized by a cupula/endolymph time constant of ≈ 0.07 s, responding only to high frequencies of head rotation. COR gains were ≈ 0 before surgery but increased to ≈ 0.15 at low frequencies just after surgery; the COR gains increased to ≈ 0.4 over the next 12 weeks. Nine weeks after surgery, the summated aVOR + COR responses compensated for head velocity in space in the 0.5-3 Hz frequency range. The gains and phases continued to improve until the 35th week, where the combined aVOR + COR stabilized with gains of ≈ 0.5-0.6 and the phases were compensatory over all frequencies. Two cynomolgus monkeys operated 3-12 years earlier had similar frequency characteristics of the aVOR and COR. The combined aVOR + COR gains were ≈ 0.4-0.8 with compensatory phases. To achieve gains close to 1.0, other mechanisms may contribute to gaze compensation, especially with the head free. Thus, while there are individual variations in the time of adaptation of the gain and phase parameters, the essential functional organization of the adaption to vestibular lesions is uniform across these species.

Assessment of the Association of Leadership Behaviors of Supervising Physicians With Personal-Organizational Values Alignment Among Staff Physicians.

Importance: Although misalignment of values between physicians and their organization is associated with increased risk of burnout, actionable organizational factors that contribute to perceived values alignment are poorly understood. Objective: To evaluate the association between the leadership behaviors of immediate supervisors and physicians' perception of personal-organizational values alignment. Design, Setting, and Participants: This survey study of faculty physicians and physician leaders at Stanford University School of Medicine was conducted from April 1 to May 13, 2019. The survey included assessments of perceived personal-organizational values alignment, professional fulfillment, and burnout. Physicians also evaluated the leadership behaviors of their immediate supervisor (eg, division chief) using a standardized assessment. Data analysis was performed from May to December 2020. Main Outcomes and Measures: Association between mean leadership behavior score (range, 0-10) of each supervisor and the mean personal-organizational values alignment scores (range, 0-12) for the physicians in their work unit. Results: Of 1924 physicians eligible to participate, 1285 (67%) returned surveys. Among these, 651 (51%) were women and 729 (57%) were aged 40 years or older. Among the 117 physician leaders evaluated, 66 (56%) had their leadership behavior independently evaluated by at least 5 physicians and were included in analyses. The mean (SD) personal-organizational values alignment score on the 0 to 12 scale was 6.19 (3.21). As the proportion of work effort devoted to clinical care increased, values alignment scores decreased. Personal-organizational values alignment scores demonstrated an inverse correlation with burnout (r = -0.39; P < .001) and a positive correlation with professional fulfillment (r = 0.52; P < .001). The aggregate leader behavior score of the 66 leaders evaluated correlated with the mean values alignment score for physicians in their work unit (r = 0.53; P < .001). Aggregate leader behavior score was associated with 21.6% of the variation in personal-organizational values alignment scores between work units. After adjusting for age, gender, academic rank, work hours, physician-leader gender concordance, and time devoted to clinical care, each 1-point increase in leadership score of immediate supervisor was associated with a 0.56-point (95% CI, 0.46-0.66; P < .001) increase in personal-organizational values alignment score. Conclusions and Relevance: This survey study's results suggest that physicians experience their organization through the prism of their work unit leader. Organizational efforts to improve values alignment should attend to the development of first-line physician leaders.

Disruption of the head direction cell signal after occlusion of the semicircular canals in the freely moving chinchilla.

Head direction (HD) cells in the rat anterodorsal thalamic nucleus (ADN) fire relative to the animal's directional heading. Lesions of the entire vestibular labyrinth have been shown to severely alter VIIIth nerve input and disrupt these HD signals. To assess the specific contributions of the semicircular canals without altering tonic VIIIth nerve input, ADN cells were recorded from chinchillas after bilateral semicircular canal occlusion. Although ADN HD cells (and also hippocampal place cells and theta cells) were identified in intact chinchillas, no direction-specific activity was seen after canal occlusions. Instead, "bursty" cells were observed that exhibited burst-firing patterns similar to normal HD cells but with firing unrelated to the animal's actual head direction. Importantly, when pairs of bursty cells were recorded, the temporal order of their firing was dependent on the animal's turning direction, as is the case for pairs of normal HD cells. These results suggest that bursty cells are actually disrupted HD cells. The present findings further suggest that the HD cell network is still able to generate spiking activity after canal occlusions, but the semicircular canal input is critical for updating the network activity in register with changes in the animal's HD.

Adaptation of the vestibulo-ocular reflex for forward-eyed foveate vision.

To maintain visual fixation on a distant target during head rotation, the angular vestibulo-ocular reflex (aVOR) should rotate the eyes at the same speed as the head and in exactly the opposite direction. However, in primates for which the 3-dimensional (3D) aVOR has been extensively characterised (humans and squirrel monkeys (Saimiri sciureus)), the aVOR response to roll head rotation about the naso-occipital axis is lower than that elicited by yaw and pitch, causing errors in aVOR magnitude and direction that vary with the axis of head rotation. In other words, primates keep the central part of the retinal image on the fovea (where photoreceptor density and visual acuity are greatest) but fail to keep that image from twisting about the eyes' resting optic axes. We tested the hypothesis that aVOR direction dependence is an adaptation related to primates' frontal-eyed, foveate status through comparison with the aVOR of a lateral-eyed, afoveate mammal (Chinchilla lanigera). As chinchillas' eyes are afoveate and never align with each other, we predicted that the chinchilla aVOR would be relatively low in gain and isotropic (equal in gain for every head rotation axis). In 11 normal chinchillas, we recorded binocular 3D eye movements in darkness during static tilts, 20-100 deg s(1) whole-body sinusoidal rotations (0.5-15 Hz), and 3000 deg s(2) acceleration steps. Although the chinchilla 3D aVOR gain changed with both frequency and peak velocity over the range we examined, we consistently found that it was more nearly isotropic than the primate aVOR. Our results suggest that primates' anisotropic aVOR represents an adaptation to their forward-eyed, foveate status. In primates, yaw and pitch aVOR must be compensatory to stabilise images on both foveae, whereas roll aVOR can be under-compensatory because the brain tolerates torsion of binocular images that remain on the foveae. In contrast, the lateral-eyed chinchilla faces different adaptive demands and thus enlists a different aVOR strategy.

Polymorphisms in KCNE1 or KCNE3 are not associated with Ménière disease in the Caucasian population.

Ménière disease (MD) is a complex disorder of unknown etiology characterized by the symptom triad of vertigo, sensorineural hearing loss, and tinnitus. Its reported incidence is 1-2 per 1,000 in Caucasians and 0.03-0.37 per 1,000 in Japanese. Doi et al. [Doi et al. (2005); ORL J Otorhinolaryngol Relat Spec 67:289-293] recently reported that two single nucleotide polymorphisms (SNPs) in KCNE1 and KCNE3 are associated with MD in Japanese subjects. Consistent with this possibility, these two genes encode potassium channels that are expressed in the stria vascularis and endolymphatic sac, respectively, and their role in ion transport suggests that they may be important in inner ear homeostasis. To establish whether a similar association exists in the Caucasian MD population, we sequenced the coding regions and exon-intron boundaries of both genes in 180 Caucasian persons with MD and 180 matched Caucasian controls. Neither of the two reported SNPs was significantly associated with MD when compared to the Caucasian controls (KCNE1, P = 0.55; KCNE3, P = 0.870). Comparison of allele frequencies between the Japanese MD population and our study population revealed no significant difference between groups (KCNE1, P = 0.90; KCNE3, P = 0.862), suggesting that the significant differences reported in the Japanese study arose from their control population. Six additional SNPs in both KCNE1 and KCNE3 were genotyped and none was associated with MD. Population stratification within our MD and Caucasian control population was excluded. Our data show that SNPs in KCNE1 and KCNE3 are not associated with MD in Caucasians.

Static and dynamic discharge properties of vestibular-nerve afferents in the mouse are affected by core body temperature.

The goal of this study was to determine the effect of changes in core body temperature on the resting discharge rate and sensitivity of vestibular-nerve afferents. Extracellular recordings were made from vestibular-nerve afferents innervating the semicircular canals in anesthetized C57BL/6 mice maintained at a core body temperature of either 30-32 degrees C (T (31)) or 35-37 degrees C (T (36)). The resting rates of regular (CV* < 0.1) and irregular afferents (CV* > 0.1) were lower at T (31) than at T (36). Sensitivity and phase were compared for rotations ranging from 0.1 to 12 Hz by calculating coefficients of a transfer function, g . t(c)S . (t(z)S +1)/(t(c)S + 1), for each afferent. The sensitivity (g) increased with CV* and with higher core body temperature. The value of the coefficient representing the low-frequency dynamics (t (c)) varied inversely with CV* but did not change with core body temperature. The high-frequency dynamics represented by t (z) increased with CV* and decreased with higher core body temperature. These findings indicate that changes in temperature have effects on the static and dynamic properties of vestibular-nerve afferents.

Association of Burnout, Professional Fulfillment, and Self-care Practices of Physician Leaders With Their Independently Rated Leadership Effectiveness.

Importance: Although leadership behavior of physician supervisors is associated with the occupational well-being of the physicians they supervise, the factors associated with leadership behaviors are poorly understood. Objective: To evaluate the associations between burnout, professional fulfillment, and self-care practices of physician leaders and their independently assessed leadership behavior scores. Design, Setting, and Participants: This survey study of physicians and physician leaders at Stanford University School of Medicine (n = 1924) was conducted from April 1 to May 13, 2019. The survey included assessments of professional fulfillment, self-valuation, sleep-related impairment, and burnout. Physicians also rated the leadership behaviors of their immediate physician supervisors using a standardized assessment. Leaders' personal well-being metrics were paired with their leadership behavior scores as rated by the physicians they supervised. All assessment scores were converted to a standardized scale (range, 0-10). Data were analyzed from October 20, 2019, to March 10, 2020. Main Outcomes and Measures: Association between leaders' own well-being scores and their independently assessed leadership behavior. Results: Of 1924 physicians invited to participate, 1285 (66.8%) returned surveys, including 67 of 117 physician leaders (57.3%). Among these respondents, 651 (50.7%) were women and 729 (56.7%) were 40 years or older. Among the 67 leaders, 57 (85.1%) had their leadership behaviors evaluated by at least 5 physicians (median, 11 [interquartile range, 9-15]) they supervised. Overall, 9.8% of the variation in leaders' aggregate leadership behavior scores was associated with their own degree of burnout. In models adjusted for age and sex, each 1-point increase in burnout score of the leaders was associated with a 0.19-point decrement in leadership behavior score (ß = -0.19; 95% CI, -0.35 to -0.03; P = .02), whereas each 1-point increase in their professional fulfillment and self-valuation scores was associated with a 0.13-point (ß = 0.13; 95% CI, 0.01-0.26; P = .03) and 0.15-point (ß = 0.15; 95% CI, 0.02-0.29; P = .03) increase in leadership behavior score, respectively. Each 1-point increase in leaders' sleep-related impairment was associated with a 0.15-point increment in sleep-related impairment among those they supervised (ß = 0.15; 95% CI, 0.02-0.29; P = .03). The associations between leaders' well-being scores in other dimensions and the corresponding well-being measures of those they supervised were not significant. Conclusions and Relevance: In this survey study, burnout, professional fulfillment, and self-care practices of physician leaders were associated with their independently assessed leadership effectiveness. Training, skill building, and support to improve leader well-being should be considered a dimension of leadership development rather than simply a dimension of self-care.

Effects of canal plugging on the vestibuloocular reflex and vestibular nerve discharge during passive and active head rotations.

Mechanical occlusion (plugging) of the slender ducts of semicircular canals has been used in the clinic as well as in basic vestibular research. Here, we investigated the effect of canal plugging in two macaque monkeys on the horizontal vestibuloocular reflex (VOR) and the responses of vestibular-nerve afferents during passive head rotations. Afferent responses to active head movements were also studied. The horizontal VOR gain decreased after plugging to <0.1 for frequencies <2 Hz but rose to about 0.6 as frequency was increased to 15 Hz. Afferents innervating plugged horizontal canals had response sensitivities that increased with the frequency of passive rotations from <0.01 (spikes/s)/( degrees/s) at 0.5 Hz to values of about 0.2 and 0.5 (spikes/s)/( degrees/s) at 8 Hz for regular and irregular afferents, respectively (<50% of responses in controls). An increase in phase lead was also noted following plugging in afferent discharge, but not in the VOR. Because the phase discrepancy between the VOR and afferent discharge is much larger than that seen in control animals, this suggests that central adaptation shapes VOR dynamics following plugging. The effect of canal plugging on afferent responses can be modeled as an increase in stiffness and a reduction in the dominant time constant and gain in the transfer function describing canal dynamics. Responses were also evident during active head rotations, consistent with the frequency content of these movements. We conclude that canal plugging in macaques is effective only at frequencies <2 Hz. At higher frequencies, afferents show significant responses, with a nearly 90 degrees phase lead, such that they encode near-rotational acceleration. Our results demonstrate that afferents innervating plugged canals respond robustly during voluntary movements, a finding that has implications for understanding the effects of canal plugging in clinical practice.

Vestibular function and vertigo control after intratympanic gentamicin for Ménière's disease.

The aim of this study was to correlate long-term vertigo control with reduction in vestibular function after intratympanic (IT) gentamicin therapy for unilateral Ménière's disease. IT gentamicin injections were given as needed to control vertigo attacks. Vertigo frequency and changes in angular vestibulo-ocular reflex (AVOR) gain (measured using magnetic search coils and manual head thrusts) and caloric weakness were assessed before and after treatment. Better vertigo control after treatment was found with >or=60% reduction in quantitative ipsilateral horizontal semicircular canal AVOR gain from pre-treatment values and/or with caloric unilateral weakness (UW) >50%. However, no correlations were found between the continuous variables of vertigo control and either gain or gain recovery, nor between gain and UW because of the large variability in vertigo control in subjects with lesser reductions in these measures.

Healing the Professional Culture of Medicine.

The past decade has been a time of great change for US physicians. Many physicians feel that the care delivery system has become a barrier to providing high-quality care rather than facilitating it. Although physician distress and some of the contributing factors are now widely recognized, much of the distress physicians are experiencing is related to insidious issues affecting the cultures of our profession, our health care organizations, and the health care delivery system. Culture refers to the shared and fundamental beliefs of a group that are so widely accepted that they are implicit and often no longer recognized. When challenges with culture arise, they almost always relate to a problem with a subcomponent of the culture even as the larger culture does many things well. In this perspective, we consider the role of culture in many of the problems facing our health care delivery system and contributing to the high prevalence of professional burnout plaguing US physicians. A framework, drawn from the field of organizational science, to address these issues and heal our professional culture is considered.

Clinical and Physiologic Predictors and Postoperative Outcomes of Near Dehiscence Syndrome.

OBJECTIVE: To identify predictors of near dehiscence (ND) or thin rather than dehiscent bone overlying the superior semicircular canal in patients with signs and symptoms suggestive of superior semicircular canal dehiscence syndrome (SCDS), as well as postoperative outcomes. STUDY DESIGN: Retrospective case-control study. SETTING: Tertiary referral center. PATIENTS: All 288 patients who underwent middle cranial fossa approach for repair of SCDS (1998-2018) were reviewed for cases of ND. Demographics, symptoms, and clinical signs including nystagmus, ocular vestibular-evoked myogenic potential (oVEMP) amplitude, cervical vestibular-evoked myogenic potential (cVEMP) thresholds, and low-frequency air-bone gap were compared before and after surgery. MAIN OUTCOME MEASURE: Presence of preoperative ND and postoperative symptoms and physiologic measures. RESULTS: Seventeen cases of ND (16 patients, 17 ears) and 34 cases (34 ears) of frank SCDS were identified. ND cases differed from frank dehiscence cases in that they were less likely to have nystagmus in response to ear canal pressure or loud sounds, OR = 0.05 (95% CI 0.01-0.25) and Valsalva, OR = 0.08 (0.01-0.67), smaller peak-to-peak oVEMP amplitudes, OR = 0.84 (0.75-0.95), and higher cVEMP thresholds, OR = 1.21 (1.07-1.37). Patients with ND had similar symptoms to those with frank SCDS before surgery, and after surgery had outcomes similar to patients with frank SCDS. CONCLUSIONS: In patients with symptoms consistent with SCDS, predictors of ND include absence of nystagmus in response to pressure/loud sounds, greater cVEMP thresholds, and smaller oVEMP amplitudes. We propose ND is on a spectrum of dehiscence that partially accounts for the diversity of clinical presentations of patients with SCDS.

Rotational responses of vestibular-nerve afferents innervating the semicircular canals in the C57BL/6 mouse.

Extracellular recordings were made from vestibular-nerve afferents innervating the semicircular canals in anesthetized C57BL/6 mice ranging in age from 4-24 weeks. A normalized coefficient of variation was used to divide afferents into regular (CV*<0.1) and irregular (CV*>0.1) groups. There were three overall conclusions from this study. First, mouse afferents resemble those of other mammals in properties such as resting discharge rate and dependence of response dynamics on discharge regularity. Second, there are differences in mouse afferents relative to other mammals that are likely related to the smaller size of the semicircular canals. The rotational sensitivity of mouse afferents is approximately threefold lower than that reported for afferents in other mammals. One consequence of the lower sensitivity is that mouse afferents have a larger linear range for encoding head velocity. The long time constant of afferent discharge, which is a measure of low-frequency response dynamics, is shorter in mouse afferents than in other species. Third, juvenile mice (age 4-7 weeks) appear to lack a class of low-sensitivity, highly irregular afferents that are present in adult animals (age 10-24 weeks). By analogy to studies in the chinchilla, these irregular afferents with low sensitivities for lower rotational frequencies correspond to calyx-only afferents. These findings suggest that, although the calyx ending on to type I hair cells is morphologically complete in mice by the age of about 1 month, the physiological response properties in these juvenile animals are not equivalent to those in adults.