Anomalous cross-modal plasticity following posterior fossa surgery: some speculations on gaze-evoked tinnitus

Blast Overpressures as a Military and Occupational Health Concern

PURPOSE

This tutorial reviews effects of environmental stressors like blast overpressures and other well-known acoustic contaminants (continuous, intermittent, and impulsive noise) on hearing, tinnitus, vestibular, and balance-related functions. Based on the overall outcome of these effects, detailed consideration is given to the health and well-being of individuals.

METHOD

Because hearing loss and tinnitus are consequential in affecting quality of life, novel neuromodulation paradigms are reviewed for their positive abatement and treatment-related effects. Examples of clinical data, research strategies, and methodological approaches focus on repetitive transcranial magnetic stimulation (rTMS) and electrical stimulation of the vagus nerve paired with tones (VNSt) for their unique contributions to this area.

RESULTS

Acoustic toxicants transmitted through the atmosphere are noteworthy for their propensity to induce hearing loss and tinnitus. Mounting evidence also indicates that high-level rapid onset changes in atmospheric sound pressure can significantly impact vestibular and balance function. Indeed, the risk of falling secondary to loss of, or damage to, sensory receptor cells in otolith organs (utricle and saccule) is a primary reason for this concern. As part of the complexities involved in VNSt treatment strategies, vocal dysfunction may also manifest. In addition, evaluation of temporospatial gait parameters is worthy of consideration based on their ability to detect and monitor incipient neurological disease, cognitive decline, and mortality.

CONCLUSION

Highlighting these respective areas underscores the need to enhance information exchange among scientists, clinicians, and caregivers on the benefits and complications of these outcomes.

Classification of Tinnitus: Multiple Causes with the Same Name

Tinnitus is spoken of as if it were a single thing, but there are many different causes, likely many different mechanisms, and many different subtypes. This article reviews a broad range of approaches to understand and demarcate different tinnitus subtypes, which will be critical for exploring and finding cures for different subtypes.

Vagus nerve stimulation paired with tones for tinnitus suppression: Effects on voice and hearing

OBJECTIVE

In individuals with chronic tinnitus, our interest was to determine whether daily low-level electrical stimulation of the vagus nerve paired with tones (paired-VNSt) for tinnitus suppression had any adverse effects on motor-speech production and physiological acoustics of sustained vowels. Similarly, we were also interested in evaluating for changes in pure-tone thresholds, word-recognition performance, and minimum-masking levels. Both voice and hearing functions were measured repeatedly over a period of 1 year.

STUDY DESIGN

Longitudinal with repeated-measures.

METHODS

Digitized samples of sustained frontal, midline, and back vowels (/e/, /o/, /ah/) were analyzed with computer software to quantify the degree of jitter, shimmer, and harmonic-to-noise ratio contained in these waveforms. Pure-tone thresholds, monosyllabic word-recognition performance, and MMLs were also evaluated for VNS alterations. Linear-regression analysis was the benchmark statistic used to document change over time in voice and hearing status from a baseline condition.

RESULTS

Most of the regression functions for the vocal samples and audiometric variables had slope values that were not significantly different from zero. Four of the nine vocal functions showed a significant improvement over time, whereas three of the pure tone regression functions at 2-4 kHz showed some degree of decline; all changes observed were for the left ear, all were at adjacent frequencies, and all were ipsilateral to the side of VNS. However, mean pure-tone threshold changes did not exceed 4.29 dB from baseline and therefore, would not be considered clinically significant. In some individuals, larger threshold shifts were observed. No significant regression/slope effects were observed for word-recognition or MMLs.

CONCLUSION

Quantitative voice analysis and assessment of audiometric variables showed minimal if any evidence of adverse effects using paired-VNSt over a treatment period of 1 year. Therefore, we conclude that paired-VNSt is a safe tool for tinnitus abatement in humans without significant side effects.

LEVEL OF EVIDENCE

Level IV.

Pathophysiology, Diagnosis and Treatment of Somatosensory Tinnitus: A Scoping Review

Somatosensory tinnitus is a generally agreed subtype of tinnitus that is associated with activation of the somatosensory, somatomotor, and visual-motor systems. A key characteristic of somatosensory tinnitus is that is modulated by physical contact or movement. Although it seems common, its pathophysiology, assessment and treatment are not well defined. We present a scoping review on the pathophysiology, diagnosis, and treatment of somatosensory tinnitus, and identify priority directions for further research. Methods: Literature searches were conducted in Google Scholar, PubMed, and EMBASE databases. Additional broad hand searches were conducted with the additional terms etiology, diagnose, treatment. Results: Most evidence on the pathophysiology of somatosensory tinnitus suggests that somatic modulations are the result of altered or cross-modal synaptic activity within the dorsal cochlear nucleus or between the auditory nervous system and other sensory subsystems of central nervous system (e.g., visual or tactile). Presentations of somatosensory tinnitus are varied and evidence for the various approaches to treatment promising but limited. Discussion and Conclusions: Despite the apparent prevalence of somatosensory tinnitus its underlying neural processes are still not well understood. Necessary involvement of multidisciplinary teams in its diagnosis and treatment has led to a large heterogeneity of approaches whereby tinnitus improvement is often only a secondary effect. Hence there are no evidence-based clinical guidelines, and patient care is empirical rather than research-evidence-based. Somatic testing should receive further attention considering the breath of evidence on the ability of patients to modulate their tinnitus through manouvers. Specific questions for further research and review are indicated.

L, B. T, M. A. P, K. K, C. M. S, G. D. S. Multisensory attention training for treatment of tinnitus

Tinnitus is the conscious perception of sound with no physical sound source. Some models of tinnitus pathophysiology suggest that networks associated with attention, memory, distress and multisensory experience are involved in tinnitus perception. The aim of this study was to evaluate whether a multisensory attention training paradigm which used audio, visual, and somatosensory stimulation would reduce tinnitus. Eighteen participants with predominantly unilateral chronic tinnitus were randomized between two groups receiving 20 daily sessions of either integration (attempting to reduce salience to tinnitus by binding with multisensory stimuli) or attention diversion (multisensory stimuli opposite side to tinnitus) training. The training resulted in small but statistically significant reductions in Tinnitus Functional Index and Tinnitus Severity Numeric Scale scores and improved attentional abilities. No statistically significant improvements in tinnitus were found between the training groups. This study demonstrated that a short period of multisensory attention training reduced unilateral tinnitus, but directing attention toward or away from the tinnitus side did not differentiate this effect.

Manganese enhanced magnetic resonance imaging (MEMRI): a powerful new imaging method to study tinnitus

Manganese enhanced magnetic resonance imaging (MEMRI) is a method used primarily in basic science experiments to advance the understanding of information processing in central nervous system pathways. With this mechanistic approach, manganese (Mn(2+)) acts as a calcium surrogate, whereby voltage-gated calcium channels allow for activity driven entry of Mn(2+) into neurons. The detection and quantification of neuronal activity via Mn(2+) accumulation is facilitated by "hemodynamic-independent contrast" using high resolution MRI scans. This review emphasizes initial efforts to-date in the development and application of MEMRI for evaluating tinnitus (the perception of sound in the absence of overt acoustic stimulation). Perspectives from leaders in the field highlight MEMRI related studies by comparing and contrasting this technique when tinnitus is induced by high-level noise exposure and salicylate administration. Together, these studies underscore the considerable potential of MEMRI for advancing the field of auditory neuroscience in general and tinnitus research in particular. Because of the technical and functional gaps that are filled by this method and the prospect that human studies are on the near horizon, MEMRI should be of considerable interest to the auditory research community. This article is part of a Special Issue entitled <Annual Reviews 2014>.

Are the non-classical auditory pathways involved in autism and PDD?

OBJECTIVE

To test the hypothesis that some of the abnormal sensory perceptions that characterize autism may be explained by an abnormal activation of non-classical (extra-lemniscal) sensory pathways.

METHODS

Twenty-one individuals, 18--45 years of age who were diagnosed with autism participated in the study. Sounds (clicks presented at a rate of 40 per second and 65 dB above the normal threshold) were applied through earphones. Electrical stimulation (100 microS rectangular impulses at a rate of 4 per second) was applied through electrodes placed on the skin over the median nerve at the wrist. The participants were asked to match the loudness of the sound with and without the electrical stimulation applied to the median nerve.

RESULTS

Electrical stimulation of the median nerve at the wrist in individuals with autism could change the perception of loudness of sounds presented to one ear through an earphone showing a statistically significant abnormal sensory cross-modal interaction.

DISCUSSION

We interpreted our results to support the hypothesis that some individuals with autism have an abnormal cross-modal interaction between the auditory and the somatosensory systems. Cross-modal interaction between senses such as hearing and the somatosensory system does not occur normally in adults. As only the non-classical (extralemniscal) ascending auditory pathways receive somatosensory input, the presence of cross-modal interaction in autistic individuals is a sign that autism is associated with abnormal involvement of the non-classical auditory pathways, implying that sensory information is processed by different populations of neurons than in non-autistic individuals.

Evidence of key tinnitus-related brain regions documented by a unique combination of manganese-enhanced MRI and acoustic startle reflex testing

Animal models continue to improve our understanding of tinnitus pathogenesis and aid in development of new treatments. However, there are no diagnostic biomarkers for tinnitus-related pathophysiology for use in awake, freely moving animals. To address this disparity, two complementary methods were combined to examine reliable tinnitus models (rats repeatedly administered salicylate or exposed to a single noise event): inhibition of acoustic startle and manganese-enhanced MRI. Salicylate-induced tinnitus resulted in wide spread supernormal manganese uptake compared to noise-induced tinnitus. Neither model demonstrated significant differences in the auditory cortex. Only in the dorsal cortex of the inferior colliculus (DCIC) did both models exhibit supernormal uptake. Therefore, abnormal membrane depolarization in the DCIC appears to be important in tinnitus-mediated activity. Our results provide the foundation for future studies correlating the severity and longevity of tinnitus with hearing loss and neuronal activity in specific brain regions and tools for evaluating treatment efficacy across paradigms.

Transcutaneous electrical nerve stimulation (TENS) of upper cervical nerve (C2) for the treatment of somatic tinnitus

Somatic tinnitus has been defined as tinnitus temporally associated to a somatic disorder involving the head and neck. Several studies have demonstrated the interactions between the somatosensory and auditory system at the dorsal cochlear nucleus (DCN), inferior colliculus, and parietal association areas. The objective is to verify the effect of transcutaneous electrical nerve stimulation of the upper cervical nerve (C2) in the treatment of somatic tinnitus. As electrical stimulation of C2 increases activation of the DCN through the somatosensory pathway and enlarges the inhibitory role of the DCN on the central nervous system, C2 TENS can be considered for tinnitus modulation. A total of 240 patients in whom tinnitus is modulated by somatosensory events (e.g., tinnitus change with rotation, retro- and antiflexion of neck) or modulated by pressure on head or face were included in this study. Both a real and a sham TENS treatment were applied for 30 min (10 min of 6 Hz, followed by 10 min of 40 Hz and 10 min of sham). Significant tinnitus suppression was found (P < 0.001). Only 17.9% (N = 43) of the patients with tinnitus responded to C2 TENS. They had an improvement of 42.92%, and six patients had a reduction of 100%.

[Tinnitus treatment: neurosurgical management]

Tinnitus is a very frequent symptom affecting 10% of the general population. It corresponds to the perception of an internal noise that can severely impair the quality of life. Tinnitus management requires a multidisciplinary approach in which neuromodulation and neurosurgery tend to play major roles. Classification of tinnitus separates objective tinnitus (i.e., tinnitus that can be heard or recorded) from the more frequent subjective tinnitus (i.e., tinnitus only perceived by the patient). Objective tinnitus is either pulsatile synchronous with heartbeat or asynchronous. In the former, appropriate radiological testing should search for a vascular abnormality as well as other neurological diseases (intracranial hypertension, Arnold-Chiari malformation, vascular loops, etc.). Asynchronous objective tinnitus generally corresponds to muscular contractions that require specific management. The pathophysiology of subjective tinnitus is more complex, showing strong analogies with postamputation pain syndromes. After peripheral middle ear or inner ear damage, auditory deafferentation could result in hyperactivity and/or functional reorganization within central auditory and nonauditory structures. This could explain the persistence of tinnitus after total hearing amputation (e.g., translabyrinthine approach for vestibular schwannoma) and associated symptoms such as hyperacusis or anxiety and depression. This central model finds strong support in animal experiments and in functional neuroimagery (PET, fMRI, MEG). Since no etiologically based therapies are currently available, severe subjective tinnitus management only targets tinnitus tolerance with sound enrichment or cognitive behavior therapy. However, in the near future better knowledge of tinnitus pathophysiology and innovative therapeutic tools could emerge from neuromodulation techniques such as repeated transcranial magnetic or epidural electric stimulation.

Dorsal cochlear nucleus hyperactivity and tinnitus: are they related?

PURPOSE

Eight lines of evidence implicating the dorsal cochlear nucleus (DCN) as a tinnitus contributing site are reviewed. We now expand the presentation of this model, elaborate on its essential details, and provide answers to commonly asked questions regarding its validity.

CONCLUSIONS

Over the past decade, numerous studies have converged to support the hypothesis that the DCN may be an important brain center in the generation and modulation of tinnitus. Although other auditory centers have been similarly implicated, the DCN deserves special emphasis because, as a primary acoustic nucleus, it occupies a potentially pivotal position in the hierarchy of functional processes leading to the emergence of tinnitus percepts. Moreover, because a great deal is known about the underlying cellular categories and the details of synaptic circuitry within the DCN, this brain center offers a potentially powerful model for probing mechanisms underlying tinnitus.

Tinnitus and Neural Plasticity of the Brain

OBJECTIVE

To describe the current ideas about the manifestations of neural plasticity in generating tinnitus.

DATA SOURCES

Recently published source articles were identified using MEDLINE, PubMed, and Cochrane Library according to the key words mentioned below.

STUDY SELECTION

Review articles and controlled trials were particularly selected.

DATA EXTRACTION

Data were selected systematically, scaled on validity and comparability.

CONCLUSION

An altered afferent input to the auditory pathway may be the initiator of a complex sequence of events, finally resulting in the generation of tinnitus at the central level of the auditory nervous system. The effects of neural plasticity can generally be divided into early modifications and modifications with a later onset. The unmasking of dormant synapses, diminishing of (surround) inhibition and initiation of generation of new connections through axonal sprouting are early manifestations of neural plasticity, resulting in lateral spread of neural activity and development of hyperexcitability regions in the central nervous system. The remodeling process of tonotopic receptive fields within auditory pathway structures (dorsal cochlear nucleus, inferior colliculus, and the auditory cortex) are late manifestations of neural plasticity. The modulation of tinnitus by stimulating somatosensory or visual systems in some people with tinnitus might be explained via the generation of tinnitus following the nonclassical pathway. The similarities between the pathophysiological processes of phantom pain sensations and tinnitus have stimulated the theory that chronic tinnitus is an auditory phantom perception.

Tinnitus and pain

Tinnitus has many similarities with the symptoms of neurological disorders such as paresthesia and central neuropathic pain. There is considerable evidence that the symptoms and signs of some forms of tinnitus and central neuropathic pain are caused by functional changes in specific parts of the central nervous system and that these changes are caused by expression of neural plasticity. The changes in the auditory nervous system that cause tinnitus and the changes in the somatosensory systems that cause central neuropathic pain may have been initiated from the periphery, i.e. the ear or the auditory nerve for tinnitus and receptors and peripheral nerves in the body for pain. In the chronic condition of tinnitus and pain, abnormalities in the periphery may no longer play a role in the pathology, but the tinnitus is still referred to the ear and central neuropathic pain is still referred to the location on the body of the original pathology. In this chapter we will discuss specific similarities between tinnitus and pain, and compare tinnitus with other phantom disorders. Since much more is known about pain than about tinnitus, it is valuable to take advantage of the knowledge about pain in efforts to understand the pathophysiology of tinnitus and find treatments for tinnitus.

The role of neural plasticity in tinnitus

There is considerable evidence that expression of neural plasticity plays a central role in the development of the abnormalities that cause many forms of tinnitus. Expression of neural plasticity can change the balance between excitation and inhibition, promote hyperactivity, and cause re-organization of specific parts of the nervous system or redirection of information to parts of the nervous system not normally involved in processing of sounds (such as the non-classical, or extralemniscal pathways). The strongest promoter of expression of neural plasticity is deprivation of input, which explains why tinnitus often occurs together with hearing loss or injury to the auditory nerve.

The Prevalence and Onset of Gaze Modulation of Tinnitus and Increased Sensitivity to Noise After Translabyrinthine Vestibular Schwannoma Excision

OBJECTIVE

To determine the prevalence and timing of onset of gaze-modulated tinnitus and increased sensitivity to noise in patients who had undergone translabyrinthine excision of a vestibular schwannoma.

STUDY DESIGN

Retrospective questionnaire study.

SETTING

University hospital departments of audiology and neurotology.

PATIENTS

A cohort of 359 patients who had undergone translabyrinthine excision of a vestibular schwannoma in the period 1997 to 2003.

INTERVENTION

Translabyrinthine excision of a unilateral sporadic vestibular schwannoma.

MAIN OUTCOME MEASURES

Patient reports and visual analogue scale measures of the timing of onset of gaze-modulated tinnitus and the presence, timing of onset, and persistence of increased sensitivity to noise after surgery.

RESULTS

Completed questionnaires were returned by 275 patients (77%), of whom 132 (48%) were men and 143 (52%) were women. Preoperative tinnitus was reported in 150 patients (55%). In 124 of these 150 (83%) the tinnitus persisted, and in 26 of 150 (17%) it abated. Of the 125 patients without preoperative tinnitus, 43 (34%) developed it postoperatively. In 167 (61%) patients of the total group of 275, postoperative tinnitus was reported. Gaze-modulated tinnitus was reported in 53 patients (19%). Somatic-evoked or -modulated tinnitus was reported in 38 patients (14%). In response to the question, "Did you notice being extra sensitive to noise after your operation?," 138 patients (50%) replied that they did. In 111 patients, this persisted. The onset of the modulation of tinnitus was distributed throughout the postoperative period, whereas the onset of increased sensitivity to noise was overwhelmingly in the first 2 months after surgery.

CONCLUSION

Gaze modulation of tinnitus after vestibular schwannoma removal was identified in 19% of patients in this series. The onset data did not convincingly argue for any specific mechanism. The prevalence of increased sensitivity to noise is surprising and may represent central hyperacusis in response to unilateral deafferentation of the auditory system.

Neural plasticity in tinnitus

Two distinctly different kinds of tinnitus occur: objective and subjective tinnitus. Objective tinnitus is caused by sounds generated in the body while subjective tinnitus is caused by abnormal neural activity that is not evoked by sound. This chapter discusses subjective tinnitus. Subjective tinnitus has many forms. In most forms of tinnitus the anatomical location of the physiological abnormality is in the central nervous system, although the sensation is often referred to one ear or both ears. The cause of most forms of subjective tinnitus is the changes that have occurred as a result of expression of neural plasticity, thus a form of reprogramming of the brain that is not to the benefit of the individual person. Tinnitus often occurs together with hearing loss, indicating that the expression of neural plasticity has been evoked by deprivation of input. Tinnitus is often accompanied by hyperacusis, and sometimes phonophobia and depression, indicating altered processing of auditory information or rerouting of information. Several studies have brought evidence that some forms of tinnitus are associated with an abnormal involvement of the nonclassical (extralemniscal, diffuse, or polysensory) auditory pathways that bypass the primary auditory cerebral cortex and provide subcortical connections to limbic structures among others. There is no general treatment for tinnitus, but there are several treatments that can alleviate or reduce the tinnitus in some patients.

Effects of trigeminal ganglion stimulation on the central auditory system

A projection from the trigeminal ganglion to the ventral cochlear nucleus (VCN) of the guinea pig was recently described. The synaptic terminals of this projection terminate in the granule and magnocellular regions of the VCN. Stimulation of this projection has been shown to result in activation of neurons of the ventral cochlear nucleus. We investigated the effect of electrically stimulating the trigeminal ganglion on the central auditory system activity using 2-deoxyglucose (2-DG) autoradiographic techniques. Electrical stimuli were applied to the left trigeminal ganglion as bipolar pulses, 100 micros per phase, at intervals of 200 ms and an amplitude of 100 microA. Negative control animals were not stimulated. A positive control animal was stimulated in the left ear using a 1 kHz tone burst with 200 ms duration and an amplitude of 80 dB SPL. 2-DG was administered by intramuscular injection. Following a 1 h incorporation period, animals were sacrificed, the brains rapidly harvested, and prepared for autoradiography using standard techniques. Autoradiographs were analyzed using computer-assisted video densitometry to determine film optical density in the central auditory regions of interest. The cerebellum was also sampled as a gray matter indifferent intra-brain control region. Results showed systematic and significant differences between 2-DG uptake in the cochlear nucleus and higher auditory centers between control and stimulated animals. Trigeminally stimulated animals showed significantly higher uptake than unstimulated animals in all auditory centers examined, especially ipsilateral to the stimulation site. The activation pattern differs qualitatively from that seen with sound stimulation in that mainly contralateral pathways are activated with sound stimulation. These results demonstrate that a projection from the predominantly somatosensory trigeminal ganglion can influence the activity of central auditory neurons in a manner distinct from acoustic stimulation, suggesting activation of non-classical auditory pathways.

[Multi-sensory interaction in tinnitus: visual evoked potentials and somatosensory stimulation]

Anomalous cross-modal interactions along the audiovestibular, visual and soma-tosensorial pathways could be the responsible for aberrant signals, clinically expressed as phantom perceptions. This results in tinnitus that can be modified by gaze movements or somatosensorial stimulation through skin, orofacial (jaw) and cervical movements. This phenomenon has also been described in some patients with acute unilateral deafferentation of the auditory peripheral system as a result of surgery to remove a tumour in the posterior fossal. Neuroimaging preliminary studies (PET, f-MRI) describe multisensorial interactions and cortical reorganisation processes in chronic tinnitus. Treatment approaches are still unknown although counselling regarding the benignity of the process and the high percentage of habituation to the symptom is the most effective framework. We present our experience in four cases.

Pathophysiology of tinnitus

Tinnitus is not a single entity but a rather diverse group of disorders. Despite symptoms that indicate the ear is the site of the pathology, there is strong evidence that most forms of severe tinnitus are caused by functional changes in the central nervous system. The changes are induced through expression of neural plasticity, some of which may have been caused initially by abnormalities in the ear or the auditory nerve. The involvement of the nonclassical ascending auditory pathway with its subcortical connections to limbic structures (the amygdala) may explain some of the symptoms of some forms of tinnitus including hyperacusis and affective disorders, such as phonophobia and depression, which often accompany severe tinnitus.

Zinc deficiency and tinnitus

OBJECTIVE

To determine if there is a correlation between serum zinc levels and audiometric performance in tinnitus patients.

METHODS

Seventy-three patients participated in this study. Patient's age was restricted to 20-59 years. All patients were examined at the otolaryngology outpatient clinic of the St. Marianna University Toyoko Hospital. The control group consisted of 38 age- and sex-matched healthy volunteers. A blood sample was taken to measure serum zinc levels. Hypozincemia was set at a level of the mean minus one S.D. in the control group. An average hearing sensitivity was calculated as the mean value of hearing thresholds at five frequencies: 250, 500, 1000, 2000, and 4000 Hz. Normal hearing was indicated when the hearing threshold at each of these frequencies was within 20 dB of normal thresholds.

RESULTS

There was no significant difference in serum zinc levels between patients with tinnitus and controls. However, patients with tinnitus who had normal hearing had significantly lower serum zinc levels compared to controls. In contrast, no significant difference in serum zinc levels was found between patients with tinnitus who had hearing loss, and controls. A significant correlation between average hearing sensitivity and serum zinc level was observed.

CONCLUSIONS

These findings suggest that zinc is involved in the generation of tinnitus, especially in patients whose hearing is relatively normal.

Expanding the biological basis of tinnitus: crossmodal origins and the role of neuroplasticity

Tinnitus is most often initiated by modality specific otopathologic disturbances affecting peripheral and central auditory pathways. However, there is growing evidence indicating that the anatomical location generating tinnitus occurs at sites different from the initial pathology. Support for this notion is found in individuals where tinnitus can be triggered or modulated by inputs from other sensory modalities or sensorimotor systems (somatosensory, somatomotor, visual-motor). The use of functional imaging methods combined with psychophysics, detailed physical examinations and questionnaire-based assessments has reinforced and validated these observations. Available data suggest that tinnitus-related crossmodal interactions are more common than previously anticipated. This communication reviews these advancements and suggests that a relatively broad multimodal network of neurons is involved in generating and sustaining the tinnitus perception in some forms of the disorder. Also implicated as part of the tinnitus experience are interactions within large-scale neural networks subserving attention, cognition, and emotion. Incorporating this knowledge into contemporary psychophysiological models will help facilitate the conceptualization of this phantom perception in a more comprehensive manner.

Gaze-evoked tinnitus following acoustic neuroma resection: a de-afferentation plasticity phenomenon?

A review of the prevalence of gaze-evoked tinnitus in patients who have undergone acoustic neuroma resection was undertaken. The relationship of gaze-evoked tinnitus to tumour size, pre-existing tinnitus, side of the lesion or direction of eye gaze was also analysed. The design was a retrospective review of patients who underwent total unilateral translabyrinthine vestibular schwannoma resection by the senior author. Patients 1 year and 5 years following surgery were assessed. Results were obtained by either direct questioning in a clinical setting or via questionnaire. Data was collected on 36 subjects at an average of 14.6 months postoperatively (range 8-20 months) and on 32 subjects at an average 62 months postoperatively (range 56-68 months). Of the first group, 13 had evidence of gaze-evoked tinnitus (36%) compared with six (19%) of the second. The presence of preoperative tinnitus was a significant variable in univariate and multivariate analyses of the presence of postoperative tinnitus and postoperative gaze-evoked tinnitus. In a reduced model multivariate logistic regression using the two groups combined, the significant independent risk factors for gaze-evoked postoperative tinnitus were female sex (P = 0.046, odds ratio = 4.3), right side surgery (P = 0.02, odds ratio = 5.5) and pre-existing tinnitus (P = 0.008, odds ratio = 24.2). Gaze-evoked tinnitus is more common than previously reported in patients undergoing vestibular schwannoma resection. The underlying pathophysiology remains unclear but neural plasticity following unilateral de-afferentation remains a viable theory. The reduced prevalence over time suggests that this phenomenon may represent an active reinnervation process within the brain, which stabilizes with time. The significance of pre-existing tinnitus suggests that mechanisms of neural reorganization modify an existing physiological pathway. Further research is required into the longevity of this phenomenon, preferably with longitudinal studies.

The non-classical auditory pathways are involved in hearing in children but not in adults

Auditory information ascends through the brainstem to the cerebral cortices in two parallel pathways, known as the classical and the non-classical ascending auditory pathways. The importance of the non-classical auditory pathway for hearing in humans is unknown but its subcortical connection to limbic structures may be important in tinnitus. In this study we show evidence that non-classical pathways are involved in loudness perception in young individuals but not in adults. We used the fact that some neurons in the non-classical auditory pathways receive somatosensory input and we determined the effect on loudness perception of monaural sounds from electrical stimulation of the median nerve at the wrist. Stimulation of the somatosensory system had the greatest effect on loudness perception in the youngest children that we studied (7-8 years) and the effect was minimal for individuals above 20 years of age. The effect was an increase in loudness in 20 of the 40 individuals we studied and a decrease in 4 individuals; 16 experienced no noticeable change in loudness during somatosensory stimulation.

Symptoms and signs caused by neural plasticity

Plastic changes in the central nervous system are associated with hyperactivity, hypersensitivity, and spread of activity including activation of brain regions that are not typically involved. Symptoms and signs such as neuropathic pain and tinnitus and hyperactive disorders such as muscle spasm and synkinesis may result from such changes in function. Plastic changes that cause symptoms of diseases can be initiated by novel stimulations, overstimulation, or deprivation of input and the induced changes in the function of central nervous system structures may persist and aggravate after these events have ceased if the condition is not reversed. Disorders that are caused by neural plasticity are potentially reversible with treatment. However, the absence of morphologic abnormalities makes diagnosis of these conditions difficult and their treatment has been hampered by lack of understanding of their pathophysiology. Here the role of neural plasticity in the pathophysiology of several disorders is reviewed.

Characteristics of patients with gaze-evoked tinnitus

OBJECTIVE

The authors describe symptoms and population characteristics in subjects who can modulate the loudness and/or pitch of their tinnitus by eye movements.

STUDY DESIGN

Data were obtained by questionnaire.

SETTING

The study was conducted at a university center and a tertiary care center.

PATIENTS

Respondents had the self-reported ability to modulate their tinnitus with eye movements.

RESULTS

Ninety-one subjects reported having gaze-evoked tinnitus after posterior fossa surgery involving the eighth nerve. Eighty-seven of them underwent removal of a vestibular schwannoma (acoustic neuroma), two had bilateral eighth nerve tumors (one underwent bilateral tumor removal; the other unilateral tumor removal), one underwent removal of a cholesteatoma, and one underwent removal of a glomus jugulare tumor. Seventeen subjects who had never had posterior fossa surgery reported gaze-evoked tinnitus. Of those with vestibular schwannomas, tumor size ranged from small (<2 cm) to large (>4 cm). The gender distribution was 48.3% male and 51.7% female. In 77% of patients, the gaze-evoked tinnitus was localized to the surgical ear or side of head; 21.8% had bilateral tinnitus that was louder in the surgical ear or side of head. In 86 of 87 subjects, loudness of tinnitus changed with eye movement. Eye movement away from the central (eyes centered) position increased the loudness of tinnitus in all 86 subjects who responded to this question. Seventy-three of 85 (85.9%) patients indicated that pitch changed with eye movement, with pitch increasing in 64/72 (88.9%) of them. Eighty-three of 87 (95.4%) patients reported total loss of hearing in the surgical ear. Seventy of 83 (84.3%) patients reported facial nerve problems immediately after surgery, 52 of 87 (60%) reported persistent facial weakness, and 16 of 87 (18.4%) patients reported persistent double vision. In those 17 subjects with gaze-evoked tinnitus and no posterior fossa surgery, the majority of respondents (14/17, 82.4%) were male.

CONCLUSIONS

Gaze-evoked tinnitus after cerebellar pontine angle surgery is more common than was previously believed. In addition, posterior fossa surgery is not a prerequisite for the development of gaze-evoked tinnitus. It is likely that gaze-evoked tinnitus is a manifestation of functional reorganization. Gaze-evoked tinnitus could result from an unmasking of brain regions that respond to multiple stimulus/response modalities, and/or from anomalous cross-modality interactions, perhaps caused by collateral sprouting.

Lateralized tinnitus studied with functional magnetic resonance imaging: abnormal inferior colliculus activation

Tinnitus, the perception of sound in the absence of external stimuli, is a common and often disturbing symptom that is not understood physiologically. This paper presents an approach for using functional magnetic resonance imaging (fMRI) to investigate the physiology of tinnitus and demonstrates that the approach is effective in revealing tinnitus-related abnormalities in brain function. Our approach as applied here included 1) using a masking noise stimulus to change tinnitus loudness and examining the inferior colliculus (IC) for corresponding changes in activity, 2) separately considering subpopulations with particular tinnitus characteristics, in this case tinnitus lateralized to one ear, 3) controlling for intersubject differences in hearing loss by considering only subjects with normal or near-normal audiograms, and 4) tailoring the experimental design to the characteristics of the tinnitus subpopulation under study. For lateralized tinnitus subjects, we hypothesized that sound-evoked activation would be abnormally asymmetric because of the asymmetry of the tinnitus percept. This was tested using two reference groups for comparison: nontinnitus subjects and nonlateralized tinnitus subjects. Binaural noise produced abnormally asymmetric IC activation in every lateralized tinnitus subject (n = 4). In reference subjects (n = 9), activation (i.e., percent change in image signal) in the right versus left IC did not differ significantly. Compared with reference subjects, lateralized tinnitus subjects showed abnormally low percent signal change in the IC contralateral, but not ipsilateral, to the tinnitus percept. Consequently, activation asymmetry (i.e., the ratio of percent signal change in the IC ipsilateral versus contralateral to the tinnitus percept) was significantly greater in lateralized tinnitus subjects as compared with reference subjects. Monaural noise also produced abnormally asymmetric IC activation in lateralized tinnitus subjects. Two possible models are presented to explain why IC activation was abnormally low contralateral to the tinnitus percept in lateralized tinnitus subjects. Both assume that the percept is associated with abnormally high ("tinnitus-related") neural activity in the contralateral IC. Additionally, they assume that either 1) additional activity evoked by sound was limited by saturation or 2) sound stimulation reduced the level of tinnitus-related activity as it reduced the loudness of (i.e., masked) the tinnitus percept. In summary, this work demonstrates that fMRI can provide objective measures of lateralized tinnitus and tinnitus-related activation can be interpreted at a neural level.

Cutaneous-evoked tinnitus. I. Phenomenology, psychophysics and functional imaging

DC00166e and acute unilateral deafferentation of the auditory periphery (auditory and vestibular afferents) can induce changes in the central nervous system that may result in unique forms of tinnitus. These tinnitus perceptions can be controlled (turned on and off) or modulated (changed in pitch or loudness) by performing certain overt behaviors in other sensory/motor systems. Clinical reports from our laboratory and several other independent sources indicate that static change in eye gaze, from a neutral head-referenced position, is one such behavior that can evoke, modulate and/or suppress these phantom auditory events. This report deals with a new clinical entity and a form of tinnitus that can be evoked directly by cutaneous stimulation of the upper hand and fingertip regions. In 2 adults, cutaneous-evoked tinnitus was reported following neurosurgery for space-occupying lesions at the base of the skull and posterior craniofossa, where hearing and vestibular functions were lost completely and acutely in one ear (unilateral deafferentation) and facial nerve paralysis (unilateral deefferentation) was present either immediately following neurosurgery or had occurred as a delayed-onset event. Herein, we focus on the phenomenology of this discovery, provide perceptual correlates using contemporary psychophysical methods and document in one individual cutaneous-evoked tinnitus-related neural activity using functional magnetic resonance imaging. In a companion paper, neuroanatomical and physiological interactions between auditory and somatosensory systems, possible mechanistic accounts and relevant functional neuroimaging studies are reviewed.

Cutaneous-evoked tinnitus. II. Review Of neuroanatomical, physiological and functional imaging studies

Cutaneous-evoked tinnitus is a clinical entity that has not been reported previously in the neurootological literature. Herein, a neuroscience framework that encompasses several distinct areas of research is used to conceptualize and help understand this phenomenon. We review normal neuroanatomical and physiological interactions between auditory and somatosensory systems in mammals. Also considered are mechanistic accounts of lesion-induced changes in the CNS following deafferentation/deefferentation of peripheral sensory or motor structures that may have a relationship to this phenomenon, as well as the role of functional imaging modalities in studying various phantom perceptions.

Presence of tinnitus indicated by variable spontaneous otoacoustic emissions

Spontaneous otoacoustic emissions (SOAE) have been widely studied in normal subjects, and there is evidence of their high frequency stability in repeated recordings. A study to determine the frequency stability of SOAE in 53 of 100 consecutive patients, who presented with tinnitus and in whom SOAE were recordable, was undertaken. Patients were divided into five aetiologically homogeneous subgroups: (i) those with normal hearing and no identified pathology, (ii) those with sensorineural hearing loss of unknown origin, (iii) those with normal hearing, but complaining of tinnitus related to head injury, (iv) those with endolymphatic hydrops, and (v) those with noise exposure. The control group consisted of 20 subjects, selected on the basis of recordable SOAE from 38 volunteers with normal hearing and no tinnitus. The prevalence of SOAE and their inter-session frequency stability (reproducibility and relative frequency shift) were analysed. In contrast to the controls, the tinnitus group had significantly increased frequency variability of SOAE (lower reproducibility and increased relative frequency shift). The prevalence of subjects with SOAE was not notably different between the controls and subjects with tinnitus, if the tinnitus group was considered in toto, but a striking 100% prevalence of bilaterally present SOAE was observed in the tinnitus subgroup with head injury.