Evaluation of white matter structures in patients with tinnitus using diffusion tensor imaging

Review and Perspective on Brain Bases of Tinnitus

In advancing our understanding of tinnitus, some of the more impactful contributions in the past two decades have come from human brain imaging studies, specifically the idea of both auditory and extra-auditory neural networks that mediate tinnitus. These networks subserve both the perception of tinnitus and the psychological reaction to chronic, continuous tinnitus. In this article, we review particular studies that report on the nodes and links of such neural networks and their inter-network connections. Innovative neuroimaging tools have contributed significantly to the increased understanding of anatomical and functional connections of attention, emotion-processing, and default mode networks in adults with tinnitus. We differentiate between the neural correlates of tinnitus and those of comorbid hearing loss; surprisingly, tinnitus and hearing loss when they co-occur are not necessarily additive in their impact and, in rare cases, additional tinnitus may act to mitigate the consequences of hearing loss alone on the brain. The scale of tinnitus severity also appears to have an impact on brain networks, with some of the alterations typically attributed to tinnitus reaching significance only in the case of bothersome tinnitus. As we learn more about comorbid conditions of tinnitus, such as depression, anxiety, hyperacusis, or even aging, their contributions to the network-level changes observed in tinnitus will need to be parsed out in a manner similar to what is currently being done for hearing loss or severity. Together, such studies advance our understanding of the heterogeneity of tinnitus and will lead to individualized treatment plans.

A large-scale diffusion imaging study of tinnitus and hearing loss

Subjective, chronic tinnitus, the perception of sound in the absence of an external source, commonly occurs with many comorbidities, making it a difficult condition to study. Hearing loss, often believed to be the driver for tinnitus, is perhaps one of the most significant comorbidities. In the present study, white matter correlates of tinnitus and hearing loss were examined. Diffusion imaging data were collected from 96 participants-43 with tinnitus and hearing loss (TIN_HL), 17 with tinnitus and normal hearing thresholds (TIN_NH), 17 controls with hearing loss (CON_HL) and 19 controls with normal hearing (CON_NH). Fractional anisotropy (FA), mean diffusivity and probabilistic tractography analyses were conducted on the diffusion imaging data. Analyses revealed differences in FA and structural connectivity specific to tinnitus, hearing loss, and both conditions when comorbid, suggesting the existence of tinnitus-specific neural networks. These findings also suggest that age plays an important role in neural plasticity, and thus may account for some of the variability of results in the literature. However, this effect is not seen in tractography results, where a sensitivity analysis revealed that age did not impact measures of network integration or segregation. Based on these results and previously reported findings, we propose an updated model of tinnitus, wherein the internal capsule and corpus callosum play important roles in the evaluation of, and neural plasticity in response to tinnitus.

Neuroanatomic Volume Differences in Tinnitus and Hearing Loss

OBJECTIVES

To investigate neuroanatomic volume differences in tinnitus and hearing loss.

STUDY DESIGN

Cross-sectional.

METHODS

Sixteen regions of interest (ROIs) in adults (43 male, 29 female) were examined using 3Tesla structural magnetic resonance imaging in four cohorts: 1) tinnitus with moderate hearing loss (N = 31), 2) moderate hearing loss only (N = 15), 3) tinnitus with normal hearing (N = 17), and 4) normal hearing only (N = 13). ROI volumes were corrected for brain size, age, and sex variations. Analysis of covariance (ANCOVA) and post hoc Tukey's test were used to isolate the effects of tinnitus and hearing loss on volume differences. Effect sizes were calculated as the fraction of total variance (η² ) in ANCOVA models and percent of mean volume difference relative to mean total volume.

RESULTS

The four cohort ANCOVA revealed tinnitus and hearing loss cohorts to have increased volume in the corona radiata (η²  = 0.192; P = .0018) and decreased volume in the nucleus accumbens (η²  = 0.252; P < .0001), caudate nucleus (η²  = 0.188; P = .002), and inferior fronto-occipital fasciculus (η²  = 0.250; P = .0001). Tinnitus with normal hearing showed decreased volume in the nucleus accumbens (22.0%; P = .001) and inferior fronto-occipital fasciculus (18.1%; P = .002), and hearing loss only showed increased volume in the corona radiata (10.7%; P = .01) and decreased volume in the nucleus accumbens (22.1%; P = .001), caudate nucleus (16.1%; P = .004), and inferior fronto-occipital fasciculus (18.3%; P = .003).

CONCLUSION

Tinnitus and hearing loss have overlapping effects on neurovolumetric alterations, especially impacting the nucleus accumbens and inferior fronto-occipital fasciculus. Neurovolumetric studies on tinnitus or hearing loss can be more complete by accounting for those two clinical dimensions separately and jointly.

LEVEL OF EVIDENCE

3 Laryngoscope, 131:1863-1868, 2021.

Reorganization of Brain White Matter in Persistent Idiopathic Tinnitus Patients Without Hearing Loss: Evidence From Baseline Data

It remains unknown whether tinnitus or tinnitus-related hearing loss (HL) could indirectly impair or reshape the white matter (WM) of the human brain. We aim to explore the possible brain WM change in tinnitus patients without HL and further to investigate their associations with clinical variables. Structural and diffusion tensor imaging (DTI) of 20 idiopathic tinnitus patients without HL and 22 healthy controls (HCs) were obtained. Voxel-based morphometry (VBM) and tract-based spatial statistics (TBSS) analysis were conducted to investigate the differences in WM volume and integrity between patients and HCs, separately. We extracted WM parameters to determine a sensitive imaging index to differentiate the idiopathic tinnitus patients from the HCs in the early stage. Correlations between the clinical variables and WM indices were also performed in patients. Compared with the controls, the tinnitus patients without HL exhibited significant decreased fractional anisotropy (FA) in the body and genu of corpus callosum (CC), left cingulum (LC) and right cingulum (RC), and right superior longitudinal fasciculus (RSLF) and increase in mean diffusivity (MD) in the body of CC in WM. Moreover, the patients also showed decreases in WM axial diffusivity (AD) in LC, left superior longitudinal fasciculus (LSLF), and right interior cerebellar peduncle (ICP) and increases in radial diffusivity (RD) in the body and genu of CC and RSLF (p < 0.05, voxel-level FWE corrected). Furthermore, the increased RD value of the genu of CC is closely associated with the tinnitus handicap inventory (THI) subscale scores. No WMV changes were detected in tinnitus patients. We combined the altered WM integrity index of body and genu of CC and LC and RSLF as an index to differentiate the two groups and reached a sensitivity of 100% and a specificity of 77.3%. Our findings suggest that tinnitus without HL is associated with significant alterations of WM integrity. These changes may be irrespective of the duration and other clinical performance. The combination of diffusion indices of body and genu of CC and LC and RSLF might be used as the potential useful imaging index for the diagnosis of persistent idiopathic tinnitus without HL in the early stage.

Age-related hearing loss and tinnitus, dementia risk, and auditory amplification outcomes

Age-related hearing loss (ARHL) or presbycusis, as the third leading cause of chronic disability in older adults, has been shown to be associated with predisposing cognitive impairment and dementia. Tinnitus is also a chronic auditory disorder demonstrating a growth rate with increasing age. Recent evidence stands for the link between bothersome tinnitus and impairments in various aspects of cognitive function. Both ARHL and age-related tinnitus affect mental health and contribute to developing anxiety, stress, and depression. The present review is a comprehensive multidisciplinary study on diverse interactions among ARHL, tinnitus, and cognitive decline in older adults. This review incorporates the latest evidence in prevalence and risk factors of ARHL and tinnitus, the neural substrates of tinnitus-related cognitive impairments, hypothesized mechanisms concerning the association between ARHL and increased risk of dementia, hearing amplification outcomes in cases with ARHL and cognitive decline, and preliminary findings on the link between ARHL and cognitive impairment in animal studies. Given extensive evidence that demonstrates advantages of using auditory amplification in the alleviation of hearing handicap, depression, and tinnitus, and the improvement of cognition, social communication, and quality of life, regular hearing screening programs for identification and management of midlife hearing loss and tinnitus is strongly recommended.

Tinnitus and the Brain: A Review of Functional and Anatomical Magnetic Resonance Imaging Studies

Purpose

The tinnitus patient population is inherently heterogeneous. Although tinnitus often co-occurs with hearing loss and is more frequent among elderly people, it affects all age groups with varying hearing sensitivity. In addition, tinnitus severity and patients' reaction to tinnitus vary across individuals. Regardless of the metrics used to measure tinnitus handicap, on one end of the severity spectrum are the patients who have managed to habituate to their tinnitus; at the other end are those who are extremely bothered by tinnitus, and often have a confluence of related comorbidities of mood disorders. Understanding the neural correlates of tinnitus while accounting for such variations could benefit clinicians, helping them modify and objectively monitor tinnitus management strategies. Brain imaging, specifically magnetic resonance imaging, is an excellent tool to study the functional and structural properties of the neural networks involved in tinnitus and tinnitus severity.

Method

In this article, we review studies that employ magnetic resonance imaging-based neuroimaging techniques including resting-state functional connectivity, voxel-based morphometry, and diffusion tensor imaging to investigate underlying functional and structural neural correlates of tinnitus to address overarching dimensions of a person's reaction to tinnitus, namely, audition, emotion, and attention.

Results

We discuss findings from brain imaging studies in the context of theories and models proposed for tinnitus generation and persistence.

Conclusion

These studies have revealed tinnitus-related alteration in the auditory, emotion, and attention neural networks. Future research is required to better understand these changes in the neural circuitry based on tinnitus comorbidities and severity, and to refine existing theoretical models.

Diffusion-based tractography atlas of the human acoustic radiation

Diffusion MRI tractography allows in-vivo characterization of white matter architecture, including the localization and description of brain fibre bundles. However, some primary bundles are still only partially reconstructed, or not reconstructed at all. The acoustic radiation (AR) represents a primary sensory pathway that has been largely omitted in many tractography studies because its location and anatomical features make it challenging to reconstruct. In this study, we investigated the effects of acquisition and tractography parameters on the AR reconstruction using publicly available Human Connectome Project data. The aims of this study are: (i) using a subgroup of subjects and a reference AR for each subject, define an optimum set of parameters for AR reconstruction, and (ii) use the optimum parameters set on the full group to build a tractography-based atlas of the AR. Starting from the same data, the use of different acquisition and tractography parameters lead to very different AR reconstructions. Optimal results in terms of topographical accuracy and correspondence to the reference were obtained for probabilistic tractography, high b-values and default tractography parameters: these parameters were used to build an AR probabilistic tractography atlas. A significant left-hemispheric lateralization was found in the AR reconstruction of the 34 subjects.

A Missing Connection: A Review of the Macrostructural Anatomy and Tractography of the Acoustic Radiation

The auditory system of mammals is dedicated to encoding, elaborating and transporting acoustic information from the auditory nerve to the auditory cortex. The acoustic radiation (AR) constitutes the thalamo-cortical projection of this system, conveying the auditory signals from the medial geniculate nucleus (MGN) of the thalamus to the transverse temporal gyrus on the superior temporal lobe. While representing one of the major sensory pathways of the primate brain, the currently available anatomical information of this white matter bundle is quite limited in humans, thus constituting a notable omission in clinical and general studies on auditory processing and language perception. Tracing procedures in humans have restricted applications, and the in vivo reconstruction of this bundle using diffusion tractography techniques remains challenging. Hence, a more accurate and reliable reconstruction of the AR is necessary for understanding the neurobiological substrates supporting audition and language processing mechanisms in both health and disease. This review aims to unite available information on the macroscopic anatomy and topography of the AR in humans and non-human primates. Particular attention is brought to the anatomical characteristics that make this bundle difficult to reconstruct using non-invasive techniques, such as diffusion-based tractography. Open questions in the field and possible future research directions are discussed.

Tinnitus and Cognition: Linked?

Tinnitus is conscious auditory perception without corresponding external stimuli resulting from neuronal activity along auditory pathway either in peripheral or central auditory system. The exact origin, pathophysiologic pathway and its treatment is still under exploration. The main risk factor is hearing loss, but can occur in patients with normal hearing. Auditory brainstem response test helps to objectify this subjective complaint and aid in locating its origin. P300 is late latency auditory evoked potential, objective marker of cognitive potential. The purpose of our study was to ascertain any significant difference in P300 latency and amplitude values in cases of sensorineural hearing loss with tinnitus when compared to controls (normal hearing subjects) and also to find any correlation between severity of tinnitus and cognition. This was a case control study conducted at the department of ENT at Safdarjung Hospital, New Delhi between September 2016 to March 2018. The study comprised of one hundred and six patients, out of which fifty five patients were having sensorineural hearing loss with tinnitus and fifty one patients were normal hearing subjects. General medical and audiological history and examination was done. Significant increase in P300 latency and decrease in P300 amplitude was found in sensorineural hearing loss with tinnitus cases on comparing with controls. The similar pattern of P300 values was observed on increasing severity of tinnitus and degree of hearing loss. Hence, results revealed the role of cognition and involvement of central auditory pathway in tinnitus generation.

The characteristics of cognitive impairment in subjective chronic tinnitus

INTRODUCTION

Subjective chronic tinnitus is a common medical syndrome with a high frequency of cognitive impairment; however, the characteristics of cognitive impairment in chronic tinnitus are poorly understood. Investigating the scope of cognitive impairment across the severity spectrum of tinnitus patients may shed light on the issue.

METHODS

A consecutive series of 207 subjective chronic tinnitus patients were classified into mild tinnitus group (n = 95) and severe tinnitus group (n = 112) by THI score (the cutoff THI scores were 37/38). These patients were assessed using the Cognitive Abilities Screening Instrument (CASI) and P300 event-related potential.

RESULTS

Although pure tone averages were not different between mild or severe tinnitus patients, severe tinnitus patients scored lower on the CASI assessment as well as almost all subdomains of CASI, particularly in items such as "short-term memory," "concentration or mental manipulation," "orientation," "abstraction and judgment," "language abilities," and "visual construction." Furthermore, compared to mild tinnitus patients, severe tinnitus patients exhibited longer N2 and P3 latencies. Finally, a correlation analysis revealed that tinnitus severity was negatively correlated with CASI score and positively correlated with N2 and P3 latencies.

CONCLUSIONS

This study reveals that tinnitus patients on the severe end of the spectrum may be at risk for serious cognitive deficits, which may not be a secondary response to disease manifestations but a primary feature of the underlying disease.

Topography of the human acoustic radiation as revealed by ex vivo fibers micro-dissection and in vivo diffusion-based tractography

The acoustic radiation is a compact bundle of fibers conveying auditory information from the medial geniculate nucleus of the thalamus to the auditory cortex. Topographical knowledge of this bundle in primates is scarce and in vivo diffusion-based tractography reconstructions in humans remains challenging, especially with the most widely used MRI acquisition protocols. Therefore, the AR represents a notable anatomical omission in the neurobiological investigation of acoustic and linguistic functional mechanisms in humans. In this study, we combine blunt micro-dissections and advanced diffusion tractography methods to provide novel insights into the topographical anatomy of this bundle in humans. Evidences from ex vivo blunt micro-dissection in three human (two right) hemispheres are compared to the 3D profile of this bundle as reconstructed by tractography techniques in four healthy adult data sets provided by the Human Connectome Project. Both techniques show the unique trajectory of the AR, a transversal course from the midline to the lateral convexity of the posterior temporal lobe. Blunt dissections demonstrated three portions of this bundle that we defined as the genu, stem, and fan, revealing the intimate relationships that each of these components has with neighboring association and projection pathways. Probabilistic tractography and ultra-high b values provided results comparable to blunt micro-dissections and highlighted the main limitations in tracking the AR. This is, to our knowledge, the first ex vivo/in vivo integrated study providing novel and reliable information about the precise anatomy of the AR, which will be important for future investigations in the neuroscientific, clinical, and surgical field.

Blast-Induced Tinnitus and Elevated Central Auditory and Limbic Activity in Rats: A Manganese-Enhanced MRI and Behavioral Study

Blast-induced tinitus is the number one service-connected disability that currently affects military personnel and veterans. To elucidate its underlying mechanisms, we subjected 13 Sprague Dawley adult rats to unilateral 14 psi blast exposure to induce tinnitus and measured auditory and limbic brain activity using manganese-enhanced MRI (MEMRI). Tinnitus was evaluated with a gap detection acoustic startle reflex paradigm, while hearing status was assessed with prepulse inhibition (PPI) and auditory brainstem responses (ABRs). Both anxiety and cognitive functioning were assessed using elevated plus maze and Morris water maze, respectively. Five weeks after blast exposure, 8 of the 13 blasted rats exhibited chronic tinnitus. While acoustic PPI remained intact and ABR thresholds recovered, the ABR wave P1-N1 amplitude reduction persisted in all blast-exposed rats. No differences in spatial cognition were observed, but blasted rats as a whole exhibited increased anxiety. MEMRI data revealed a bilateral increase in activity along the auditory pathway and in certain limbic regions of rats with tinnitus compared to age-matched controls. Taken together, our data suggest that while blast-induced tinnitus may play a role in auditory and limbic hyperactivity, the non-auditory effects of blast and potential traumatic brain injury may also exert an effect.

Neuroanatomical Alterations in Tinnitus Assessed with Magnetic Resonance Imaging

Previous studies of anatomical changes associated with tinnitus have provided inconsistent results, with some showing significant cortical and subcortical changes, while others have found effects due to hearing loss, but not tinnitus. In this study, we examined changes in brain anatomy associated with tinnitus using anatomical scans from 128 participants with tinnitus and hearing loss, tinnitus with clinically normal hearing, and non-tinnitus controls with clinically normal hearing. The groups were matched for hearing loss, age and gender. We employed voxel- and surface-based morphometry (SBM) to investigate gray and white matter volume and thickness within regions-of-interest (ROI) that were based on the results of previous studies. The largest overall effects were found for age, gender, and hearing loss. With regard to tinnitus, analysis of ROI revealed numerous small increases and decreases in gray matter and thickness between tinnitus and non-tinnitus controls, in both cortical and subcortical structures. For whole brain analysis, the main tinnitus-related significant clusters were found outside sensory auditory structures. These include a decrease in cortical thickness for the tinnitus group compared to controls in the left superior frontal gyrus (SFG), and a decrease in cortical volume with hearing loss in left Heschl’s gyrus (HG). For masked analysis, we found a decrease in gray matter volume in the right Heschle’s gyrus for the tinnitus group compared to the controls. We found no changes in the subcallosal region as reported in some previous studies. Overall, while some of the morphological differences observed in this study are similar to previously published findings, others are entirely different or even contradict previous results. We highlight other discrepancies among previous results and the increasing need for a more precise subtyping of the condition.

Probabilistic Fiber-Tracking Reveals Degeneration of the Contralateral Auditory Pathway in Patients with Vestibular Schwannoma

BACKGROUND AND PURPOSE

Vestibular schwannomas cause progressive hearing loss by direct damage to the vestibulocochlear nerve. The cerebral mechanisms of degeneration or plasticity are not well-understood. Therefore, the goal of our study was to show the feasibility of probabilistic fiber-tracking of the auditory pathway in patients with vestibular schwannomas and to compare the ipsi- and contralateral volume and integrity, to test differences between the hemispheres.

MATERIALS AND METHODS

Fifteen patients with vestibular schwannomas were investigated before surgery. Diffusion-weighted imaging (25 directions) was performed on a 3T MR imaging system. Probabilistic tractography was performed for 3 partial sections of the auditory pathway. Volume and fractional anisotropy were determined and compared ipsilaterally and contralaterally. The laterality ratio was correlated with the level of hearing loss.

RESULTS

Anatomically reasonable tracts were depicted in all patients for the acoustic radiation. Volume was significantly decreased on the hemisphere contralateral to the tumor side for the acoustic radiation and diencephalic section, while fractional anisotropy did not differ significantly. Tracking did not yield meaningful tracts in 3 patients for the thalamocortical section and in 5 patients for the diencephalic section. No statistically significant correlations between the laterality quotient and classification of hearing loss were found.

CONCLUSIONS

For the first time, this study showed that different sections of the auditory pathway between the inferior colliculus and the auditory cortex can be visualized by using probabilistic tractography. A significant volume decrease of the auditory pathway on the contralateral hemisphere was observed and may be explained by transsynaptic degeneration of the crossing auditory pathway.

Frontostriatal Gating of Tinnitus and Chronic Pain

Tinnitus and chronic pain are sensory-perceptual disorders associated with negative affect and high impact on well-being and behavior. It is now becoming increasingly clear that higher cognitive and affective brain systems are centrally involved in the pathology of both disorders. We propose that the ventromedial prefrontal cortex and the nucleus accumbens are part of a central 'gatekeeping' system in both sensory modalities, a system which evaluates the relevance and affective value of sensory stimuli and controls information flow via descending pathways. If this frontostriatal system is compromised, long-lasting disturbances are the result. Parallels in both systems are striking and mutually informative, and progress in understanding central gating mechanisms might provide a new impetus to the therapy of tinnitus and chronic pain.

Altered interhemispheric functional coordination in chronic tinnitus patients

Purpose. Recent studies suggest that tinnitus may be due in part to aberrant callosal structure and interhemispheric interaction. To explore this hypothesis we use a novel method, voxel-mirrored homotopic connectivity (VMHC), to examine the resting-state interhemispheric functional connectivity and its relationships with clinical characteristics in chronic tinnitus patients. Materials and Methods. Twenty-eight chronic tinnitus patients with normal hearing thresholds and 30 age-, sex-, education-, and hearing threshold-matched healthy controls were included in this study and underwent the resting-state fMRI scanning. We computed the VMHC to analyze the interhemispheric functional coordination between homotopic points of the brain in both groups. Results. Compared to the controls, tinnitus patients showed significantly increased VMHC in the middle temporal gyrus, middle frontal gyrus, and superior occipital gyrus. In tinnitus patients, a positive correlation was found between tinnitus duration and VMHC of the uncus. Moreover, correlations between VMHC changes and tinnitus distress were observed in the transverse temporal gyrus, superior temporal pole, precentral gyrus, and calcarine cortex. Conclusions. These results show altered interhemispheric functional connectivity linked with specific tinnitus characteristics in chronic tinnitus patients, which may be implicated in the neuropathophysiology of tinnitus.

Diffusion imaging of auditory and auditory-limbic connectivity in tinnitus: preliminary evidence and methodological challenges

Subjective tinnitus, or “ringing in the ears,” is perceived by 10 to 15 percent of the adult population and causes significant suffering in a subset of patients. While it was originally thought of as a purely auditory phenomenon, there is increasing evidence that the limbic system influences whether and how tinnitus is perceived, far beyond merely determining the patient's emotional reaction to the phantom sound. Based on functional imaging and electrophysiological data, recent articles frame tinnitus as a “network problem” arising from abnormalities in auditory-limbic interactions. Diffusion-weighted magnetic resonance imaging is a noninvasive method for investigating anatomical connections in vivo. It thus has the potential to provide anatomical evidence for the proposed changes in auditory-limbic connectivity. However, the few diffusion imaging studies of tinnitus performed to date have inconsistent results. In the present paper, we briefly summarize the results of previous studies, aiming to reconcile their results. After detailing analysis methods, we then report findings from a new dataset. We conclude that while there is some evidence for tinnitus-related increases in auditory and auditory-limbic connectivity that counteract hearing-loss related decreases in auditory connectivity, these results should be considered preliminary until several technical challenges have been overcome.

A brain centred view of psychiatric comorbidity in tinnitus: from otology to hodology

Introduction. Comorbid psychiatric disorders are frequent among patients affected by tinnitus. There are mutual clinical influences between tinnitus and psychiatric disorders, as well as neurobiological relations based on partially overlapping hodological and neuroplastic phenomena. The aim of the present paper is to review the evidence of alterations in brain networks underlying tinnitus physiopathology and to discuss them in light of the current knowledge of the neurobiology of psychiatric disorders. Methods. Relevant literature was identified through a search on Medline and PubMed; search terms included tinnitus, brain, plasticity, cortex, network, and pathways. Results. Tinnitus phenomenon results from systemic-neurootological triggers followed by neuronal remapping within several auditory and nonauditory pathways. Plastic reorganization and white matter alterations within limbic system, arcuate fasciculus, insula, salience network, dorsolateral prefrontal cortex, auditory pathways, ffrontocortical, and thalamocortical networks are discussed. Discussion. Several overlapping brain network alterations do exist between tinnitus and psychiatric disorders. Tinnitus, initially related to a clinicoanatomical approach based on a cortical localizationism, could be better explained by an holistic or associationist approach considering psychic functions and tinnitus as emergent properties of partially overlapping large-scale neural networks.

Neuroanatomical abnormalities in chronic tinnitus in the human brain

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We review brain anatomical studies of tinnitus.

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We evaluate the “gating mechanism” in light of the evidence from these studies.

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We discuss the results and the possible causes of disparity between findings.

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Overall, the evidence for structural abnormalities in tinnitus is unconvincing.

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We identify methodological concerns and suggest strategies for future research.

In this paper, we review studies that have investigated brain morphology in chronic tinnitus in order to better understand the underlying pathophysiology of the disorder. Current consensus is that tinnitus is a disorder involving a distributed network of peripheral and central pathways in the nervous system. However, the precise mechanism remains elusive and it is unclear which structures are involved. Given that brain structure and function are highly related, identification of anatomical differences may shed light upon the mechanism of tinnitus generation and maintenance. We discuss anatomical changes in the auditory cortex, the limbic system, and prefrontal cortex, among others. Specifically, we discuss the gating mechanism of tinnitus and evaluate the evidence in support of the model from studies of brain anatomy. Although individual studies claim significant effects related to tinnitus, outcomes are divergent and even contradictory across studies. Moreover, results are often confounded by the presence of hearing loss. We conclude that, at present, the overall evidence for structural abnormalities specifically related to tinnitus is poor. As this area of research is expanding, we identify some key considerations for research design and propose strategies for future research.

Left hemisphere fractional anisotropy increase in noise-induced tinnitus: a diffusion tensor imaging (DTI) study of white matter tracts in the brain

Diffusion tensor imaging (DTI) is a contemporary neuroimaging modality used to study connectivity patterns and microstructure of white matter tracts in the brain. The use of DTI in the study of tinnitus is a relatively unexplored methodology with no studies focusing specifically on tinnitus induced by noise exposure. In this investigation, participants were two groups of adults matched for etiology, age, and degree of peripheral hearing loss, but differed by the presence or absence (+/-) of tinnitus. It is assumed that matching individuals on the basis of peripheral hearing loss, allows for differentiating changes in white matter microstructure due to hearing loss from changes due to the effects of chronic tinnitus. Alterations in white matter tracts, using the fractional anisotropy (FA) metric, which measures directional diffusion of water, were quantified using tract-based spatial statistics (TBSS) with additional details provided by in vivo probabilistic tractography. Our results indicate that 10 voxel clusters differentiated the two groups, including 9 with higher FA in the group with tinnitus. A decrease in FA was found for a single cluster in the group with tinnitus. However, seven of the 9 clusters with higher FA were in left hemisphere thalamic, frontal, and parietal white matter. These foci were localized to the anterior thalamic radiations and the inferior and superior longitudinal fasciculi. The two right-sided clusters with increased FA were located in the inferior fronto-occipital fasciculus and superior longitudinal fasciculus. The only decrease in FA for the tinnitus-positive group was found in the superior longitudinal fasciculus of the left parietal lobe.

Neuroanatomical correlates of tinnitus revealed by cortical thickness analysis and diffusion tensor imaging

INTRODUCTION

Tinnitus is a poorly understood auditory perception of sound in the absence of external stimuli. Convergent evidence proposes that tinnitus perception involves brain structural alterations as part of its pathophysiology. The aim of this study is to investigate the structural brain changes that might be associated with tinnitus-related stress and negative emotions.

METHODS

Using high-resolution magnetic resonance imaging and diffusion tensor imaging, we investigated grey matter and white matter (WM) alterations by estimating cortical thickness measures, fractional anisotropy and mean diffusivity in 14 tinnitus subjects and 14 age- and sex-matched non-tinnitus subjects.

RESULTS

Significant cortical thickness reductions were found in the prefrontal cortex (PFC), temporal lobe and limbic system in tinnitus subjects compared to non-tinnitus subjects. Tinnitus sufferers were found to have disrupted WM integrity in tracts involving connectivity of the PFC, temporal lobe, thalamus and limbic system.

CONCLUSION

Our results suggest that such neural changes may represent neural origins for tinnitus or consequences of tinnitus and its associations.

Tinnitus: network pathophysiology-network pharmacology

Tinnitus, the phantom perception of sound, is a prevalent disorder. One in 10 adults has clinically significant subjective tinnitus, and for one in 100, tinnitus severely affects their quality of life. Despite the significant unmet clinical need for a safe and effective drug targeting tinnitus relief, there is currently not a single Food and Drug Administration (FDA)-approved drug on the market. The search for drugs that target tinnitus is hampered by the lack of a deep knowledge of the underlying neural substrates of this pathology. Recent studies are increasingly demonstrating that, as described for other central nervous system (CNS) disorders, tinnitus is a pathology of brain networks. The application of graph theoretical analysis to brain networks has recently provided new information concerning their topology, their robustness and their vulnerability to attacks. Moreover, the philosophy behind drug design and pharmacotherapy in CNS pathologies is changing from that of "magic bullets" that target individual chemoreceptors or "disease-causing genes" into that of "magic shotguns," "promiscuous" or "dirty drugs" that target "disease-causing networks," also known as network pharmacology. In the present work we provide some insight into how this knowledge could be applied to tinnitus pathophysiology and pharmacotherapy.

Neuroanatomical changes due to hearing loss and chronic tinnitus: a combined VBM and DTI study

Subjective tinnitus is the perception of sound in the absence of an external source. Tinnitus is often accompanied by hearing loss but not everyone with hearing loss experiences tinnitus. We examined neuroanatomical alterations associated with hearing loss and tinnitus in three groups of subjects: those with hearing loss with tinnitus, those with hearing loss without tinnitus and normal hearing controls without tinnitus. To examine changes in gray matter we used structural MRI scans and voxel-based morphometry (VBM) and to identify changes in white matter tract orientation we used diffusion tensor imaging (DTI). A major finding of our study was that there were both gray and white matter changes in the vicinity of the auditory cortex for subjects with hearing loss alone relative to those with tinnitus and those with normal hearing. We did not find significant changes in gray or white matter in subjects with tinnitus and hearing loss compared to normal hearing controls. VBM analysis revealed that individuals with hearing loss without tinnitus had gray matter decreases in anterior cingulate and superior and medial frontal gyri relative to those with hearing loss and tinnitus. Region-of-interest analysis revealed additional decreases in superior temporal gyrus for the hearing loss group compared to the tinnitus group. Investigating effects of hearing loss alone, we found gray matter decreases in superior and medial frontal gyri in participants with hearing loss compared to normal hearing controls. DTI analysis showed decreases in fractional anisotropy values in the right superior and inferior longitudinal fasciculi, corticospinal tract, inferior fronto-occipital tract, superior occipital fasciculus, and anterior thalamic radiation for the hearing loss group relative to normal hearing controls. In attempting to dissociate the effect of tinnitus from hearing loss, we observed that hearing loss rather than tinnitus had the greatest influence on gray and white matter alterations.

A diffusion tensor imaging study on the auditory system and tinnitus

Tinnitus is an auditory percept in the absence of an external sound source. Mechanisms in the central nervous system are believed to be key in the pathophysiology of tinnitus. Diffusion tensor imaging (DTI) is an MR imaging technique that allows in vivo exploration of white matter tissue in the human brain. Using a probabilistic DTI approach, we determined the characteristics of fiber tracts from the inferior colliculus to the medial geniculate body up to the primary auditory cortex. We also investigated the connections between the auditory system and the amygdala, which may be involved in some forms of tinnitus. White matter tracts were characterized by three quantities: the mean fractional anisotropy, the weighted mean fractional anisotropy and the path strength. All these quantities are measures of the patency of white matter tracts. The most important finding is an increased patency of the white matter tracts between the auditory cortex and the amygdala in tinnitus patients as compared to healthy controls.