Magnetic resonance spectroscopy features of Heschl's gyri in patients with unilateral acoustic neuroma: preliminary study

An MR spectroscopy study of temporal areas excluding primary auditory cortex and frontal regions in subjective bilateral and unilateral tinnitus

Previous studies indicate changes in neurotransmission along the auditory pathway in subjective tinnitus. Most authors, however, investigated brain regions including the primary auditory cortex, whose physiology can be affected by concurrent hearing deficits. In the present MR spectroscopy study we assumed increased levels of glutamate and glutamine (Glx), and other Central Nervous System metabolites in the temporal lobe outside the primary auditory cortex, in a region involved in conscious auditory perception and memory. We studied 52 participants with unilateral (n = 24) and bilateral (n = 28) tinnitus, and a control group without tinnitus (n = 25), all with no severe hearing losses and a similar hearing profile. None of the metabolite levels in the temporal regions of interest were found related to tinnitus status or laterality. Unexpectedly, we found a tendency of increased concentration of Glx in the control left medial frontal region in bilateral vs unilateral tinnitus. Slightly elevated depressive and anxiety symptoms were also shown in participants with tinnitus, as compared to healthy individuals, with the bilateral tinnitus group marginally more affected. We discuss no apparent effect in the temporal lobes, as well as the role of frontal brain areas, with respect to hearing loss, attention and psychological well-being in chronic tinnitus. We furthermore elaborate on the design-related and technical obstacles of MR spectroscopy.

Asymmetry of Gray- and White-Matter Volume and Metabolites in the Central-Vestibular System in Healthy Individuals

This study was designed to determine whether there was an asymmetry of structure and neurochemical activity of the interhemispheric vestibular-cortical system between healthy individuals and patients with vestibular failure. Previous studies have identified differences in gray-matter-volume (GMV) and white-matter-volume (WMV) asymmetry in the central-vestibular system and in concentrations of brain metabolites in the parietal lobe 2 (PO₂) between patients with vestibulopathy and healthy controls. However, a comparison of the left and right sides in the healthy controls has not been made conclusively. This study included 23 healthy right-handed volunteers, and was carried out between March 2016 and March 2020. A three-dimensional T1-weighted image was used to calculate the GMV and WMV of the central-vestibular network on both sides, and proton magnetic resonance spectroscopy (H1MRS) was employed to analyze the brain metabolites in the PO₂ area. The relative ratios of N-acetylaspartate (NAA)/tCr, tNAA/tCr, glycerophosphocholine (GPC)/tCr, Glx/tCr, and myo-inositol/tCr were quantified from the proton-MRS data. GMV and WMV differed significantly between the right and left vestibular-cortical regions. The GMVs of the right PO₂, caudate, insula, and precuneus were significantly higher than those of the same locations on the left side; however, in the Rolandic operculum, the GMV on the left was significantly higher than on the right. In the PO₂, Rolandic operculum, thalamus, and insula, the WMV on the left side was higher than on the right side of the corresponding location. However, the right caudate and precuneus WMV were higher than the left at the same location. In the H1MRS study, the Glx/tCr and GPC/tCr ratios on the left side were significantly higher than on the right. In comparison, the NAA/tCr and tNAA/tCr ratios showed contrasting results. The NAA/tCr ratio (r = -0.478, p = 0.021), tNAA/tCr ratio (r = -0.537, p = 0.008), and Glx/tCr ratio (r = -0.514, p = 0.012) on the right side showed a significant negative correlation with the participants' age. There was no relationship between GMV and metabolites on either side. Brain structure and concentrations of brain metabolites related to the vestibular system may differ between the two hemispheres in healthy individuals. Therefore, the asymmetry of the central-vestibular system should be considered when performing imaging.

Lower glutamate and GABA levels in auditory cortex of tinnitus patients: a 2D-JPRESS MR spectroscopy study

We performed magnetic resonance spectroscopy (MRS) on healthy individuals with tinnitus and no hearing loss (n = 16) vs. a matched control group (n = 17) to further elucidate the role of excitatory and inhibitory neurotransmitters in tinnitus. Two-dimensional J-resolved spectroscopy (2D-JPRESS) was applied to disentangle Glutamate (Glu) from Glutamine and to estimate GABA levels in two bilateral voxels in the primary auditory cortex. Results indicated a lower Glu concentration (large effect) in right auditory cortex and lower GABA concentration (medium effect) in the left auditory cortex of the tinnitus group. Within the tinnitus group, Glu levels positively correlated with tinnitus loudness measures. While the GABA difference between groups is in line with former findings and theories about a dysfunctional auditory inhibition system in tinnitus, the novel finding of reduced Glu levels came as a surprise and is discussed in the context of a putative framework of inhibitory mechanisms related to Glu throughout the auditory pathway. Longitudinal or interventional studies could shed more light on interactions and causality of Glu and GABA in tinnitus neurochemistry.

Human Auditory Cortex Neurochemistry Reflects the Presence and Severity of Tinnitus

It is not known why tinnitus occurs in some cases of hearing damage but not others. Abnormalities of excitation-inhibition balance could influence whether tinnitus develops and its severity if it does. Animal models of hearing damage, which also produce tinnitus based on behavioral evidence, have identified abnormalities of GABAergic inhibition, both cortically and subcortically. However, the precise relationships of GABA inhibitory changes to tinnitus itself, as opposed to other consequences of hearing damage, remain uncertain. Here, we used magnetic resonance spectroscopy to non-invasively quantify GABA in the left (LAC) and right (RAC) auditory cortices of a group of 14 patients with lateralized tinnitus (eight left ear) and 14 controls matched for age, sex, and hearing. We also explored the potential relationships with other brain metabolites (i.e., choline, N-acetylaspartate, and creatine). The presence of tinnitus was associated with a reduction in auditory cortex GABA concentration. Regardless of tinnitus laterality, post hoc testing indicated reductions that were significant in RAC and nonsignificant in LAC. Tinnitus severity and hearing loss were correlated positively with RAC choline but not GABA. We discuss the results in the context of current models of tinnitus and methodological constraints.

SIGNIFICANCE STATEMENT

Permanently affecting one in seven adults, tinnitus lacks both widely effective treatments and adequate understanding of its brain mechanisms. Existing animal models represent tinnitus that may not be distinguishable from homeostatic responses to the auditory insults used to induce it. Human studies can be well controlled in this regard but are usually not (with few even matching control subjects for hearing loss) and are limited in scope as a result of relying solely on non-invasive recording techniques. Here, we exploit recent advances in non-invasive spectroscopic techniques to establish, in a human study tightly controlled for hearing loss and hyperacusis, that tinnitus is associated with a significant reduction in auditory cortex GABA concentration, which has implications for understanding and treatment of the condition.