Brazilian journal of otorhinolaryngology | 2021
Does tinnitus lead to chaos?
Abstract
Tinnitus, the perception of sound in the ears or the head without an actual external origin, is an influential problem in public health, negatively affecting the life of about 15% of the world’s adult population. Considering the prevalence of this disorder, and the need for reliable treatment, much research has been done to identify the mechanisms and eventual management strategies for tinnitus. While some articles consider the deafferentation of the auditory bottom-up tract with or without hearing loss as the main culprit, others focus on the deficiency of the top-down noise cancelling mechanism or even both factors simultaneously. The neural deafferentation in the central auditory structures resulting from cochlear damage makes changes in the central auditory tract including tonotopic map reorganization, hyperactivities in auditory cortex and thalamus and increased neural synchronization, especially in areas related to the auditory cortex that are affected by hearing loss. It seems that as a result of any decrease in auditory inputs and imbalance between excitation and inhibition, a compensatory mechanism is activated resulting in increased neural spontaneous activity and synchronization. Although increased activities in the auditory brain areas and tonotopic map reorganization seem to be the main reasons for tinnitus, it cannot always be the case. Since tinnitus is not present during sleep conditions, some brain areas related to cognition must be involved in the perception of tinnitus. For a conscious perception of acoustic stimuli, in addition to the activity of the main auditory center in the brain, other centers such as parts of the prefrontal lobe, parietal lobe, cingulate and insula must be activated. These structures comprise two main networks: the perception network including anterior and posterior cingulate cortex and some parts of parietal and frontal lobes, and Precuneus and the salience network covering anterior and posterior parts cingulate cortex and anterior insula. Some variations in the activity of these networks has been proven by electroencephalogram/ magnetoencephalogram investigations in tinnitus patients. The thalamocortical dysrhythmia theory states that the low frequency brain waves such as theta and delta increase due to the auditory deafferentation and high frequency brain waves i.e. gamma band increase as a result of the reduced inhibition from the thalamus to cortex. The same studies indicate the involvement of more areas of the brain including the parahippocampus, hippocampus as well as amygdala, which are parts of the learning network. On the other hand, distress has been proven to be another reason for tinnitus, increasing the likelihood of tinnitus when combined with hearing loss. Thus, it appears that the generation and perception of tinnitus can be resulted from the activation of a global network including perception network, salience network, learning network and distress network. As a conclusion, the network theory could be a perfect explanation for tinnitus generation. At first, it was assumed that the networks would be completely coincidental i.e., the node regions in each network were randomly connected and all nodes had the same importance in each network. Later on, the scale-free networks were presented. In these networks, there are more obvious interconnections among the nodes, which in turn increases the potential of the whole unit. In the scale-free network theory, any harm to the major hubs would result in the general deficiency and harm to the entire network. Today, it has become clear that the brain does not follow a single network law, but multiple networks are simultaneously activated based on the number of its functions and stimuli. Therefore, since multiple scale-free networks are probably activated simultaneously in tinnitus, the management approaches by acoustical and electrical neuromodulation must have been effective because major hubs can be disrupted or affected. However, researches have shown that this type of tinnitus management has had only a temporary and limited effect for a small percentage of patients, showing inefficiency of network model. So, it appears that a new mechanism for tinnitus might be considered. Different studies have shown that the human brain is not a linear system, but rather a dynamic and nonlinear one. Chaos is a phenomenon concerning nonlinear systems. According to chaos theory, in a deterministic nonlinear and dynamic system that has apparent irregularity and random states, a small change in the input can bring about major changes in the output, because of the presence of underlying patterns, interconnectedness, constant feedback loops, repetition, self-similarity, fractals, and self-organization in this system. Any small change in the human auditory sys-