The role of cognitive processes in vestibular disorders
Equilibrium of the body is the result of perfect control of the relationship between the body and the environment in every life condition. Equilibrium is an essential function and hence cannot be entrusted to a single organ or apparatus. The function requires an entire balance system consisting of a set of communicating structures and processes. At every moment, the central nervous system (CNS) processes the inputs received from various receptors in the human body. Key steps in processing the different inputs involve recognition of the single afferences, comparison and integration of the multitude of afferences and their modulation to regulate the gain on the whole system. Furthermore, the CNS stores the experiences and finally programmes the responses to adapt to the ongoing reality. The vestibular system plays a critical role in maintaining the balance function and, therefore, its inputs must be integrated with the highest brain functions. From an anatomo-physiological perspective, the existence of cognitive-vestibular interaction is supported by the presence of neuronal projections between the vestibular centres in the brainstem and both the cerebral and cerebellar cortex. However, the vestibular system is connected to the whole CNS. In humans, galvanic and caloric vestibular stimulation activates several cortical regions. The so-called vestibular cortex is a multisensorial cortex because it receives not only vestibular inputs but also somatosensory and visual inputs. The vestibular system is tightly connected to the limbic system, which regulates emotions, homeostasis and storage of experiences. More specifically, the vestibular inputs are basic to the topokinetic memory of the hippocampus.Acute lesions affecting the peripheral receptors or nerves produce a sudden change in information reaching the balance system, which results in serious symptoms. Hence, all balance disorders lead to a significant degree of handicap and emotional disturbance for the patient.The evolution of vestibular symptoms is related to the ability of the CNS to adapt to the development of new and dangerous conditions. The plasticity of the CNS permits adaptation to both physiological and pathological conditions in every condition of life. Cognitive resources play a continuous role in the reorganization of the balance function during adaptation to vestibular loss. Despite the fact that the adaptation is usually satisfactory, a ‘vestibular cicatrix’ remains forever. The vestibular cicatrix is characterized by topokinetic memory disorganization at a high level of information processing, spatial working memory deficit, alerting state with abnormal levels of anxiety and depression, substitutional saccadic eye movements if the head is turned quickly in the direction of the damaged ear, errors in cognitive tasks and fewer attentional resources. Vestibular adaptation is not constant in time and is dependent on many individual factors (e.g. age, gender, emotions, stress).Vertigo has a negative impact on the quality of life, affects everyday activities and is associated with cognitive impairment. Hence, treatment of vertigo assumes great significance. Treatment must target the entire CNS and should aim to facilitate the vestibular adaptation process. It is preferred that the patient is not confined to bed after the first two days in order to allow the early initiation of dynamic experiences, which are vital for adaptation without running the risk of activating the mechanism of fear and traumatic memory.While selecting a drug for therapy, the plastic and cognitive neuronal mechanisms activated by vestibular deficits must be taken into account. If the use of symptomatic drugs in the past was simply a kind of ‘siesta strategy’, it is expected that in future we will rely on a more dynamic strategy, a ‘mind-energy strategy’, i.e. a vertigo treatment that would also involve a cognitive approach. Adaptation mechanisms require activation of neurons, synapses and neuronal metabolism, particularly in mitochondria. Thus, we need drugs that enhance neuronal metabolic activity, neurotransmission and facilitate the transfer of information between the two hemispheres. It may be particularly useful to counsel patients about the disease and its likely consequences.An excessive alerting state can be prevented by providing patients (and their families) with a simple but detailed explanation of how symptoms are provoked and about the process of adaptation following a balance disorder. In the first days following the vertigo attack, the new dynamic experiences are fundamental for the patients, who need to stay active instead of totally passive. For these patients, the use of sedative drugs should be limited, while the use of nootropic and/or neuromodulator drugs and possibly anti-anxiety drugs in anxious or stressed subjects should be continued. If symptoms persist, factors that interact with the processes of adaptation should be identified and limited, thus reducing stress. The other sensory problems that affect the visual or proprioceptive apparatus should be removed and should have vicarious compensatory activity. Possibilites of reducing the high levels of anxiety, depression and psychological conditions that interact negatively should be considered. The use of nootropics and/or neuromodulator drugs should be continued and a customized rehabilitation training should be implemented.
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