Olfactory Dysfunction Following Moderate to Severe Traumatic Brain Injury: A Systematic Review and Meta-Analysis

Traumatic brain injury-induced inflammatory changes in the olfactory bulb disrupt neuronal networks leading to olfactory dysfunction

Approximately 20-68% of traumatic brain injury (TBI) patients exhibit trauma-associated olfactory deficits (OD) which can compromise not only the quality of life but also cognitive and neuropsychiatric functions. However, few studies to date have examined the impact of experimental TBI on OD. The present study examined inflammation and neuronal dysfunction in the olfactory bulb (OB) and the underlying mechanisms associated with OD in male mice using a controlled cortical impact (CCI) model. TBI caused a rapid inflammatory response in the OB as early as 24 h post-injury, including elevated mRNA levels of proinflammatory cytokines, increased numbers of microglia and infiltrating myeloid cells, and increased IL1β and IL6 production in these cells. These changes were sustained for up to 90 days after TBI. Moreover, we observed significant upregulation of the voltage-gated proton channel Hv1 and NOX2 expression levels, which were predominantly localized in microglia/macrophages and accompanied by increased reactive oxygen species production. In vivo OB neuronal firing activities showed early neuronal hyperexcitation and later hypo-neuronal activity in both glomerular layer and mitral cell layer after TBI, which were improved in the absence of Hv1. In a battery of olfactory behavioral tests, WT/TBI mice displayed significant OD. In contrast, neither Hv1 KO/TBI nor NOX2 KO/TBI mice showed robust OD. Finally, seven days of intranasal delivery of a NOX2 inhibitor (NOX2ds-tat) ameliorated post-traumatic OD. Collectively, these findings highlight the importance of OB neuronal networks and its role in TBI-mediated OD. Thus, targeting Hv1/NOX2 may be a potential intervention for improving post-traumatic anosmia.

Medicolegal aspect of loss of smell and olfactory event-related potentials

Background

It is not straightforward to objectively evaluate the olfactory dysfunction that occurs following forensic incidents. The olfactory event-related potentials method, based on electrophysiological records, may provide objective data in the evaluation of posttraumatic anosmia cases from the medicolegal perspective. This study, where a quantitative evaluation of the cases with the complaints of olfactory sensation disorder was performed using the olfactory event-related potentials test, aims to identify the factors that should be considered in the evaluation of olfactory dysfunction from the medicolegal perspective.

Results

This study first evaluated the complaints of 98 patients admitted because of posttraumatic impaired smell and then administered electrophysiological odor tests on the patients. Because of this, the relationship between the EEG responses of the cases and the olfactory disorder was examined. Of the 98 cases that participated in the study, 68 (69.4%) were male and 30 (30.6%) were female. Of all cases, 53 (54.1%) had complaints of not being able to smell at all, 14 (14.3%) had complaints of reduced smell, whereas, in addition to the existing complaints of olfactory dysfunction, 44 (44.9%) of them had complaints of taste perception and 18 (18.3%) reported having vision disorders. 21 of 37 cases who reported being unable to smell during the test turned out to be anosmic. Furthermore, 16 cases stated that, though having had a response in the odor test, they had no sense of smell following the test.

Conclusions

Although it seems possible to prove that there is a relationship between the olfactory event-related potential test and the diagnosis of anosmia, there is still ongoing research on its use in clinical practice. Performing both subjective and electrophysiological tests together to detect olfactory dysfunctions that occur after a forensic incident enable provide more reliable results in diagnosis.