Although there were no complaints of dizziness and no significant differences in subjective psychological stress between the residents of Tokyo and Osaka prefectures, more severe equilibrium dysfunction was detected in the Quake group, indicating that repetitive exposure to aftershocks can evoke equilibrium dysfunction 4 months after a major earthquake.
The low- and high-frequency band powers for the CoG in the antero-posterior direction were positively and negatively correlated, respectively, with the current anxiety state as reflected by STAI-state scores in the Quake group. Specifically, positive correlations between all stabilometric parameters and the STAI-state score were observed under the EC condition in the Quake group. Thus, equilibrium dysfunction in the Quake group may also have been affected by psychological stress due to repetitive exposure to aftershocks. It has been suggested that vestibular–corticolimbic interaction occurs via two separate neural pathways at the higher cortical and the medullary levels19
. Anxiety may enhance equilibrium dysfunction both in healthy subjects15
and in patients with vestibular dysfunctions such as Ménière disease27
, in which an increase in the high-frequency band power for CoG has been found, suggesting that vestibular–corticolimbic interaction could be promoted by the neural pathway at the higher cortical level. However, equilibrium dysfunction exhibiting the increase in the low-frequency band power of the CoG in the Quake group, likely caused by bilateral inner ear disturbance due to repetitive physical shaking, may have been modulated by vestibular–corticolimbic interaction through the other neural pathway at the medullary level.
The parabrachial nuclei receive input from the vestibular nuclei as well as the insular, amygdala, and bed nuclei of the stria terminalis, which are involved in emotional processing18
. In particular, the bed nuclei of the stria terminalis are involved in longer-lasting emotional responses such as anxiety29,30
. Anxiety is typically integrated with vestibular information in the corticolimbic area31
; however, it may also be translated by autonomic neurotransmission. Somatic senses such as startle stimuli often form an autonomic reflex via association with psychological stress, called a startle response32
. Physical shaking may be associated with autonomic regulation; thus, contextual conditioning may have been formed by repetitive aftershocks experienced by the Quake group individuals. Animal neuroanatomical findings indicate that the vestibulo–parabrachial pathway, which reciprocally integrates not only vestibular and corticolimbic but also autonomic information, is present in the brain stem19
. Even considering that the autonomic center is involved in the reticular formation of the medulla, humans may also integrate autonomic, vestibular, and corticolimbic information at the medullary level, potentially enhancing inner ear disturbance.
Visual information is also combined with vestibular information at both the higher cortical and brain stem levels33
at the superior colliculi of the mesencephalon and the tegmentum of the pons, in which secondary vestibular neurons make synaptic connections with the oculomotor nuclei and the abducens nuclei, respectively34
. The medial and lateral rectus muscles, which are innervated by the somatic motor nerves of the oculomotor and the abducens, respectively, rotate the eyes when the head turns35
. This visual compensation supporting equilibrium function may arise from an oculovestibular reflex together with a possible neural connection associated with visuo-vestibular interactions at the higher cortical level36
. Therefore, equilibrium dysfunction is amplified when the eyes are closed.
The current study has several potential limitations. First, our results may demonstrate regional differences in the effects of aftershocks on equilibrium function; however, the cross-sectional study protocol did not exclude individual differences in evaluating the specific impacts of this major earthquake on equilibrium function. Second, we were not able to examine direct relationships between the clinical symptoms of dizziness and equilibrium dysfunction. To investigate the effects of the earthquake on equilibrium function, we excluded patients with mental disorders, including those who had experienced dizziness and/or clinical levels of anxiety, because dizziness patients may simultaneously have several anxiety symptoms as one of the common sequelae of dizziness8
. For similar reasons, subjects were included only if their anxiety and depression symptoms did not reach threshold levels for the respective screening criteria. Third, because the airborne radiation doses in both regions were not noticeably different between the period from March 11 to July 6 in 2011 (Tokyo: average, 0.071 μSv/h; Osaka: 0.043 μSv/h) and previously (Tokyo: 0.028–0.079 μSv/h; Osaka: 0.042–0.061 μSv/h) (Ministry of Education, Culture, Sports, Science & Technology in Japan, 2011: The database of monitoring information of environmental radioactivity level
), individual acute doses were not estimated in the current study. Thus, there is room for further research into the effects of radiation exposure on equilibrium function. In addition, although equilibrium function could be influenced by daily activities and environmental factors such as commuter traffic and fitness habits, it was difficult to control these lifestyle variables. We were able to recruit participants from groups that had broadly similar living environments. Finally, we did not observe significant differences in psychological effects from the earthquake as measured by the STAI and BDI. However, it is uncertain whether these inventories are able to specifically evaluate post-earthquake effects. Further research on the nature of post-disaster psychological suffering would be valuable.
We have provided evidence that repetitive exposure to earthquakes causes dysfunction in the inner ear. In addition, the current study indicates potential effects of autonomic stress on equilibrium function. Equilibrium dysfunction caused by earthquakes may be affected by physical invasion and automatic stress, as well as interactions between them, through vestibular neural integration. The present results contribute to risk management of mental and physical health after major earthquakes with aftershocks, and may allow development of a new empirical approach to disaster care during and after such events. Medical treatment for equilibrium dysfunction such as a rehabilitation of balance control37
and pharmacological treatment for vestibular dysfunction38
could alleviate psychological distress, including anxiety symptoms of earthquake victims, without direct psychological support. Disaster victims often prefer not to discuss post-disaster trauma, but may be willing to seek treatment for their physical symptoms. Medical approaches to equilibrium dysfunction may be helpful for earthquake disaster victims, although further trials are needed.