Subjective visual vertical in first time seafarers experiencing sea sickness.

Objectives: Sea sickness, a form of motion sickness, is a common condition among sailors on various sailing vessels, primarily due to their roll and pitch movements. While the exact neurobiological mechanisms remain unclear, the most widely accepted explanation is the sensory conflict theory, which identifies two main sources of conflict: discrepancies between visual and vestibular inputs and conflicts within the vestibular system itself. This study aims to evaluate otolith-mediated verticality perception in first-time seafarers on board a naval frigate ship using the Subjective Visual Vertical (SVV) test. Methods: This observational study was conducted on board a naval frigate and involved two groups. The case group consisted of 50 newly recruited sailors experiencing their first sea voyage. The SVV test was conducted at three time points: prior to embarkation, during an active episode of sea sickness, and after disembarkation. A control group of 50 healthy, experienced sailors-regular seafarers with no history of vestibular symptoms underwent the same SVV testing at corresponding intervals. The results from both groups were collected and analyzed to assess changes in verticality perception related to sea exposure and sea sickness. Results: Analysis of the SVV test data from the 50 newly recruited sailors (case group) demonstrated a statistically significant difference compared to the control group of experienced sailors. During an active episode of sea sickness, the case group exhibited a notable tilt and deviation in SVV angles, indicating altered verticality perception. These deviations reduced significantly 24 hours post-disembarkation, suggesting an improvement in verticality perception once the motion stimulus was removed. Conclusion: This distinctive study assessed otolith-mediated verticality perception using the SVV test in first-time seafarers experiencing sea sickness aboard a naval frigate. Our findings emphasize the functional importance of the otolith organs in maintaining spatial orientation during maritime motion exposure. The observed SVV deviations support the subjective vertical conflict theory, highlighting the role of vestibular-visual mismatch in the development of sea sickness. However, our findings more likely reflect transient functional disturbance or overstimulation rather than a pre-existing utricular asymmetry alone. Based on the pattern of SVV deviation and recovery, we propose that the utricle may play a more significant role than the saccule in inducing sea sickness.