A comparison of subjective and objective measures of vestibular adaptation in rotary chair among Indian military aircrew

Objectives Vestibular adaptation time (VAT) refers to subjective assessment of vestibular stimulation in motion environment. While simple, the method is subjective and, hence, is naturally deemed inferior to the objective methods of assessment. Vestibulo-ocular reflex-induced nystagmus, however, can well be used as a quantified measure in a Rotary Chair. Time constant (TC) has conventionally been defined as the time taken for nystagmus to dissipate to 37% of its peak value. Hence, TC becomes an obvious, objective, and more reliable measure for vestibular adaptation. The objective of this study is to compare the VAT and TC in aircrew and determine differences in outcome while using these two different methods. Material and Methods Fifty-two healthy male military aircrew with mean age 25.7 ± 4.25 years and median flying experience of 325 h were enrolled in this study after obtaining written informed consent. In the rotary chair, velocity step test was performed, in which the subjects were rotated at an angular velocity of 100°/s. On reaching this constant angular velocity, subjects reported perceived cessation of rotation (per-rotatory). Subsequently, on stopping the chair, the subjects perceived continued rotation in the opposite direction (post-rotatory). The test was then repeated by rotating the subjects in opposite direction. The period from beginning of horizontal rotation till cessation of perceived motion was recorded as the VAT. Simultaneously, the time taken for induced nystagmus to decay to 37% of its peak was recorded by video-oculography and noted as TC. Values of VAT and TC were recorded in per-rotatory and post-rotatory phases in both left and right directions and were statistically analyzed to assess any differences in the outcomes. Results Mean VAT for the right and left rotation was found to be 28.22 ± 6.95s and 27.74 ± 6.55s, while mean TC for the right and left rotation was found to be 19.50 ± 5.01s and 18.59 ± 3.99s. Mean VAT was found to be 27.98 ± 6.74s and mean TC was found to be 19.05 ± 4.54s. Bland–Altman graph found 95% of the data points within the upper and lower limits of agreement. Conclusion In our study, a higher mean VAT of around 28s and a mean TC of 19s can be attributed to the varying onset and offset rates and angular velocities being used in different studies. Results indicate an offset of 8.925s between values of VAT and TC. However, Bland–Altman plots indicate good agreement and ICC indicates mild reliability between the two tests. This signifies that although there is good agreement between the values of the two tests, further studies may be required to ascertain whether the two parameters may be used interchangeably as a measure of vestibular adaptation.