Development of a Clinical Examination to Evaluate Gravity Perception during Static Head Roll Tilt.

Clinicians frequently encounter patients experiencing a floating sensation of unknown cause, referred to as floating dizziness. We developed a clinical examination to quantify gravity perception as a first step to test our hypothesis that a gravity perception disturbance causes floating dizziness. Our preliminary experiments revealed a linear relationship between head-tilt angle (HTA) and head-tilt perception (HTP) at a head roll tilt of <30°, with correlation coefficients of 0.991 and 0.999 between HTA and HTP in the whole body and head-tilt conditions, respectively. Using these observations, we established a method to evaluate the HTP gain (HTPG), which is the slope of a linear regression line produced by the relation of subjective visual vertical (SVV) with HTA during head tilt, as an index of gravity perception. We named this method head-tilt SVV (HT-SVV). When HTPG was>1, the subjects' HTP overestimated their HTA and vice versa. HT-SVV measurements of 329 healthy subjects gave an average and standard deviation (1.02±0.12) and reference value (0.80～1.25) for HTPG, a difference between left and right HTPG (4.7%±3.7%, <10.0%), and an absolute value for SVV in the upright head position (1.1°±0.8°, <2.5°). The next step is to make the HT-SVV system easier and faster to use as well as to collect more data from patients experiencing dizziness, including floating dizziness, to clarify the clinical significance of HT-SVV.