The aim of this study was to measure the neutral position of direction-changing apogeotropic positional nystagmus (heavy cupula of the horizontal semicircular canal) and persistent direction-changing geotropic positional nystagmus (light cupula of the horizontal semicircular canal). We conducted a prospective case series study on 31 patients with heavy cupula (12 males, 19 females; mean age, 64.3 years) and 33 patients with light cupula (10 males, 23 females; mean age, 60.9 years). We measured the angle of the neutral position in patients with heavy cupula (θ1) and that in patients with light cupula (θ2) using a large protractor. The mean value and standard deviation of θ1 was 31.6 ± 22.4°, minimum value was 5°, and maximum value was 89°. The mean value and standard deviation of θ2 was 44.4 ± 20.5°, minimum value was 5°, and maximum value was 85°. θ2 was significantly greater than θ1 (p < 0.05). The neutral position varies widely. Some patients exhibit a great angle (more than 40°); therefore, examiners should make patients adopt a completely lateral position in the supine head roll test and should confirm the direction of nystagmus in order to avoid mistaking positional nystagmus for spontaneous nystagmus.
Horizontal semicircular canal Supine head roll test Heavy cupula Light cupula Buoyancy
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