Subjective Visual Vertical (SVV)

The subjective visual vertical (SVV) signifies the visually determined gravitational vertical. The measurement of SVV is clinically used as a method to assess the degree of dysfunction in the otolith, primary vestibular nerves, and central graviceptive pathways. Perception of vertical, however, does not depend solely on the function of those pathways, but is affected by various factors such as visual information, head position relative to gravity, and linear acceleration forces acting on the gravity vector. In addition, it undergoes a kind of 'vestibular compensation', that is, the tilt of SVV decreases within days to weeks even if the function of the organ responsible for the tilt does not restore function. In examining the patient with vertigo or dysequilibrium, the physician must be well acquainted with those factors affecting SVV and its natural course. This article also referred to the difference between SVV tilts and room tilt illusions. The former are usually stable and chronic signs and manifest as a continuum of angle of tilt up to abut 30°. In contrast, the latter occur paroxysmally or transiently in 90° steps. In clinical practice, the measurement of SVV is not so commonly carried out as compared to the recordings of nystagmus. It is expected, however, that SVV be more widely taken into account in clinical practice because the tilt of SVV and the occurrence of nystagmus do not share the same anatomical structures, thus SVV can provide a greater understanding of the patient's complaints and underlying pathologies.