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The work has been performed at the University of Tampere, Lääkärinkatu 1, 33520 Tampere.
Ilmari Pyykkö
Hearing and Balance Research Unit, University of Tampere
Itäinen Puistotie 12 A 1
FI–00140 Tampere (Finland)
E-Mail [email protected]
Lea J, Pothier D (eds): Vestibular Disorders. Adv Otorhinolaryngol. Basel, Karger, 2019, vol 82, pp 32–38
DOI: 10.1159/000490269
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Videonystagmography and Posturography
Carolyn Falls
University Health Network, Toronto, ON, Canada
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Abstract
Videonystagmography (VNG) and posturography are two vestibular assessment techniques that are still in use today. VNG: VNG allows clinicians to observe and record eye movements in real-time. Compared with electronystagmography (ENG), VNG tracings are more detailed and can capture subtle clinical findings. The utility of the monothermal caloric screen has been proposed in various studies. When appropriate cut-offs are used, the monothermal screen can be completed with a low false-negative rate. Air is often used as a medium for caloric testing, though not without some controversy. When air and water are compared, the response magnitudes for air are consistently smaller than for water. However, the overall classification of the result appears to be generally the same regardless of the caloric medium used. Posturography: Concerns have been raised about the reliability and validity of posturographic measurements. Approaches and analysis techniques vary widely, and there is little consensus regarding the best approaches for assessment or interpretation. There is a need for standardized protocols. The portable and low-cost Nintendo Wii Balance Board (WBB) has inspired a surge in publications looking at the validity of the system for quantifying posturography and for balance training; preliminary findings are very promising.
© 2019 S. Karger AG, Basel
Introduction
It has been over 100 years since Róbert Bárány first described the caloric test and was subsequently awarded a Nobel Prize for his research on the vestibular apparatus [1]. Since then, laboratory tests for vestibular assessment have evolved significantly with improvements in technology and the emergence of new techniques. Two methods that are rooted in some of the early approaches to vestibular assessment, videonystagmography (VNG) and posturography, are still in use today and are the subject of this chapter.
Videonystagmography
VNG is a technique for recording eye movements for the purpose of assessing patients with suspected vestibular dysfunction. Infrared video cameras and digital video image analyses are used to isolate the location of pupil(s) so that a tracing of eye movement can be generated. The emergence and mass adoption of VNG is, in itself, a relatively recent advance. Before the pupil-tracking algorithms for VNG became ready for clinical use, the preferred technique for monitoring eye movements was electronystagmography (ENG). ENG involves the use of electrodes placed close to the eyes to measure the corneoretinal potential (CRP). Voltage changes in the CRP from the difference between the positively charged cornea and the negatively charged retina are amplified, and subsequently captured within the resultant tracing.
VNG testing has some distinct advantages over ENG. In particular, VNG testing allows clinicians to observe the patient’s eye movements in real-time; with ENG, eye movements have to be inferred from the tracings, particularly for components of the test that are completed without fixation. With VNG, there is an option to record the video for documentation and later review. In addition, VNG testing does not rely on the CRP, the magnitude of which can be impacted by the level of illumination within the testing room [2] and other factors such as retinal health [3]. The quality of ENG tracings can be further diminished by noise (ambient or from muscle activity/blinks), though VNG is not immune to artifact [4].
With VNG, it is also possible to recognize torsional eye movements. Currently, torsional eye movements are identified most successfully when observed directly from a video recording, though most VNG systems now offer algorithms for detecting torsional movement with varying degrees of reliability. The ability to identify torsional eye movements can be helpful in recognizing benign paroxysmal positional vertigo (BPPV) [5], one of the most common causes of vertigo. Torsional analysis is not possible with ENG [6].
VNG systems can generally sample at higher rates than ENG and do not require low frequency filtering; the additional detail from the superior signal processing allows for the ability to recognize more subtle clinical findings [6]. For example, early studies looking at peripheral and central impairments in patients with mild traumatic brain injuries have yielded promising preliminary results
