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 [3, 7], suggesting that VNG may hold promise as a tool for identifying and tracking the progress of these patients. There are potential downsides to the additional detail that VNG tracings provide: it may not always be clear whether a subtle clinical finding is significant. There is a risk of over-diagnosis if all clinical findings are treated as pathological indiscriminately. In a publication by Martens et al. [8], measureable nystagmus was detected in at least one of the 6 testing positions for 88% of participants with no history of vestibular complaints. However, the nystagmus was generally of low velocity (≤5 deg/s for horizontal movements); for the Dix-Hallpike, the nystagmus was not paroxysmal [8]. There is a need for more research looking at VNG results in normal populations to establish better norms.
It is worth noting that ENG is still considered to be an acceptable approach for vestibular assessment. Some populations are more challenging to evaluate successfully with VNG due to the physical fit of the goggles (e.g., small children). In addition, some patients find it difficult to keep their eyes open without blinking excessively; with ENG, the patient’s eyes can remain closed for most of the test.
Monothermal Calorics
The traditional caloric test is administered bithermally, wherein the patient’s ears are irrigated with both a warm and cool stimulus and then analyzed [9]. Unfortunately, the full bithermal battery is time-consuming (and therefore, costly) to administer and caloric testing can be a source of patient discomfort. Recent studies have examined the potential utility of a monothermal screen by retrospectively comparing the results of the warm and/or cool screen to the results from the bithermal test. Most have shown that a monothermal warm screen can be used to reduce the testing time and patient discomfort for a large proportion of patients and can be accomplished with a very low false-negative rate [10–14]. To optimize the predictive accuracy of the monothermal test, a warm screen is preferable to a cool screen [10–15].
To guarantee the value of the monothermal warm screen, a high sensitivity (low false-negative rate) is essential. The specificity for the monothermal warm screen is generally quite low [13]; this is of little consequence because patients whose inter-ear difference (IED) values exceed the determined normal criterion for the warm screen continue to complete the entire bithermal test. According to a recent systematic review of the literature by Adams et al., when a bithermal unilateral weakness (sometimes referred to as canal paresis) cut-off of 20 or 25% is assumed, an IED criterion set at ≤15% for the monothermal screen achieved a low false-negative rate (≤5%) and reduced the number of patients requiring the full bithermal caloric test to between 49 and 57% [16]. Establishing a minimum SPV value (<11 deg/s) for each irrigation [16]