href="#ulink_2f398a12-7741-57b9-baf0-4a06b1e0a147">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] and excluding/adjusting for significant spontaneous nystagmus [12] may also improve the sensitivity of the results, though further investigation is needed.
Air versus Water Calorics
Compared with water caloric irrigation systems, air caloric systems have some practical benefits: they tend to be more portable than water systems, they reach the desired testing temperatures more rapidly, and they do not require a nearby water source. In addition, air calorics can be performed safely even when a tympanic membrane perforation or other contraindications to water calorics (e.g., presence of mastoid cavity) are present [17]. These factors have likely contributed to the adoption of air caloric systems in many centers.
However, some questions have been raised about the use of air as a caloric stimulus; to date, no standards for air calorics have been approved by the American National Standards Institute (ANSI) [18]. One concern with air calorics is the potential for unexpected sources of variability. Even the size of the speculum that is used can influence the SPV values obtained; smaller speculums (2.5 mm) generate nystagmus that is nearly twice that of the SPV values generated with larger (4 mm) speculums on average [19]. It appears that the speculum size has an impact on the speed of the air flow as it enters the ear canal.
The general consensus in the literature indicates that air calorics yield unilateral weakness values that are comparable to water calorics [20–22]. However, the literature also indicates that the average SPV values obtained with water as the caloric medium are significantly larger than the values obtained with air [20, 22–24]. This raises questions about whether air and water caloric mediums should be considered as interchangeable when the response magnitudes achieved from the two techniques are clearly different. Moreover, from a statistical standpoint, small differences between SPV values are amplified when the average SPV values are lower overall, an element of Jongkees’ formula [25] that makes lower SPV values more vulnerable to skewed results. Indeed, one study comparing water and air caloric media found that almost 6% of the normal group had abnormal caloric results with air while none had abnormal results with water [20]. Recent publications investigating water and air as caloric mediums have identified new considerations and potential approaches that may help in generating equivalent responses across the two approaches [19, 21].
Posturography
Posturography pertains to the functional evaluation of postural control and stability, which is largely mediated by the interaction between sensory systems (visual, vestibular, somatosensory), and can be assessed in both static and dynamic conditions. Approaches to posturography vary from simple bedside clinical tests to computerized systems that can quantify postural control/stability over various static and dynamic conditions (e.g., computerized dynamic posturography; CDP). Though CDP is considered the “gold standard” for posturography, the large size and high cost of CDP has limited the number of centers that are equipped with it.
Considerations for Future Research
An in-depth discussion of the areas within the posturography literature that are in need of further exploration, standardization, and consensus, of which there are many, falls outside the scope of this chapter. However, recent advances within posturography cannot be explored as a topic without acknowledging the need to fill the many gaps within the literature. Comprehensive reviews of these issues have been published in recent years [26, 27].
