Читать книгу Vestibular Disorders - Группа авторов - Страница 43

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.