By Marcello Cherchi, MD PhD
For patients
The body’s balance system is responsible for controlling certain eye movements, so the study of eye movements can sometimes reveal information that is helpful in understanding and diagnosing conditions that affect a person’s equilibrium. This is why, even if you do not have a “visual problem,” your doctor may still suggest checking one or more studies of eye movements.
For clinicians
Humans are visual creatures. About half of all the brain’s pathways are related in some way to vision (Patil et al. 2023). Consequently, the body provides significant supportive mechanisms for vision to execute its tasks well.
Among these supportive mechanisms for vision is the vestibular system. There are several reasons for this. First, in order for the visual system to process stimuli effectively it needs to maintain an object of interest on the fovea — and it needs to stay on target despite perturbations from head/body movement. Second, although the visual system is wonderfully capable, in some respects it is slow; for example, it takes nearly 100 msec for information from the retina to reach the visual cortex in the occipital lobe, which is far too slow for a visual tracking system to function effectively. Third, the vestibular system is fast, and easily capable of providing information about rotational and linear acceleration — information which is helpful in reorienting the eyes in response to movement of the head and body. Thus, the vestibular system’s advantage (speed) can overcome the visual system’s deficiency (slowness) to mediate eye movements that achieve one of the goals of vision (foveation).
For this reason, a substantial proportion of the vestibular system’s circuitry is devoted to eye movements. Eye movements can serve as a proxy for understanding some (not all) of the vestibular system’s function and dysfunction. The valuable tome by Leigh and Zee (Leigh and Zee 2015), and a growing literature regarding eye movements, are a testament to this.
Some of our tools for evaluating eye movements are mature technology. Other tools are more primitive. New tools and techniques will probably emerge that will advance our understanding of the vestibular system — and there is plenty of work remaining to be done (Cherchi and Yacovino 2021).
There are several well-established methods for studying eye movements, to which entire textbooks have been dedicated (Duchowski 2003). Several of these are relatively common in clinical practice. Some methods are becoming obsolescent (such as electronystagmography). Others remain restricted to research settings (such as scleral search coils). Each system has advantages and disadvantages.
- Electronystagmography (ENG). This was the earliest available method. It is gradually being replaced by videonystagmography (VNG).
- Videonystagmography (VNG). This is currently the most commonly employed clinical method for quantification of eye movements.
- Video Frenzel oculography (VFO). This is similar to VNG, but only includes the “raw video” (without any computational post-processing to quantify eye movements), and thus is used for bedside testing.
- Video head impulse testing (vHIT). This shares some characteristics with VNG, but uses faster frame rates of image acquisition because it is intended to capture very rapid eye and head movements.
- Magnetic scleral search coils. This is the most sophisticated and accurate method of recording eye movements, but is cumbersome and restricted to research settings.
Note that although the conventional terms “videonystagmography” (VNG) and “electronystagmography” (ENG) imply that these tests only measure nystagmus, in fact they evaluate many aspects of oculomotor function beyond simply patterns of nystagmus; nevertheless, we will continue to use the conventional terms VNG and ENG even though their utility extends beyond what the names suggest.
We have chosen to include discussion of ENG because it probably remains the most common instrumented oculomotor test worldwide as it is less expensive (albeit technically more demanding), though it should be noted that in many countries it is rapidly becoming obsolete as it is supplanted by VNG which is easier to perform (albeit more expensive). ENG and VNG, while not identical (Pietkiewicz et al. 2012), overlap significantly in their ability to detect pathology (Bielinska et al. 2018; Gananca et al. 2010).
References
Bielinska M, Polaczkiewicz A, Pietkiewicz P, Milonski J, Olszewski J (2018) Concurrent ENG and VNG recording in healthy people – preliminary report. Otolaryngol Pol 72: 45-49. doi: 10.5604/01.3001.0011.7251
Cherchi M, Yacovino DA (2021) Dysfunction along the continuum of vestibulocochlear anatomy, and the corresponding spectrum of clinical presentation: how little we know, and what else we need to learn. Hearing, Balance and Communication: 1-12. doi: 10.1080/21695717.2021.1975984
Duchowski AT (2003) Eye tracking methodology : theory and practice. Springer, New York
Gananca MM, Caovilla HH, Gananca FF (2010) Electronystagmography versus videonystagmography. Braz J Otorhinolaryngol 76: 399-403.
Leigh RJ, Zee DS (2015) The neurology of eye movements, 5th edition. edn. Oxford University Press, Oxford ; New York
Patil SA, Grossman S, Kenney R, Balcer LJ, Galetta S (2023) Where’s the Vision? The Importance of Visual Outcomes in Neurologic Disorders: The 2021 H. Houston Merritt Lecture. Neurology 100: 244-253. doi: 10.1212/WNL.0000000000201490
Pietkiewicz P, Pepas R, Sulkowski WJ, Zielinska-Blizniewska H, Olszewski J (2012) Electronystagmography versus videonystagmography in diagnosis of vertigo. Int J Occup Med Environ Health 25: 59-65. doi: 10.2478/s13382-012-0002-1
