||Bryan Ward, MD, Department of Otolaryngology - Head & Neck Surgery, Division of Neuro-otology, The Johns Hopkins School of Medicine; Daniel R. Gold, D.O. Departments of Neurology, Ophthalmology, Neurosurgery, Otolaryngology - Head & Neck Surgery, The Johns Hopkins School of Medicine
||Electronystagmography (ENG), and videonystagmography (VNG) or videooculography (VOG) are a collection of tests of eye movements that are performed either using surface electrodes around the eye (ENG) or with video goggles (VNG, VOG) (Figure 1). For all clinical intents and purposes, VNG and VOG are equivalent and these terms are commonly used interchangeably. While VNG and ENG typically refer to a battery of ocular motor/vestibular tests including calorics, VOG is a term that may be used more generically for all video eye tracking. For the remainder of this section, ‘VNG' can be used to refer to VOG as well. ; ; Regardless, these methods all allow the recording of the position of the eye over time and are useful for quantitatively evaluating eye movements and nystagmus. The tests differ only in the use of the recording technique. ENG relies on the presence of a corneo-retinal dipole to produce a tracing as the eye moves left and right or up and down. VNG uses infrared lighting and video cameras to track and record the position of the eye's pupil over time by thresholding the image to accentuate the darkest object on the image (i.e. the pupil) (Figure 2). ENG cannot provide information on torsion of the eye; newer VNG systems are now providing an assessment of torsion, but can be unreliable. ; ; Both ENG and VNG typically include a series of tests that commonly includes measuring ocular motor function (saccades, spontaneous nystagmus with and without visual fixation, gaze holding, smooth pursuit), as well as the effect of head position on eye movements, and the caloric test (see separate Caloric Testing section). Other provocative testing can be added to this battery such as assessing response to optokinetic stimuli, head-shaking, hyperventilation, pressure applied to the ear canal (Hennebert's sign, if nystagmus and vertigo are evoked), or loud sounds (Tullio phenomenon, if nystagmus and vertigo are evoked). Participants are seated or lying down during testing. The testing requires an examiner, but is semi-automated, systematically progressing through the tests, typically using a commercial software system. The first test is an assessment of spontaneous nystagmus in light and in darkness (Figure 1). A peripheral vestibular nystagmus can be suppressed in light with fixation, whereas central peripheral or other varieties of congenital or acquired nystagmus may not change or may even intensify in light with fixation. Participants will follow a moving target (laser point or light-emitting diode) for the saccade, smooth pursuit, and gaze testing. The targets will move at different velocities and amplitudes and the accuracy of the eye movements are assessed by tracking the movements of the eyes relative to the target. Laboratory standards are used to determine the accuracy and timing of saccades and pursuit, and the results are provided in graphical format (see Figures 3 and 4 for examples). The effects of head position on nystagmus is commonly performed, as is Dix-Hallpike testing, assessing for benign paroxysmal positional vertigo (BPPV). While these tests can be performed as part of the bedside exam, the recording equipment provides a quantitative assessment. This testing is also usually performed in conjunction with caloric testing, which requires quantified eye movements. Several medications can interfere with ENG/VNG testing, particularly vestibular suppressants like benzodiazepines and antihistamines (meclizine, dramamine), and it is usually recommended that these drugs be stopped for at least 24 hours prior to testing.