Electronystagmography (ENG)/Videonystagmography (VNG)
OVERVIEW
Electronystagmography (ENG)/videonystagmography (VNG) utilizes two-dimensional (2D) eye movement recordings to indirectly examine the following: (1) peripheral vestibular function (e.g., low frequency horizontal semicircular canals (SCCs) and afferent pathway), (2) status of central compensation, and (3) central vestibulo-ocular pathway function.
• ENG: utilizes bitemporal electrode array. Electrodes are placed at outer canthus of each eye (channel one), above and below one eye (channel two), and ground electrode (forehead) to record changes in corneoretinal potential (CRP) with eye movements. The CRP signal is derived from an electrical charge difference between the cornea (positive charge) and the retina (negative charge). Monocular electrode array (channel one and channel two electrodes around one eye) is used in cases of disconjugate eye movements.
• VNG: utilizes infrared goggles with infrared diodes and mirrors to reflect the eyes to track pupil position and angle of gaze (Figure 5–1). VNG goggles typically have two cameras, but monocular recording may be performed.
With the 2D systems, eye movement signals are translated into a representation of horizontal and vertical channel eye movements, but not torsional (Table 5–1).
Figure 5–1. Model wearing infrared binocular infrared VNG goggles.
Table 5–1. Two-Dimensional (2d) Eye Deflection Patterns and Nystagmus Interpretations
Note: Oblique eye movements include eye deflections of both horizontal and vertical channels (example: leftward and upward eye movements are shown as a downward deflection of the horizontal channel and an upward deflection on the vertical channel).
ENG/VNG is a test battery that includes oculomotor testing followed by screening measures for benign paroxysmal positional vertigo (BPPV; see Chapter 4), positional nystagmus tests, and finally caloric irrigations. When BPPV is suspected, it is recommended to start with BPPV screening measures (Bhattacharyya et al., 2017). Additional tests may be included in the battery such as headshake, hyperventilation, and mastoid vibration testing (discussed in Chapter 3). This chapter will focus on the standard procedures within the ENG/VNG: (1) oculomotor examination, (2) positional nystagmus testing, and (3) caloric irrigations.
Patient Preparation
The following steps are recommended for general patient setup:
1. Verify compliance with pretest instructions. Medications for control of nausea and vertigo may suppress vestibular responses and alcohol may cause abnormal positional nystagmus. McCaslin (2013) stresses review of the patient’s current medication list to identify adverse effects on central and peripheral vestibular function:
• Anticonvulsants: brainstem and cerebellar signs, sedation
• Antidepressants and benzodiazepines: brainstem and cerebellar signs, sedation, inhibition of central compensation
• Antivertigo, nausea, and antihistamine medications: sedation effect, inhibition of central compensation
• Aminoglycosides and chemotherapeutics: ototoxic, neurotoxic, and vestibulotoxic agents
• Antihypertensives: presyncope with position changes; orthostatic hypotension
2. Perform otoscopy. Anatomic abnormalities and tympanic membrane perforations present challenges performing caloric irrigations. Obstructive cerumen should be removed prior to caloric irrigations.
3. Complete bedside testing (see Chapter 3) to verify full range of motion of extraocular muscles and conjugate eye movements. Performing bedside tests assists in understanding how the patient follows instructions and may provide the examiner with areas of focus to quantify during objective testing.
4. Position the patient on the examination chair or table and confirm that the patient is sitting at the correct distance from the visual target-based equipment settings. If the patient is too close or too far, resultant faulty calibration may lead to over/underestimating eye movements during testing.
5. Clean the appropriate electrode sites with an alcohol wipe or other solution (e.g., NuPrep with gauze pad) when performing ENG and place electrodes.
6. Ask the patient to remove all eye make-up prior to VNG testing.
7. Place the infrared VNG goggles on the patient and adjust the cameras to focus on the pupils. Ensure that the eye(s) are centered horizontally, and the pupil crosshairs and/or threshold are adjusted to adequately track the pupil(s) during testing. If the patient is visually impaired (e.g., blind) accurate calibration may not be possible, and only inferences about vestibular function (caloric irrigations) can be determined during testing (McCaslin, 2013).
8. Turn off or dim lights in the room prior to testing; verify absence of light for fixation removed testing (i.e., place cover over goggles).
9. Instruct the patient on the calibration procedure (e.g., saccade task). Re-instruct the patient and repeat calibration if necessary; use the default calibration option if the patient is unable to achieve acceptable calibration. Frequent calibration is needed when performing ENG due to changes in CRP over time.
Operator Preparation
The examiner should be positioned at a workstation adjacent to the ENG/VNG equipment, in a room that is large enough to adequately move around the patient. The use of a remote control allows the operator to stay near the patient at all times for safety reasons. The operator should instruct the patient on: (1) the different parts of the test battery, (2) mental tasking during testing to improve the quality and strength of the VOR response (see Appendix B), and (3) the necessity to report any symptoms provoked during testing. If breaks are needed, calibration may need to be repeated. The operator must ensure the following for optimal data collection:
• Eye traces are centered and patient’s eyes are still (i.e., not wandering around). Note: Keep in mind Alexander’s law during testing (i.e., if the patient’s line of gaze is in the direction of nystagmus beat, it will enhance the response). The patient’s line of gaze should be in center gaze position (looking straight ahead) unless instructed to follow a visual target or change their line of gaze during oculomotor procedures.
• Thresholds are adjusted and crosshairs are stable to minimize artifact in eye movement recordings.
• The patient is alert during the procedure with a minimum of eye blink contamination.
• The patient’s head is stable and upright.
Finally, Shepard, Shubert and Eggers (2021) stress that the operator must watch the eye movement videos rather than solely relying on nystagmus tracings to report presence of fine nystagmus and to observe torsional nystagmus. All nystagmus collected should be reviewed to determine presence of slow phase velocity of nystagmus, and artifact (e.g., blinks) must be removed.
TEST PROTOCOLS, ANALYSES AND INTERPRETATIONS
Oculomotor Examination
ENG/VNG examination begins with examination of oculomotor control when the head is still. This portion of the examination is imperative to screen for central (i.e., brainstem/cerebellum) involvement and for aid interpreting peripheral vestibular findings and physiologic responses (e.g., caloric irrigations).
During the saccade test, the patient is asked to follow a visual target (e.g., red light) moving eyes only and not anticipate the target movements. A random paradigm of target movements with +/− 30° range of subtended arc about the horizontal or vertical plane is used to elicit reflexive saccades (Shepard et al., 2021). The saccade procedure of choice is individual eye recording to evaluate for disconjugate eye movements such as internuclear ophthalmoplegia (INO; Figure 5–2). Typically, reflexive saccades are performed during a 30-second recording (or more if necessary) and the parameters of velocity, accuracy and latency are analyzed.
• Velocity: plot of peak to peak velocity of the eye movement during the trajectory from point to point
• Accuracy: the percentage of the distance the eye moved in its first single movement relative to that of the target as a function of eye movement excursion
○ Values = 100% indicates that the eyes moved the same distance as the target in a single major excursion
○ Values > 100% indicate that eye movements overshot the target
○ Values < 100% indicate that the eye movements undershot the target
• Latency: represents the lapse in time (milliseconds) from initiation of the target to the initiation of eye movement to the new target
Figure 5–2. Saccade test results demonstrating bilateral INO. Top panel shows sample raw eye tracings. Bottom panels show individual eye performance for velocity, accuracy, and latency, respectively. Significantly reduced velocity and borderline abnormal accuracy observed for left and right eye adduction movements (dark gray dots in shaded region). Normal latency values shown for both eyes.
Normal performance is indicated when 50% or more of the recorded saccades fall within normal region for velocity, accuracy and latency (Shepard et al., 2021). Abnormalities should be repeatable and consistent (McCaslin, 2013); repeat testing is required if any of the saccade parameters are outside of the normal range. Consistent abnormalities may relate to brainstem and/or cerebellar involvement (Table 5–2). Age-related normative data is not necessary for routine clinical analysis.
Smooth Pursuit Test
During the smooth pursuit test, the patient is asked to strictly follow horizontal sinusoidal movements of a visual target with eyes only. Practice may be required to follow rather than anticipate the target movements. The target’s peak excursion is fixed at 15–20° subtended arc and increases in frequency from ~0.2–0.6 Hz (Shepard et al., 2021). Individual eye recording is not as clinically revealing as saccade evaluation; however, it is of clinical importance to compare performance to age-referenced normative data, which is provided with most computerized systems (Shepard et al., 2021). The outcome parameters of interest include gain, asymmetry, and observation of saccadic pursuit.
• Gain: comparison between sinusoidal eye movements and target movements. The gain value usually is described as the peak eye velocity/peak target velocity
• Asymmetry: percentage difference in velocity gain for left or right eye moving rightward and leftward.
Table 5–2. Abnormalities of Saccade Parameters
Note: Modified from Balance Function Assessment and Management, Third Edition (pp. 199) by Gary P. Jacobson, Neil T. Shepard, Kamran Barin, Robert F. Burkard, Kristen Janky, and Devin L. McCaslin. Copyright © 2021 Plural Publishing, Inc. All Rights Reserved.
INO = internuclear ophthalmoplegia, MLF = medial longitudinal fasciculus, PPRF = paramedium pontine reticular formation, RIMLF = rostral interstitial nucleus of the medial longitudinal fasciculus
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