VNG/ENG Testing

Introduction

Dizziness is a common complaint but challenging to evaluate due to its complexity, subjectivity, and broad differential. Following a comprehensive history and physical examination, dizziness can often be attributed to the peripheral vestibular system, central nervous system, or cardiovascular system.[1][2] When a peripheral vestibular etiology of dizziness is suspected, videonystagmography (VNG) or electronystagmography (ENG) can be performed to evaluate the dizziness further.

VNG/ENG measures nystagmus, which are rhythmic, rapid eye movements with a slow and fast phase, to provide objective information about the peripheral vestibular system. Nystagmus is named for its fast phase and is described by its rotary, vertical, or horizontal directionality.[3][4] Peripheral vestibular system abnormalities lead to an asymmetry that disturbs the vestibulo-ocular reflex (VOR) pathway, resulting in nystagmus.[4] 

ENG is one of the oldest methods employed in detecting and evaluating abnormal eye movements in a dizzy patient and utilizes electrodes placed above and below the eyes and on the temples to track corneal-retinal potentials recorded with a polygraph tracer. The most significant advantage of ENG over VNG is its efficacy in patients who cannot open their eyes or have significant visual impairment. However, ENG cannot measure rotational or torsional nystagmus and may be negatively impacted by retinal pathology. In current clinical practice, VNG has largely replaced ENG. VNG utilizes magnifying goggles with a computerized tracer to track the center of the pupil; eye movements may be video-recorded during testing. VNG offers increased reliability compared to ENG and can better recognize torsional and subtle nystagmus. However, certain patients, including young children, may be intolerant of the magnifying goggles.[5]

Procedures

VNG and ENG comprise 7 separate but essential tests, including the positioning test, positional test, gaze test, bithermal caloric test, saccade test, tracking test, and optokinetic test. An audiologist typically performs VNG and ENG, but these tests can be conducted by any healthcare provider specializing in hearing and balance disorders.

Positioning Test

The positioning test, or Dix-Hallpike maneuver, is performed by rotating the patient's head 45° laterally from neutral and briskly transitioning the patient from a seated to supine position while maintaining head rotation. This movement should result in neck extension over the end of the examination table with the examiner supporting the patient's head. The patient's eye movements should be observed for at least 20 and as much as 60 seconds. Nystagmus is a positive response to the Dix-Hallpike maneuver. Historically, a positive response would warrant repeating the maneuver to document fatigue of the nystagmus; this is no longer warranted in all circumstances. However, the maneuver should be repeated with the head turned to the opposite side.

Patients without vestibular disease will not demonstrate nystagmus with neck extension while supine.[6] Nystagmus that is geotropic and torsional, fatigueable, associated with vertigo, and lasts less than 1 minute typically indicates benign positional paroxysmal vertigo (BPPV).[7] The direction of the nystagmus coincides with the side affected. Nystagmus that is vertical, nonfatigueable, is not associated with vertigo, and lasts more than 1 minute may indicate a central nervous system disease.[8] The etiology of BPPV is displaced otoconia shifting endolymphatic flow within the semicircular canals. Patients with BPPV typically describe episodic room spinning lasting a few seconds that is triggered by head movements and commonly occurs when rolling over in bed. The Dix-Hallpike maneuver is considered the diagnostic gold standard of BPPV. 

Positional Test

The positional test differs from the positioning test by determining if head positions without head movements induce nystagmus. During the positional test, eye movements are observed for at least 30 seconds with the head in a neutral position while supine and with the head neutrally positioned, turned left, and turned right while seated.[8] A positive positional test is defined as intermittent, persistent, direction-fixed, or direction-changing nystagmus in any head position. A positive positional test does not localize the laterality of dysfunction or differentiate between vestibular and central causes of vertigo. 

Gaze Test

The gaze test assesses for nystagmus in the absence of vestibular stimulation. The gaze test is performed by recording eye movements during primary gaze while looking straight ahead, in addition to looking up, down, 30° to the right, and 30° to the left. Each eye position should be held for at least 30 seconds. Spontaneous nystagmus that does not decrease in intensity with visual fixation (maintaining gaze on a single position) suggests a central system etiology of dizziness.[5]

Caloric Test

Caloric testing evaluates each labyrinth independently through stimulation or suppression of the vestibular system by introducing a warm (stimulation) or cold (suppression) stimulus to the external auditory canal. Thermal transfer across the tympanic membrane changes the density of the endolymph within the ipsilateral labyrinth, causing cupula deflection and resulting in either stimulation or suppression of the vestibular afferent pathway. 

When performing caloric testing, the patient should be supine with the head of the examination table elevated to 30°. This position places the horizontal semicircular canal in the vertical plane; the horizontal semicircular canal is stimulated due to its orientation and proximity to the external auditory canal.[9] Historically, each ear was irrigated with warm water at 44 °C and cold water at 30 °C; air stimulus has largely replaced water stimulus and is required when testing a patient with a tympanic membrane perforation.[10]

In health, cold irrigation induces nystagmus with the fast-beating component moving toward the contralateral ear, and warm irrigation induces nystagmus with the fast-beating component moving toward the ipsilateral ear.[11] A popular mnemonic used to remember the directionally is "COWS" (cold opposite, warm same). Peak slow-phase velocity of the caloric responses is measured, and each ear should elicit a similar response. A unilateral caloric weakness may indicate a peripheral vestibular dysfunction on the side that elicits a weaker response. The inability to suppress caloric nystagmus with visual fixation suggests a central dysfunction. 

Saccade Test

The saccade test tracks nonvestibular eye movements. Saccades are voluntary, quick eye movements that bring images of new stimuli to the fovea. The saccade test is performed by having the patient track a computer-controlled visual stimulus across a horizontal plane by only moving their eyes; latency, velocity, and accuracy are measured.[5] Latency describes the time between the movement of the stimulus and the movement of the eyes. Velocity corresponds to the speed of eye movement when arriving at a new stimulus. Accuracy measures the over- or undershoot of the focus on the stimulus. Deficits in these measurements can signify basal ganglia disorders, supranuclear palsy, and cerebellar disorders, respectively.[8][4]

Tracking Test

The tracking test also evaluates nonvestibular eye movements by having the patient track a computer-generated visual stimulus that moves in a sinusoidal pattern of varying frequency. While tracking abnormalities may indicate vestibulocerebellar impairment, multiple pathways are involved in the generation of smooth pursuit; localizing a lesion is difficult.[5][8][4]

Optokinetic Test

The optokinetic test is a nonvestibular eye movement test similar to the tracking test. However, during optokinetic testing, the patient follows a visual target with their eyes while simultaneously sustaining low-frequency head movements. Many disease processes that cause tracking abnormalities also lead to deficits in optokinetic testing because smooth tracking pathways contribute to generating optokinetic responses.[5][8][4]

Indications

Obtaining a thorough medical history and performing a comprehensive physical examination are frequently all that is necessary to correctly diagnose the etiology of dizziness or vertigo in most patients. However, if the diagnosis remains unclear or confirmation of a diagnosis is required, performing VNG or ENG may be beneficial. These evaluations may also be performed to monitor therapeutic responses or to provide quantitative, reproducible data for medicolegal claims. Neither VNG nor ENG is an indicated assessment for patients with hearing loss without dizziness or vertigo.

Potential Diagnosis

While VNG and ENG provide insight into vestibular pathology, neither typically offers a specific diagnosis. These tests are helpful to determine the site and extent of the lesion and the level of compensation.[12] However, when paired with supplementary information, VNG/ENG can help diagnose the following disease processes:

• Acoustic neuroma
• Benign paroxysmal positional vertigo 
• Vestibular neuronitis 
• Ménière disease 
• Superior canal dehiscence
• Vestibular migraine
• Central vestibular disorder.

Normal and Critical Findings

Positioning Test

In health, the Dix-Hallpike maneuver produces no observable nystagmus. However, in patients with BPPV, right-beating torsional, geotropic nystagmus will be observed when the Dix-Hallpike maneuver is performed on the right, indicating right-sided posterior semicircular canal pathology; when the Dix-Hallpike maneuver is performed on the left, left-beating torsional, geotropic nystagmus indicates left-sided posterior semicircular canal BPPV. Purely vertical nystagmus elicited by the Dix-Hallpike maneuver indicates a central nervous system abnormality. 

Positional Test

Performing a positional test on a healthy patient yields no nystagmus. A positive positional test elicits nystagmus, which may be intermittent, persistent, direction-fixed, or direction-changing. However, the type of nystagmus observed during positional testing does not discriminate between peripheral and central nervous system lesions. Despite this lack of specificity, positional testing remains valuable due to its ability to indicate possible vestibular dysfunction in patients with dizziness or vertigo.

Gaze Test

Gaze testing performed in health elicits no nystagmus. Abnormalities during gaze testing are frequently observed shortly following a unilateral vestibular insult. The direction of the fast phase of the nystagmus typically indicates a peripheral lesion opposite that side. For example, a patient who experiences right-sided horizontal nystagmus while gazing to the right with the head in a neutral position has a left peripheral deficit. Nystagmus that decreases in intensity with visual fixation also suggests a peripheral system cause. Nystagmus that does not decrease in intensity with visual fixation suggests a central cause.

Caloric Test

In a healthy patient, cold irrigation induces nystagmus with the fast-beating component moving towards the contralateral ear, and warm irrigation induces nystagmus with the fast-beating component moving towards the ipsilateral ear. Data collected during caloric testing permits the calculation of the duration, latency, frequency, amplitude, and velocity of the nystagmus. The peak slow-phase velocity yields 4 values:

• RC: right, cold irrigation
• RW: right, warm irrigation
• LC: left, cold irrigation
• LW: left, warm irrigation.

In a healthy patient, the peak slow-phase eye velocity should be the same in both right and left directions. The total response (TR) is the absolute sum of all appropriately directed responses given by the equation,

TR = RC + RW + LC + LW.

A difference in peak slow-phase eye velocity between the right and left sides is calculated using the Jongkee formula,

Unilateral weakness (UW) = [(RC+RW)-(LC+LW)]/TR x 100.

A UW of greater than 25% indicates a unilateral peripheral weakness of the side of the weaker response.[13]

Saccade, Tracking, and Optokinetic Tests

The saccade, tracking, and optokinetic tests measure nonvestibular eye movements. An abnormal response during any of these tests indicates a central lesion. Fixed or random sudden rapid eye movements (saccades), the inability to smoothly track a stimulus, and nystagmus produced by moving objects indicate a central nervous system abnormality. A saccade velocity of less than 275 °/s is considered pathological. The normal range of saccade accuracy is between 80% and 134%. Saccade latency values of greater than 260 ms are considered abnormal. Smooth pursuit is described by gain, defined as the ratio of eye velocity to target velocity; the normal range is defined as 0.9 to 1.0 for target velocities less than 20 °/s.[14]

Interfering Factors

Due to the episodic symptomology of specific vestibular pathologies, such as Ménière disease and BPPV, VNG/ENG may not detect a vestibular abnormality when the patient is asymptomatic, leading to a falsely negative result. Additionally, various medications, including but not limited to benzodiazepines, antihistamines, opioids, and barbiturates, may suppress vestibular responses, leading to a false negative result; patients are advised to discontinue these medications 72 hours before VNG/ENG testing. Lastly, the plethora of equipment utilized during VNG/ENG testing must be properly cleaned, calibrated, and maintained to ensure accurate testing results.[15] 

Complications

VNG/ENG testing is a simple and safe procedure; very few complications are documented in the literature. Dizziness, nausea, and vomiting may be induced by testing; these adverse effects typically resolve within minutes to hours. To limit these adverse effects, patients should be advised to eat a light meal before testing and preemptively arrange for safe transportation upon leaving the testing site.[16]

Patient Safety and Education

Approximately 90 minutes are required to complete the VNG/ENG testing procedures. Patients should be instructed to discontinue medications that can interfere with the procedure 72 hours before testing, including antihistamines, tranquilizers, sedatives, antidepressants, muscle relaxants, aspirin, opioids, and barbiturates. Alcohol and marijuana should not be consumed in the 72 hours before testing. Mascara and contact lenses may interfere with the procedure and should be removed before testing.

Clinical Significance

VNG and ENG testing are tools used in assessing and diagnosing disorders related to the vestibular system and, as such, can play a significant role in clinical decision-making. These tests help evaluate the function of the inner ear and vestibular pathways, objectively documenting peripheral vestibular function and detecting central deficits not otherwise apparent during physical examination. The results obtained from VNG and ENG testing can guide the interprofessional healthcare team in developing effective treatment strategies tailored to the specific vestibular disorder diagnosis. For example, a patient with hearing loss who undergoes VNG or ENG testing demonstrating unilateral vestibular weakness may benefit from magnetic resonance imaging to evaluate for underlying acoustic neuroma.[2][17] Contrarily, VNG or ENG testing results that suggest a central nervous system lesion may benefit from consultation with a neurologist. 

VNG and ENG testing are also valuable tools in monitoring the progression of vestibular disease and response to therapeutic interventions. These noninvasive tests enable targeted interventions and improve patient care through disease-specific treatments.

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Disclosure: Gabriele Noreikaite declares no relevant financial relationships with ineligible companies.

Disclosure: Ryan Winters declares no relevant financial relationships with ineligible companies.

Disclosure: Carl Shermetaro declares no relevant financial relationships with ineligible companies.