This study is sued to evaluate patients with dizziness and vertigo. This test provides an objective assessment of the oculomotor and vestibular systems. The vestibular system monitors the position and movements of the head to stabilize retinal images. This information is integrated with the visual system and spinal afferents in the brain stem to produce vestibulo ocular reflex.
Standard ENG test battery consists of three parts:
1. Oculomotor evaluation
2. Positional / positioning testing
3. Caloric stimulation of vestibular system
ENG is the most widely used lab test to assess vestibular function. A normal ENG do not mean that the patient has a normal vestibular function. ENG cannot be used to determine the specific site of the lesion, but it can be used along with the history, symptoms and other test results to aid in the diagnosis. ENG evaluation can be useful in ruling out potential causes of dizziness. Only a third of patients who underwent EEG could be diagnosed with the site of the lesion.
Otoscopic examination should be performed before ENG is started. Presence of discharge / perforation of ear drum are contraindications for ENG. Presence of fluid in middle ear limits effective stimulation of labyrinth.
Many medications can affect test results. Patient should discontinue all medicines unless contraindicated a couple of days before the test.
ENG equipment varies from basic to advanced. Advanced versions provide computerized stimulus genertion, acquisition and interpretation. The basic models use a strip recorder for acquisition of responses.
Oculomotor stimulus generation can be accomplished with manually controlled objects. Gaze and saccadic testing can be performed using dots placed on the wall / ceiling at specified places (10, 20 and 30 degrees in each direction). Optokinetic testing can be conducted with a rotating drum with stripes of contrasting colors and smooth pursuit can be testing using a swinging pendulum.
For this purpose the physician can choose to use only one channel to evaluagte horizontal eye movements, but a minimum of 2 channels are recommended. If three electrodes are used then horizontal and vertical movements of the eye can be recorded.
Eye movements can be measured using potential difference between cornea and retina. Cornea is relatively electropositive to the retina. Electrodes can be placed around the patient's eyes to record corneal - retinal potential differences. By placing electrodes on both horizontal and vertical axis around the eyes, tracings are produced of the eye movements on both the axes.
In video nystagmography horizontal and vertical tracings of eye movements can be recorded by a camera tracking the pupil fo the eye. The camera uses infrared technology, these tracings can be made with the patient is complete darkness thereby eliminating any visual fixation points. The system may use a camera for one or both eyes. Main advantage of VNG is that the eye movements can be recorded and stored so that the physician can study it in leisure.
The visual system provides information about whether the environment is stationary or mobile. Stabilization of visual information is made possible by foveation. The oculomotor portion of ENG assesses eye movement function for various command eye movements in the absence of vestibular stimulation. For evaluation of oculomotor function, the patient is presented with various visual stimuli while the eye movements are recorded.
Saccades / Calibration test:
This test evaluates the saccadic eye movement system. Saccadic system is reponsible for rapid eye movements and refixation of the target to the fovea. For this test dots are placed on the wall / ceiling at specified distances from each other (10, 20 and 30 degrees off the centre) and the patient is instructed to look back and forth between the dots while the head is kept fixed. The rapid movements of the eye is recorded. Modern ENG system have computer generated stimuli.
Hypermetric saccades - In some patients it may be noted that eye overshoots the target and is followed by a correction. This is known as hypermetric saccades. Patients who exhibit hypermetric saccades may have ocular dysmetria which is suggestive of CNS lesion at the level of cerebellum.
Hypometric saccades - In this type patient undershoots the target. Sometimes this undershooting could be normal. It should be reproducible and must occur frequently for it to be considered abnormal. Hypometric saccades are seen in basal ganglia pathology.
Multistep saccades - This occurs when a patient undershoots the target and then attempts to correct it with multiple smaller saccades. This is seen in patients with CNS pathology.
Ocular flutter - in this type there is spiky overshoot. The patient overshoots the target several times with a short duration between overshoots. This condition is seen in CNS pathology.
Latency disorders - This is due ot prolongation of saccades. Saccades characterised by short latency are due to an artifact or due to the ability of the patient to anticipate the position of the target. If there is prongation of latency then patient's inattention should be ruled out. Prolongation of more than 400 ms in attentive and co operative patients could be suggestive of CNS lesions.
Asymmetrical latencies can occur in patients with lesions in the occipital or parietal cortex. In these patients saccades in one direction may be normal while there could be prolongation of saccades in the opposite direction.
Velocity issues - If saccadic slowing is observed then drowsiness / drug effects should be ruled out. Saccadic slowing is also associated with oculomotor weakness, degenerative conditions and basal ganglia pathology.
Gaze testing - This is conducted to evaluate the presence of nystagmus in the absence of vestibular stimulation. Three parameters are studied:
1. Presence / absence of spontaneous nystagmus
2. Presence / absence / exacerbation of nystagmus with addition of off centre gaze tasks to stress the system
3. Fixtion suppression of spontaenous nystagmus
In gaze testing the patient is instructed to look straight ahead and then fixate on a target 30 degrees to the right, left, up and down. Fixation is maintained for approximately 30 seconds in centre gaze and 10 seconds in eccentric gaze.
For evaluation of spontaneous nystagmus, eliminating possiblity of suppression is important. Fixation suppression can be eliminated by having the patient's eyes open in the dark room with Frenzels glasses used to eliminate optic fixation. When electrode based systems are used then spontaneous nystagmus may be evaluated with the patient's eyes closed. Patient can also be distracted byt asking them to participate in mental taks (arithmetic calculations etc).
Square wave jerks - This is the most common abnormality found with eyes closed (in the absence of optic fixation). Healthy person may also exhibit square wave jerks. The frequency of square wave jerks increase with age. In young patients it may be considered abnormal if they occur frequently indicating cerebellar disorder.
Presence of spontaneous nystagmus may indicate a central / peripheral pathology. Presence of nystagmus with visual fixation is always diagnostically significant.
Indicators for central pathology include:
1. Vertical nystagmus
2. Nystagmus suppressed by visual fixation
3. Non direction changing nystagmus
4. Nystagmus exacerbated by gazing in the direction of the fast phase
Unilateral gaze paretic nystagmus - appears with eccentric gaze in one direction. Elicited nystagmus beats in the direction of the gaze. This is consistent with CNS pathology.
Bilateral gaze paretic nystagmus - When the patient gazes to the right, nystagmus is elicited that beats to the right; when the patient gazes to the left then left beating nystagmus occurs. This pattern suggests CNS pathology.
Bruns nystagmus - is a combination of unilateral gaze paretic nystagmus and vestibular nystagmus which is evidenced as nystagmus in both directions of a gaze that is asymmetrical. Bruns nystagmus is associated with extra axial mass lesions on the side of the gaze paretic nystagmus.
Congenital nystagmus - has a spiky appearance and increases with lateral gaze. Congenital nystagmus may decrease in velocity or completely disappear in the absence of visual fixation.
Rebound nystagmus - This is characterized by a burst of nystagmus that lasts for 5 seconds and begins when the eyes return to the centre gaze. If present brain stem / cerebellar lesions should be suspected.
Smooth pursuit tracking:
This is useful in following targets within the visual field. Tracking can be evaluated both horizontally and vertically. Vertical tracking as a rule is not as smooth as the horizontal even in healthy subjects. When interpreting this in extremes of age care should be taken.
In smooth pursuit tracking the patient is instructed to follow a sinusoidal moving target. The target can be a pendulum, or computer generated stimulus.
For this the patient must track multiple stimuli. These may take the form of stripes on a rotating drum or a stream of lighted dots across the light bar. Stimuli can be moved at a rate of 300, 400, or 600 per second in each direction. Eye movements that are generated by moving fields resemble nystagmus. Symmetry of response should be looked for. If responses are not symmetrical, CNS pathology may be suspected.