Screening tests for objective diagnoses
What tests should be used for screening and for objective diagnostic testing?
Most but not all tests assess the VOR. In the clinic the head impulse test, in which the head is rapidly rotated 20° to either side while the patient fixates on a central target, might show an abnormality.
Look for one or two compensatory saccades. The test is only reliable if the patient is elderly, can relax enough to allow the clinician to perform the test, and has a large decrease in VOR function.
Head shaking might be useful to elicit vertigo, but it has not been shown to be reliable.
The Dix–Hallpike maneuver is pathognomonic for BPPV, if the patient has a classical pattern of nystagmus beating upward in pitch, and ipsilaterally in horizontal and roll directions.
For a patient who cannot perform the Dix–Hallpike maneuver, side lying may be used instead. For either test, expect to see a delay of a few seconds followed by a burst of nystagmus lasting several seconds.
Tandem walking for 10 steps with eyes open is not reliable; tandem walking for 10 steps with eyes closed might be useful if the patient has no other neurologic deficit, peripheral neuropathy, musculoskeletal deformities, or weakness. Standing balance testing with the Romberg test, with eyes closed on an unstable surface such as compliant foam, is reliable, unless the patient has peripheral neuropathy, musculoskeletal deformities, or weakness.
Patients under the age of 55 years should be able to perform the test for 25 seconds. Patients aged 60 to 79 years should be able to perform the test for 9.5 seconds. Patients aged 80 years and older should be able to perform for 4 to 5 seconds.
The most reliable screening test, however, is a history of true vertigo, as opposed to lightheadedness, nausea, or some other form of dizziness.
The objective diagnostic battery of tests assessing the vestibular system, often referred to as ENG for the original recording technique of electro-oculography, includes cervical vestibular evoked myogenic potentials (C-VEMP), ocular evoked myogenic potentials (O-VEMP), low-frequency rotational tests in darkness in a rotatory chair, Dix–Hallpike maneuvers, and bithermal caloric tests. In C-VEMP a tone is played in the ear while the ipsilateral sternocleidomastoid muscle is recorded with electromyography as it is stressed by lifting the head against gravity or pushing against a hand.
A very slight relaxation of the muscle should be observed in response to sound as indicated by the response at the p13–n23 waveform. If the patient is responsive to sound at a very low threshold, however, the response might be consistent with superior canal dehiscence.
VEMP is also considered a test of saccular function. The absence of a C-VEMP response might be consistent with loss of inferior vestibular nerve function, if a response is absent.
This response is inhibitory, uncrossed, descending, and sacculocollic.
In O-VEMP electrodes are placed on the face around the eyes and are used to measure a tiny response to auditory stimulus or tap in the contralateral eye muscles, primarily the inferior oblique, as the patient looks upward to stress the eye muscles. The response is seen at n10.
This response is excitatory, crossed, ascending, and utriculo-ocular. Impaired O-VEMP responses may indicate superior vestibular nerve impairment or central impairment.
The uses of these evoked potentials are still being worked out.
In the rotatory chair the VOR and other eye movements—saccades, smooth pursuit, and optokinetic nystagmus—are usually recorded with VOG. If, however, VOG is not available or the patient cannot tolerate the goggles, ENG can be used.
Saccades, pursuit, and optokinetic responses are used to test the integrity of the brain stem and cerebellar mechanisms that control eye movements including the VOR. Impaired saccades and other eye movements may be an early indicator of multiple sclerosis.
These eye movements and the VOR should be conjugate. Disconjugate eye movements may indicate internuclear ophthalmoplegia or some other central deficit.
Patients should also be observed for the presence of spontaneous nystagmus and gaze-evoked nystagmus.
The rotatory chair can be used for low- or high-frequency testing. It is most comfortable and least disturbing for the patient when used for low-frequency testing. The VOR is linear in the range of 0.1 to 7.0 Hz.
Therefore, testing is usually performed below and above 0.1 Hz, such as 0.0125, 0.05, and 2.0 Hz with sinusoidal stimuli ±60°. Higher-frequency tests with steps of velocity, rather than sinusoids, are useful when bilateral vestibular impairment is suspected.
Patients who are claustrophobic may become stressed during testing. Therefore, technical staff must be supportive but firm in convincing such a patient to complete the test.
Other patients may become drowsy. For that reason patients are usually asked to play word games during testing to maintain consistency of eye movements.
For objective diagnostic testing Dix–Hallpike maneuvers should be performed using VOG or ENG and recorded on the computer for later analysis.
Other positional tests such as side lying can also be performed.
Bithermal caloric testing uses changes in the temperature of the external ear and tympanic membrane to cause a change in the temperature of the middle ear and eventually inner ear, causing flow of the endolymph in one labyrinth only.
Caloric testing is highly reliable in detecting unilateral peripheral vestibular loss. The alternate binaural bithermal caloric test is usually performed while the patient is lying supine with the neck flexed 30° to bring the lateral canal into the earth vertical position.
Then, each lateral canal can be tested separately. Typically irrigations are performed at 30° C and 44° C in each ear, using water for the best stimulus, or air if the patient has a perforated tympanic membrane.
Cold and warm caloric stimuli elicit opposite current flows and therefore opposite responses. Remember the side of stimulation and the response using COWS: cold, opposite, warm, same.
So, for example, left cold irrigation elicits right-beating nystagmus and left warm irrigation elicits left-beating nystagmus. To perform the test correctly the patient must have all four irrigations.
The four measures are combined in the Jonkees formula, shown below, to calculate relative weakness of one side using the velocity of the slow phase of nystagmus, which represents the VOR, where RC, RW, LC, and LW indicate the peak slow-component velocity of nystagmus from right cool, right warm, left cool, and left warm irrigations.
100×[(LC+LW)−(RC+RW)/(LC+LW+RC+RW)]=% caloric paresis100×[(LC+LW)−(RC+RW)/(LC+LW+RC+RW)]=% caloric paresis100×[(LC+RW)−(RC+LW)/(LC+LW+RC+RW)]=% directional preponderance100×[(LC+RW)−(RC+LW)/(LC+LW+RC+RW)]=% directional preponderance
A weakness of 25% or greater is usually considered to be abnormal and indicative of impaired peripheral vestibular function.
Directional preponderance of greater than 30% indicates an abnormal asymmetry between right-beating nystagmus (right warm and left cool) and left-beating nystagmus (left warm and right cool) evoked by the caloric irrigations.
Directional preponderance is a nonspecific sign of vestibular dysfunction.
A total eye velocity of less than 30° may indicate a bilateral impairment or loss. The bithermal caloric test response approximates the low-frequency response in the rotatory chair, to frequencies below 0.1 Hz, but the two tests elicit different characteristic patterns of nystagmus and should be used to complement each other.
The disadvantage of caloric testing is interindividual variability of caloric vestibular responses.
Computerized dynamic posturography measures the integrity of the vestibulo-spinal tracts and the use of visual–vestibular interaction.
It is not a purely vestibular test but may give some insight into the patient’s status and may be informative if a patient has intact VOR responses, such as an elderly person who is subsequently diagnosed with presbystasis.
The test battery, which is the computerized version of the clinical Romberg test, has the patient stand on a movable force platform that measures the change in postural sway under the patient’s feet.
The patient is tested under six conditions: eyes open (control condition), eyes closed with stationary force platform, eyes open but with movement of the visual surround in phase with postural sway (sway referenced) to “fool” the system and make visual information useless, eyes open with sway-referenced force platform motion, eyes closed with sway-referenced force platform motion, and eyes open with sway-referenced motion of the force platform and of the visual surround.
In normals performance decreases across the different conditions. In patients significantly decreased performance on conditions five and six is most likely to be consistent with a vestibular impairment.
Normal performance on conditions five and six but abnormal performance on conditions one or two might be consistent with malingering.
CT scans (high resolution fine cuts of temporal bones) are essential for diagnosis of superior canal dehiscence syndrome but not useful for diagnosis of other disorders.
MRI is the gold standard test for diagnosis of acoustic neuroma, multiple sclerosis, cerebrovascular infarct or hemorrhage, intracranial tumor, and cerebellar or brain stem degeneration.
Other peripheral vestibular disorders are not diagnosed with imaging technologies.