Posted by: Thixia | June 23, 2008

Dizziness, Vertigo, and Imbalance Section 4 of 8

VESTIBULAR DIAGNOSTIC TESTS

 

Evaluation of the patient with dizziness begins with careful history taking and complete physical examination, including vestibular examination.  In the course of evaluating patients with vestibular and balance disorders, additional tests that are commonly considered include audiometry, vestibular tests, blood tests, CT, and MRI.  These tests, especially vestibular tests, must be tailored according to the history and physical findings. 

The yield of MRI in patients younger than 50 years is low ( <1%).  The incidence of an acoustic tumor or other brainstem and posterior-fossa lesions also are low.  Clinical judgment, careful neurotologic examination, and audio and vestibular studies are often helpful in obviating MRI.

Of importance, results of these tests are not diagnostic in the medical sense.  For example, unilateral vestibular loss can be due to vestibular neuronitis or an acoustic tumor.  Therefore, clinicians must avoid the temptation to interpret the results as indicating pathologic entities.  Physicians who are responsible for the medical interpretations of these results should also have the proper training and background in neurophysiology and electrophysiology to be able to use these results effectively.  They also must be aware of the limitations and variability inherent in such tests.

The most common vestibular tests are ENG, rotating chair test or sinusoidal harmonic acceleration (SHA), and computerized dynamic posturography (CDP).

ENG testing

The standard ENG test battery is composed of saccadic, gaze, pursuit, optokinetic-eye movement, head-shake nystagmus, positional nystagmus, positioning nystagmus, and bithermal caloric tests.

Saccadic test

The saccadic test is used to evaluate voluntary fast-eye movements.  The neural substrate of the saccadic system includes the frontal eye fields, brainstem reticular formation, oculomotor nuclei, and cerebellum.  The test should be performed by recording each eye separately, especially if dysconjugate eye movements are suspected.  A single-channel saccadic test does not provide meaningful clinical information and should be used only as a calibration signal for horizontal eye movements.  Common saccadic abnormalities include dysmetria, slow saccadic velocity, and dysconjugate saccades.

Gaze test

The gaze test is used to evaluate the ability to generate and hold a steady gaze without drift or gaze-evoked nystagmus.  The neural substrates of the gaze system are similar to those of the saccadic system.  A direct current ENG recording is used to distinguish electronic from pathologic drift.  The most common abnormalities detected by the gaze test are gaze-evoked nystagmus and rebound nystagmus due to cerebellar disease.

Pursuit eye-movement test

 

Pursuit eye movements prevent slipping of an image on the retina while the patient is tracking moving objects.  The neural substrate of the pursuit system includes parietal cortex, brainstem reticular formation, cerebellum, vestibular nuclei, and oculomotor nuclei.  Pursuit abnormalities occur with brainstem and cerebellar lesions.

Test for optokinetic nystagmus

Optokinetic nystagmus (OKN) is a complex CNS reflex initiated by moving images on the retina.  OKN supplements pursuit and vestibular eye movements to stabilize retinal images during constant-velocity head motion.  The cortical origin is the parietal lobes.  Vestibular nuclei, accessory optic tract, inferior olivary nucleus, cerebellum, and oculomotor nuclei participate.  OKN abnormalities are seen in deep parietal-lobe lesions.  OKN testing can also be used to identify subtle ocular motor abnormalities, such as incomplete internuclear ophthalmoplegia.

Test for head-shake nystagmus

Head movements produce vestibular responses with an extremely short latency ( <15 ms).  Oculomotor responses are slower than this, with latencies approaching 100-200 msec.  The compensation for this temporal discrepancy is the ability of the central vestibular system to maintain a memory of head motion, so that eye movements can be accurately matched to head movement.

This capability is referred to as velocity storage, which is usually impaired with unilateral vestibular deficit and uncovered by the head-shake test.  The test if performed by 20 cycles of low amplitude, high-velocity active or passive head movements followed by observation for nystagmus.  This is done in both the horizontal and vertical direction.  Observation must be done with suppression of visual fixation, with Frenzel goggles or an infrared video system.  Head-shake nystagmus is seen with uncompensated, unilateral vestibular hypofunction of any cause.

Positional test

Positional testing is performed by recording eye movements without visual fixation in 3 cardinal positions: supine, head right, and head left.  Direction-fixed or changing positional nystagmus is usually peripheral and an objective sign of vestibular asymmetry, even if it is present in only 1 head position.

Dix-Hallpike positioning test

Positioning nystagmus is a classic finding in patients with BPPV.  It is elicited by positioning the patient rapidly from sitting to the head right, left, and center supine positions; by recording the induced nystagmus; and by noting the patient’s symptoms.  Hyperextension of the neck is not necessary and should be avoided.  Two ENG channels are required to determine the direction of the torsional component of the nystagmus.  ENG is less sensitive than clinical observation of benign positioning nystagmus because ENG is insensitive to record torsional BPPV components.  In the authors’ opinion, ENG should not be used to evaluate patients for BPPV nystagmus.

Bithermal caloric test

Barany introduced the caloric test in 1903.  Since then, it has been the time-honored vestibular test in clinical neurotology.  The caloric test remains the standard for evaluating unilateral vestibular deficit.  However, it is a limited and nonphysiologic test of the vestibular system.  Literature about the caloric test is extensive; therefore, only a brief description of the test and its interpretation are provided here.

The traditional caloric test is performed with the patient lying with the head elevated 30° Cold (30°C) and warm (44°C) water are used to irrigate each ear, 1 at a time.  Cold irrigation is an inhibitory stimulus, and warm irrigation is excitatory.  The direction of postcaloric nystagmus is determined by the quick phase direction and is easily remembered by using the mnemonic COWS: cold opposite and warm same, (ie, quick phase away from or toward the irrigated ear).

The 3 most important findings from the caloric test are unilateral weakness, bilateral weakness, and FFS of caloric-induced nystagmus.  The first 2 abnormalities are due to peripheral vestibular disease, and the third is due to central cerebellar disease.

Rotating chair test, or SHA

Barany introduced rotational testing in 1907.  In clinical practice, the rotation test lagged behind the caloric test.  However, with the advancement of computer technology, rotational chair-test systems were developed in the late 1970s and continue to evolve.  They are now used in several vestibular testing laboratories.

The test is used to evaluate the integrity of the VOR in the low- (0.1-0.32 Hz) or high-frequency (1-4 Hz) ranges.  The measured parameters are VOR gain, phase (latency), and symmetry.  The test is most useful in determining residual vestibular function and the degree of central vestibular compensation.

An alternative to the rotating chair test is the active head-rotation test, which is used to evaluate VOR gain in the high-frequency range.  This test is substantially less expensive and more practical than the chair test.  Active head rotation involves recording head and eye position while the patient actively turns his or her head from side to side at progressively faster frequencies.

CDP test

Dynamic posturography has become an integral part of vestibular testing in many vestibular test centers.  The clinical application of posturography in neurotology was introduced in the 1970s.  CDP system consists of a computer-controlled platform and visual booth used to evaluate both sensory and motor components of balance.  The sensory test is most clinically useful, especially in peripheral lesions, vestibular rehabilitation, and medicolegal cases.  Posturography is not a substitute for a careful gait examination and probably is of more value in rehabilitation than in diagnosis.

Clinical yield of vestibular tests

Several observations discussed below are drawn from a database of 8,000 patients who underwent the 3 tests (ENG, SHA, CDP) in 1985-1995 under direct supervision of 1 of the authors.

First, the raw data tracings should be viewed and evaluated, particularly those acquired by using computerized systems, and clinicians should not relying on computerized analysis generated by the system software, even if the raw data are merely noise.

Second, oculomotor findings are frequently overinterpreted, and unnecessary neurologic investigations and MRI studies result.  In the database describe above, the yield for abnormalities of central eye movements, saccadic dysmetria, saccadic pursuit, asymmetric optokinetic response, and gaze-evoked nystagmus was less than 5%.  Therefore, ENG readers are advised to cautiously interpret eye movements.  Novice ENG users sometimes read oculomotor results as being normal for several years while they store their pattern for more-accurate interpretation as their experience increases.

Third, ENG system prints outs only horizontal and vertical eye movements and is therefore insensitive to record pure torsional eye movements often seen with BPPV.  Video-based ENG (VNG) has the advantage of depicting and digitally recording pure torsional nystagmus for storing and reediting of the captured video signals.

Fourth, findings on chair and dynamic posturography are infrequently abnormal, and their routine use is probably not cost-effective.

Finally, most abnormalities on vestibular testing can be gleaned from vestibular examination carefully conducted in the office setting.

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