Diagnostic Testing

Infrared Video Nystagmography (VNG) and Electro-nystagmography (ENG)--Tests Mostly for Horizontal Canal Function:

Thanks to new computer technology, eye movements can be tracked precisely under several circumstances. ENG records eye movements with electrodes that are attached to the skin around the eyes. VNG records eye movements directly using infrared cameras placed in front of the eyes. Both nystagmography methods are different ways of measuring the same thing. Video nystagmography is generally more precise and is the method of choice in most modern facilities. Nystagmography records small jerking eye movements called "nystagmus." Normally, these jerking eye movements occur whenever the inner ear sends signals to the brain that signify that the body is moving relative to the visual environment (for example, while spinning in a rotating chair or while standing on a merry-go-round). Usually, nystagmus also occurs whenever the patient experiences vertigo caused by vestibular disease. During nystagmography testing, the eye movements are recorded in several different head positions in order to diagnosis positional vertigo. Positional vertigo can be caused by canalithiasis of any one of the six semicircular canals. In this regard, nystagmography can be an indirect clue for vertical semicircular canal function or dysfunction. Caloric testing, however, is a test only of horizontal canal function. During caloric testing, each ear is irrigated with warm and cool water. This stimulates the horizontal semicircular canal and induces nystagmus. The intensity of the nystagmus is measured and this reflects function of this canal. The advantage of caloric testing is that the right and left horizontal canals are isolated and therefor are tested separately. In other tests of canal function, the left and right sides are tested simultaneously and a single side cannot be isolated.

Vestibular Autorotation Test (VAT)--A Test of Horizontal, Posterior, and Superior Canal Function:

The ability of the eyes to remain focused on an object of interest during head movement is known as the vestibulo-ocular reflex (VOR). The VOR, for example, allows us to comfortably read a book while sitting in car that is traveling down a bumpy road. This complex reflex is mediated by both inner ear and brain stem centers. Abnormalities of the VOR will cause instability of visual images and this can lead to intense dizziness and nausea. The VAT is a computer-based system that provides quantitative measurement of this reflex. The VAT is an indirect method for testing horizontal and vertical semicircular canal function. The disadvantage of VAT testing is that the left or right sides cannot be isolated during testing. Also, because both the posterior and superior canals are "vertical," their functions cannot be tested separately as both may be stimulated during vertical head movements.

Dynamic Visual Acuity (DVA)--A Test of Horizontal, Posterior, and Superior Canal Function:

This is a computerized method of determining the level of visual blurring induced by head movement in the horizontal or vertical planes. Unlike VAT testing, dynamic visual acuity is a method of evaluating the functional, rather physiologic integrity of the vestibulo-ocular reflex. The disadvantage of DVA testing is that the left or right inner ears cannot be isolated during testing. Like the VAT, the DVA cannot distinguish between posterior and superior canal function.

Subjective Verticality--A Test of Utricular Function:

Subjective verticality is considered a test of utricular function. During this test, the patient stands in a completely darkened room. The patient is asked to adjust a dimly lighted bar so that it is positioned vertically (i.e., perpendicular to the floor). Many patients tilt the bar toward the inner ear that is involved in utricular dysfunction. This is because their eyes are rotated toward the side with the problem and these patients perceive the world as tilted to that side. Our center has developed an inexpensive apparatus to measure subjective verticality. This will be available soon for purchase from our web site.

Vestibular Evoked Myogenic Potentials (VEMPs)--A Test of Saccular and Posterior Canal (or inferior vestibular nerve) Function:

VEMPs are a study of saccular function via stimulation of the vestibulocollic pathway. During this study, electrodes are attached to the patient's neck over the sternocleidomastoid muscle and the ear on the same side is stimulated with clicking or tone sounds at a level of 95 dB. Electrical potentials from the muscle are amplified and filtered for analysis. VEMPs can be useful in the diagnosis of Ménière's disease. Additionally, the VEMP study can be useful for predicting the onset of paroxysmal positional vertigo (of the posterior canal type) after an attack of vestibular neuritis. Both the saccule and the posterior canal are innervated by the inferior vestibular nerve. In order for the VEMP to be recorded, the inferior vestibular nerve must be intact. If the VEMP is intact, then the innervation to the posterior canal is intact and BPPV caused by posterior canalithiasis can occur. An absent VEMP is a sign that the patient will not go on to have symptoms from posterior canalithiasis. In this sense, the VEMP is an indirect way to measure posterior semicircular canal function. An advantage of VEMP testing is that the right and left sides are isolated during testing.

Audiometry--A Test of Cochlear and Auditory System Function:

During audiometry, earphones are placed over the ears and the patient presses a button whenever a tone is heard. Several different frequencies and intensities of sound are tested. The presence or absence of hearing loss is important in determining the etiology of dizziness. If hearing loss on the side of vestibular dysfunction is detected, this suggests involvement of the cochlea or auditory nerve.

Auditory Brain Stem Responses (ABRs)--A Test of the Complete Auditory Pathway:

ABRs are a method of determining the integrity of the auditory portion of the eighth cranial nerve. This test is done by stimulating the ears with sound and then recording electrical potentials produced by the brain with electrodes placed on the scalp. The entire pathway for hearing from the ear to the brain can be studies with this technique. ABRs are used most commonly to screen for acoustic neuromas. They also can be used to localize lesions in the brainstem (for example in stroke and multiple sclerosis).

Oculomotor Testing--A Test of Brain Function:

During oculomotor testing, infrared video cameras are used to record eye movements with very high precision. Typically, the oculomotor evaluation involves several sub-tests: saccades, smooth pursuit tracking, gaze-evoked nystagmus, and optokinetic nystagmus. During each of the tests, the patient watches lights projected on a screen while the eye movements are recorded and analyzed by a computer. Certain abnormalities of eye movements then can be used to diagnose abnormalities in specific brain regions. Oculomotor testing is a subject of intensive research at our center.

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