env_vng errors - Common Errors in ENG/VNG Kamran Barin,...

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Common Errors in ENG/VNG Kamran Barin, Ph.D. Ohio State University Medical Center, Balance and Dizziness 7/17/2006 During electronystagmography (ENG) or videonystagmography (VNG), the examiner often faces a critical decision. Do the test results accurately reflect the physiologic and pathologic status of the patient or do they represent artifacts caused by technical errors? The answer is not always simple and wrong decisions can have serious consequences. For example, artifacts can cause the examiner to either miss an abnormal finding or misidentify a normal result as abnormal. To identify technical errors, the examiner must have a basic knowledge of the capabilities and limitations of the ENG or VNG equipment. Also, the examiner must be familiar with the underlying physiology of the vestibular function tests. Technical errors must be suspected when results can not be explained by any known physiology or pathology of the vestibular and oculomotor pathways. Although technical errors in ENG and VNG can be due to various causes, there are a few errors that are more common than others. In this article, I will describe how to identify some of these common errors and ways to avoid them. 1. Failure to perform physical examination of eye movements Examiners must perform a thorough examination of the eye movements before the actual ENG or VNG testing. Failure to do so, can cause a number of different errors in different subtests. First, the physical examination can reveal if the eyes movements are conjugate. With the standard electrode arrangement in ENG, horizontal movements of the right and left eyes are averaged based on the assumption that both eyes move identically. Similarly, most VNG systems also average the movements detected by the right and left cameras. In VNG, both horizontal and vertical eye movements undergo the averaging process. Failure to recognize disconjugate eye movements will result in faulty measurement of eye movements throughout the test because averaged signals do not accurately reflect actual movements of either eye. Depending on the type of disconjugate eye movements, the examiner must change the standard protocol to record either movements of one eye only or to record movements of both eyes independently . An example of a case where eye movements must be recorded from one eye only, includes a patient who has significant reduction in the range of motion of one eye. This is usually caused by misalignment of the eyes (strabismus). Another example is a patient with one prosthetic eye. To record from one eye only in ENG, the examiner should move the electrode from the temple on the side of the affected eye to the bridge of the nose and place the vertical electrodes around the unaffected eye (Figure 1A). In VNG, the procedure for recording from one eye only, depends on the manufacturer of the equipment. Some VNG systems allow monocular recording. In others, one camera may have to be turned off manually by decreasing the brightness or other similar procedures.
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This note was uploaded on 04/11/2010 for the course AUD 831 taught by Professor Drlisakoch during the Spring '10 term at A.T. Still University.

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env_vng errors - Common Errors in ENG/VNG Kamran Barin,...

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