Last week we discussed the benefits and advantages of the ENG/VNG exam over other testing techniques. This week, we discuss the limitations associated with this test, which include:
(1) The inability to record (except visually) torsional or rotary eye movements common in benign paroxysmal positional vertigo (BPPV): With ENG we were limited to recording in just the X-Y axis, up and down, left or right. The nystagmus associated with the most common and treatable vestibular disorder, BPPV, moves in a torsional or rotational manner and can not be recorded accurately with most ENG or VNG. With VNG, you can visualize this rotary eye movement, but most VNG systems do not record the rotary component.
(2) The lack of a wide-range stimulus in caloric testing: The normal functioning Vestibular Ocular Reflex (VOR) functions over a wide range of head speeds, from no movement up to as fast as you can move your head. The oculo-motor and positional portion of the ENG/VNG exam are not measures of VOR function, and the caloric test evaluates the VOR response to a stimulus that is analogous to a very slow head movement, roughly equivalent to one 360 degree horizontal rotation of the head over 240 seconds. So, out of all the movements you can make with your head, slow or fast, up or down, the ENG/VNG exam only provides information related to very slow horizontal head movements.
(3) Evaluation of the horizontal canal only through caloric testing: see above
(4) Minimal information about the patient’s level of central compensation: Following an acute vestibular injury, nystagmus is typically present for days to weeks due to asymmetrical output between the two labyrinths. The process of cerebellar clamp causes the nystagmus to diminish over a relatively short time period. Once the nystagmus are resolved, there is no finding on a ENG/VNG exam that would allow the examiner to make a judgment regarding how well the patient is compensating from the injury. The ENG/VNG exam (unlike rotational chair) is not a test of the dynamic VOR and does not measure gain, phase or symmetry of the VOR, therefore, no judgment regarding level of central compensation can be made.
(5) No information about the patient’s functional abilities: Objective measures such as Computerized Dynamic Platform Posturography (CDP), or subjective self inventories such as the Dizziness Handicap Inventory can provide insight into what a patient is able or unable to do. The ENG/VNG exam provides no such information.
Bottom Line: A normal ENG/VNG examination cannot be interpreted as normal vestibular function, and should not be described as a comprehensive vestibular or balance evaluation.
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