ENG and Rotational Testing: Interview with Gary Jacobson, PhD
Douglas L. Beck, AuD, speaks with Dr. Jacobson about auditory and vestibular electroneurodiagnostics and more.
Academy: Hi, Gary. Good to speak with you.
Jacobson: Hi, Doug. Thanks, good to speak with you, too.
Academy: Gary, your two books on balance are classics. I wonder if you can tell from 1993 to 2008, what changed as far as balance assessment and management.
Jacobson: There have been many changes since the first book was published. Vestibular-evoked myogenic potentials were undiscovered. Otolith function testing, in general, was in its infancy and these topics are covered in detail in the new book (by both Dr. Adolfo Bronstein and Drs. Faith Akina and Owen Murnane). Also, rotational testing is covered in detail (by Dr. Robt. Brey and his Mayo Clinic colleagues) in the 2008 book. As you know, when rotational tests are done well, they provide tremendous insight into the functioning of the vestibular system, and we came up a little short in the 1993 book. In the 2008 book, there are extended sections about positional vertigo, too, and remediation of positional vertigo. Richard Gans and Richard Roberts wrote those chapters, which are among the functional chapters in the book.
Academy: I recall the university I was with in 1993 (or so) buying our first rotary chair. There wasn’t much in the clinical literature back then, and in retrospect, it’s quite amazing we were able to buy it! We had to evaluate the rotary chair system based on manufacturer’s brochures, case reports, and good will. Then, once we had it installed and we took the course on how to use it, it was difficult to get referrals, and reimbursement was a bit of a nightmare! However, the test itself was solid.
Jacobson: Right-and it is even more so today. Rotary chair tests allow you to assess the function of the vestibular system across a section of its spectrum of response. It makes it possible for one to determine how “dense” is a bilateral vestibular system impairment, and it assists us in the identification central vestibular system impairment.
Academy: And all central impairments cannot be picked up via caloric tests, right?
Jacobson: Right. We are all familiar with failure of fixation suppression, which is a central sign that is observed during caloric testing. However, that is not the only pattern of central vestibular system impairment and some of these other patterns we have described. Unfortunately, caloric testing provides us with an assessment of the lowest frequency in the spectrum of response of the vestibular system akin to testing auditory sensitivity at 125 Hz and inferring from that threshold the rest of the audiogram. I can already hear you asking where I came up with that idea. Well, if you think about the velocity plots of nystagmus (from ICS) and think of that as half cycle of a sine wave lasting 120 seconds, and then invert it and create the whole sine wave by attaching the two halves together, you’d have one cycle every 240 seconds or so, and then take the reciprocal of that and you get a frequency of ~.004 Hz or a whole cycle every four minutes, and that’s just not a great diagnostic analysis of the complete system!
Academy: Excellent example. The vestibular system does respond to an entire spectrum of stimuli, much like the auditory system, and when you only stress and test one small portion, it’s hard to get the “gestalt.”
Jacobson: Sure. Therefore, to really get a good analysis, it takes multiple tests, looking at different functions. In other words, it takes more than an ENG test that reflects the vestibular system to the lowest possible frequency. In fact, it’s hard to even imagine what real or natural motion or movement might correspond with 0.003 Hz. It is just not a realistic or pragmatic representation of vestibular activity, but it is the gold standard for the detection of unilateral peripheral vestibular system impairment. Therefore, to me, low frequencies become the “Achilles Heel” of the vestibular system; often it’s where damage is first identified, whereas in the auditory system, damage is usually detected in the high frequencies first.
Academy: So the caloric test is diagnostic, as long as it is used within certain constraints, such as looking for unilateral peripheral vestibular system impairment?
Jacobson: Right. However, in bilateral weakness, the ENG can give a clinician a false sense that there is no function when, in fact, there is function remaining at high frequencies (in the same way that with a precipitous high frequency hearing loss there may be low and middle frequency auditory sensitivity intact). This patient may not generate nystagmus in the rotary chair at .01 but may generate normal gain (aka normal nystagmus velocity) at higher frequencies (e.g. .16 Hz, .32 Hz). One of the other applications for rotational testing is detecting central vestibular system impairments. These patterns have included normal nystagmus peak velocity but abnormal timing of this compensatory eye movement (i.e., abnormal phase) and decrease in nystagmus velocity with increases in rotational frequency (the opposite of what we expect to see) that is equivalent to a "decruitment" pattern for the vestibular system.
Academy: And there is no way that can be apparent on a caloric test?
Academy: These machines are big and space is scarce in most clinics. Any chance these machines can be, or, will be made smaller?
Jacobson: Well, the use of video goggles that record eye movements and are light tight have increased the likelihood that we will be able to do these recordings in dim room light without a lightproof enclosure. These improvements have really changed my world from measuring nystagmus beats by hand 30 years ago (do students even know we used to do measurements by hand?).
Academy: Gary, I know we have to end this soon, but if you only had one repositioning maneuver you could apply to a given patient with Benign Paroxysmal Positional Vertigo (BPPV), which would you choose?
Jacobson: Well, assuming everything is exactly right and classic for a posterior semicircular canal canalithiasis, (i.e., the patient reports having 30 seconds or so duration violent dizziness episodes, and they demonstrate a positive Dix-Hallpike in the head dependent left position), then the “Modified Epley” maneuver would be my choice.
Academy: And that is still not common knowledge across ERs in the United States?
Jacobson: No, it is not and it is crazy when you consider how many patients probably present to the ER thinking they are having a stroke or a heart attack. These patients undergo neuroimaging tests, non-invasive blood flow and cardiac tests and, in the end, come to us as outpatients where we find they have a self-limiting problem that can be cured in less than 10 minutes in most cases.
Academy: Okay, Gary. Thanks for your time. It’s always a pleasure chatting with you.
Jacobson: Thank you, too, Doug.
Gary Jacobson, PhD, is a professor in the Department of Hearing and Speech Sciences at Vanderbilt University. He is also the director of the Division of Audiology at the Vanderbilt Bill Wilkerson Center for Otolaryngology and Communication Sciences. Dr. Jacobson is co-editor of The Handbook of Balance Function Testing (1993, published by Mosby) and Balance Function Assessment and Management (2008, published by Plural).
Douglas L. Beck, AuD, Board Certified in Audiology, is the Web content editor for the American Academy of Audiology.