Interview with Kamran Barin, Ph.D., Consultant, GN Otometrics
CAROLYN SMAKA: Hi Dr. Barin, it's good to chat with you again. I wanted to talk with you about a subject that is gaining recognition, but may be unfamiliar to some people. Will you give us a brief history of what the head impulse test (HIT) is and what it is measuring?
KAMRAN BARIN: The head impulse test was introduced by Drs. Michael Halmagyi and Ian Curthoys and their associates in Australia. The group is well known in the vestibular area. They introduced this procedure as a bedside test that evaluates the function of the semicircular canals and the vestibulo-ocular reflex (VOR).
Kamran Barin, Ph.D.
The HIT is very easy to do. The tester watches the patient's eyes as they make a quick head motion to one side or the other. You watch to see if the eyes are moving smoothly or there are quick jumps. Most normal individuals keep their eyes fixated on a target in front of them. Patients with a vestibular abnormality, specifically semicircular canal dysfunction, will have extraneous eye movements. When you move the head toward the damaged semicircular canal in the same plane, their eyes will fall behind the head movement. They can't keep up. Then, the eyes make a quick jump to redirect the gaze toward the target. Those are called catch-up saccades.
The bedside HIT a test for experienced users who can look at the patient's eyes and determine if there are any catch-up saccades. Unfortunately, many patients with abnormal VOR don't show this pattern because they learn to move their eyes during the head motion. They send their eyes ahead so that by the time the head motion is over, their eyes are on target.
SMAKA: There's no apparent catch up.
BARIN: Exactly. So the Australian group developed a device that was comparable to the accuracy of using the scleral search coil system. The scleral search coil is like a contact lens that you put in the eye with a coil embedded in it. And you put the patient in a magnetic field and it measures the eye movements. A bite bar measures the head movement.
Simultaneous testing using the ICS Impulse and scleral search coils.
BARIN: It's a gold standard for measuring high-speed, accurate, three-dimensional eye movements. Unfortunately, it's not a very good test for routine clinical use because it is uncomfortable to place the contact lens on the cornea and keep it from moving.
So they developed a video goggle system where you replace the scleral search coil with cameras that can measure the eye movement, and you embed a sensor in the goggles that can measure the head movements as well. That prototype has been in use for four or five years. Otometrics has collaborated with Halmagyi and Curthoys and developed video goggles based on their prototype. It is called the ICS Impulse.
Otometrics ICS Impulse goggles
It is designed to detect peripheral vestibular abnormalities. It evaluates each ear separately just like the caloric test, which most of the other tests do not allow you to do. But at the same time, the head impulse test gives absolute responses, whereas caloric testing uses differences between right and left ears. If you have reduction of function on both sides, you may not pick up on that in the caloric test. You need an additional test, such as the rotation test. The ICS Impulse allows you to compare the right and left ears, so you will know if one side or both sides have abnormalities.
Another advantage of giving absolute responses rather than relative measures between the right and left sides, is that you can look at function over time. For example, if the patient has Meniere's disease and is receiving gentamicin injections in the ear, you can see over time if the function is declining or not. Right now, we just use serial audiograms to monitor that. Similarly, if someone has vestibular neuritis and the function returns, you'll see over time that the response is improving. It therefore allows us capabilities that we did not have before.
There is some exciting research underway with head impulse testing. You know, one of the difficulties we have with severe vertigo is deciding if the patient has a cerebellar infarct or a vestibular problem. If it's likely to be a cerebellar infarct, then it is an emergency and the treatment must start immediately. On the other hand, if they have a vestibular problem, the condition is less urgent. A handful of centers are currently studying acute vertigo in the emergency room, one through Johns Hopkins and two or three centers in the UK. A few reports have shown, such as a 2008 study from Johns Hopkins, that if the head impulse testing is abnormal, it's very unlikely that the patient is experiencing a cerebellar infarct. In that case, the need for unnecessary hospitalization and maybe even some procedures such as for an MRI can be reduced. This is only in the research stage, so at this point, everyone will go through the standard diagnostic and treatment protocol, but the 2008 study shows promise for the future.
SMAKA: Using the ICS impulse, does it take long to administer head impulse testing?
BARIN: All together, from patient entry to reporting the data, takes less than 10 minutes.
SMAKA: Can the goggles be used for other applications as well?
BARIN: The goggles can be used for other tests because video recording is incorporated into the goggles. The video recording can be used for the Dix-Hallpike maneuver.
It has the option of recording at various frame rates and playing back the eye movements in slow motion. So you can easily do the Dix-Hallpike maneuver because the goggles are snug, as opposed to using VNG goggles that can move on the patient's face during the maneuver. These goggles stay on the patient's face and allow one to observe and record torsional eye movements. Another benefit is that the head impulse test allows you to test the posterior and anterior canals. You do not have that option with the caloric test. By moving the head in the plane of the vertical canals, you can get a complete assessment of the function of all three semicircular canals, in each ear separately.
SMAKA: Dr. Barin, it was good seeing you today. Thanks for sharing this information on the head impulse test and the ICS Impulse device. Otometrics has a wealth of information on this topic including articles and videos on their website, www.icsimpulse.com as well as on www.headimpulse.com Hope to talk to you again soon!
BARIN: Thanks, Carolyn.
For more information about Otometrics, visit www.otometrics.com or the Otometrics web channel on AudiologyOnline.