FAQ - GN Otometrics
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1. In coils the difference was reported at 15 It is hypothesized that the slower velocity head impulses used with goggles results in a lower difference between leftward and rightward head impulses This effect most likely arises from physiology ICS Impulse BALANCE Question Answer How is Gain calculated for ICS Impulse a START There are 175 samples per analyzed impulse We take each sample from the start to the zero cross for the head movement We sum the amplitudes of each of these samples This gives us one value for the head We repeat the process for the eye movement giving us one value for the eye Gain eye sum divided by the head sum Note that for the sake of this calculation the eye tracing is first de saccaded In summary our present gain calculation is based on area under the de saccaded eye velocity vs area under the head velocity Why do we not tilt the head for The horizontal canal planes are pitched up just a little in the head by about 21 degrees Curthoys et al 1977 So to get them exactly horizontal you would have the head pitched down by 21 degrees But it does head impulse testing so that the not really matter because the strength of the stimulus decreases as a cosine function of head pitch So semi circular canals will be straight being 20 degrees off has hardly reduced to strength of the stimulus to the horizontal canals like when one walks should We F inf i d and Fi 2 Curthoys I S2. normal on the left side Why is this This is often seen and has been documented in published articles The VOR gain to the healthy side does decrease a bit but the normal range is large so you don t detect that decrease In Manzari L Burgess AM MacDougall HG Curthoys IS Objective verification of full recovery of dynamic vestibular function after superior vestibular neuritis Laryngoscope 2001 Rapid Communication In this article a patient when in attack the VOR gain for the healthy side is 0 8 still in the normal range but on recovery the VOR returned to 1 0 Why are gains for head impulses to one side higher than to the other side There can be many reasons for that The gain of each side can be affected by factors affecting just that labyrinth There will be natural variations Another reason is since the data is recorded only from the right eye it is slightly biased gains are higher on the right than on the left lan Curthoys calculated the asymme try ratio for 90 healthy subjects data collected by Manzari with prototype and unpublished and found that the difference was very small 2 7 This difference was reported in coils Prog Brain Res 2008 171 195 8 Inter ocular differences of the horizontal vestibulo ocular reflex during impulsive testing Weber KP Aw ST Todd MJ McGarvie LA Pratap S Curthoys IS Halmagyi GM Department of Neurology Royal Prince Alfred Hospital Sydney Australia konrad weber bluewin ch
3. Blanks H I 8 Markham C H 1977 or more information please read and see Figure 2 Curthoys I S Blanks H I arkham C H tilt the head to compensate for the SEMICIRCULAR CANAL FUNCTIONAL ANATOMY IN CAT GUINEA PIG AND MAN Acta Otolaryngol 83 angle of the canals 258 265 GN Otometrics Europe 45 45 75 55 55 info gnotometrics dk GN Otometrics North America 1 800 289 2150 sales gnotometrics com ot O m et r C S www otometrics com www icsimpulse com MADSEN AURICAL ICS Specifications are subject to change without notice Copyright GN Otometrics 2012 08 7 26 8801 EN 04 Part no 7 26 88001 EN
4. ICS Impulse www icsimpulse com Se otometrics MADSEN AURICAL ICS ICS Impulse BALANCE Question Answer Why is ICS Impulse superior to Visual Observation e ICS Impulse can identify covert saccades e Validates that the head impulse is performed properly Both sensitivity and specificity are 95 for ICS Impulse Sensitivity is estimated at 70 for visual obser vation Reduction in false negative identifying patients as normal who are truly abnormal Objective Analysis with normative data Documented head impulse test results Better patient comfort during testing because smaller head impulses can be used 150 200 deg s with superior diagnostic accuracy Weber KP Aw ST Todd MJ McGarvie LA Curthoys IS Halmagyi GM Head impulse test in unilateral vestibular loss vestibulo ocular reflex and catch up saccades Neurology 2008 70 6 454 463 How is Head Impulse testing similar and different from Caloric testing HIT e Ear specific e Detects in cases of peripheral vestibular loss in Lateral SCC LARP RALP in Anterior and Posterior SCC e Tests at High Frequencies 4 5 Hz which is stimuli replicating the patient s everyday situations e Stimulus does not persist between tests Caloric e Ear specific e Detects in cases of peripheral vestibular loss in Lateral SCC e Tests at Low Frequencies 0 025 Hz e Stimulus can persist between irrigations especially if not performed properly How does Impuls
5. ation of the convergence system may interfere with the VOR In addition a close target adds a linear component e the eyes moves sideways relative to the dot this effect becomes smaller with increasing distance of the target From a technical point of view there may be some parallax of the laser relative movement of the laser dot in the field because it is attached to the right side of the goggles Therefore it is a good idea to have at least 1 m of distance What happens if the frame rate drops below 219 If the frame rate drops below 219 during data collection the software will reject that head impulse A frame rate below 219 is problematic because the data collection will not be accurate e g peak velocity gain measurements etc What is the difference between overt and covert catch up saccades e Overt occur after the head movement and can sometimes be seen using visual observation e Covert occur during the head movement and cannot be seen using visual observation Where did the default normative data cutoffs come from MacDougal HG Weber KP et al 2009 The video head impulse test Diagnostic accuracy in peripheral vestibulopathy Neurology 73 1134 1141 What is the difference between the collected and accepted data These are 2 completely separate algorithms The collected algorithm is on line during data collection and less detailed These are the numbers that display in the collection window left righ
6. e assist in perfor ming proper head impulses and making sure only quality data is analyzed Performing a proper head impulse is important in order to properly diagnosis the patient Impulse displays training curves that assist in demonstrating how a good head impulse looks The operator feedback tells you if the head impulse is good or poor If it is poor it tells you why too slow or too much overshoot There are also 2 algorithms that assess if the head impulse is properly performed If it is not then the head impulse will be rejected The analysis algorithm looks at both the head and eye movement If the patient is not staring at the fixation dot the head impulse will be rejected All of these features ensure that only quality data is represented in the analysis This increases your comfort in providing a proper diagnosis Why is it important to calibrate each patient Why should default cali bration only be used if the patient cannot be properly calibrated If possible you should always perform calibration If the patient cannot see the laser calibration dots or cognitively cannot perform the task take the default calibration values Remember the patient has to be able to see the fixation dot or stare at something during the test Default calibration A value is 21 Based on 25 patients range 24 51 left and 42 73 right By using the default calibration you could be introducing as much as 10 variability to the data collection res
7. patient and have them fixate on your nose when using ICS Impulse This method of testing will result in higher gains 3 If you have ruled out 1 amp 2 it could be that the patient has Meniere s See Manzari et al Rapid fluctuations in dynamic semicircular canal function in early Meniere s disease Eur Arch Otorhinolaryngol 14 Dec 2010 Why collect varying velocities Recording of different velocities was interesting from a scientific point of view because it told us that velocities of about 150 200 deg s are necessary to detect a unilateral deficit reliably The only reason for different velocities in clinics is that the 3D plots look much nicer What is the best velocity to use du ring testing when trying to identify unilateral deficits reliably 150 200 deg s The lower limit is based on the following publication figure 2 Head impulse test in unilate ral vestibular loss vestibulo ocular reflex and catch up saccades Weber KP Aw ST Todd MJ McGarvie LA Curthoys IS Halmagyi GM Neurology 2008 Feb 5 70 6 454 63 The upper limited was based on experience from the team in the article The more vigorous the head impulse the more likely you will get bumps amp artifacts with a video goggle system ICS Impulse BALANCE Question Answer Why should the patient be 1 meter from the fixation dot Having a very close target activates convergence i e the eyes have to cross in order to look at the dot Activ
8. t accepts and rejects and is displayed in the collected column in the test info window All the data not just the collected data goes thru a second algorithm the Analysis algorithm and this is more detailed and off line and the results are displayed in the accepted column in the test info window and what is included in the 2D and 3D analy sis The numbers for the collected and accepted may not always add up to the same number because the raw data is being analyzed twice by 2 different algorithms Why would the collection number be larger than the analysis Due to the way impulses are found peak detection in analysis vs velocity threshold in collection if the operator performs an impulse with a peak of around 50deg sec this impulse will be counted by collection but may not be counted by analysis Why would the analysis number be larger than the collection Collection impulses are 250 samples and analyzed impulses are 175 samples there is a small window whereby if you perform two impulses quickly between 100 and 200 samples apart the collection code will see this as one impulse and the analysis code might see it as two impulses It is important to perform the head impulse wait and look to verify that your head impulse matches the training curve and the operator feedback is green Can I export data in a format I can use for research Yes you can export patients and choose ASCII Test Results or ASCII Raw Data ASCII Test Res
9. ults This is why it is important to calibrate the patient if possible however this variability is better than not being able to test at all Why is performing unpredictable head impulses important Why do we not want the patient to be able to anticipate the head impulse We do not want the patient to try to compensate for their disorder Randomizing head impulse direction mainly affects the catch up saccade pattern If the head impulses are completely predictable patients will do better in making covert saccades compensating In order to show the deficit which is the purpose of the test it s better to randomize the order Unless the catch up saccades are very early lt 100ms they will not interfere with gain measurements Why am I getting high gain values High gain values could be one or a result of a combination of the below issues 1 Slippage of the goggles make sure strap is very snug that the cable from the goggles is clipped to the patient s right shoulder or collar with some slack and there are not gaps between the foam cushion and the patient s face See section 3 4 of the user manual for pics of a good and poor goggle placement You can access the manual from the bottom left corner of the software 2 Patient is too close to the fixation dot If they are closer than 1 meter you will see increased gains due to convergence of the eyes VOR gains will go up with close distances Never stand in front of the
10. ults exports the accepted data shown in the analysis windows ASCII Raw Data exports all data collected even the data rejected by the collection and analysis algorithms How many head impulses should perform The default is 20 This was recommended by Drs Halmagyi and Curthoys as a starting point Once the tester is confident in their ability to perform proper head impulses with very few rejects then this number can be reduced How large are the video files See Ch 4 in the manual Video Record Playback for detailed information on eye video room video and combined video sizes Can you test a patient that has Strabismus Gain vs 3D Tracing What should I look at Yes as long as it s not a paralytic strabismus You need to cover one eye the one that is not recorded to make sure the patient is always fixating with the same eye BOTH Do not only rely on the gain graph to determine if the patient is within normal limits or has an ab normal VOR response You must look at the traces It is easier to see catch up saccades in the 3D analysis If the gain is in the normal range but the 3D analysis clearly shows catch up saccades then the patient is exhibiting an abnormal response This is sometimes seen with patients where their covert saccades are right on top of the peak velocity of the head trace See blog posts at www headimpulse com to learn more We tested a patient with right vestibular neuritis but the gain was
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