Different clinicians have varying approaches to caloric testing, including in what determines “common practices” in the United States and elsewhere. We asked our in-house audiologist, Dr. Terri Ives, to detail some of those variations. Her report follows.
Dr. Terri Ives
The caloric test allows separate evaluation of one horizontal semicircular canal at a time by measuring the resulting nystagmus response in each eye representing each ear at low frequency (~0.003 to 0.005 Hz). There are subtle differences in the testing techniques throughout the United States and greater differences as one travels the world. For as long as this test has been in use, one would expect consistency from one clinic to the next, from one country to another. This is, however, not the case.
First, we will examine the stimuli delivery methods and how they affect the test results. Next, we will examine how the software analysis can change one’s final results and, finally, how the examiner can affect the test results.
The bithermal caloric test is based upon delivering a temperature change to the horizontal semicircular canal in the inner ear. Frequently, the temperature stimulus delivered to each ear’s horizontal canal is unequal because the air or water temperature is altered due to: dissimilar ear canal sizes, torturous ear canals, obstructive foreign bodies, e.g. cerumen (ear wax), a middle ear prosthesis, or perforations of or missing eardrums. The result can be a false unilateral weakness (UW) or directional preponderance (DP).
Either air or water irrigators can be used to stimulate the inner ear, with a handful of manufacturers selling them. There are still some clinics that own and use a closed-loop water irrigator system, but the Brookler-Grams system is no longer commercially manufactured. No matter the stimulus, technique is very important to the quality of the test.
Air versus Water The American National Standards (ANSI) Institute established that water temperatures be 30º Celsius (C) for cool and 44ºC for warm irrigations corresponding to 7ºC above and below normal body temperature with less than ±0.5°C variation at the irrigation tip. ANSI has not approved standards for air irrigators. The British Society of Audiology (BSA) has established standards for water irrigation temperatures to be the same as ANSI with less than ±0.4ºC variation at the irrigation tip, and air stimulus irrigation temperatures of 24º and 50ºC for cool and warm air respectively.
In addition to the problems listed above with irrigation in general, air irrigation requires a particularly adept clinician who is careful to aim the tip directly at the tympanic membrane to cause proper temperature stimulation. It is helpful to have an air delivery method that allows the clinician to both place the delivery tube close to the tympanic membrane and view the placement at the same time. Additionally, we are not aware of an air irrigation system that measures air temperature at the irrigation end of the tube. The stimulus temperature once it reaches the tympanic membrane can be quite different, distorting the quality of the test.
Flow rate (volume) and delivery time (duration) of the irrigation are also important. ANSI recommends 200 ml ±20 ml/min for 40 sec ±1 sec for water irrigators. The BSA recommends 250 ml ±10 ml/min for 30 sec for water irrigators or 8 liters of air ±0.4 over 60 seconds for air irrigators. The force of the delivery system varies with each device, which could lead to differences in temperature stimulus, but this is not regulated at this time.
Analysis In reality, many clinics and societies, organizations, and studies use and recommend other flow rates and delivery times. For example, the normative data developed by Barber and Stockwell was based on 250mL and 30 seconds.
The importance of using proper normative data is an important point, particularly for such a traditional test as the caloric. We often assume that the data should match up between clinics, but unless one is using the same test protocol, irrigation parameters and analysis methods, and the same quality technique, the results can be vastly different. Neuro Kinetics devices are flexible and allow clinicians to collect and analyze data in almost any way they choose. While this flexibility is a great asset, the clinician must be aware that he or she must match previously established data for what is considered a bilateral weakness, a hyperactive response, and an abnormal UW or DP with the same methods for the protocol, irrigation and analysis. Neuro Kinetics recommends that each clinic ensures that its stimulus delivery protocol is analyzed using normative data collected using that same protocol before adopting normal/abnormal criteria.
The test procedure or protocol also varies from clinic to clinic and country to country. Neuro Kinetics believes that it is critical to test with eyes open in total darkness due to Bell’s phenomenon. For a caloric using video (VNG), a dark setting is less of a problem, but is virtually impossible with electrodes (ENG). Alerting tasks are important during the caloric test to prevent the patient from suppressing nystagmus. If the patient is not adequately alerted or is over tasked, the nystagmus results will be affected.
Both ANSI and BSA have recommended performing warm irrigations first with the right ear irrigation followed by the left ear irrigation. The nystagmus response declines from the first irrigation to the final irrigation. This has been attributed to central adaptation mechanisms. There is no correction factor for this central adaptation mechanism at this time in the Unilateral Weakness or Directional Preponderance formulas. There is research that has indicated that the reduction in nystagmus over the course of irrigations is due to the reduction of the cornea-retinal potential, which occurs with ENG but not VNG. Therefore, as long as a clinic is consistent about always beginning with the same temperature at that clinic, there is no significant difference in VNG if you begin with warm or with cool.
Both ANSI and BSA agree and recommend a 5-minute waiting period between each irrigation. What constitutes the end of a recording, however, is not defined. It is common to analyze the data between caloric irrigations, giving the patient time to recover, improving patient comfort and making the best use of the clinician’s time between irrigations.
The Fixation Suppression Test, or Index, is an inconstantly used technique. While fixation is widely used in the United States and Europe, it is deployed in different ways. One common method is to watch the nystagmus build during the caloric irrigation, after the peak nystagmus occurs, the patient is asked to fixate at center gaze and the nystagmus is recorded for at least 5 seconds and then the fixation is removed to see the nystagmus return. The nystagmus should decrease by at least 60% by 2 seconds post fixation (not decreasing by 60% or more is a central finding). There is wide variation on what is defined as abnormal. Some clinics use 60%, others 70%, while others consider 50–60% as borderline. Neuro Kinetics recommends that one analyze the three strongest beats together in the 5-second peak nystagmus period prior to fixation and compare this to the three strongest beats together in the 5-second period 2 seconds after fixation. Further, compare all four suppression tests for outliers and use two from each direction of nystagmus, rather than have only one from each direction of nystagmus and risk having an outlier cloud the data. The Neuro Kinetics VNG has chosen to set an abnormal Fixation Index at 60% or greater. As with all independent clinical findings, it is the recommendation of Neuro Kinetics that an abnormal Fixation Index be evaluated by the clinician as part of the total clinical picture.
Neuro Kinetics’ I-Portal®-VOG (pat.) has a light cover with a patented built-in fixation light within the cover to make completion of the Fixation Suppression Test simple.
Algorithms (the analytical math behind the scene) are built into all VNG/ ENG analysis packages. The analysis in each package allows the clinician to either override or adjust certain key variables to facilitate the clinician’s overall assessment if the clinician feels it is necessary. Neuro Kinetics believes that these adjustments must be made with caution as a data override can result in incorrect clinical conclusions.
Neuro Kinetics gives the clinician the ability to change the variables without altering the underlying algorithms, which have been peer reviewed and vetted over many years. The final analysis for Unilateral Weakness (UW) and Directional Preponderance (DP) are dependent on both the algorithms in the software and the diagnostic skills of the clinician. Irrigation-induced nystagmus does not typically occur until approximately 15 seconds after the test has begun. If there is spontaneous nystagmus showing between 0 and 15 seconds, how the software adjusts or does not adjust for this baseline shift will alter the final analysis for UW. Neuro Kinetics has chosen not to adjust for spontaneous nystagmus–caused baseline shift. If the clinician does all four irrigations, the calculation of UW automatically takes into account spontaneous nystagmus baseline shift. Directional Preponderance is a measure of bias in the vestibular system of the patient. Therefore, the effect of spontaneous nystagmus on DP is showing the clinician a bias in the system which is an abnormality in the patient that should not be removed with baseline shift adjustment software. Neuro Kinetics recommends that a clinician should neither apply an adjustment for spontaneous nystagmus (baseline shift) nor do only one temperature screening calorics. Only applying the UW calculation and DP calculation with all four irrigations will give accurate data when spontaneous nystagmus is present.
Calculation of the Peak Slow Phase Velocity is a critical analytic component that also varies from one analytic package to another, resulting in different outcomes with different manufacturers. If the calculation is based on the single strongest nystagmus beat, or even cluster of 3 beats that are outliers, the results can be misrepresentative. Neuro Kinetics recommends that the peak caloric response for each irrigation be calculated by averaging the Slow Phase Velocity of the 3 nystagmus beats with the highest velocities that occur in sequence. The Neuro-Kinetics I-Portal® VNG uses this method to lower the chance of error, and gives the clinician the power to set the Peak Slow Phase Velocity manually should they determine that the algorithms chose the wrong peak due to a cluster of outliers as well as remove outlier nystagmus beats.
Not removing outlier nystagmus beats from the trace will result in different Velocity calculations in all analytic packages and sometimes from one version to another. This makes it very difficult to precisely compare results for UW and DP between manufacturers. Lastly, if one irrigation is vastly different from the other irrigations, the results will show an abnormality. It is recommended that the clinician repeat the irrigation to verify that a technical error in delivery of the stimulus did not occur.
No matter how well the clinician executes a caloric test or the software analyzes the data, there are sources of variability in both technique and data quality. This variability, and the lack of agreement in what constitutes normal and abnormal, makes comparison of results from one clinic to another and one device to another nearly impossible.
Dr. Ives is Neuro Kinetics’ clinical director. She may be reached at TIves@neuro-kinetics.com or 412-963-6649 (Eastern U.S.).