Ask an Expert

Get the answers to all those challenging questions.
orSubmit a question

Ask an Expert

The Ask an Expert Forum is not designed as a first line support function. If you are a user of Interacoustics equipment and have an immediate request related to your instrumentation, then we recommend you contact you local Interacoustics representative.

The Ask an Expert forum is designed to deal with clinical questions related to Interacoustics products and their technologies. Once submitted, questions will be reviewed by an Interacoustics Academy representative and an answer will be made. Alternatively your question might be forwarded to your local Interacoustics representative for processing.

Lastly, the Ask an Expert is not an instant service and answers may take a few weeks to appear. We will however e-mail you with the outcome of your question. Thank you for your question.

Submit a question

Disclaimer: By sending your e-mail address you agree that Interacoustics A/S is allowed to share your e-mail address with other relevant affiliates within our group of companies and that Interacoustics A/S or its affiliates may contact you in relation to the topics related to the question you have asked. If you live in a country which is governed by GDPR, your details will only be used to in relation to answering the question you submit and will be deleted immediately following this interaction.

Balance

I've heard of the 'cerebellar clamp' that quickly causes a temporary suppression in vestibular activity from both sides after a unilateral insult. Is that right?

Answer: It sounds like you might be referring to the ‘cerebellar shutdown’ hypothesis. This hypothesis concerns the process of static vestibular compensation following a unilateral vestibular insult whereby loss of afferent sensory input from the vestibular periphery leads to vertigo, nystagmus and postural symptoms. The process of compensation, a very clear example of neural plasticity, restores neural activity in vestibular reflexes despite a lesion, and thereby alleviates static (head still) symptoms of imbalance. This process takes several days in mammalian species and likely involves several mechanisms such as expression of synaptic receptors, altered cell membrane excitability and rebalancing between inhibitory and excitatory neural circuitry. The cerebellar shutdown hypothesis suggests a bilateral drop in activity takes place within hours of a unilateral lesion, in at least some brainstem vestibular nuclei. If true, one would expect an alleviation of symptoms and a bilateral loss of postural muscle tone, which is not typically observed. For a more detailed overview please see Smith and Curthoys (1989)


References and caveats
Smith, P.F. and Curthoys I.S. (1989) Mechanisms of recovery following unilateral labyrinthectomy: a review. Brain Research Reviews 14, 155-180

January 2016

Your question seems to relate to the various options for rotational testing i.e. measuring the VOR in response to a rotational stimulus.

The modern balance clinic offers various complementary ways in which to provide a rotational stimulus to the vestibular system such as calorics, rotatory chair and video Head Impulse Testing. A common question relates to which test option should one select under different circumstances if they are all a rotational stimulus.

The present question ties into this wider one, and an analogy sometimes given to convey the way in which the test options are complementary is the use of the audiogram – restricting oneself to one or another would be rather like only testing hearing using one pure tone frequency instead of several across the audiometric spectrum.

As a starting point this seems a good analogy, and the below figure shows the spectrum of (head) rotational frequencies over which the different vestibular tests operate.

But at some level the analogy does break down and perhaps this is the source of your confusion. Let us unpack matters a little.

The frequency of sound (or number of cycles per second) is coded by the cochlear in a tonotopic way – different locations on the cochlear respond best to different frequencies. However, the vestibular region of the ear that sense rotation (semi-circular canals) decompose the stimulus in different semi-circular according to direction, not frequency (different semi-circular canals respond best to rotations in their plane of orientation). 

So you might say the word “frequency” is context specific. In terms of frequency of head motion, we are by extension talking about the strength of the stimulus. A low frequency head rotation would produce a low acceleration (weak stimulus) and a high frequency head rotation would produce a high acceleration (strong stimulus) while in hearing the strength of the stimulus is denoted by intensity. So, in a sense one could argue that a better analogy between vestibular and hearing systems would be to relate frequency of head rotation to a sound intensity, not frequency. Then you might visualise how caloric testing (which simulates a low frequency head rotation and low acceleration) might be analogous to testing one’s hearing with only a low-level sound. Increasing the head rotation frequency such as with the rotational chair or vHIT increases the head acceleration, which is analogous to increasing the sound intensity. 

If you like feel free to watch the webinar which generated this question.

Advanced Calorics

May 2017

For caloric testing, I am just thinking if we can do cold or warm water alone, why we need to do bithermal beside the benefit of better averaging.

Answer: Well your question already encapsulates one of the key reasons why one might need to perform all four irrigations (binaural & bithermal) i.e. that while mono-thermal screening reduces test time and predicts the bithermal result with high sensitivity and specificity but with certain criteria attached (e.g. the absence of spontaneous nystagmus and overall response size above a certain threshold).

The part about response size being above a certain threshold may be what you mean by “better averaging” – both monothermal results would need to be above a threshold in order to exclude bilateral canal paresis, otherwise the overall low figures could lead to a false positive.

Another reason to perform all four irrigations is measuring directional preponderance, which may give additional clinically relevant information.

You mention either cold or warm water alone – please do note that these concepts apply to both water irrigation and air insufflation (the other key method of caloric stimulation). You might also note that warm water (or air) stimulation is recommended as a mono-thermal caloric screening stimulus but cold water (or air) might be lead to lower accuracy in predicting the bithermal result (Lightfoot et al. 2008)


Learn more about caloric testing by completing this 15 minute e-learning course 

References and caveats
Lightfoot, G., Barker, F., Belcher, K., Kennedy, V., Nassar, G., and Tweedy, F. (2008) The derivation of optimum criteria for use in the monothermal caloric screening test. Ear Hear 30 (1), pages 54-62

May 2017

Yes VEMPs can be used in a number of ways and, like a number of vestibular diagnostic tests associated with Meniere’s disease, the result and the interpretation depend to a large extent of the point along the disease cycle at which the test is performed. For example, as part of a test battery one use of VEMPs would be to determine the extent of residual otolith function, where reduced VEMP amplitudes or absent VEMPs might be expected as the condition becomes more advanced. However, another interesting application is the VEMP tuning curve, which considers the VEMP amplitude across a range of stimulus frequencies. 

In the healthy ear the VEMP (similar concepts apply for both the cervical and ocular VEMP) tuning curve shows a maximal response at around 500 Hz and a progressively reducing amplitude as stimulus frequency increases. However, in a diseased ear the tuning shifts so that the response becomes dominant at around 1000 Hz (Murofushi et al. 2017; Sandhu et al., 2012). The exact mechanism for this shift is unclear but may be attributable to the altered fluid dynamics of the fluid-distended vestibule. Clearly this technique depends on the overall disease stage (e.g. if a VEMP is absent, suggestive of later stages, then a tuning curve cannot be plotted). The postulated mechanism for a tuning curve shift might also suggest that a positive finding should not occur during a remission period even if the patient was a true positive.

To assist you with differentiating Meniere’s disease the Interacoustics Academy has made these tables which suggest expected test results across the test battery in meniere’s disease patients. 

 

To learn more about cVEMP we recommend you watch the following webinar

References and caveats

Murofushi, T., Tsubota. M., Suizu, R.  and Yoshimura, E. (2017) Is Alteration of Tuning Property in Cervical Vestibular-Evoked Myogenic Potential Specific for Ménière’s Disease? Front. Neurol. 8 Article 193.

Sandhu, J., Low, R., Rea, P., and Saunders, N. (2012) Altered frequency dynamics of cervical and ocular vestibular evoked myogenic potentials in patients with Ménière’s disease. Otol Neurotol 33. Pages 444-449.


Disclaimer 
The information in the figures are a resource tool based on the needs of medical professionals and students that allows quick access to the typical assessment findings in a range of common vestibular disorders. The resource was developed to provide fast, easy-to-use, and always available information which can aid in reaching the correct diagnosis. The information contained within is provided as an information resource only, and should not be used as a substitute for professional diagnosis and management.

December 2017

I gather vHIT can be used as a screening tool before I send patients for further testing like VNG or VEMP. I am thinking of doing so but worried we may miss some early stage vestibular problems.

Answer: vHIT can be used as a screening tool, or perhaps a more appropriate term is ‘triage’. If you would like to learn more about vestibular triage please note this topic was raised in another thread here.  If you see an abnormality on vHIT then you can interpret the results to decide whether or not further testing is necessary, and what those tests might be. Very often they may involve VNG and VEMP, but equally there are audiological tests, the caloric and rotatory chair tests, and tests of otolith function other than VEMPs, such as SVV.  Therefore if used appropriately you should not miss signs of abnormalities.

February 2017

Firstly perhaps we should make a distinction between the so called “static” and “dynamic” compensations for prolonged unilateral vestibular weakness.

The static form of compensation arises spontaneously and its results can be observed in the gradual cessation of spontaneous nystagmus and postural asymmetries in the ensuing days after the onset of (abrupt) unilateral vestibular weakness. 

Dynamic compensation may take a number of forms such as a change in VOR gain during head movement, but for high velocity head rotations there appears to be a change in the latency of the corrective saccade such that their latency decreases. In the context of the video Head Impulse Test, this would appear as an increase in the number of covert saccades (i.e. so early that they occur during a head rotation and cannot be seen by the naked eye of the tester).

The saccade helps to keep the patients eye on the target of interest – a role normally fulfilled by a smooth eye movement produced by the vestibular ocular reflex. One or a series of saccades would compensate for a deficient VOR by ballistic eye movements. An introduction to this emerging literature is provided by MacDougall and Curthoys (2012).

Figure 1 shows a real-world example – a patient with a fixed unilateral weakness of the left ear following vestibular neuritis which affected the superior branch of the vestibular nerve. Lateral vHIT impulses taken shortly after the vestibular event show a reduced gain and a mixture of covert and overt saccades, whereas lateral vHIT impulses taken a different points downstream, after vestibular compensation, show apparently fewer overt saccades and more covert saccades.


To learn more about interpreting vHIT we recommend you watch the following webinar

 

References and caveats
MacDougall, H.G. and Curthoys, I.S. (2012) Plasticity during vestibular compensation: the role of saccades. Frontiers in Neurology 3 (21) Pages 1-9. 

September 2017

There are a range of vestibular assessment services ranging from local, community based triage services, to specialist and even supra-specialist centres (sometimes known as tertiary centres). 

1. Vestibular triage service 

To triage means to provide some relatively quick assessment procedure on patients, which allows the clinician to determine what (if any) further assessment is needed as well as deciding upon the urgency. 

Balance patients may present to their hospital from family doctors, from other medical disciplines (e.g. geriatrics or emergency room) or from the independent sector (e.g. hearing aid dispensaries) and in each of these primary care places, some form of vestibular triage is the means by which their symptoms are assessed and appropriate referrals are triggers for presentation at the hospital.

A vestibular triage usually consists of routine audiological, vestibular and tinnitus assessment. The vestibular part of this assessment typically includes some testing of vestibular reflexes; an eye movement examination. Some basic vision and gait tests may also be carried out.

2. Specialist centre

Whilst the triage process might identify some patients who can be managed locally (for example, patients with Benign Paroxysmal Positional Vertigo, BPPV) others with more complex needs or with medical ‘red flags’ would be referred to a secondary level of care, typically the ENT or neurology departments at their local hospital. A combination of ENT specialists, audiologists, physiotherapists, neurologists and others will work together to provide a more comprehensive assessment service, and will usually apply more specialist vestibular assessment techniques such as caloric testing, advanced (e.g. speech) audiometry, posturography testing, and evoked potentials.

3. Supra specialist

The most complex cases would be referred to tertiary centres with additional test facilities such as rotatory chairs, particle repositioning chairs, advanced evoked potentials and advanced rehabilitation via virtual reality. 


References and caveats
For more information please see the following articles:
Kaski (2015) A practical approach to managing the dizzy patient Prescriber 26 (12), pages 27 – 30
Provision of Adult Balance Services: A Good Practice Guide. UK Department of Health, 2009

January 2017

Well there are certainly guidelines, as opposed to rules.

Perhaps it would help if we refer to the pathophysiology for interpreting such guidelines.

Patients with a dehiscent semi-circular canal (either the superior canal or another) exhibit a reduction in the impedance of the endolymphatic fluid to the flow of mechanical (sound) energy. In other words, sound energy moves more readily through the canal than normal – part of the so-called third window syndrome. (i.e. the dehiscence acts as a third “window” to the inner ear, the first and second being the oval and round windows which direct movement of mechanical energy through the inner ear fluid). A schematic of this effect is shown below.

Figure 1 shows a schematic of the normal configuration of the ear. Sound energy enters the inner ear via the stapes footplate inserting into the oval window. Inwards movements of the footplate result in a corresponding outwards movement of the round window to allow sound vibrations to propagate along the cochlear duct. Some amount of sound energy might propagate through the vestibular system (and can be used to elicit a VEMP), although normally the fluid impedance is high in this region so the sound intensity would have to be high in order to elicit a VEMP.

Figure 2 shows a dehiscent superior semi-circular canal. The effect of this third window is an increase in sound energy propagation and an increased cervical and ocular VEMP amplitude. 

 That is, although the VEMP itself is a test of otolith function, anatomically the sound energy introduced at the oval window during the VEMP procedure would propagate through the vestibule on route to the dehiscent canal. Simultaneously, less sound energy flows through the cochlear duct, which results in a related diagnostic phenomenon – pseudoconductive hearing loss.

Therefore, a larger than normal c/o VEMP amplitude may be expected at normal sound stimulation levels (e.g. 90-100 dB nHL). A related outcome to VEMP testing is that a response may still be present at lower sound levels than normal i.e. the VEMP threshold is reduced. A third possible outcome is that there will be an interaural asymmetry in VEMP amplitude (and threshold) – the affected ear should show an abnormally high amplitude and low threshold although this would only be the case if one ear was affected. If both ears were affected then the tester might see enlarged VEMP amplitudes and low thresholds but no significant interaural asymmetry.

For more information, including an overview of management of patients, please see Minor (2005)


References and caveats
Minor, L.B. (2005) Clinical manifestations of superior semicircular canal dehiscence. Laryngoscope 115, pages 1717 – 1727.

August 2017

The Interacoustics Eclipse has several features to assist you with this technical challenge related to the cVEMP.

Firstly the instrument is equipped with an “EMG monitor” – in other words the system measures the myogenic activity associated with muscle contraction and provides visual feedback via a computer monitor so that the patient can maintain a target level of contraction. To avoid asymmetries of course the same target is used when testing the left and right cVEMP. The below figure (Figure 1) shows an example of this. Here the patient is having the right cVEMP measured, hence is gazing over the left shoulder to contract the right sternocleidomastoid muscle. The visual feedback consists of the scroll bar (at the bottom), which is colour coded and hovers in the green zone when the correct muscle tone is obtained. If either too much or too little muscle tone occurs (which can happen momentarily in some patients) then the scroll bar will go up or down into red zones accordingly, and the system will pause the averaging of these trials until the correct muscle tone is again obtained.

In addition to the scroll bar there is a dial (top right of screen) indicating test progress (% of requisite number of accepted sweeps) and this can help guide the patient as to how much longer they need to keep up their efforts. The top left shows a chart which plots the EMG amplitude (y-axis) over time. A consistent muscle contraction should produce a horizontal line.

After cVEMPs for each ear are collected the clinician can compare the EMG graphs to “eyeball” these charts and ensure there is a similar mean EMG, and overlapping variance. For example the below figure (Figure 2) shows the result after completion of right ear measures.

The visual feedback to the patient provides a way to help prevent muscle asymmetries from occurring. However, if asymmetries do occur then the Interacoustics Eclipse features a further tool to account for any muscle asymmetries and this provides a way to ensure valid interaural comparisons of saccular function despite muscle asymmetries; the feature is known as EMG scaling.

The VEMP amplitude is scaled in proportion to the tonic EMG activity, which is calculated from the pre-stimulus period. So, low muscle contraction would produce a scaled up VEMP and high muscle contraction would produce a scaled down VEMP according to the following equation.

 

To learn more about cVEMP we recommend you watch the following webinar

 

October 2017

The video Head Impulse Test (vHIT) produces a rotational stimulus to the head and then assesses the resulting eye movement via the Vestibulo-Ocular Reflex (VOR). The rotation stimulus is applied in the anatomical planes of the respective semi-circular canals hence each stimulus is best detected by one pair of canals; each canal is a coplanar pair with another canal in the other ear.

“Lateral” refers to a head rotation in the horizontal plane, usually with the head bowed forward by approximately 30°. This best stimulates the horizontal canals of each ear (which are a co-planar pair).

“RALP” means Right Anterior, Left Posterior and refers to a head rotation in an orientation which best stimulates this co-planar pair. “LARP” means Left Anterior, Right Posterior and refers to a head rotation in an orientation which best stimulates this co-planar pair. The below figure is a schematic showing a top-down view of the head, which visualises the orientation of the semi-circular canals of each ear.

August 2017

It is a very positive step to use these questionnaires alongside the VNG tests to support your diagnosis, guide decision-making and chart progress of patients.

Let us touch on the DHI first.

This is a widely used tool for quickly capturing the impact of dizziness. The answers to the 25 questions are marked to give a total score (from 0 to 100 points), which provides an indication of the handicapping effect of dizziness. However, the questions are also groups into three domains (physical, functional and emotional) and referring to the responses in each of these domains may help the clinician to gauge the area of most handicap, and thus help to understand where they might place the emphasis in terms of rehabilitation strategies.

If the DHI is being used as an outcome measure (i.e. to chart progress e.g. after a programme of vestibular rehabilitation therapy), then a change of 18 points in the overall score is needed for the clinician to consider this a true change.

It is important to note that the scores someone provides on the DHI will not always relate closely with the evidence of peripheral vestibular dysfunction that is indicated by the vestibular test battery. For example, someone may have a high degree of handicap and negative impact on their quality of life, and yet have little or no apparent vestibular dysfunction. 

The second tool that you mentioned is the VRBQ. This can be used similarly to the DHI, in that is can be used to provide a snapshot of the overall status at the beginning of a treatment programme (which might well be at the point of vestibular assessment), and it can be repeated after a treatment programme (Vestibular Rehabilitation) to assess any changes in the patient’s self-reported status.

The questionnaire has two halves. The first assesses the symptoms (and the responses can also be broken down into 3 domains, the dizziness, anxiety and motion related aspects). The second half assess the impact of dizziness upon quality of life. The scoring provides a percentage scale, where 0% is no deficit compared with the patient’s own normal state, and 100% is the maximum deficit. 

A change of 7% in the overall score is needed for the clinician to consider this a true change. Changes of 6% and 9% are needed for the clinician to consider a true change in the Symptoms and Quality of Life halves, respectively


References and caveats
Jacobson, G.P. and Newman, C.W. (1990) The development of the dizziness
handicap inventory. Archives of Otolaryngology Head Neck Surgery, 116 pages 424 - 427.

Morris, A., Lutman, M., and Yardley, L. (2008) Measuring Outcome from Vestibular
Rehabilitation, Part I: Qualitative development of a new self-report measure. International Journal of Audiology, 47 pages 169-77.

Morris, A., Lutman, M., and Yardley, L. 2009. Measuring Outcome from Vestibular
Rehabilitation, Part II: Refinement and validation of a new self-report measure.
International Journal of Audiology, 48 pages 24-37.

November 2017

Like all evoked potentials, where time-locked stimulus averaging is involved then the number of trials is only indirectly relevant. The metric that is directly relevant is residual noise – this these needs to be low enough for to make a conclusive interpretation of the results and, usually, more trials produce a lower residual noise.

The cVEMP is a relatively large amplitude evoked potential so, even with a relatively low number of trials the signal-to-noise ratio is often high enough to make a conclusive interpretation.  The default number of sweeps set in the Interacoustics Eclipse instrument software is 200 and under the vast majority of circumstances this should be sufficient to achieve a suitably low residual noise in the averaged trace (i.e. to make interaural comparisons, to assess latency and amplitude and to differentiate “response present” and “response absent”).

As mentioned, although these concepts are transferable to other forms of evoked potential the cVEMP is a special case in one sense, and that is that the response relies on muscle contraction by the individual actively flexing their sternocleido mastoid muscle. Therefore, the tester cannot increase the number of trials (hence test time) without the penalty of fatigue and reduced contraction, which could render the results inconclusive. It seems 200 trials achieves a good compromise with this in mind. Needless to say that if necessary several runs, with breaks in between, could be combined into a grand average with sufficiently low residual noise.

To learn more about cVEMP we recommend you watch the following webinar

August 2017

Regarding which side is considered the cause of a pathological SVV measurement. I've watched the webinar you've uploaded online and it really clarifies a lot of things in regards to how the equipment works and how to interpret the results. However I’m a bit in doubt about how you interpret overestimations?

Answer: In discussing the interpretation we decided to offer a starting point as the webinar is aimed at an introductory level. It offers some description of the underestimations that might occur following common peripheral vestibular disorders i.e. the discussion focussed on the “classic” unilateral weakness in both acute and various chronic phases but stops short of considering interpretation of overestimations.

When considering overestimation results, one point we would strongly advise is that SVV is not only assessing otolith (utricular) function, but as a behavioural test will also be influenced by subjective factors, and of course any central neurological disorders involving the neural connections from each ear to the brainstem (e.g. vestibular nuclei) and also the motor control of the eyes. From this you might quickly see how a variety of central lesions and disorders might also influence the SVV results, and some of these might be associated with overestimations. For example traumatic brain injuries that cause dizziness and postural instability, plus demyelinating diseases affecting the central vestibular circuits and cerebellar abnormalities, and ischemia / infarcts. On this note, unilateral cerebellar lesions have been found to produce contraversive ocular tilt results, and one might speculate that the SVV findings in a case like this might appear as an overestimation in head tilt conditions to one side;  contraversive tilt occurs with a unilateral cerebellar lesion and ipsiversive with cerebellar stimulation. It would be interesting to learn whether certain irritative disorders (e.g. Vestibular Paroxysmia) might also lead to an overestimation of SVV.


To learn more about SVV we recommend you watch the following webinar

References and caveats
Mossman S, Halmagyi GM. Partial ocular tilt reaction due to unilateral cerebellar lesion. Neurology 1997;49:491–493

December 2017

Vestibular neuritis is a condition where dizziness is caused due to an infection (mainly viral) of the vestibular nerve. The vestibular nerve has two branches which innervate the inner ear vestibular structures: The superior branch and the inferior branch. It is important not to forget that the neuritis can affect either branch individually or both branches at the same time.  In order to understand the pattern of test results found in acute patients which have either superior vestibular neuritis or inferior vestibular neuritis it is important to know which organs each nerve synapses to. 

Have a look at the image below. The Superior vestibular (Grey) nerve connects to lateral semi-circular canal, anterior semi-circular canal and the utricle. Whereas the inferior vestibular (Black) nerve has connections to the saccule and the posterior semi-circular canal.  

Now that we know the inner ear anatomy with relation to each vestibular nerve, we now need to know which piece anatomy each diagnostic test measures. The video head impulse test can measure the function of each of the semi-circular canal independently and their corresponding vestibular nerves, the cVEMP measures the function of the saccule and the inferior vestibular nerve and the oVEMP measures the function of mainly the utricle and the superior vestibular nerve.  Therefore if a patient presents with a neuritis on the left superior nerve then you should expect the following results 

The Video head impulse test will show reduced VOR gain and catch up saccades in the lateral and anterior canals. Whereas the posterior vHIT should reveal normal test findings. The cVEMP will be normal as it only tests the inferior nerve but the oVEMP will be abnormal as the superior nerve needs to be intact to record a response from the utricle. 

To assist you with differentiating between inferior vestibular neuritis and superior vestibular neuritis the Interacoustics academy has made these useful tables which show expect tests results in acute patients. 


Disclaimer 
This resource is a tool based on the needs of medical professionals and students that allows quick access to the typical assessment findings in a range of common vestibular disorders. The resource was developed to provide fast, easy-to-use, and always available information which can aid in reaching the correct diagnosis. The information contained within is provided as an information resource only, and should not be used as a substitute for professional diagnosis and management. 

December 2017

In order to answer this question it is easiest to look at an example. Below you can see a graph which displays horizontal eye movements for a five second period. On the graph is a red and blue line which tells the examiner which eye is being recorded: red = right eye and blue = left eye. You can see on the graph that there appears to be some nystagmus (involuntary eye movement) present. But which direction is it beating?

Well in order to answer this, it is firstly important to know that jerk nystagmus (as displayed above) comprises of two parts: a slow phase which causes the eyes to drift in a certain direction and a fast phase which bring the eyes back to primary position (central position).  It is convention to refer to nystagmus by describing the direction of the fast phase.  In the example we have we can now label the slow phase and the fast based on the duration of the eye movement in each direction.  We can see the eye drifts slowly to the right (upwards on the graph) for a duration of ≈ 1 second and then quickly moves back to centre by moving to the left (downwards on the graph). Therefore by using the naming convention described above we can identify that in this example the fast phase is moving the eye in a leftwards (downwards direction on the graph) and thus we call this left beating nystagmus. In VNG we typically measure in two channels so we can record left beating nystagmus, right beating nystagmus, up beating nystagmus, and down beating  nystagmus for each eye. 

 

December 2017

How is SVV (β) defined? It seems to me that the "angle of deviation" (Δ) would be what is considered the SVV.  The study which provides the normative data used for the confidence interval defines SVV as "the set angle, as measured with reference to the true vertical", but this almost seems like the definition of "angle of deviation".

Answer: I believe the SVV (β) is defined simply as the angle at which the viewer sets the luminous line. For example, if the viewer set the line at 0° then it would be parallel with gravity. If they set the line -15° then it would be tilted to their left by 15 degrees and so on.

So the question then becomes how is 0°, -15° etc… defined? This is defined by a sensor in the instrument. Without going into the technicalities, there is a self-aligning transducer in the instrument which defines vertical using gravity. (the same principle as a plumb line.)

So referring to the article by Schoenfeld and Clarke (2011), which is the one I believe you’re quoting from in your question, I guess what they mean by the phrase “with reference to the true vertical” is at what angle did the viewer set the line relative to the true vertical as measured by the self-aligning transducer. They aren’t referring to the "angle of deviation" (Δ) with this phrase, so far as I can tell. 

What appears somewhat counterintuitive at first, and is perhaps the source of your puzzlement, is that the instrument displays β as a value relative to the head tilt angle (α). However, the head tilt angle must be defined by reference to the self-aligning transducer (in addition to the luminous line), so these two values are related by the same reference.


To learn more about SVV we recommend you watch the following webinar

References and caveats
Schönfeld U., Clarke A. H. – 2011: A Clinical Study of the Subjective Visual Vertical during Unilateral Centrifugation and Static Tilt; Acta Otolaryngol 131(10):1040-50 

December 2017

That is a good question as you could argue that either approach for displaying the results is acceptable.

Of course both values are provided/calculated, so your question perhaps relates to why the graphical display is plotted with angle of deviation [Δ] not SVV [β].

One consideration is that Δ and not β is used for reasons of ease of interpretation in the various different static head tilt conditions. 

That is, if you showed either Δ or β for the upright head condition it would be inconsequential as both Δ and β would give the same value, but in the various head tilt conditions that the cSVV instrument enables you to measure, these two measures will give different values (since β is displayed relative to head tilt angle, α). 

So, take a “normal & healthy” result – you should always end up with a Δ of 0° irrespective of head tilt which, when plotted, produces a horizontal line intercepting the y-axis of the graph. 

However, if β were plotted on the y-axis then a “normal & healthy” result would not give a horizontal line as the β result would not always be 0°. Instead the data would produce a line at 45° (since β is displayed relative to α), and perhaps this (or deviations from this normal pattern) is a little harder to read (when “eyeballing” the data).

As it happens, β rather than Δ results are displayed by Schönfeld and Clarke (2011) and this enables a simple illustration. For example see the below figure from this reference, which shows the SVV findings in the case study of a 59 year old female in the acute and chronic phase of a left unilateral vestibular loss. The left panel in the figure shows the results with Δ plotted on the y-axis (similarly to how it would appear in the cSVV instrument) and the right panel shows the same findings but now with β plotted on the y-axis (although please note different scales used in these two plots).

Plots of SVV data showing SVV angle of deviation (left panel) and SVV angle (right panel), both with ranges for normal subjects and data from a case with a left unilateral vestibular deficit.

To learn more about SVV we recommend you watch the following webinar

References and caveats
Schönfeld U., Clarke A. H. – 2011: A Clinical Study of the Subjective Visual Vertical during Unilateral Centrifugation and Static Tilt; ActaOtolaryngol 131(10):1040-50 

December 2017

vHIT alone cannot easily be used as a tool to decide if someone is suitable for rehab as there are many factors to consider, although it is an integral part of the test battery. 

Once rehabilitation has commenced then a further application of vHIT is a tool to measure status central compensation at various points throughout the rehabilitation pathway. If rehabilitation is successful then catch up saccades should group together and reduce in latency. At this point there isn’t any recommendations of when a patient has reached their optimum state following rehab but there is a lot of researchers looking into this.

February 2017

The answer here depends on the stimulus and recording parameters, and guidelines for these may vary e.g. between different regions. For this reason it is often recommended that vestibular clinics gather their own normative data.  The British Society of Audiology guidelines indicate a value of < 8 º / sec for all 4 irrigations as indicative of bilateral weakness (BSA 2010) whereas another commonly used criteria is that the total response should be 20 °/sec or greater. Zapala et al (2008) present an extensive literature review of the ranges shown in different studies, and provide a description based on several thousand cases with caloric response data.

Please note that the law of large numbers applies when considering the interaural ratios (canal paresis and directional preponderance). The low overall numbers in a case of bilateral caloric weakness make these ratios unstable so should be ignored when this is the case. 


References and caveats
Zapala, D.D., Olsholt, K.F., and Lundy, L.B. (2008) A comparison of water and air caloric responses and their ability to distinguish between patients with normal and impaired ears. Ear and Hearing 29 (4) pages 585-600.

September 2017

The mono-thermal (“screening”) version of the caloric test refers to stimulating at one temperature on each ear. In many cases the results from two irrigations will be sufficient to inform the tester without the need for all four irrigations associated with the alternating binaural bithermal (ABB) test. This is useful both for saving time and sparing the patient as much discomfort as possible (Adams et al 2016).

With the mono-thermal screen when the interaural asymmetry falls below a certain minimum value (with the overall response size above a certain value amongst other criteria) it is statistically unlikely that two further irrigations will alter the conclusion (the “screen” has returned a “pass” outcome).

However if asymmetry results fall above a certain value (in addition to other criteria), the “screen” has returned a “refer” outcome i.e. four irrigations are needed to fully characterise the vestibular status. The two existing results are used with the two additional results to provide a calculation of caloric weakness and directional preponderance within the same session; i.e. the results of screening are not gathered separately to the ABB test.

There has been interest in the possible order effects in the bithermal caloric (e.g. Lightfoot, 2004) as well as the use of cool or warm stimuli as part of the mono-thermal screening process. The rationale generally given for using the warm stimulus instead of the cool is as follows: firstly we must consider the possibility of a spontaneous nystagmus produced by a unilateral caloric weakness. Both cool and warm stimuli should reveal this scenario. However, if the unilateral weakness was also accompanied by directional preponderance (often associated with a spontaneous nystagmus) then the directional preponderance will be expected to interact with the caloric nystagmus either constructively or destructively. Secondly we must consider that the warm stimulus has an excitatory effect whereas the cool stimulus has an inhibitory effect. That is, the warm stimulus causes utriculo-petal deflection of the cupula in the horizontal semi-circular canal, which leads to an increase in neural firing in the associated vestibular nerve. The cool stimulus draws the cupula in the opposite direction and leads to a decrease in neural firing on the associated vestibular nerve.

The relevance of this is that a combination of caloric weakness in one direction and directional preponderance in the opposite direction can interact to produce similar sized responses with monothermal cool stimuli (indicating no apparent asymmetry when one does exists). The reason is that a cool (inhibitory) response may not overcome the interaction effects of the directional preponderance on the caloric induced nystagmus. However, this is less likely to occur with warm (excitatory) stimuli. [On the contrary, it should enhance the asymmetry.]  In terminology used for describing test performance, the cool stimuli are more likely to lead to a false negative. Minimising such false negatives is a priority, but in so doing, one would inevitably increase the number of false positives. The specificity of the cool test would therefore become lower (than the warm), reducing the benefit in terms of timesaving and minimising patient discomfort.

Lightfoot et al (2009) report that in practice around half of patients are spared the ABB test when using monothermal warm stimuli, whereas only around one quarter of patients are spared using the cool stimuli (higher false positive rate).

 

___________________________________________________________________________________________________________________________________________________________________________________________________

References

Adams, M., et al (2016) Monothermal Caloric Screening Test Accuracy A Systematic Review. Otolaryngology– Head and Neck Surgery 154(6) 982–996

Lightfoot, G (2004). The origin of order effects in the results of the bi-thermal caloric test. Int J Audiol; 43:276-282.

Lightfoot G., et al (2009). The derivation of optimum criteria for use in the monothermal caloric screening test. Ear Hear;30:54-62.

October 2018

Our clients say

„As a student, it has been a great supplement to what we learn in class, I expect that I will continue to make use of these excellent resources. I love the whole concept of Interacoustics Academy.“

Doctoral Student of Clinical Audiology Graduate Assistant to Director, Audiology Clinic University of South Florida, Tampa Dept. of Communication Sciences & Disorders Luke Wassermann

Earn CEU or CPD points

Access educational topics that enhance your knowledge and improve your clinical efficiency. Currently available from: AAA. You can still claim independent points for other professional bodies.

Learn more

Extensive Documentation

Looking for help? Take a look at our support center to find extensive documentation. From tips on getting started to how to test guides — everything can be found here.

Learn more

The views expressed on this website are the professional opinions of the author(s) and are not necessarily those of Interacoustics A/S. This website is intended for the sole use of professionally trained healthcare professionals and Interacoustics A/S assumes no liability for the failure by healthcare professionals to conduct standard, professional behavior.
Find ways to buy our products: Visit an Interacoustics sales office, call 45-6371-3555, or find a distributor.
Features and/or functions may not be available for all countries or all areas and product specifications are subject to change without prior notification.