vHIT Flashcards
Video Head Impulse Testing:
what does vHIT measure?
tests measure the function of the vestibulo-ocular reflex (VOR), which is the connection between the vestibular system and the oculomotor muscles. Able to pick up on overt saccades
what’s the difference between an overt and covert saccade?
The first is an overt saccade. This is when the saccade occurs after the head comes to a stop. These are visible to the naked eye and detectable in the bedside test. However, there are some patients that learn to initiate the saccades during head movements. These are called covert saccades because they happen so quickly that they are not visible to the naked eye.
What is the video head impulse test?
The video head impulse test (vHIT) is a quantitative assessment of the VOR. In vHIT, eye movements are recorded and analyzed by high-speed cameras and head movements are measured by motion sensors embedded in goggles.
how do you do the vHIT?
- Calibration: (Eye Calibration: The goggle includes a built-in laser array that projects a five-dot pattern onto the wall in front of the patient. The patient is instructed to look at each of the dots in turn as directed by the examiner. This process provides a plot of the eye position relative to the five points, which can be reviewed and used throughout the test)
Head calibration: Head movements can be passive, where the clinician moves the patient’s head, or active, where the patient moves their own head. shaking the head yes and then no. - Lateral Head movements: The head impulses should be small amplitude (less than 15 degrees) and high velocity (more than 150 degrees per second).
- vertical head impulse: For vertical head impulses, the test can be performed in the plane of the right anterior, left posterior (RALP) canals or in the plane of the left anterior, right posterior (LARP) canals. The head is then impulsed either downward and 45 degrees to the right (right anterior stimulation) or upward and 45 degrees to the left (left posterior stimulation).
The impulses should be performed in a similar fashion to the horizontal method. However, a slower velocity of 100 degrees per second is acceptable in the vertical planes.
what is a normal vHIT result?
The first pattern is the normal vHIT pattern which is represented by the absence of significant catch-up saccades and VOR gains that are greater than 0.8 bilaterally.
abnormal response
covert or overt saccade.
to consider saccades what should we look out for?
Frequency
Direction
Latency
Velocity
Frequency
First, how many of the impulses produce significant catch-up saccades and at what head velocities?
Direction
Second, are the catch-up saccades in the same or opposite direction as the VOR eye movements?
Latency
Third, are they short-latency or long-latency saccades and what is the intersaccadic latency?
When considering the latency, concentrate on the latency of the first saccade for each head impulse. The value for short-latency saccades is usually less than 225 milliseconds and higher than that for long‑latency saccades.
Some systems give you the values and allow you to edit the profiles if something is not right. The measured latencies depend on the saccade algorithm and norms may be specific to each system.
Velocity
Currently, most devices characterize saccades using the peak-saccade velocity. But this may not be the best measure because the peak-saccade velocity is related to the amplitude and duration of saccades, especially when there’s more than one saccade during each impulse. For example, if the duration is large, then the peak velocity may be lower.
In addition, both the peak velocity and latency are affected by filtering that’s necessary to condition the eye velocities. So, it’s conceivable that a different parameter – namely cumulative saccade amplitude – may be a more stable parameter for characterizing the catch-up saccades. But it’s not being used currently.
what is a unilateral loss?
(look at notes app)
A unilateral loss is epresented by the presence of significant catch-up saccades on one side along with asymmetric gain (usually less than 0.8 for side of lesion). This pattern indicates a unilateral lesion involving the ipsilateral semicircular canal or its branch of the vestibular nerve. Catch-up saccades may also be present for impulses away from the side of lesion, but they are not as large, and they usually start at higher head velocities.
Spontaneous nystagmus in vHIT
Unlike catch-up saccades, spikes for spontaneous nystagmus can occur before or after head impulses. Also, the velocity of spontaneous nystagmus fast phases is usually much smaller than the velocity of catch-up saccades. In acute lesions, fast phases of spontaneous nystagmus will be intermixed with catch-up saccades for impulses toward the side of lesion. But they will be in the opposite direction of typical catch-up saccades for impulses away from the side of lesion
Acute unilateral lesions
In this case, you have the characteristics of a unilateral loss as discussed earlier but the patients will also have spontaneous nystagmus and these fast phases of spontaneous nystagmus will be intermixed with catch-up saccades.
what are some limitations of doing the bedside head impulse test instead if using vHIT?
decreased accuracy.
Mean sensitivity of 46% and specificity of 94%
* Very subjective
– Relies on examiners ability to interpret and detect overt saccades
– Early results were very variable due to technique and experience
* Difficult to detect covert saccades occurring during head movement (Weber et
al 2008)
* No feedback as to how well impulses are delivered or at which frequency
What does RALP mean and how do you test?
“LARP” means Left Anterior, Right Posterior and refers to a head rotation in an orientation which best stimulates this co-planar pair.
keep eyes centred in the middle, but move head to the right a little. small but quick movements going up and down. these movements are unpredictable. then relax dn put head back to centre.
What does RALP mean and how do you test?
Right Anterior, Left Posterior and refers to a head rotation in an orientation which best stimulates this co-planar pair.
keep eyes centred in the middle, but move head to the left a little. small but quick movements going up and down. these movements are unpredictable. then relax dn put head back to centre.
