Session 7 - vHIT Flashcards
vHIT, Video Head Impulse Test
What is vHIT?
and objective efficient means of testing the vestibular system that provides immediate results regarding side specific vestibular dysfunction
what are the problems with traditional HIT?
1 -subjective results as it’s the examiners opinion whether or not an abnormal eye movement has occurred in response to head movement
2 -the human eye can only detect the more obvious eye movements that occur after head movements (known as Overt Saccades)
3- the human eye isn’t capable to detecting eye movements that occur within the head movement (known as Covert Saccades)
why is vHIt better than traditional HIT?
1 - vHIT using EyeSeeCam allows the professional to accurately and objectively assess the dizzy patients for vestibular dysfunction on the 1st office visit without the time and expense of ENG saving the patient and healthcare professional time and money
2- the EyeSeCam is objective, portable and reliable and easy to use making vHIt ad obvious choice as a starting point for Vestibular Assessment
what does vHIt measure?
vHIt is a measure of the VOR in response to head movement
-a patient with a healthy vestibular system should be able to keep their eyes focused on a object even when head is in motion
which is faster: VOR or pursuit?
VOR, remember shaking the paper vs shaking your head
What is Catch-up saccade?
in patients, with vestibular dysfunction, when the head moves, the eyes will move with the head, requiring a corrective movement back to the target = inability to remain focus
how does vHIT tell us a patient has catch-up Saccade?
-vHIT captures this abnormal eye movemnt, analyses and puts it into a digital graph which is easily read and understood
what equipment is used in vHIT?
dedicated goggles made of 2 major components:
1- Inertial Measurement Unit (IMU)= is a combined Gyroscope and Accelerometer that measures head movement WHILE
2- Infrared Camera captures records eye movement
How is vHIT perfromed?
1- secure goggles
2-perform brief calibraton of eye movement
3- exmainer asks patient to maintain eye focus on a stationary object
4-the examiner manipulates the pateint head with quick precisie head movements
when putting the goggles on for vHIt, what do you ensure?
the goggles must be fitted securely to avoid slippage
how long does vHIT take to perform?
the entire process should take less than 10 mins to perform and provides immediate side specific results
what does the EyeSeeCam software do?
-its designed to perform intricate and repeatable calculations of the relationship between head and eye movements to determine whether there is a weakness in 1 or both vestibular systems
- shows an analysis of instantaneous gain. specifically at 40, 60 and 80milli seconds
- the software provides and analysis of velocity regression meaning a calculation is derived form a comparison of eye and head velocity in the time range of 0-100 milli seconds after impulse onset, to give a quick estimate of symmetry of symmetry between vestibular systems
how does vHIT represent its calculations?
-as tracings which clearly show the presence of overt and covert saccades
what is the clinical application of vHIT?
- to differentiate between no vestibular dysfunction and peripheral vestibular dysfunction and central vestibular dysfunction
what does a vHIT showing no vestibular dysfunction mean?
normal horizontal semi-circular vestibular dysfunction at high velocity head movements between -4 Hz
what is the next step after seeing either peripheral or central vestibular dysfunction from vHIT?
referral for more advance testing such as:
- Video Nystagmography
- Electrocohleography
what are the clinical benefits of EyeSeeCam?
- Quick, side specific information regarding vestibular function
-Portable equipment allows for bedside testing for non ambulatory patients. Further recommendation for care can be made based on vHIT results
- easily tolerated by most patients as its quick test that utilises natural head movements
-utilises natural head movements that emulate normal, daily activities particularly in 1-4 H range (such high frequency info can only be attained by vHIT
-accurate data interpretation and storage allows for objective reporting ad comparsions of vestibular nerve function to measure vestbular compenstation over time to see effectiveness of vestibular therapies
- can be used as a pre-operative and post operative evaluation of vestibuar function
which patients may not be able to tolerate the vHIT?
- (those who are those with cervical issues with the neck or spine
- those who cannot relax enough for quick head movements required
- patients who are visually impaired as they won’t be able to see the target
what are some unique features of EyeSeeCam?
1- Lightweight goggles with built-in laser calibration.
2- Switchable left/right camera with ball-and-socket design.
3- Instantaneous gain plots at 40, 60, and 80ms for detailed data analysis.
4- Velocity regression analysis for frequency range assessment.
5- USB-driven for easy plug-and-play setup.
6- Generates PDF reports for quick review.
which is normal overt or covert?
neither
what are the benefits of vHIT?
- vHIT is a quick, well tolerated test
- vHIT yields side specific information
- vHIT guides further recommendations and treatments
- vHIT increases your efficiency by saving both time and money
What happens in our balance systems when we
move?
- Head movement activates semicircular canals: Detects rotational motion.
- Signals travel via vestibular nerve: Superior/inferior branches send signals to the brainstem.
- Central processing: Vestibular nucleus complex and cerebellum integrate signals with visual and proprioceptive input.
- Reflex output: Activates pathways like the vestibulo-ocular reflex (VOR) for stable vision and vestibulospinal reflex (VSR) for balance.
what is the purpose of the Head Impulse Test?
- its described as the bedside test of semi-circular canal function
- it examines the ability to maintain fixation on a target without the generation of abnormal eye movements during an extremely fast angular acceleration of the head
What is the mechanism of head impulse testing?
- Angular acceleration (thrusting head) is applied during head movements.
- Asymmetry occurs between inhibitory and excitatory neural firing patterns in the vestibular system. - excitation on the side you’re thrusting the head to and inhibits the other side to 0 (no neural activity).
- The angular reflex (a component of the VOR), controlled by the semicircular canals, compensates for head rotation to maintain stable vision.
It only tests for VOR because its too fast for other tests like optokinetic.
on which side would you expect a abnormal catch up if the patient had bilateral vestib abornmality?
both sides
what are the limitation of the bedside head impulse technique?
- Sensitivity and Specificity: Mean sensitivity is 46%, specificity is 94%.
- Subjectivity: Relies on the examiner’s ability to interpret and detect overt saccades, leading to variability based on technique and experience.
- Covert Saccades: Difficult to detect covert saccades that occur during head movement.
- Lack of Feedback: Provides no feedback on the quality or frequency of head impulses.
- easy to miss people with small vestibular abnormalities
Why is vHIT considered accurate for assessing vestibular function?
- Sensitivity and Specificity: vHIT has high sensitivity and specificity, validated against scleral coils with comparable specificity.
*Reasons for Accuracy:
- High-speed camera captures precise eye movements.
- Gyroscopes measure head movement accurately.
- Sophisticated software provides detailed analysis.
What is the overview of vHIT?
*vHIT is a simple test.
*Procedure:
- Involves passive, unpredictable, and abrupt head turns.
- Uses angular acceleration to reach peak head velocity, followed by immediate deceleration.
*Normal Response: In a healthy subject, both eyes move to compensate for head movement, maintaining stable vision.
How are all six semicircular canals assessed during vHIT?
- RALP (Right Anterior Left Posterior): Assessed by head tilts stimulating the right anterior and left posterior canals. (diagonal tilting)
- Lateral Canals: Assessed by lateral head movements (side-to-side). (horizontal movement)
- LARP (Left Anterior Right Posterior): Assessed by head tilts stimulating the left anterior and right posterior canals. (diagonal in opposite plane)
What are the parameters for testing the lateral semicircular canals during vHIT?
- Displacement: 10°–20°ensuring appropriate stimulation without excessive motion. (this isnt a of of movement)
- Peak Head Velocity: 100°/s – 250°/s high enough for VOR stimulation but not too rapid for patient safety.
- Peak Head Acceleration: 1000°/s² – 2500°/s² providing sufficient angular acceleration to activate the lateral canals effectively.
How are the anterior and posterior semicircular canals tested during vHIT? LARP and RALP
*The head is tilted 35°–45°, and the patient maintains gaze at a fixation point during small, abrupt head movements.
- Amplitude: 10°–15°.
- Velocity: 100°–150°/s.
*Movements:
- Downward stimulates the anterior canal.
- Backward stimulates the posterior canal.
what are limitations of vHIT testing?
- patient with any neck injury ar unsuitable
- patients who are visually impaired cant do it because they cant wear glasses because of goggles and have to see the target
How many impulses are required during vHIT testing?
- Each impulse: Represents a separate measure of vestibular function. so you should be able to see a abnormality of the first impulse
- Repetition: Improves confidence in results.
- Recommendation: 20–30 impulses are suggested by some manufacturers.
- Key Focus: Prioritize quality of impulses over quantity.
What are common pitfalls during vHIT testing?
- Goggle Slippage: Movement of the goggles can affect accuracy.
- Predictable Head Movements: Reduces test reliability as the patient may anticipate the motion. (non rhythmic like an audiogram)
- Patient Positioning: Sitting too close to the target can distort results as eyes can cross over due to the target being too close.
What should be analyzed during vHIT testing?
- Focus on more than just saccades or gain.
- Both are complementary indicators of vestibular loss and should be interpreted together for accurate assessment.
are overt and covert saccades normal?
- they are both abnormal and mean there is a vestibular abnormality.
- its has some diagnostic properties as it can tell us about any compensation strategies, how long ago someone had a vestibular episode? whether they started to compensate for that? it can indicate management plans?
What does the absence of saccades indicate?
The absence of saccades in a patient with impaired VOR could indicate a severe abnormality in the vestibular system, central nervous system, or visual pathways.
What is the difference between overt and covert saccades? and how do they look on the trace?
- Overt Saccades: Occur after head movement.
- Covert Saccades: Occur during head movement.- they are harder than to spot than the overt as by the time the head movement stops, the eyes will be on the target.
how do you read a vHIT trace?
-the blue is the head movement
- the green is the eye movement
- if the green movement is after the blue, its and overt sacade
would small lines on the vHIT trace indicate abnormality?
no, they are random deviation eye movements, saccades would be lots of them, constant (around the same time), clustered
What aspects of saccades should be analyzed during vHIT?
- Presence or Absence: Check if saccades are present or absent.
- Latency: Measure the time delay before the saccades occur.
- Consistency: Evaluate how consistent the saccades are across impulses.
what does the gain in vHIT measure?
the difference between the head movements and the eye movements (ideally 1:1 ratio so should be 1 but this is not always the case due to things like visual acuity)
What are the normal and abnormal gain values for vHIT?
*lateral Canals:
- Normal: >0.8 – <1.2
- Abnormal ->Unilateral Loss: 0.1 – 0.8
- Abnormal - Bilateral Loss: <0.1
*Vertical Canals:
- Normal: >0.7 – <1.2
- Abnormal - Unilateral Loss: 0.1 – 0.7
- Abnormal - Bilateral Loss: <0.1
what does a low gain tell us?
the eye wasn’t keeping up with the head movement and the saccade is the corrective mechanism that is implemented to get the eyes back onto the target
how is the middle hexagon thing read with the vHIT results?
so it tells us the gain.
- if the line is reaching the outside line then the gain is 1
- the inside line is 0.5
- if its marked in green, its a normal gain and has fallen within the normal parameters
what is the diagnostic benefit of vHIT testing?
e.g. caloric give info about ONLY the lateral SCC and the nerves that supply it and takes 1 hour
BUT
vHIT assesses all 6 SCC and their afferent pathways in around 15 mins
- can be used when other tests cant be done like calorics cant be done with middle ear abnormalities
- non evasive, paediatrics
- we use it early on to give us info on whether their history matches up with the vHIT and gives indication on what further testing we need to do -MDT with MDT
