Part 17 - HIT, vHIT

Question 1

What does the horizontal head impulse test assess?

Answer 1

A test that assesses the integrity of the horizontal VOR (Semicircular canal ipsilateral to the direction of head turn)

• aka Head Thrust
• most important test of the vestibular system

Question 2

What is the main difference in Calorics vs HIT test?

Answer 2

HIT is high frequency stimulus

Caloric is low frequency (~0.003 Hz)

Question 3

What is higher: the sensitivity or the specificity of the HIT test?

Answer 3

The specificity (94%)
- the sensitivity is low (46%), which has more to do with the abilities of the tester

Question 4

What is the advantage of vHIT over HIT?

Answer 4

vHIT tests all 6 SCCs (in pairs)

Question 5

Do “LARP” and “RALP” head impulses result in horizontal or vertical saccades?

Answer 5

Vertical

Question 6

What is the equation for VOR gain?

Answer 6

VOR gain = eye velocity/head velocity

Normal = 0.79 - 1.2 (horizontal)
0.7 - 1.2 (vertical)

Question 7

Which is faster: VOR or Saccade? Why?

Answer 7

VOR (7-15ms)

Saccades are longer (100 ms) because they are initiated at the “supervisor” (High Level Oculomotor), so they take longer than a reflex

Question 8

What are the two types of saccades?

Answer 8

Overt saccades - occurs after head and eye movement is finished

Covert Saccades (not visible) - occurs when head is still in motion

• possibly followed by small or large overt corrections
• sign of faulty VOR

Question 9

If your vHIT results show the eyes moving before the head, what might be the cause?

Answer 9

Slippage/fitting of the glasses

Question 10

What are covert anti compensatory saccades?

Answer 10

Saccades in the opposite direction that you would expect during vHIT

• sometimes seen in malingering
• reinstruct

Question 11

What do you need to remember to do before vHIT?

Answer 11

Calibration and calibration check

Question 12

How many impulses should you do in each direction for vHIT?

Answer 12

20

If pt is unable to do 20, then at least 10 and report

Question 13

What is the cutoff for normal asymmetry?

Answer 13

< 25% = normal

Question 14

Does a low left gain indicate a deficit in the left side in Menier’s in crisis?

Answer 14

No - it’s just that the inhibitory action of the right side is hyperactive

Question 15

What type of neuritis is indicated by a normal horizontal and anterior, and abnormal posterior vHIT test?

Answer 15

Superior vestibular neuritis

Question 16

Why is vHIT better than calorics?

Answer 16

• can be performed in pt with acute vertigo
• doesn’t rely on high level of patient alertness
• physiological stimulus
• well tolerated
• normally lit room
• fully portable
• fast (5-10 mins)
• all 6 SCCs
• accurate with bilateral SCC loss (symmetry and canal-specific gain)
• young children accept vHIT easier (younger than 6 yrs old)
• normal response range is tight
• can be given repeatedly (used c/ Meniere’s and gentamicin ototoxicity)
• Calorics still useful