|
|
Head-Impulse Test (HIT) and Head Heave test (HHT)
This material is intended for clinicians and vestibular scientists.
Page last modified: March 19, 2012
The HIT is a bedside technique used to diagnose reduction in vestibular function in one ear vs. the other. It is one of several bedside methods offering similar information. The table below provides our opinions regarding their general clinical characteristics.
The term "HIT" is generally used for a sudden rotational movement about the up-down axis of the head. The term "impulse" is used to roughly denote that this is a very sudden movement- -similar to an "impulse" of velocity.
A similar "impulse" can also be delivered about a linear axis -- this is generally called the "head heave test", with the word "heave" denoting that the movement is lateral along the interaural axis. In both cases, the trajectory of motion is an "impulse" -- of rotational velocity in one case, and of linear velocity in the other, so the nomenclature could be improved.
| Test | Sensitive | Specific | Vulnerability to bias | Durability | Clinical Utility |
| Head-Impulse Test, head-heave test | Middle | High | High | high | Low |
| Head-Shaking test | High | Middle | Low | low | Medium |
| Vibration Test | Middle | Middle | Low | very high | High |
| head heave test | Unknown | Unknown | High | Unknown | Probably low |
Bottom line: Practically, the HIT test is just not reliable -- it doesn't always work (i.e. see movie below) and it is vulnerable to bias. The vibration test is a better choice.
HIT (head impulse test) -- rotational version.
Click here to see movie of HIT test in a person with a vestibular nerve section (on the left side). While there is little question that this person has a very significant vestibular lesion, as caloric testing documented only a 4 deg/sec response in the left ear (right ear had 40), we find it hard to appreciate the 10:1 difference in vestibular responsiveness from the HIT recording.
The patient's head is rapidly rotated by the examiner (abruptly and with high acceleration) about 20 degrees to the right or left. The patient is told to fixate on the examiners nose. After the head stops, the examiner watches the patient's eye to see whether or not a refixation saccade is needed to get the patient's eye on the examiners nose. A reliable and significant refixation saccade is judged as positive.
This test requires a subjective judgement on the part of the examiner, who also controls the stimulus.
The HIT test also is vulnerable to prediction as patients who know which way their head will be turned, can generate "covert" saccades. (Weber et al, 2008). Another name for these saccades are "vestibular catch-up saccades". (Tian and Crane, 2000).
The HIT test requires rapid head movements, which may cause neck pain in persons with arthritis. So far, we are unaware of any reports of carotid or vertebral dissection due to the HIT, but we would expect that it would have a similar prevalence of vascular compromise as do similar chiropractic maneuvers. Heart block has been rarely reported after the HIT (Ullman and Edlow 2010).
HHT (head heave test)
The HHT is so far not used often at the bedside. The head is moved linearly along the interaural axis (Kessler et al, 2007). The heave is about 5-6 cm in excursion. There are some practical problems with this procedure as it is difficult to confine the head to a single direction, and also (perhaps), some safety concerns incurred by very rapid acceleration/acceleration movements similar to those that occur from chiropractors or in whiplash injuries.
Like the HIT, the head-heave test depends on a subjective judgement of the examiner, and is likely affected by prediction (although the test is very little studied).
Allowing the patient to rotate their own head (active head rotation) is not as sensitive as the passive HIT as patients can use prediction to normalize their performance (Black et al. 2005).
Instrumentation (scleral eye coil recording) can make this test capable of detecting disturbances of vertical canals (Cremer et al, 1998). Practically however, it is not possible to use scleral eye coils in routine clinical practice. Similarly, motorized devices can be used to automate the test (Aalto et al, 2002). Again, this is impractical for clinical use.
The HIT, like head-shaking nystagmus, is a method of measuring asymmetries in vestibular gain. The asymmetry in vestibular gain was first observed by Ewald (Ewald 1892), and is referred to as Ewald’s second law. In its specific form it states that ampullopetal endolymph flow in the horizontal canal causes a greater response than ampullofugal endolymph flow (Ewald 1892; Baloh and Honrubia 2001). In its general form it states that excitation is a relatively better vestibular stimulus than is inhibition (Leigh and Zee 2006). Ewald’s second law is thought to be due to the inability of inhibitory stimuli to decrease vestibular nerve firing rates to less than zero (Baloh, Honrubia et al. 1977; Hain and Spindler 1993).
 |
| Bilateral mathematical model of vestibular processing from Hain (Hain, Fetter et al. 1987). This early model does not include acceleration in the canal transfer function, but otherwise contains the general features described below. |
We can take these ideas and convert them into a more quantitative form. We will not do this formally, but just try to get the general concepts converted into a mathematical form. First, firing rate in the vestibular nerve is mainly proportional to head velocity, but there is also a component due to head acceleration. So one can (approximately) say that:
Firing rate = K0+Kv*head-velocity + Ka * head-acceleration
(Vestibular physiologists would shudder at this oversimplification). In the figure above, the acceleration term has been left out -- so to model the HIT, one would need to add an extra term to the canal transfer function as well as to the central processing in order to take out the acceleration sensitivity (so that eye velocity doesn't become proportional to head acceleration).
Continuing, because firing rate has a lower limit of 0 spikes/second, there is a saturation of the overall response for the nerve. In the diagram above, this is the little box with the curved line (nonlinearity) inside of it.
Net firing rate = saturation*Firing rate
The vestibular system is hooked up in "push-pull" so that the firing rate for one side is subtracted from the other. In other words:
central firing rate = right-left.
This is the circle with the +- in the center. When both vestibular nerves are working, net firing rate (right-left) shows a saturation in both direction.
When just one vestibular nerve is working, there is a strong response for rotation in that excites the remaining nerve (i.e. contralateral to the lesion for the lateral canal), and a weaker response for rotation for rotation towards the lesion.
Other machinery in the model above that are not necessary to cover right now are the cross-connected boxes that implement velocity storage. This is needed to explain head-shaking nystagmus, but irrelevant to the HIT.
This (fairly) simple theory explains the HIT in quantitative terms.
The HIT is reported to be very sensitive to complete vestibular loss but insensitive to mild or moderate vestibular loss (Beynon et al, 1998; Hamid 2005; Harvey et al, 1997). In our clinic setting, we think the HIT test is poorer than the vibration test for localization, as it can fail and is vulnerable to bias.
As a general comment, because this test depends so heavily on either subjective judgments on the part of the examiner, or inaccessible technology (scleral eye coils), its utility is probably limited to use by "vestibular experts".
The HIT appears to be most useful as part of a rapid battery of bedside tests (as described above). (Mandala et al. 2008). By combining several quick clinical tests, the "dizzy doctor" can quickly determine the status of the inner ear. We do not advise reliance on the HIT test.
| © Copyright April 6, 2012 , Timothy C. Hain, M.D. All rights reserved. Last saved on April 6, 2012 |