Otolith Testing: Ocular Counter Roll Test

This video is taken from the webinar: Beyond bedside testing: The importance of objectifying the ocular counter roll test. In this section, Dr. Kamran Barin will describe the different tests of otolith function and will explain the principles of, and how to perform, the ocular counter roll test, using a case study to demonstrate abnormal results.

You can read the full transcript below.

Otolith function tests

This brings us to the clinical tests of the otoliths in the order of most commonly used. They include VEMPs (oVEMPs and cVEMPs), subjective visual vertical (SVV) test, ocular counter-rolling (OCR) test, and off-axis or eccentric rotation.

There are other methods that have not made it to widespread clinical use. For example, linear sleds or tilted rotation methods are mostly research tools. Same goes with the galvanic stimulation.

VEMPs

Let's briefly discuss VEMPs before we move on to the other tests. VEMPs are short-latency myogenic potentials that are evoked in response to loud sounds. We now know that their origin is the vestibular system and not the auditory system.

Cervical VEMPs

Cervical VEMPs or cVEMPs are inhibitory responses that originate from the saccule and travel to the inferior portion of the vestibular nerve to the vestibular nuclei, and then through the medial vestibulo-spinal tract, and finally activate the ipsilateral neck muscle.

Ocular VEMPs

Ocular VEMPs or oVEMPs are excitatory responses that are mediated primarily to the utricle, superior portion of the vestibular nerve, vestibular nuclei, medial longitudinal fasciculus, and finally activate the contralateral eye muscle.

Because of the few strands of the nerve fibers from the saccules that travel to the superior portion of the vestibular nerve, some people dispute the notion that oVEMP is strictly a test of the utricles. But over the years, a general consensus has emerged that the oVEMPs are primarily a test of utricular pathways.

Advantages and disadvantages to VEMP testing

Like any other test, there are advantages and disadvantages to VEMP testing.

The advantages of VEMPs include the fact that they provide independent assessment of each side. That's important. They can be measured with the same equipment used for auditory evoked potentials. VEMPs are site-of-lesion diagnostic tests meaning that their findings depend only on the presence or absence of lesion, and they do not change with compensation.

That's of course an advantage but it can also be considered as a disadvantage because we always like to know not only just the presence of a lesion, but also the level of compensation to that abnormality. Other disadvantages include the fact that the stimuli are not physiologic and also the fact that there's a high degree of variability between individuals and between tests.

What is the OCR test?

Let's talk about other otolith tests that are the focus of this presentation. The ocular counter-rolling or the OCR test consists of tilting the head toward either the right or left shoulder in the role plane.

As you can see in this video, the head starts upright and then tilted left and held there for several seconds. The head is then returned to the upright position and then tilted to the right. Three-dimensional eye movements including torsion are recorded throughout the test.

Physiology of the OCR test 

The movements that you just saw provoked the otolith-ocular reflex, which causes compensatory eye movements by generating static torsion of the eyes in the opposite direction.

Unlike the VOR, the gain of otolith-ocular reflexes is only about 0.2, which means the eye movements are not fully compensatory. The head tilt may also generate small, vertical skew deviation, which shows up as the ipsilateral eye moving upward and the contralateral eye moving downward.

The origin of skew deviation is not fully understood but it's possible that they originate from the saccules whereas the torsion is likely to originate from the utricles. Notice that the hair cell activation and deactivation is more complex than we see in the canals.

In other words, some hair cells in one utricle are in excitation mode and others are in inhibition mode. And a different set of hair cells are in excitation mode and inhibition mode on the other side.

In a person with normal otolith function, there is no static torsion in the upright head position and the responses for the right and left head tilts are approximately symmetrical.

OCR responses

Here you see a video of torsional eye movement recordings in response to head tilts. This is from the Interacoustics VisualEyes™ software during the ocular counter roll test. First the head is upright. And after a few seconds, the head is tilted to the left. Again after a few seconds, the head is brought back to the upright position and tilted to the right.

The response consists of two components.

The dynamic component is the transient nystagmus that begins during or shortly after the head tilt and lasts for several seconds. This component is mediated primarily by the vertical semicircular canals with minimal contribution from the otoliths.

The static component of OCR is the persistent torsion of the eye position that lasts as long as the head is tilted. This component is mediated by the otoliths and their central pathways.

As you can see here, the responses to the rightward and leftward head tilts are approximately symmetrical in this individual.

Normative OCR values

The normative values for the OCR test are provided by a study in 2017 from the Hopkins group [1]. The mean static torsion for a 30-degree head tilt is approximately 4.65 degrees. Although we're not measuring the head tilts, it's easy for the examiner to practice and passively move the head to approximately 30 degrees during each trial.

Pathology of the OCR test

In patients with a unilateral otolith lesion, the changes in the excitation and inhibition pattern of hair cells affects perception of vertical alignment. In the upright head position, these patients will have static torsion of the eyes toward the side of lesion and skew deviation as if the head was tilted contralateral to the side of lesion.

In addition, the ocular tilt response may become somewhat asymmetrical with smaller responses for head tilts toward the side of lesion.

Abnormal OCR results

Here you see the ocular counter roll test results of a patient with complaint of rocking sensation and feeling of being on a boat.

The oVEMPs show smaller responses for the right side compared to the left side. The OCR responses for the left head tilt are larger than those for the right head tilt. This asymmetry is consistent with the oVEMP results and signifies a right utricular abnormality. The clinical utility of the OCR test, as this particular case indicates, is the ability to provide a quantitative method and document these types of abnormalities.

Clinical testing from the same study that I mentioned before shows that the total amplitude of static torsion is significantly reduced in unilateral vestibular abnormalities. The reduction of the amplitude is even more severe for bilateral lesions.

This study shows that the asymmetries were not statistically significant. That means the test is unlikely to show you the side of lesion. The reason may be that in OCR testing, we don't know what the alignment of the eyes was before the lesion.

What we measure is the torsion from the baseline of the upright head position. In that position, the lesion has already caused static torsion of the eyes, and we don't know what the alignment was before the lesion. I will come back to this topic a little bit later on.

Clinical applications of OCR

So the clinical utility of the OCR test. It provides a fast measure of otolith function. It can detect abnormalities of the otolith-ocular pathways, but it's not always site-specific. The test procedure is simple and can be performed with video goggles. And if your device can measure torsional eye movements, then you can quantify the test results.

The OCR test results correlate well with VEMPs. The OCR abnormalities usually are severe in the acute lesions but improve with compensation. By combining the OCR and VEMP findings, one can detect both the presence of an abnormality and the level of compensation for it.

References

[1] Otero-Millan, J., Treviño, C., Winnick, A., Zee, D. S., Carey, J. P., & Kheradmand, A. (2017). The video ocular counter-roll (vOCR): a clinical test to detect loss of otolith-ocular function. Acta oto-laryngologica, 137(6), 593–597.