The eye movements that are mediated by the semicircular canals through the vestibulo-ocular reflex (VOR) are well-known and used in many clinical tests of vestibular function.
These reflexive eye movements are generated whenever there is an asymmetry in the right-left paired semicircular canals.
The asymmetry can be induced by caloric or head rotation stimuli or can result from a peripheral vestibular lesion involving one of the semicircular canals or its afferent neural pathways.
There is a similar reflex that originates from the otolith organs and generates eye movements in response to right-left otolith asymmetries6.
These eye movements, also known as ocular tilt reaction (OTR), can be observed during the ocular counter roll (OCR) test4.
In this test, the patient begins with the head upright and then the head is tilted toward the right or left shoulder in the roll plane (Figure 1).
The movement can be done manually by the examiner or by the patient.
One can also use an instrumented chair to move the whole body instead of rotating the neck.
Figure 1: Eye movements in response to head tilts in the roll plane.
The two components of OCR eye movements
The OCR eye movements that follow the head tilt consist of two components (Figure 2)5.
The dynamic component is a transient component that begins during or shortly after the head tilt and lasts for several seconds. This component is meditated primarily by the vertical semicircular canals with minimal contribution from the otoliths3.
The static component of OCR is a persistent shift of the eye position that lasts as long as the head is tilted. This component of OCR eye movements is mediated by the otoliths and their central pathways6.
Figure 2: Dynamic and static ocular counter-rolling (OCR) responses.
The typical static OCR responses are depicted in Figure 1.
It consists of approximately equal torsion of both eyes in the opposite direction of the head tilt.
In normal individuals, the amount of torsion is proportional to the head tilt and approximately equal in both eyes.
However, unlike the eye movements that are generated by the semicircular canals, the amplitude of otolith-mediated eye movements is significantly lower at approximately 10-20% of the head tilt5.
In addition to the torsional eye movements, static OCR responses also include vertical eye movements with the upward shift of the ipsilateral eye and downward shift of the contralateral eye.
This vertical misalignment of the eyes is known as the skew deviation.
In this case, the skew deviation is a normal reaction to the asymmetry of the otolith pathways induced by the head tilt.
However, spontaneous skew deviation is often caused by central lesions and less frequently by the peripheral vestibular lesions involving the otoliths or their pathways1.
Until recently, routine clinical implementation of the OCR test has been hampered because of the difficulties in the measurement of torsional eye movements.
However, a robust method for measurement of torsional eye movements has been developed and implemented in the VisualEyesTM software which should allow more widespread use of the OCR test.
Clinical application of OCR
The static component of OCR is considered a test of the peripheral and central otolith pathways1,3,5.
As such, it is expected that the OCR findings will be correlated with other tests of otolith function.
In particular, the subjective visual vertical (SVV) test has been used extensively as a perceptual measure of the static OTR.
Similarly, correspondence between the vestibular-evoked myogenic potential (VEMP) findings and the OTR has been demonstrated in both normal individuals and in patients with vestibular abnormalities6.
In one study by Otero-Milan et al (2017), patients with peripheral vestibular lesions had a smaller amplitude of static torsion in response to head tilts compared to that in normal individuals5.
Compared to unilateral lesions, the amplitude of the response was smaller for bilateral lesions5.
However, the study did not find a significant difference between the responses for head tilts toward or away from the side of lesion.
This contradicts findings of another study by Lim et al (2017) and is inconsistent with the SVV findings in patients with otolith abnormalities.
The discrepancy may be due to the small sample size and because Otero-Milan et al (2017) used only one angle of head tilt (30°).
SVV findings show consistently higher underestimation of the tilt angle for head tilts toward the side of lesion.
The SVV findings have been shown to change with compensation.
Similarly, the OCR responses are expected to reflect the state of vestibular compensation.
One should interpret the OCR findings with caution because both peripheral and central abnormalities are known to affect the OTR1.
In fact, OTR findings in many central patients include triad symptoms of skew deviation, tonic ocular torsion, and head tilt.
These abnormalities are common in brainstem, thalamus, and other central otolith pathway lesions.
One possible distinction is that central lesions seem to produce disassociated abnormalities in the right and left eyes.
Furthermore, the tonic ocular torsion can be either ipsilateral or contralateral to the side of lesion1.
In summary, the ability to record torsional eye movements with high level of accuracy allows us to utilize the OCR test to assess the otoliths and their central connections.
About the authors
Kamran Barin, Ph.D., is Assistant Professor Emeritus, Department of Otolaryngology-Head & Neck Surgery and Department of Speech & Hearing Science, The Ohio State University. He established and served as the Director of Balance Disorders Clinic at the Ohio State University Medical Center for over 25 years until his retirement in June 2011. He received his Master’s and Doctorate degrees in Electrical/Biomedical Engineering from the Ohio State University. He has published over 80 articles and book chapters and has taught national and international courses and seminars in different areas of vestibular assessment and rehabilitation.
Michelle Petrak, Ph.D., is the Director of Clinical Audiology and Vestibular Research for Interacoustics. Her primary role is development and clinical validation of new technologies in the vestibular and balance areas. She is located in Chicago where she is a licensed private practice clinical audiologist at Northwest Speech and Hearing (NWSPH). Dr. Petrak received her doctorates in Electrophysiology (1992) and Biomolecular Electronics (1994) from Wayne State University and her Masters in Audiology in 1989. Her special areas of expertise include vestibular and balance assessments and management of the dizzy patient. Dr. Petrak is involved with new innovative product developments, clinical evaluations of new protocols, and publishing, teaching and training on the management of patients with dizziness. She continues to lecture extensively nationally and internationally, and she has numerous articles published in the hearing industry journals. She also participates on the committees for several doctoral students as support for the research projects.
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