Anatomy and Physiology: The Otolith System
The vestibular otolith organs consist of the saccule and utricle. The saccule lies inferior to the utricle in the vestibule. The utricle and saccule are in contact with one another anatomically, but do not have direct connections to each other.1 Endolymph is communicated between the utricle and saccule via the endolymphatic duct and with the cochlea via the ductus reuniens.1 The saccule is primarily located in a vertical plane under the stapes footplate.2 The utricle is primarily oriented horizontally to the horizon and near the anterior opening of the horizontal semicircular canal.1
The sensory membrane of each organ is called the maculae. The maculae are covered by the otolithic membrane which consists of otoconia. Otoconia are composed of calcium carbonate crystals. The stereocilia of the macular haircells project into the otolithic membrane. The stereocilia are oriented along a center line called the striola. The striola curves through the center of the maculae. On each side of the striola, the kinocilia are oriented in opposite directions facing the striola in the utricle and turning away from the striola in the saccule. Type I haircells are located in abundance the closer you get to the striola. Due to this opposite orientation of the kinocilia and the curvature of the striola in both otolith organs, they are sensitive to multiple directions of linear acceleration.1
Orientation of the stereocilia on the macula of the Saccule facing away from the striola and on the Utricle macula facing the striola.
The otolith organs receive their blood supply from the Anterior Inferior Cerebellar Artery via primarily the Anterior Vestibular Artery for the Utricle and the Posterior Vestibular Artery for the Saccule. Innervation of the Utricle derives from the Superior Division of the Vestibular Nerve. The Saccular innervation derives from the Inferior Division of the Vestibular Nerve.3
The physiology of the otolith organs confirms that they are linear acceleration sensors. In other words, they sense the direction of gravitational force. They respond to both static tilt and dynamic linear acceleration. Therefore, if the head orientation is changed relative to gravity or gravitational forces acting upon the head, the otoconia and endolymph movement causes the activation of the macular haircells. In the utricle, sitting or standing with the head held in a normal neutral position causes a baseline firing rate which changes with head tilt and linear acceleration. A head tilt or linear acceleration in the direction of the test ear causes an increase in firing of haircells located medially to the striola and a decrease in those haircells located laterally to the striola.4 The utricle striola is curved so that tilts of the head or linear accelerations forward and backward also cause haircell activation and inhibition. The otolith organs are not only sensitive to simple displacement but also to how fast (velocity) that displacement occurs. Exactly which cells of the utricle cause the related vestibule-ocular reflexes and postural muscle tone changes is under intensive investigation by many researchers.
General Overview: Otolith Evaluation
Diagnostic evaluation of the utricle organs of the vestibular system requires specialized equipment to separate utricle function from saccule and semi-circular canal function as well as the right sided utricle from the left side utricle. The horizontal semi-circular canal function is clinically evaluated by sinusoidal rotation tests, caloric stimulation, head thrust, or vestibular evoked potentials. Anterior and Posterior semi-circular canal function is typically not evaluated clinically, but can be evaluated with head thrusts in the planes of the canals. Saccular function is clinically evaluated with Vestibular Evoked Myogenic Potentials. Utricular function can be clinically evaluated using Subjective Visual Vertical tests and Dynamic Unilateral Centrifugation. Thus, we can currently delineate through standard clinical tests abnormalities in the horizontal semicircular canal, saccule and utricle.
Background: Dynamic Unilateral Centrifugation
The Dynamic Unilateral Centrifugation test was developed by Andrew Clarke et al 5,6,7,8,9 and further refined by Floris Wuyts et al.10 It is the only test that both delineates utricle function from both saccule and semicircular canal function and gives utricle laterality information in a quick and comfortable protocol for the patient.
The Mechanics of Dynamic Unilateral Centrifugation
During Dynamic Unilateral Centrifugation test the patient is seated comfortably in a chair that is accelerating slowly (4-6 deg/sec²) up to a constant speed of 300 to 400 degrees per second. Typically, after 30-60 seconds the horizontal semicircular canal vestibular ocular reflex (VOR) ceases to respond. The semicircular canals respond to changes in acceleration but do not sense constant velocity. This part of the reason why you do not notice that an airplane is still smoothly flying forward once it reaches and maintains a constant speed for about a minute. The initial reduction of the horizontal canal semicircular response is accomplished in a dark enclosure to remove visual cues that would cause illness. The patient watches a visual suppression laser dot to suppress the VOR. The suppression laser dot reduces significantly the sensation of dizziness while the horizontal canal VOR response is declining.
Once the horizontal semicircular canal VOR responds has vanished, the Neuro Kinetics Neuro Otologic Test Center (NOTC) Dynamic Centrifugation begins positioning the patient for testing. The patient’s chair is moved right or left of center axis (translation) by the NOTC motors, requiring no entering of the clinician to manually move the chair. Unilateral stimulation is generated by translating the patient’s chair slowly over 30 seconds to the right by 3.85 cm from the original center vertical rotation axis (fig.1, profiles 1, 2, 3). The right off-axis position is then held for 30 seconds. This places the left utricle on axis and the right utricle off axis (fig. 1, off-axis right). The patient, if they have normal utricle function, may feel a sensation of the chair tilting outward as they translate with the chair to the side. Remember, throughout the test the patient is maintaining a constant speed of 300 or 400 degrees per second.
The patient is then translated in the chair in 30 seconds to the center (fig.1, profiles 4, 5, 6), waiting at the on center-axis position for 30 seconds. In the final test position, the patient is translated in the chair to the left in 30 seconds at 3.85 cm off axis (fig.1, profiles 7, 8, 9) and this position is held for 30 seconds. When the patient is translated to the left, the right utricle is on axis and the left utricle is off axis. While in each off axis position and center, the patient sets a subjective visual vertical line and the torsional rotation of their eyes is measured. All the while, the patient is maintained at a constant velocity.
Once each side is tested, the patient is translated in the chair back to on center-axis location (center fig.1, profiles 10, 11, 12). The suppression light is again turned on and the chair velocity is gradually and comfortably slowed to a stop.
The Physiology of Dynamic Unilateral Centrifugation
The utricle on the same side of the chair translation movement has centrifugal force and gravitational force acting on it. The combination of these forces is called Gravity Inertial Acceleration (GIA). GIA causes ocular counter-rolling (the eyes have ocular torsion) to the opposite direction of the chair movement in normal patients. Thus, if the chair moves to the right, the right utricle has the forces of GIA acting upon it which causes a left ocular counter-rolling (referenced from the patient), and visa versa for a leftward movement of the chair. The side chair translation produces a unilateral centrifugal acceleration on the off-axis utricle of .2 to .3 g depending on chosen constant chair velocity of 300 to 400 deg/sec. It also gives a corresponding tilt of the GIA of 12 to 24 degrees at the stimulated utricle that is equal half of GIA at the center of the head (6 to 12 degrees).
|Left (negative) Ocular Counter-Roll
Chair is translated to patient’s right
|Right (positive) Ocular Counter-Roll
Chair is translated to patient’s left
Evaluation of Ocular Counter-Roll
The induced utricular ocular counter-roll is evaluated by two separate methods with the Neuro Kinetics NOTC Dynamic Centrifugation test.
The first method is using Subjective Visual Vertical (SVV) to evaluate the lateral utricle response of ocular counter-rolling in response to the gravity inertial acceleration (GIA) force generated in the Dynamic Centrifugation test. The patient sets a Subjective Visual Vertical line during full speed rotation while in each off-axis position (lateral right, lateral left and center). It is reflective of the ocular counter-roll because the patient’s eyes are turned in the head to offset the GIA and they will set the line to match this ocular turn. SVV reflects the neurologic response of the vestibular system up through the thalamus and vestibular cortex.8
The second method is to monitor and plot eye torsion (the induced ocular counter-rolling response) directly as a function of GIA via Neuro Kinetic’s 4D VOG I-Portal® System. Eye torsion as a function of GIA is a direct ocular-utricle reflex and reflects lower neurologic vestibular function.8
Example of Eye Torsion (Ocular Counter-Roll) as a function of GIA. This example shows a normal, symmetrical eye torsion (Ocular Counter-Roll) response. When GIA is positive it is acting on the right utricle and the eye torsion induced is negative in direction (leftward). When GIA is negative it is acting on the left utricle and the eye torsion induced is positive in direction (rightward).
Example of Eye Torsion (Ocular Counter-roll) as a function of GIA. This example shows a right utricle weakness. As the GIA is positive (chair moves right) there is very little torsion or torsion in the wrong direction (positive plotted rightward torsion), and as GIA is negative (the chair moves left) there is significant eye torsion in the rightward direction (plotted positive on the graph).
Example of Subgective Visual Vertical result. This example shows a normal, symmetrical responds.
What are Normal and Abnormal Evaluation Findings?
A normal utricle would sense GIA and cause an ocular counter-roll (eye torsion) when in either off axis position. If the chair is off-axis to the patient’s right side inducing a left ocular counter-roll, the SVV line will be set with the top of the line tipped to the left (negative). At 300 deg/sec chair constant velocity and 3.5 cm right off-axis placement of the test utricle, this angle varies between -2.7 to -11.0 degrees.7 Leftward eye torsion (induced OCR, referenced from the patient) varies between -0.8 to -2.1 degrees.10
If the chair is off-axis to the patient’s left side inducing a right ocular counter-roll the SVV line will be set with the top of the line tipped to the right (positive). For 300 deg/sec chair constant velocity and 3.5 cm left off-axis placement of the test utricle, this angle varies between +2.9 to +13 degrees.7 Rightward eye torsion (induced OCR, referenced from the patient) varies between +0.8 to +2.1 degrees.10
If the patient has a utricular weakness, they will set the SVV line, while off axis in the direction of the weakness, with less tilt than reported above because the damaged utricle is not properly sensing the GIA and sending that information to the thalamic and vestibular cortex levels of the brain. The torsional eye response (induced OCR) would reflect less torsion than the degrees reported above. The Utricle-Ocular response is not receiving enough input from the off-axis utricle’s damaged response to GIA to induce the response and thus eye torsion.
Why should I choose the Neuro Kinetics I-Portal NOTC Dynamic Unilateral Centrifugation Test?
- No other test system reduces patient discomfort by first suppressing and eliminating the horizontal canal VOR response prior to individual utricle testing.
- No other test system monitors both neurologic levels of lateral utricle function in the same test by using both Subjective Visual Vertical responses and eye torsion as a function of GIA.
- No other test system allows you to confirm patient accuracy and thus rule out malingering with eye torsion as a function of GIA.
- The I-Portal Dynamic Unilateral Centrifugation Test gives easy to understand clinical results.
- The I-Portal Dynamic Unilateral Centrifugation Test can be customized for research protocols.