Our technique of SVV assessment at certain angles of head tilt offers a standardized protocol to detect disturbances of the graviceptive function with higher redundancy compared to non-dynamic methods representing an easy-to-perform technique in clinical practice. The main advantage of this technique is its higher information content and its simple clinical applicability. As shown in our manuscript, this technique has already been applied for the detection of treatment response to botulinum toxin in patients with cervical dystonia.
Basically this method can be used for the assessment of graviceptive dysfunction in a variety of diseases. The application of this method in patients with specific cortical lesions may help to localize cortical areas involved in graviceptive processing. Fixation of the patient's head at the desired angle is crucial for the method.
To further avoid bias data, we recommend a randomized protocol considering tilt angle to avoid a learning effect. Install the patient in a stable chair with a backrest and a head-fixation unit. The fixation unit maintains the patient's head in a stable and defined position.
It consists of an elastic headband and a U-shaped headrest which can be fixed to each other using an adhesive strap. The headrest can be adjusted in the desired inclination angle by aligning it along the scale of a goniometer, which is attached to the chair's backrest. At the beginning of the experiment, adjust the headrest at a zero-degree inclination at sub-occipital height.
Place the elastic headband on the patient's head and fix it with a screw on the back. Connect the two adhesive straps on the headband and on the headrest with each other. Mount the SVV unit with the fixation device on the chair in front of the patient.
Through a connected potentiometer, adjust the position of the light bar in the roll plane exactly opposite to the patient's head and at the same level as the patient's eyes. Connect the SVV unit to the electrical connection underneath the chair. Place the potentiometer in the patient's left hand and instruct them on how to perform the SVV setting.
To calibrate, tilt the light bar 30 degrees to the right or left relative to the absolute vertical and ask the patient to adjust it to the vertical position under visual control. This serves to self-calibrate the patient and to check the visual motor ability of the patient. Open the examination protocol for simultaneous entry of the SVV estimates.
Close the cabin door so that the patient is in complete darkness throughout the experiment. Ask the patient via the intercom system to tilt the light bar in the starting position 30 degrees to the right or to the left. After a waiting period of 15 seconds, instruct the patient to adjust the light bar from the starting position until it reaches the subjective vertical.
The patient is not under time pressure and can still correct the set position at any time. The patient confirms the setting verbally via the intercom system. Enter the tilt angle shown on the display in degrees in the protocol.
In total let the patient adjust the SVV in six passes with the starting position of 30 degrees randomized. Undo the initial head fixation by disconnecting the adhesive straps. Loosen the headrest and adapt the tilt position according to the protocol, 15 degrees or 30 degrees to the right or to the left.
Fix the headrest in this position firmly. Fix the patient's head with the elastic headband to the headrest. Ensure that this head tilt is tolerable for the patient and adapt the height of the headrest if needed.
Instruct the patient to maintain this head position during the trial. Close the cabin door and perform the trial as in the neutral head position. Upon completion of the trial, undo the head restraint and adjust the headrest according to the randomized head tilt position given by the protocol.
Close the cabin door again and perform the same procedures until all SVV settings in all head tilts have been recorded. In this study, SVV measurement was performed in 13 healthy individuals at a mean age of 52.8 years. The absolute tilt of the SVV from the adult vertical at zero-degree head position showed an SVV median of 1.33.
At a head tilt of 15 degrees, an SVV median of 1.66 was achieved and the measurements of the SVV at a head tilt of 30 degrees yielded an SVV median of 5.33. The method was also used to analyze SVV tilts in 32 patients suffering from cervical dystonia. Assessment of the SVV at the patient's habitual head posture revealed major deviations from the actual vertical with a median of 2.65 degrees.
In comparison to healthy individuals at their habitual head posture, approximately zero-degree head tilt, the patients'response was significantly impaired with a median difference of negative 1.34 degrees. Three weeks after injection of botulinum toxin, the patients'SVV estimates in habitual head position and at 30-degree head tilt did not differ anymore from those of the controls. Exact positioning of the patient's head is crucial for the experiment.
We highly recommend using a randomized protocol for the tilt angles in order to avoid a detection bias. We ourselves use this technique to evaluate the effects of botulinum toxin injection on the verticality perception in patients with cervical dystonia.
