Equipment used for vestibular exploration and rehabilitation: High Frequency Compatible Rotatory chair
The rotatory chair is an essential tool for vestibular reeducation. It is a chair whose axis of rotation passes by the vertical axis of rotation of the head. Rotation of the chair should be as smooth as possible. In other words, any resistance should be reduced to a minimum to be able to rotate the chair while expending the least amount of energy, with speeds ranging from 10°/s to over 400°/s at a steady pace.
It should be comfortable, allow the subject to keep still when rotating at high speeds, with a head rest in order to avoid any unnecessary movement. There should be a seatbelt, a foot rest with a gadget to stop the subject’s leg from changing position during high-speed rotations.
It should be mounted onto a stand that provides it with a great deal of stability:
- regardless of the speed of rotation,
- regardless of the force required to make it rotate,
- regardless of the position that the subject is required to be in the chair.
The chair is where the patient will spend the whole session; it is used for the questions and answer sessions, it is used for evaluations and it is used for the reeducation.
Movements during a clockwise rotation clockwise rotation (1): the endolymphatic liquid moves in the opposite direction (2).
The slow phase nystagmus is in the direction of the flow of the liquid (3)
The fast-phase saccadic eye movement (4) in the opposite direction.
The “+” sign indicates the side stimulated during said rotation.
(in Functional Exploration in ENT Courtat et al Masson Ed.)
The observation of the spontaneous nystagmus (provoked) is carried out using magnifying glasses. These have evolved over time. The first models, or Bartel spectacles, were magnifying glasses used to increase the size of the eye. These developed into Frenzel goggles. These were the same magnifying glasses but mounted into a frame with two small lamps attached on either side to shine light directly onto the eyes. The lamps mounted on the side are not really visible to the subject unless the eye is particularly eccentric. These goggles are supposed to make things more comfortable during observation and thanks to the magnifying effect, eye movements with very little amplitude can be observed. Furthermore, they are supposed to prevent ocular fixation and therefore avoid inhibition of any peripheral nystagmic response.
The miniaturisation and advances made in technology allow us to use a small video-surveillance camera that films in infrared, and is therefore invisible to the eye (Erik ULMER). Adapting these cameras to proper frames and enabling to work total darkness enables us to avoid any form of visual feedback. These video goggles enable us, with the help of a monitor, to observe the eye movements of a subject when all they see is black. This new tool has allowed us to make a great deal of progress in the observation of signals provoked by anomalies in the vestibular chain.
Indeed, the absence of visual feedback results in an increase in the sensitivity of the observation and a decrease in the response threshold to stimulation. Thus the placing of a video mask on the subject when seated will allow us to observe a “downbeat” spontaneous nystagmus which is none other than the consequence of a descending vertical movement undertaken when sitting down. The increased performance of the tools should mean increased levels of expertise of the observer, subject to only coming across subjects with disorders!
This tool has allowed us to perfect an evaluation protocol of the canalo-ocular function. This enables us to state that with VR, we act according to what we see in the moment that the subject is present. This precision in the therapeutic approach enables us to adapt any manipulation to the state of the subject in a more appropriate manner and to "pin" the signs on the symptoms. Said evaluation protocol consists of the following:
- observation of spontaneous signs in all the position of the gaze,
- observation of the counter-rolling eye movements,
- rotatory chair impulse test at constant speed,
- rapid rotatory head test,
- reflex control test.
This tool is often part of the equipment used by ENT specialists for oculometry testing. It comprises a metal bar with electroluminescent diodes mounted on top. These diodes are mounted one next to another and are controlled by an EPROM by which we are able to reproduce the different eye movements. These movements are as follows:
- random (in both time and amplitude) saccadic eye moment,
- slow eye pursuit movements with speeds of between 10°/s to 90°/s. Pursuit can be sinusoidal or triangular,
- the ramps; in other words a semi-pursuit in one direction or the other, either sinusoidal or triangular.
This bar is used for the treatment of combined movement of the head and eyes. It can be used in conventional kinesitherapy for the mobilization of the spine using a different route from the one used habitually.
In different research laboratories, or those specialising in the exploration of the vestibular function, the opto-kinetic generator consists of a rotatory chair for exploration within a large drum. Positioned on the wall of the drum are vertical test bars. When the drum begins to rotate around its vertical axis, the subject undergoing examination is presented with a series of alternating black and white vertical bars that the subject has to focus on.
We have already shown that to trigger the optokinetic reflex, the whole visual field has to be stimulated and the subject must be upright before any reflex anomalies manifest themselves. To stimulate the whole visual field, and after a great number of attempts, we commissioned the construction of a so-called planetarium projector to convert the room being used for vestibular exploration and reeducation into a “planetarium”. The planetarium is a ball, perforated with holes with an approximate 7º gap between each one. This angle roughly equates to the angle of the drum tests used in the past. In fact, this value is useful for calculating but is not envisaged for reeducation. The “planetarium” is mounted on a system of three axes that allow for all possible combination of patterns during testing. The speed of rotation of the “planetarium” along its own axis should be able to range from just a couple of degrees a second to 120°/s. The presentation of the tests should be carried out seamlessly, smoothly, regardless of the speed being used. The generator should be able to be positioned in such a way that during eye pursuit testing, gain should be equal to one. The subject should be positioned at a minimum distance of two metres from the nearest screen. The room where stimulation is being carried out should be in complete darkness and the screen, positioned immediately before the eyes of the subject should not have any details that might attract the subject’s attention.