It is crucial, if we wish to have a comprehensive understanding of the vestibular function as a whole, to carry out an OCULOMOTRICITY EVALUATION. We do not believe that without these tests that a vestibular examination can be considered to be truly complete.
Before dealing with the detailed analysis of the testing, a couple of reminders about physiology might be required.
Binocular sight requires that the position of both maculae must strictly be maintained opposite the target image. This implies a synergy of actions between the different eye muscles. Table 3 contains the different actions of the extra-ocular muscles.
The muscles have a double attachment:
- For each eye, they are organised in opposing pairs,
- Between the eyes, they articulate as synergistic couples.
These two coupling systems are governed by the rules for reciprocal innervation (Sherrington’s law and Hering’s law). A summary of these elements appear in figure 21.
All these systems share the same effector pathway. They originate from the pontic region for horizontal movement, and the mesocephalic region for vertical movement.
Conversely, the initiator centres and the pathways that like one to the next are different, depending on whether it is a saccadic, or pursuit eye movements or an optokinetic reflex
The study of saccadic eye movement brings with it a wealth of clinical lessons to be learned. This pre-programmed ballistic eye movement is examined while asking the subject to follow a fleeting LED light, without intermediary trace, from right to left, along an LED ramp, with an amplitude of + /- 20°. The software records the eye movement and facilitates treatment and analysis.
Different parameters have to be examined:
- The general appearance of the curve allows us to observe an initial saccadic eye movement (re-fixation) which corresponds to 90% of the definitive amplitude. This is followed, around 130 msec. later, by a second saccadic eye movement (corrective) that leads the eye directly to the target.
- Latency of the saccadic eye movement is a crucial parameter. It equates to the delay between the start of the stimulus and that of the re-fixation saccadic eye movement. This is normally less than 250 msec. This stays the same for peripheral disorders, although latency increases with central disorders (Vitte et al., 1983).
- The maximum speed of saccadic eye movement should also be examined. It is a direct function of the amplitude. For an amplitude of 40°, maximum speed reaches 400°/sec. This measurement is important as it brings to light subclinical oculomotor paresis. For example, a disorder of outer-left oculomotor nerve (VI) will result in a slowing down of the saccadic eye movement of the left eye when looking to the left.
- Precision of the saccadic eye movement is defined as the rapport between the amplitude of the re-fixation saccadic eye movement and the overall amplitude of the stimulus. This is normally over 80%. Remember that this parameter is controlled by the cerebellum.
Slow eye pursuit can be comprehensively analysed using VNG. Stimulus is a slow LED ramp movement (0.4 Hz) delivered with an amplitude of +/-20° in a continuous manner. The subject is asked to focus on the target and follow it using their central or foveal vision).
It must be studies using the traces:
- The general appearance of the curve should be a smooth pursuit, with no saccadic eye movement therefore,
- Gain is calculated as the speed of the eye compared to the speed of the target. For normal subjects, this parameter should be over 0.7 (Freyss et al., 1984) and identical from one side to the other.
We believe that the study of the optokinetic nystagmus is essential.
Indeed, ”the optokinetic system is constantly paired with the vestibular system» (Waespe et al., 1977). However, for a comprehensive study, the whole visual field must undergo stimulation (Baloh and Honrubia, 1990). This is why we use total visual field generator (360°) with mobility along 3 axes. This type of stimulus alone enables us to carry out stimulations in circular per-rotational vection. Other stimuli (flat screen or LED rod, for example) only look at the foveal pursuit system and not the optokinetic system.
Indeed, one should clearly distinguish between a consecutive nystagmus and optokinetic stimulation (foveal pursuit) of a nystagmus in response to the triggering of the optokinetic reflex. The latter is only effective for testing the optokinetic system. This difference can clearly be seen in the trace that appears in the following diagram.
Recording eye movement during horizontal optokinetic eye movement (trace by A. Sémont). If we look at the trace below, we note that up to the 12-second mark, the speed of the slow-phase of the nystagmus almost reaches the same value as that of the stimulus, corresponding to the response associated with the optokinetic pursuit system, any longer than that, then the average speed falls. The pursuit system has been replaced by the optokinetic system on its own.
For our examination we use horizontal and vertical stimuli of 20°/sec. and 40°/sec. respectively. These are provided by a planetarium projector (figure 25) that stimulates the whole visual field. For a speed of 40°/sec., the normal frequency of a nystagmus is 2.9 Hz +/-0.5 while the average speed during the slow phase is 28.7°+/- 4.3 (Vitte et al., 1994).