Electronystagmography in Migraine Equivalent Syndrome

Objective The aim of the study was to determine the efficacy of electronystagmography testing in the diagnosis of vertigo in children with migraine equivalent syndrome. Methods The investigation included 20 patients, aged between 5 and 14 years and affected with an “migraine equivalent syndrome” (group A). The patients, head blocked, sat on a “Tönnies rotatory chair” which was placed in the middle of a rotatory cylindrical chamber (2 metres in diameter and 1.9 metres in height). The rotatory cylinder was driven by a direct current engine which turned it clockwise and counterclockwise, up to 200 degrees/sec., and its internal area was covered with 32 black vertical contrasts. As a control group, 50 healthy children were identified. All the subjects underwent the rotatory vestibular stimulation by Stop test, to optokinetic stimulation and to contemporary rotatory vestibular and optokinetic stimulation (VVOR). Results For the analysis of the results, we have considered nystagmus mean gain and direction of visual-vestibular-ocular-reflex (VVOR) nystagmus. In group A, all the children presented a VVOR nystagmus homodirectional to vestibular-ocular-reflex (VOR). In the control group, all the subjects presented a VVOR nystagmus homodirectional to optokinetic nystagmus. Conclusions In the healthy patients, VVOR nystagmus is always homodirectional to OKN and indicates the optokinetic system prevalence on VOR. The presence of a VVOR nystagmus homodirectional to VOR indicates the absence of the optokinetic system prevalence due to a central nervous system (CNS) modification and highlights a CNS disease. Our data highlight a possible correlation between CNS disorders and migraine equivalent syndrome.

Nystagmus

Nystagmus is involuntary eye oscillations initiated by slow eye movements that drive the eye away from the target. In contrast, saccadic dyskinesia consists of involuntary, fast eye movements that take the fovea off target. Nystagmus usually arises from lesions in the 1. Vestibulo-ocular system (VOR) 2. Gaze-holding system 3. Smooth pursuit and optokinetic system. 1. Pendular versus jerk ■ Pendular (see A in the figure below): both phases are slow eye movements. ■ Jerk (see B, C, and D in the figure below): one phase consists of fast eye movements (quick phase), and the other consists of slow eye movements. By convention, the direction of nystagmus is named after the direction of quick phases that return the eye to the target. 2. Plane: horizontal, vertical, torsional, or combined form (e.g., rotary, elliptical) 3. Conjugacy ■ Conjugate: Both eyes move in the same direction with similar amplitude and frequency. ■ Disconjugate: Both eyes move in the same direction with different amplitude and frequency (e.g., internuclear ophthalmoplegia). ■ Disjunctive: The eyes move in opposite directions (e.g., oculomasticatory myorhythmia seen in Whipple’s disease).

Independent visual threshold measurements in the two eyes of freely moving rats and mice using a virtual-reality optokinetic system

Slow horizontal head and body rotation occurs in mice and rats when the visual field is rotated around them, and these optomotor movements can be produced reliably in a virtual-reality system. If one eye is closed, only motion in the temporal-to-nasal direction for the contralateral eye evokes the tracking response. When the maximal spatial frequency capable of driving the response (“acuity”) was measured under monocular and binocular viewing conditions, the monocular acuity was identical to the binocular acuity measured with the same rotation direction. Thus, the visual capabilities of each eye can be measured under binocular conditions simply by changing the direction of rotation. Lesions of the visual cortex had no effect on the acuities measured with the virtual optokinetic system, whereas perceptual thresholds obtained previously with the Visual Water Task are. The optokinetic acuities were also consistently lower than acuity estimates from the Visual Water Task, but contrast sensitivities were the same or better. These data show that head-tracking in a virtual optokinetic drum is driven by subcortical, lower frequency, and contralateral pathways.

Response variability and information transfer in directional neurons of the mammalian horizontal optokinetic system

This study is concerned with how information about the direction of visual motion is encoded by motion-sensitive neurons. Motion-sensitive neurons are usually studied using stimuli unchanging in speed and direction over several seconds. Recently, it has been suggested that neuronal responses to more naturalistic stimuli cannot be understood on the basis of experiments with constant-motion stimuli (de Ruyter van Steveninck et al., 1997). We measured the variability and information content of spike trains recorded from directional neurons in the nucleus of the optic tract (NOT) of the wallaby, Macropus eugenii, in response to constant and time-varying motion. While the NOT forms part of the mammalian optokinetic system, we have shown previously that the responses of its directional neurons resemble those of insect H1 in many respects (Ibbotson et al., 1994). We find that directional neurons in the wallaby NOT respond with lower variability and higher rates of information transmission to time-varying stimuli than to constant motion. The difference in response variability is predicted by an inhomogeneous Poisson model of neuronal spiking incorporating an absolute refractory period of 2 ms during which no subsequent spike can be fired. Refractoriness imposes structure on the spike train, reducing variability (de Ruyter van Steveninck & Bialek, 1988; Berry & Meister, 1998). A given refractory period has a greater impact when firing rates are high, as for the responses of NOT neurons to time-varying stimuli. It is in just these cases that variability in experimentally observed spike trains is lowest. Thus, differences in response variability do not necessarily imply that different models are required to predict neuronal responses to constant- and time-varying motion stimuli.

The pigeon optokinetic system: Visual input in extraocular muscle coordinates

The generation of compensatory eye movements in response to rotational head movements involves the transformation of visual-optokinetic and vestibular signals into commands controlling the appropriate eye muscles. Previously, it has been shown that the three systems (optokinetic, vestibular, and eye muscle) share a similar three-dimensional reference frame. In this report, we suggest that a peculiarity in the structure of the horizontal recti in pigeons demonstrates that the optokinetic system is organized with respect to the eye muscles rather than the vestibular canals. Measurements of the orientation of the plane for each of the lateral and medial recti were obtained. These were compared with the direction preferences of optokinetic neurons responsive to horizontal motion, namely “back” units in the nucleus of the basal optic root (nBOR), “forward” units in the pretectal nucleus lentiformis mesencephali (LM), and “vertical axis” (VA) Purkinje cells in the flocculus. The average direction preference of LM neurons excited in response to forward (temporal to nasal) visual motion, and VA Purkinje cells in response to optokinetic motion in the ipsilateral visual field was approximately parallel to the visual horizontal. This corresponded to the orientation of the medial rectus, which was also approximately parallel to the visual horizontal. The average direction preference of nBOR neurons excited in response to backward (nasal to temporal) visual motion, and VA Purkinje cells in response to optokinetic motion in the contralateral visual field was approximately 20–30 deg down from the visual horizontal. The orientation of the lateral rectus was also approximately 20–30 deg down from the visual horizontal. These data suggest that the incoming optokinetic signals are organized with respect to the outgoing extraocular muscle commands.