NYSTAGMUS AND ITS MECHANISM

V.A.Filin, A.V.Filin

Moscow, 2005

We have been studying a mechanism of saccadic eye movements more than 35 years (since 1967). We used photoelectronic device for exact remote eye movements' registration (fig.1).

Fig. 1. The photoelectronic device for eye movements recoding

All-round researches were carried out in norm and in pathology (weak vision, blindness, myopia, strabismus, brain tumors, paresis and oculomotor nerves paralysis, 410 patients in total were examined) (fig.2).

Fig.2 Recording type of eye movements (horizontal component) with fixation of a steady dot (right eye) by a subject. Upper recording is for right eye, lower one is for left eye. Beam deviation upwards corresponds to eye movement to the right, downwards - to the left. Calibration of vertical line is 10, calibration of horizontal line is 1 sec time, "t" is interval between saccades, "A" is saccadic amplitude

Basing on the data obtained we drew up a concept about saccade automaticity. According to the concept a saccade is generated without any external stimuli in its own rhythm. Every person has his own pattern of saccade automaticity. Saccade parameters, particularly amplitude and orientation, modulate in dependence on external and internal conditions (amplitude - from 2/ up to 15°).

The most important evidence of concept of automaticity of saccades is the fact that all subjects have saccades and the most part of intersaccadic intervals is in the range of 0,2-0,6 sec. This is so called operating mode of automaticity of saccades and it was the same for subjects with normal vision, for weak-eyed and blind patients, strabistic patients and for subjects with brain pathologies.

It is impossible to explain by accidental coincidence this common mode of generating of saccades for so many subjects differ in great extent (norm - weak-eyed; norm - strabismus; norm - brain tumor). This is meant some mechanism exists working both in norm and in pathology. In our opinion this common mechanism is the automaticity of saccades. Another forcible argument for automaticity of saccades is the coincidence of curves of intersaccadic intervals distribution under different conditions of eye movements recording: during fixation of real and virtual dot in the darkness; during adults and newborn children sleep; when free seeing a picture. All the curves had unimodular distribution with maximum in the range of 0,2-0,6 sec (fig.3). These facts clear demonstrate that saccadic system operates as a single organism generated the same quantity of saccades in the same time.

Fig. 3. - Distribution of intervals between saccades under various observation conditions. 1 and 2 - during babies' and adult's quick sleep respectively (470 and 2343 intervals between saccades processed, 7 and 3 subjects); 3 and - fixation of a steady and virtual dots respectively (2362 and 1215 intervals between saccades processed, 11 and 8 subjects); 5 - free seeing of a picture (2000 intervals between saccades processed, 5 subjects); X-axis - intervals between saccades, s, Y-axis - number of measurements, %

Thus unlike the idea about the determined character of the most of saccadic eye movements, we propose the concept of automaticity of saccades as basic law of saccadic activity and as the background for variable oculomotor activity. Micro- and macrosaccades, nystagmus, fast eye movements are special cases of automaticity of saccades. The concept of automaticity of saccade make it possible to change radically the conception of mechanism of saccadic eye movements and forced us to revise critically some generally used theses. Nystagmus is a disturbance of saccade automaticity (V.A. Filin, 1987). Unfavourable factors such as mechanical compression of the saccadic centre structures by tumour, space accelerations, motion disease, intoxication disturb a normal mode of operation of the saccadic centre resulting in marked eyes oscillations known as nystagmus in clinical practice (fig.4).

Fig 4. Samples of records of eye movements of patient with varolian bridge tumor and left side paralysis of sight (age 30 years old, OD mobility: outside - 400, inside - 00; OS: outside - 00, inside - 420; visual acuity OD - 0,1; OS - 0,6). 1 - fixation of immovable point be right eye and changing of fixation points; 2 - fixation of virtual point in the darkness; 3 and 4 - deviation of eye in extreme right and left positions respectively, right eye (3) and left eye (4) is recorded. Other signs are the same as for fig.2

Thus, nystagmus is a special case of saccadic automation (fig.5).

Fig. 5. Average values of frequency (f), amplitude (A) and coefficient of asymmetry (K) of saccades in norm (N) and in patients with different levels of oculomotor system affection: M - muscles, BIP - basal isolated paralysis, ST - supratentorial affection, IP - intratube, EP - extratube affection, FID - fixation of immovable dot, FVD - fixation of virtual dot

We can see in practice the transformation of automaticity of saccades into nystagmus. When diverting eyes to the left the number of saccades directed to the left and their amplitudes are increased (fig.6), and as a result we can see these saccades by our own eyes. When fixing a dot located straight in front of eyes the number of saccades was the same, but they were directed evenly to the right and to the left.

Fig. 6. Samples of records of one subject's eye movements when fixing a dot located straight in front of eyes (1) and at 200 and 400 to the left respectively. Arrows show the location of eyes. Other signs are the same as for fig.2

Hence the nystagmus of an extreme position is not some special type of eye movements. It is usual automaticity of saccades, but orientation of these saccades is changed and their amplitude is increased.

Fig. 7. Samples of records of postrotatory nystagmus and involuntary saccades after rotation of a subject in Barani armchair and after recoding eye movements without the dot of fixation: 4 - 20 turns in 40 sec from the right to the left; 5, 6 - continue of 4 record. Eye movements recorded under small value of temporary constant, and as a result saccades on this record look as sharp vertical lines. Other signs are the same as for fig.2

Postrotatory nystagmus also smoothly transfers into automaticity of saccades (fig.7), and after drinking some alcohol the automaticity smoothly transfers into nystagmus of alcohol intoxication (fig. 8).

Fig. 8. Samples of records of eye movements when fixing an immovable dot before (record 1) and after drinking 150 ml of vodka (after 20 and 40 minutes respectively). Other signs are the same as for fig.2

On the basis of above mentioned, a conservative point of view notwithstanding, one can affirm that the primary source of nystagmus genesis is a quick phase, and the secondary source is the slow one.

Thus, an approach to the nystagmus mechanism from the position of the primary quick phase radically changes the attitude of practitioners - neuropathologists, otoneurologists and ophthalmologists - to accepted symptom being a nystagmus.

The concept of automaticity of saccades has given us possible to create new scientific direction that is the videoecology (Filin, 1989, 1997).

Conclusion
In norm automaticity of saccades is generated by saccadic center. Nystagmus is the consequence of disturbance of automaticity of saccades. There are various reasons of the disturbance: mechanical compression of the saccadic center structures by tumor, space accelerations, motion disease and intoxication. As a result of the disturbance amplitude of saccades is increase, and most of them are directed in one side. Thus nystagmus is an instance of the automaticity of saccades.