|Year : 1984 | Volume
| Issue : 3 | Page : 127-131
A new approach in the study of saccadic eye velocity
Prem Prakash, Pradeep Sharma, V Menon, DK Gahlot
Dr. R.P. Centre. for Ophthalmic Sciences, ARMS, New Delhi, India
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, New Delhi
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Prakash P, Sharma P, Menon V, Gahlot D K. A new approach in the study of saccadic eye velocity. Indian J Ophthalmol 1984;32:127-31
|How to cite this URL:|
Prakash P, Sharma P, Menon V, Gahlot D K. A new approach in the study of saccadic eye velocity. Indian J Ophthalmol [serial online] 1984 [cited 2020 Dec 2];32:127-31. Available from: https://www.ijo.in/text.asp?1984/32/3/127/27404
Comparison of saccadic velocity in patients with ocular motility-defects with those of normal subjects has been suggested to be a simple yet accurate way of uncovering paresis of an extra-ocular muscle[']. Several authors have compared the saccadic velocities of cases of established ocular motility defects with saccadic velocities of normal subjects. But there is a marked intersubject variability in the saccadic velocity even in normal subjects,,. Not only an inter subject variability, but also a variability in the saccadic velocity of the same subject at different times of recording has been observed. In such a circumstance it is very difficult to distinguish a normal subject with "normally low" saccadic velocity from one with some ocular motility defect. To detect subtle defect in extraocular muscles at an early stage, the comparative saccadic velocities have to be considered. The aim of this article is to suggest a definite indicator to evaluate the comparative/relative extraocular muscle functioning and to assess its value on normal subjects and in cases of lateral rectus palsy.
The comparative saccadic velocities have to be considered. The aim of this article is to suggest a definite indicator to evaluate the comparative/relative extraocular muscle functioning and to assess its value in normal subjects and in cases of lateral rectus palsy.
| Material and methods|| |
Horizontal ocular saccades were generated by voluntary refixation method (VRM) and recorded electroculographically in ten normal subjects, and ten cases of lateral rectus palsy whose ophthalmic and orthoptic examination had been done. Mingograf-800, a multichannel precision recording device with preamplifiers, amplifiers and ink-jet oscillograph was used with the following parameters :
Time Constant = 2.5 Sec.
Frequency 30 Hz.
Sensitivity = 500 Microvolts.
Paper speed = 50 mm/Sec.
The calibration was set such, that an excursion of 30° by the eye gave a deflection of 15 mm on the recording paper.
5 skin electrodes were used, one at each canthus on either side and a neutral electrode on the central forehead.
During recordings, one eye was made to maintain fixation, while the other was covered with a rubber shield, these recordings were taken with either eye fixating alternately.
The subject was made to fixate alternately at the two fixed spots so that the eye moved 30° in the process. The alternate refixation was regulated by a metronome.
The saccadic velocity was calculated as a mean of 3 consecutive readings from the recording by measuring its slope with a Moore cg Wright vernier protractor, at the mid saccade under the desk magnifying device (X 7). Subsequently the saccadic velocity were calculated with the help of a digital calculator (Casio-2600 fx.) by the already described method.
| Observations|| |
[Figure - 1] shows saccadic eye movements in a case of right lateral rectus palsy by voluntary refixation. The under acting saccades, both in amplitude as well as velocity are conspicuously discernible.
[Table - 1] shows the marked variability observed in the saccadic velocities of normal subjects. However it was also noted that the saccadic velocities of different muscles in the same individual did not have much variation.
[Table - 2] shows the saccadic velocities of the lateral recti of the cases with lateral rectus palsy. The lower saccadic velocities of the clinically paretic muscles are evident but these could not be characterised in statistical terms for the simple reason that the standard deviation among normal subjects was so wide that the lower limit of the normal range with a difference of 2 standard deviation (M-2 s.d.) would be ridiculously, below zero. Thus even the clinically diagnosed cases with apparently poor saccades could not be classified as paretic with statistical certainty. With this problem in view, a new concept, that of S. quotient has been evolved.
The S. Quotient is the ration of the saccadic velocity of one muscle of the right eye over the saccadic velocity of the similar muscle of the
figuratively as the S. Quotient of the medial recti. This ratio was calculated for each set of muscles both with right eye fixating and with left eye fixating.
The S. Quotients were calculated as described above for all the ten normal subjects. The mean of the S. Quotients for each muscle group was calculated and the range of S. Quotients with a variation of 2 standard deviations M ± 2 s.d.) was fixed [Table - 3].
Further S. quotients were calculated for the lateral rectus palsy group and these were compared with the normal range as fixed above. It was noted [Table - 4] that nine of the ten cases of lateral rectus palsy clearly showed abnormal S. quotient with left eye fixation and eight out of ten with right eye fixation. Not only was a palsy of the concerned lateral rectus muscle evident but also evident was the medial rectus overaction of the contralateral synergist in some cases. Out of the two cases of lateral rectum palsy which showed normal S. quotients, one (Case L. 3) was a case in which muscle transposition surgery and resection-recession procedures had been done earlier. The other (Cases L-10) was a case of recovering lateral rectus palsy in which presumably the saccadic function, being subserved by fast-twitch fibers, had recovered enough though ocular deviation had still persisted to some extent indicating a disparity between abduction paralysis and ocular deviation.
[Figure - 2] shows the mean S. quotient of the medial and lateral recti in cases of lateral rectus palsy. It clearly depicts that the S. quotients of lateral recti are either too low or high depending on whether the right lateral rectus is affected. The S. quotient of medical recti also show a non-uniform pattern indicating a medial rectus overaction in the corresponding medial rectus (Contralateral synergist).
| Discussion|| |
As has been earlier suggested that interpretation of individual saccadic velocities are not conclusive in determining its function, comparisons of the saccadic velocities of the like muscles of the two eyes has been devised in the form of S. quotient.
The S. quotient highlights the comparative saccadic velocities of like-muscles of the two eyes. This being a ratio, obviously, obviates the intersubject variation, it also by-passes the common technical errors, if any, in the saccadic velocity. It has been observed that these S. quotients correlate well in normal subjects. being close to unity, and can be very useful for comparison of saccadic velocity of different subjects, normal or abnormal or of the same subject at different lines.
Earlier Metz had used the concept of a percentage function of one muscle as compared to that of its antagonist. The S. quotient is scientifically more appropriate as the comparison is here being made in the functioning of the similar muscles of the two eyes.
It should be however noted that in the concept of S. quotient, the stress is on the asymmetry between the involvement of the two eyes. Thus, in a theoretical possibility of equal involvement of the muscles of the two eyes, the S. quotient will appear to be fallacious. But in such rare cases there has to be a bilateral identical decrease of saccadic velocities of the affected muscles of the two eyes, which in itself will be striking as compared to saccadic velocities of the other muscles. In such evident cases there is hardly any need of a sensitive parameter like S. quotient, and moreover such bilaterally identical involvement will be rarely encountered. However, it is suggested for such rare circumstances that the ratio of saccadic velocity of one muscle (the suspected palsied muscle) with the mean of the saccadic velocities of other three muscles may be taken into consideration.
The S. quotient seems to be a highly satisfactory index by which statistical significance can be attached to the abnormal functions of a muscle, and the comparisons can be made between the saccadic velocities of different individuals and in the same individual at different times. For the same reason they seem to be sensitive indicators of the subclinical paresis.
| Summary|| |
A study of saccadic velocities was conducted in a group of 10 normal subjects and 10 cases of lateral rectus palsy. There was wide variation in normal so much so that the saccadic velocities of paretic cases were within the limits of standard deviation. Therefore a new approach in such studies is suggested in the form of S. Quotients, which is a ratio of saccadic velocity of like-muscles of the two eyes of the same individual.
| References|| |
Metz. H.S., Scott A.B., Meara D.M., Steward H.L., Arch. Ophthalmol. 84:453-460-1970.
Boghem D.M., Trooost B.T., Daroff R.B., Dell osso L.F. Birkctt J.E., : Invest, Ophthalmol, 13:619-523 1974.
Taumer R, Lemb M, Namislo M,: Albrecht Von Graefes Arch. Klin.Ophthalmol 200 (2) 163-74, Aug, 1976. As quoted in Excerpta Medical Ophthalmology, 1977.
Prakash P, Sharma P, Menon V, Gahlot D.K. (1983) Proceedings of the 43rd All India Ophthalmological Society Meeting, Pune.
Scgmidt D, Abel L.A., Dell' Osso L.F, Daroff R.B, Aviat. Space Environ. Med. 50 (4):393-5 April, 1979.
Gay A.J., Newman N.M. Keltner J.L., Stroud M.H, (pp 27-46) in Eye Movement Disorders, St. Louis C.V. Mosby, 1974.
Virdi P.S. (1978) Proceedings of the III meeting of International Strabismological Association, Kyoto, Japan, Edited by Reinecke. R.D.
Metz. H.S, Arch. Ophthalmol 97 (5):901-3 May, 1979.
[Figure - 1], [Figure - 2]
[Table - 1], [Table - 2], [Table - 3], [Table - 4]