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ORIGINAL ARTICLE
Year : 1984  |  Volume : 32  |  Issue : 3  |  Page : 127-131

A new approach in the study of saccadic eye velocity


Dr. R.P. Centre. for Ophthalmic Sciences, ARMS, New Delhi, India

Correspondence Address:
Prem Prakash
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, New Delhi
India
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Source of Support: None, Conflict of Interest: None


PMID: 6519726

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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

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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



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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[2],[3],[4]. 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[5]. 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 Top


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[6].


  Observations Top


[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[7].

[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 Top


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[8] 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 Top


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 Top

1.
Metz. H.S., Scott A.B., Meara D.M., Steward H.L., Arch. Ophthalmol. 84:453-460-1970.  Back to cited text no. 1
    
2.
Boghem D.M., Trooost B.T., Daroff R.B., Dell osso L.F. Birkctt J.E., : Invest, Ophthalmol, 13:619-523 1974.  Back to cited text no. 2
    
3.
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.  Back to cited text no. 3
    
4.
Prakash P, Sharma P, Menon V, Gahlot D.K. (1983) Proceedings of the 43rd All India Ophthalmological Society Meeting, Pune.  Back to cited text no. 4
    
5.
Scgmidt D, Abel L.A., Dell' Osso L.F, Daroff R.B, Aviat. Space Environ. Med. 50 (4):393-5 April, 1979.  Back to cited text no. 5
    
6.
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.  Back to cited text no. 6
    
7.
Virdi P.S. (1978) Proceedings of the III meeting of International Strabismological Association, Kyoto, Japan, Edited by Reinecke. R.D.  Back to cited text no. 7
    
8.
Metz. H.S, Arch. Ophthalmol 97 (5):901-3 May, 1979.  Back to cited text no. 8
    


    Figures

  [Figure - 1], [Figure - 2]
 
 
    Tables

  [Table - 1], [Table - 2], [Table - 3], [Table - 4]



 

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