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ORIGINAL ARTICLE
Year : 1984  |  Volume : 32  |  Issue : 3  |  Page : 133-138

The phenomenon of alternate saccadic underaction in essential alternating convergent squint


Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, Ansari Nagar, New Delhi, India

Correspondence Address:
Prem Prakash
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, New Delhi-110 029
India
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Source of Support: None, Conflict of Interest: None


PMID: 6519727

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How to cite this article:
Prakash P, Sharma P, Menon V, Gahlot D K. The phenomenon of alternate saccadic underaction in essential alternating convergent squint. Indian J Ophthalmol 1984;32:133-8

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Prakash P, Sharma P, Menon V, Gahlot D K. The phenomenon of alternate saccadic underaction in essential alternating convergent squint. Indian J Ophthalmol [serial online] 1984 [cited 2020 Dec 2];32:133-8. Available from: https://www.ijo.in/text.asp?1984/32/3/133/27405



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Ocular Saccades have been widely studied in incomitant strabismus,[1],[2],[3],[4],[5][6],[7],[8],[9],[10],[11] but the con­comitant strabismus has somehow been ignored probably because the saccades were not expected to be affected in concomitant strabismus. However certain studies have shown significant findings in fixation and pursuit movements even in concomitant strabismus.[12] The inadequacy of the saccadic velocity per se for the detection of ocular motility defects has been severely felt,[13] and to overcome this deficiency, the role of S. quotients has already been described in a study of lateral rectus palsy and normal sub­jects.[14] The aim of this article is to highlight the phenomenon of alternate saccadic under action in essential alternating con­vergent squint.


  Material and methods Top


Methodology used has been the same as described earlier.[14] In view of comparing the saccades of a particular muscle, a new quotient has been devised named P. quotient which is defined below:­.

P. Quotient

The P. quotient is the ratio of the saccadic velocity of one muscle with right eye fixating over the saccadic velocity of the same muscle with left eye fixating.

RMR saccadic velocity with Right eye fixating.

i e. RMR saccadic velocity with Left eye fixating.

is the p. quotient of Right Medial Rectus (RMR).



  Observations Top


[Figure - 1] shows the saccades in a case of essential alternating convergent squint. The first part of each figure is with right eye fixat­ing and the next with left eye fixating. A strik­ing change in the saccades of the left eye, both of the left medial rectus as well as of left lateral rectus, show an under action, both in the amplitude as well as the velocity. This under action is however transitory because as soon as the fixation is assumed by the left eye, the right eye saccades are reduced and there is a total recovery of the saccades of the left eye. This phenomenon has been named as "phenomenon of alternate saccadic under action" (PASU).

The saccadic velocities were calculated by the method described earlier.[14] Since no definite conclusions could be drawn from the absolute values of saccadic velocities, the S and P. quotients of the saccades were calcu­lated to assess the saccadic movements.

Abnormal S. quotients [Table - 1] are those, with variation greater than 2 standard deviations (s.d.) from the mean S. quotients as observed in normal, described earlier.[14] 5 cases showed high abnormal S. quotients of the medial and lateral recti with the right eye maintaining fixation, suggesting the under action of saccades executed by the left horizon­tal recti. 3 cases showed the reversal when the left eye took up fixation, by showing a varia­tion of standard deviations less than the nor­mal mean. S. quotient for the respective muscle groups. This indicates the under action of saccades of the right eye when the left eye maintains fixation (i.e. reverse of the earlier observation).

[Figure - 2] shows the mean S. quotient of the medial and lateral recti obtained with the right eye fixating, compared with those obtained with the left eye fixating. The strik­ing asymmetry which exists between the sac­cades, as obtained with the two eyes alter­nately fixating is obvious.

For still more accurate analysis the P. quotients of the various muscle groups (i.e. the four horizontal recti) of the alternators (ACS) were compared with those of normal. [Table - 2] shows normal values of the P. quotients of the four horizontal recti with the mean, the stan­dard deviation and the upper and lower limits of the range with a variation of 2 s.d. from the mean (p=0.05)

[Table - 3] shows values of P. quotient of the essential alternating convergent squint cases, compared with P. quotients of the respective muscles. The abnormal P. quotients varying more than 2 s.d. from the normal mean were seen in 9 cases for the right medial rectus (RMR) 6 cases for the right lateral rectus (RLR), 6 cases for left medial rectus (LMR) and 3 cases of the left lateral rectus (LLR).

[Figure - 3] shows the mean P. quotients of two horizontal recti of right eye compared with the mean P. quotients of the horizontal recti of the left eye. Again, the asymmetry between the saccades executed by the two eye is easily discernible.

Uniform results could not be obtained in the various muscle groups, partly due to the variation observed in the execution of the sac­cades at different times even by the same individual and the same muscle; and partly due to the limited accuracy of our method of calculating saccadic velocities mathematical­ly. But still the observation of the pheno­menon of alternate saccadic under action (PASU) even in a few cases is too striking to be questioned.


  Comments Top


The definite observations of asymmetrical saccadic movement of the two eyes in alter­nating convergent squint(ACS) have been demonstrated for the first time. Such a phenomenon has not been observed in the normal subjects. Moreover this asymmetry is guided by the fact whether the right eye or the left eye is maintaining fixation. It is as if the fixating eye becomes the "dominant" eye and is responsible for the saccadic under action of the non-fixating eye.

It may be concluded from this pheno­menon of alternate saccadic under action (PASU) that primarily the muscles are not at fault. It must rather be the innervational fac­tors which bring about this reversible under action with the change of fixation bet­ween the two eyes.

Quere[15] had described similar disturban­ces in the optokinetic nystagmus in nearly half of his cases of infantile strabismus and also showed abnormal dominance of the fix­ating eye in functional esotropia.[16] Mitsui[17] also recorded irregularities in the non fixating eye movements which showed improvement when the fixation was altered.

Present study not only substantiates these observations but also attempts to establish a definite asymmetrical and reciprocal relation­ship between the "monocular centres" of the two eyes. A working hypothesis is being sug­gested to explain the genesis of PASU.


  A working hypothesis Top


It appears that normally the functional "binocular cortical junction" is established by the coordinate action of the "monocular centres" of the two sides acting in coordina­tion with each other.[18] Normally the coor­dination appears to go well so that equal and opposite impulses are exchanged between the two moncular cortical centres and impulses received from the higher centres are shared equally between them i.e. there exist a 1:1 relationship between the two monocular cen­tres [Figure - 5]. If there is an interference in the establishment of the "binocular cortical junc­tion" due to one or the other reason, 1:1 relationship in the monocular cortical cen­tres is replaced by an asymmetric relation­ship, say 1:2 relationship [Figure 6]. Normally a visual stimulus would have resulted in a con­jugate movement manifesting equally in the two eyes but in a case of incoordination, the same visual stimulus would result in a "dis­conjugate" movement as the impulses driving the two eyes would be unequally distributed (1:2 relationship). It could be assumed that in such a case the conjugacy has not been fully evolved, and so gives rise to a monocular dominance of the fixating eye and causes a suppression of the movement of the non fixating eye both on abduction and adduc­tion. If the conditions are such that each eye can fixate alternately, there would be an alter­nation in the pattern of "disconjugacy": a reciprocal relationship between the two mon­ocular centres. This would result in the phenomenon of alternate saccadic under ac­tion (PASU). Evidently this interference has to occur in early childhood during the plastic stage of development of the optomotor reflexes but at present it is difficult to ascribe a definite cause leading to this interference.


  Summary Top


This study was conducted in 10 cases of essential alternating convergent squint (ACS). An important observation was made in these cases (ACS), which showed saccadic under action of both horizontal recti of the covered eye in both amplitude and velocity as compared to the saccades of the fixating eye. Moreover, there was a total reversal of sac­cadic under action of the covered eye when uncovered, on changing fixation from one eye to the other. This phenomenon has been des­cribed as the "phenomenon of alternate sac­cadic under action" (PASU). The S and P. quotients newly described have sensitively shown the saccadic asymmetry between the two eyes, which was not evident from the sac­cadic velocity value per se.

 
  References Top

1.
Metz H.S., Scott A.B.. O'Meara D.M., Stewart H.L. Ocular Saccades in Lateral Rectus palsy. Arch. Ophthalmol. 1970.84: 453-460.  Back to cited text no. 1
    
2.
Metz H.S.. Scott A.B., and O'Meara D.M.. Saccadic eye movemnt in myasthenia gravis Arch. Ophthalmol 1972, 88: 9-11.  Back to cited text no. 2
    
3.
Metz H.S., Scott W.E., Madson E. Scott A.B. Sae­cadic velocity and active force studies in blow out frac­tures of the orbit, Am. J. Ophthalmol. 1974.78: 665-6711.  Back to cited text no. 3
    
4.
Metz H.S. Ill Nerve palsy. I Saccadic velocity studies, Ann. Ophthalmol 1973, 5: 526-8.  Back to cited text no. 4
    
5.
Metz H.S., Scott A.B.. Scott W.E. Horizontal Sac­cadic velocities in Duanc s syndrome, Am. J. Ophthal­mol. 1975.80: 901-906.  Back to cited text no. 5
    
6.
Metz H.S. Saccadic velocity measurements in Inter­nuclear Ophthalmoplegia. 1976. 81: 296-299.  Back to cited text no. 6
    
7.
Rosenhaum A.L., Carlson M.R.. Gaffney R., Verti­cal saccadic velocities determination in superior oblique palsy. Arch. Ophthalmol. 1977. 95: t{2I-823.  Back to cited text no. 7
    
8.
Metz H.S.. Rice L.S. Human Eve movements following Horizontal rectos muscle disinsertion. Arch. Ophthalmol., 1973, 90: 265-267.  Back to cited text no. 8
    
9.
Metz H.S., Jantpolsky A., Change in Saccadic velocities following rectus muscle transposition, J. Pediatr Ophthalmol 1974, 2: 129-134.  Back to cited text no. 9
    
10.
Metz H.S.. Saccadic velocity studies in mechanical restriction of ocular motility. Strabismus: Proceedings of the lit meeting of International strahismological Association. Kyoto. Japan. 1978, 53-60.  Back to cited text no. 10
    
11.
Metz H.S.. Scott A.B.. O'Meara D.M.. Saccadic velocities in infants and children, Ac. J. Ophthalmol., 1971. 72: 1130-1135.  Back to cited text no. 11
    
12.
Prakash P., Grover A.K., Khosla P.K.. Gahlot D.K.. Ocular Motility in alternating squint (An electro Oculographic study) British Jour. ophthalmology 1982, 66:_258-_263.  Back to cited text no. 12
    
13.
Prakash P.. Sharnta P. Menon V. Gahlot D.K. Saccadic Eye Velocity in normal subjects and Correla­tion between VRM and ORM. Proceedings of the 43rd Meeting of All India Ophthalmological Society Pune, 1983.  Back to cited text no. 13
    
14.
Prakash P.. Sharma p.. Menon V.. Gahlot D.K. new approach in the study of saccadic eye velocity, the role of S. Quotient. (1984) LJ.O. 32: 127-131.  Back to cited text no. 14
    
15.
I5. Quere M.A.. The Disturbance of OKN in the infantile strabismus. Ann. Oculist (Paris) 1971 (Fre.) 204: 1-12.  Back to cited text no. 15
    
16.
Quere M.A.. la. dominance anormale de 1'oeil fix­atcur dands les esotropics fonctionalles Arch Ophthal­mol (Paris) May 1972. 32: 381-9 (Fre.) as quoted in Excerpts Medica. ophthalmology 1973.  Back to cited text no. 16
    
17.
Mitsui Y.. Hiraik. Takashima I.. Masuda K., EOG of squint and its relation to the etiology of squint Acta Soc. Ophthalmol Jpn. 1977. 81/11: 1787-1794.  Back to cited text no. 17
    
18.
Keiner G.B.J. New viewpoints on the origin of the squint. The Hague. Martinus Nijhoff, 1951.  Back to cited text no. 18
    


    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5]
 
 
    Tables

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



 

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Material and methods
Observations
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A working hypothesis
Summary
References
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