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   Table of Contents      
ARTICLES
Year : 1984  |  Volume : 32  |  Issue : 6  |  Page : 461-466

Saccadic underactions in concomitant convergent squint


Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS New Delhi-29, India

Correspondence Address:
P Sharma
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS New Delhi-29
India
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Source of Support: None, Conflict of Interest: None


PMID: 6599885

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How to cite this article:
Sharma P, Prakash P, Menon V. Saccadic underactions in concomitant convergent squint. Indian J Ophthalmol 1984;32:461-6

How to cite this URL:
Sharma P, Prakash P, Menon V. Saccadic underactions in concomitant convergent squint. Indian J Ophthalmol [serial online] 1984 [cited 2019 Aug 21];32:461-6. Available from: http://www.ijo.in/text.asp?1984/32/6/461/30841

Table 1

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Table 1

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Ocular Saccades have recently been studied with encouraging results in concomitant stra­bismus. Significant findings in fixation and pursuit movements have been described earlier [1]. lacomitant movements in concomi­tant convergent squints nave been described by other workers [2],[3]. We have earlier described the phenomenon of alternate saccadic under­action (PASU) in essential alternating conver­gent squint [6]. The inadequacy of saccadic velocity per se for quantitative study of ocular motility defects has also been highlighted [5] and to overcome that difficulty the role of S. quotients [6] and P. quotients [4] has already been established. The aim of this article is to highlight the saccadic underactions, observed not only in the essential alternators but also in unilateral convergent squint, and to em­phasise the role of reflexologic theory [7], in the causation of concomitant convergent squint.


  Material and methods Top


Horizontal ocular saccades were generated by voluntary refixation method (VRM) electroculographically in subjects with essential alternating convergent squint and unilateral convergent squint on Mingograph 800.

The Paper speed was 50 mm/sec. and 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 take 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 two fixed spots so that the eye moved 30° regulated by a metronome in the process for voluntary refixation method. The saccadic velocity was calculated from the E. O. G. recording and the S. quotients calculated as described earlier [4],[5],[6].


  Observations Top


The electroculographic records by the voluntary refixation method (VRM) of a case of alternating convergent squint (essential) in [Figure - 1]ii shows the saccadic underaction of both the lateral and medial recti of the non fixating (left) eye when the right eye fixates. However the movement of left eye recovers fully when the left eye takes up fixation whereas the right eye then shows the saccadic under­action in its horizontal recti movements [Figure - 1] iii.

The [Figure - 2]ii and iii are the electro-oculo­graphic recordings by VRM in a case of left convergent squint (essential). Here the sacca­dic underaction is observed in the horizontal recti of the left eye when the right eye fixate but on the left eye takes up fixation, the eye movements of the left eye recover fully but the corresponding saccadic underaction of the non fixating (right) eye is observed.

The [Figure 3]ii and iii are the EOG recor­dings of a normal subject by VRM to show the normal saccadic movements showing no underactions of the non fixating eye move­ments.

The [Figure 4]i and ii are EOG recordings of a case of right lateral rectus palsy by VRM showing the evident paresis of the right late­ral rectus muscle and slight overaction of the contralateral synergist. However their is no change in the amplitude and slope of the sac­cades on the change of fixation from the right to the left eye.

The table shows the S. Quotients compu­ted from the saccadic velocities of the hori­zontal recti as recorded by V.R.M. (see foot­note for the description of S. Quotients). The S. Quotients being a ratio of like muscles, is normally close to unity (as of the normal subject). In case of lateral rectus palsy (right) the S. quotients of lateral recti show obvious underaction (Paresis) which remains unchan­ged on changing fixation from right to left eye. Similarly the S. quotients of the medial recti show no change on alternation of fixa­tion. However, in the case of alternate con­vergent squint, the S. Quotients of both medial and lateral rectus on right fixation are significantly high suggesting an underaction of the left horizontal recti (as expected from the [Figure - 1][Figure - 2]). The ratio interestingly rever­ses to its reciprocal figure on the alternation of fixation suggesting the underaction of the right horizontal recti.

Moreover, the S. quotients of the left convergent squint show the underaction of the left horizontal recti on right fixation but on left fixation the ratio reverses to normal suggesting absence of underaction of the right eye when the amblyopic eye takes up fixation.


  Discussion Top


The Saccadic underactions observed in the alternating as well as unilateral convergent squint highlight the underlying 'dis-conjugacy' in the so called concomitant squint, however this is in no way similar to the incomitance of the paralytic squint. Whenever there is a paresis of an extraocular muscle, the concomi­tance is disturbed apparently, but since the Hering's law holds good, there still remains equal distribution of nerve impulses resulting in the overaction of the contralateral syner­gist. This is evident from the [Figure 4]. There is no difference between the S. Quotients of the muscles whether the right eye takes up fixa­tion or the left in case of lateral rectus palsy.

But in the case of concomitant convergent squint there is evidently a difference in the S. quotients depending on the fixation by the dominant eye or the other. The 'dis-conjugacy' seen in these cases is either reciprocal (in alternators) or unilateral or reversible to `conjugacy' on the change of fixation from the dominant eye to the amblyopic eye (in unilateral squint). This clearly shows two things, one that the extraocular muscle unit (lower motor neuron and the concerned muscle) is unaffected which is so in case of the paralytic strabismus indicating that the defect lies at a higher level and secondly that the Hering's law does not hold good in con­comitant convergent squints.

The solution regarding the visual outcome has been stated to be of 3 ways.

a. Binocular vision due to mutual har­mony.

b. Alternating vision due to opposed and irreconcilable impulses, which pre­vail in turn.

c. Suppression and amblyopia due to the inhibition of one of the two impulses.

Regarding the motor outcome, the squint is just one manifestation, the static one the other is in the movements of the eye. The latter has not been satisfactorily studied or emphasised, though irregularities in the move­ments of non-fixating eye have been observed by Keiner [7], Quere [2] Mitsul [3] and by us. [4],[5] Keiner was lead to believe that a functional coupling of the two eyes or the binocular junction was absent in the concomitant squint cases. Our earlier work as also this study suggests a disturbed binocular junction rather than its absence.

Going hand-in-hand with the visual outcome the motor outcome is simultaneously affected i. e.

a. Binocular conjugate movements as in normal and paralytic squints.

b. Alternating disconjugate movements as in alternating convergent (concomitant) squint.

c. Unilateral disconjugate movements as in unilateral convergent (concomitant) squint,

To explain the underlying mechanism of the phenomenon of alternate saccadic under­action (PASU) we had suggested a hypothesis [4]

The phenomenon of unilateral saccadic underaction (PUSU) in unilateral squint cases have further substantiated our hypothesis, A new neurophysiological model for eye move­ments is being proposed.

In the light of these observation the role of reflexologic theory is emphasised and that ultimately determines not only the visual outcome but also the outcome of surgical cor­rection of such cases. It has to be emphasised that in planning the squint surgery in such cases it is not only sufficient to study the static positions of the two eyes but also to study the innervational status of the two eyes (i. e. the (dynamic or kinetic status of the two eyes).

This consideration would also explain why there is so much of unpredictability in the squint surgery particularly in such cases.


  Summary Top


Saccadic underactions of the non fixating eye have been observed in concomitant con­vergent squints. There was alternation of underaction in the essential alternators but in cases of unilateral concomitant squints the underaction was only seen in the non-fixating squinting eye. Comparison with the observa­tions in normal subjects and lateral rectus palsy cases suggests underlying dis-conjugacy. These observations highlight the role of the reflexologic theory in the causation of squint.

 
  References Top

1.
Prakash, P, Grover A.K. Khosla P.K Gahlot D.K 1982, Brit, J. Ophthalmol 66: 258  Back to cited text no. 1
    
2.
Quere M. A., 1972. Arch. Ophthalrnol (Paris) 32:38  Back to cited text no. 2
    
3.
Mitsui Y, Hirai K, Takashima I, Masuda K, 1977, Acto Soc. Ophthalmol Jpn. 81 : 1787.  Back to cited text no. 3
    
4.
Prakash, P., Sharma P, Menon V, Gahlot, D.K, 1983, Proc IXth Asia Pacific Academy of Oph­thalmology, Hong Kong, March 1983. pp. 1775,  Back to cited text no. 4
    
5.
Prakash P, Sharma P, Menon V, DK Gahlot 1983. Proc 42nd All India Ophthalmology Society, Pune.  Back to cited text no. 5
    
6.
Prakash P, Sharma P, Menon V, Gahlot, DK 1984 Ind. J. Opothalmol 32, 127.  Back to cited text no. 6
    
7.
Keiner G.BJ, 1951. New View Points on the origin of Squint. The Hauge, Martinus Nijhoff,  Back to cited text no. 7
    


    Figures

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

  [Table - 1]



 

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