The phenomenon of alternate saccadic underaction in essential alternating convergent squint

Prem Prakash; Pradeep Sharma; V Menon; DK Gahlot

ORIGINAL ARTICLE

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

The phenomenon of alternate saccadic underaction in essential alternating convergent squint

Prem Prakash, Pradeep Sharma, V Menon, DK Gahlot
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

Source of Support: None, Conflict of Interest: None

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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 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 2023 Dec 10];32:133-8. Available from: https://journals.lww.com/ijo/pages/default.aspx/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

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

[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

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

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

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

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