Indian Journal of Ophthalmology

: 1991  |  Volume : 39  |  Issue : 4  |  Page : 170--173

Effect of aniseikonia on fusion

Pradeep Sharma, Prem Prakash 
 Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, New Delhi 110 029, India

Correspondence Address:
Prem Prakash
Dr. R.P Centre for Ophthalmic Sciences. AIIMS New Delhi - 110029


Physiological aniseikonia is the basis of stereopsis but beyond certain limits it becomes an obstacle to fusion. It is not well established as to how much aniseikonia can be tolerated by the fusional mechanism. Different tests under different testing conditions have given a wide range of variation. On the synoptophore we had observed tolerance upto 35% aniseikonia in some cases. Under more physiological conditions on a polaroid dissociation stereoprojector we observed lesser baseline fusional vergences but tolerance in about 70% of the cases upto 30% aniseikonia while 25% could tolerate even 35% aniseikonia. However we realise that these indicate the maximal potential and not the symptom free tolerable limits.

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Sharma P, Prakash P. Effect of aniseikonia on fusion.Indian J Ophthalmol 1991;39:170-173

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Sharma P, Prakash P. Effect of aniseikonia on fusion. Indian J Ophthalmol [serial online] 1991 [cited 2023 Sep 26 ];39:170-173
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Physiologic aniseikonia or the disparity of images of the two eyes within the limits of sensory fusion forms the fundamental basis of stereopsis and so is an asset. But disparity beyond the limits of sensory fusion, termed anomalous aniseikonia becomes a liability to motor fusion. The efforts of motor fusion are to overcome this liability to retain visual alignment within the limits of sensory fusion and regain the fruits of fusion and stereopsis. How much aniseikonia can really be overcome by the motor fusion has not been well established. Earlier studies as far back in 1932 by Ames[1] put the limit at 5% and this has been further supported by recent studies[2]. However other studies [3],[4],[5],[6],[7] have indicated tolerance of 15� 20% aniseikonia, Goel and Singhal[8] observed tolerance in one third of their cases having 10% aniseikonia. Some studies have shown stereopsis in one third of their cases with more' than 22.3% aniseikonia[9].

We observed that upto 35% aniseikonia was amenable to fusion on the synoptophore[10] In this study our aim is to evaluate the fusional tolerance on the stereoprojector with polaroid dissociation and compare it with our observation on the synoptophore.


20 normal subjects with 6/6 vision in both eyes with no significant refractive error (less than � 0.25 D.S.) and no other ocular findings were included for the study. Base line fusional vergences: adduction and abduction were measured on a polaroid stereoprojec�tor with the help of fusion slides having equal images (aniseikonia = 0)

The polaroid stereoprojector is a twin slide projector with attachments of polaroid filters of opposite orien�tation on either of them. The slides are projected on a special aluminium screen which has a high polish to maintain the polarized character of the images. The subject who is seated at 5 meters distance from the screen is made to wear polaroid spectacles so that each of his eyes can see the projection of only one of the two projectors. One of the projectors is kept steady while the other is rotated to simulate the convergence or divergence of the images. A scale below the screen measures the amount of excursion of the image in prism dioptres. Fusional vergences for adduction and ab�duction can be recorded by evaluating the maximal excursion on the screen allowing single binocular vision.

The fusion slides had a clown with a ball in one hand and blowing horn in the other. A set of equal size slides (Aniseikonia = 0) were used for measuring baseline fusional vergences. The aniseikonic slides consisted of progressively larger sized images at 5% intervals viz. 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%. Retaining the slide 'O' in one eye the aniseikonic slides were shown to the other eye and the fusional vergence recorded at each step. The maximal aniseikonic slide which could maintain fusional vergence was taken as the indicator of maximally tolerated aniseikonia. The same was repeated by showing the slide 'O' to the other eye and aniseikonic slide to its fellow.

In a similar manner base line fusional vergences were recorded on the synoptophore followed by evaluating the maximally tolerated aniseikonic slide on the synoptophore by each eye as described earlier [10]


[Table 1] shows the base line fusional vergences for adduction and abduction evaluated on the polaroid projector compared with those measured on the synoptophore. The adduction vergences ranged from 2.0 prism dioptres to 6.0 prism dioptres (mean 2.92 prism dioptres) compared with a mean of 15.2 prism dioptrse on the synoptophore. The abduction vergences ranged from 1.0 prism dioptre to 3.0 prism dioptres (M=2.16 p.d.) compared with 5 prism dioptres on the synoptophore.

[Table 2] shows the maximum aniseikonic slide that could be tolerated by each eye to maintain its baseline fusional vergences as shown in [Table 1]. They are compared with the observations on the synoptophore. Almost all cases could control 20% aniseikonia, progressively lesser number of cases could maintain their fusional vergences at higher levels of aniseikonia. Only about a quarter of the cases could control their fusion at 35% level of aniseikonia. None of the cases could control at the 40% aniseikonic slide. A comparison of the findings on polaroid projector with those on synoptophore shows poorer fusional control on the polaroid projector.

Another notable observation is that the control for adduction and abduction did not differ on the polaroid projector whereas the adduction control was almost twice effective on the synoptophore as compared to abduction vergences.

It may be noted from [Table 2] that there is no correlation between the maximum tolerance of aniseikonia and the fusional vergences indicating that the subject may have 35% aniseikonic control but poor vergences or vice versa.

A statistical analysis of the percentage of subjects tolerating different levels of aniseikonia [Table 3] indicates that the difference of adduction versus abduction on the polaroid projector are not significant�ly better on the synoptophore compared to that on the polaroid projector.


It is probably for the first time that fusional vergences have been evaluated in real space with an instrument which causes so little dissociation with the help of polaroid filters. The fusional vergences are strikingly low as compared to those on the synoptophore. This is because the role of central fusional mechanism is reduced in distance measurements with the polaroid system. Moreover there is no role of artifactual proximal and accommodative convergences which always intervene in the measurements on the synop�tophore. This may also explain the little difference between the adduction and abduction vergences on the polaroid projectors. It may be noted that these observations are not comparable with the vergences measured by prisms for distance as in that case the stimulus for fusional mechanisms is the shift of the entire field covered by the prism [11],[12].

For evaluating the tolerance of aniseikonia, the main�tenance of baseline fusional vergences at different levels of aniseikonia have been studied and the maximum aniseikonia which could be tolerated to maintain the baseline fusional vergences have been used as a direct indicator of tolerance of motor fusion. It should be noted that the range of fusional vergence has no correlation with the maximum tolerance of aniseikonia meaning thereby that a person with 35% aniseikonic tolerance could have relatively poor fusional vergence and vice versa. Moreover fusional vergences do not really reflect the control over a period of time (fusional sustenance). Thus it is possible for a person to have a high aniseikonic control but it does not necessarily mean good control over a period of time (good sustenance) and he may be symptomatic due to aniseikonia.

Our observations suggest that control of aniseikonia is quite good in normal subjects. The mean tolerance of aniseikonia being 30%, which is possible in about 75% of cases and about 25% of cases can control even 35% aniseikonia. However as no relationship between fusional vergence and aniseikonic tolerance is observed it is likely that these figures suggest the maximal potential but do not ensure a symptom free existence with aniseikonia. More studies on these lines are required to evaluate the symptomless sustenance of fusional vergences in the presence of aniseikonia.


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