Comparative Study between Synoptophore and Cover Test with Prisms at Far Vision

The stereoscope invented by Charles Wheatstone in 1830 used for oculomotoras and sensory evaluation. Its main use is to assess and treat strabismus, also used in vision therapy for the treatment of amblyopia [1]. It simulates far vision conditions and presents separate images for each eye. It studies oculomotoras and sensory capabilities mainly in strabismus patients [2]. Although the measure of the deviation angle is usually performed with prism bar or loose prisms, it is worth considering the synoptophore, given its characteristics.


Introduction
The stereoscope invented by Charles Wheatstone in 1830 used for oculomotoras and sensory evaluation. Its main use is to assess and treat strabismus, also used in vision therapy for the treatment of amblyopia [1]. It simulates far vision conditions and presents separate images for each eye. It studies oculomotoras and sensory capabilities mainly in strabismus patients [2]. Although the measure of the deviation angle is usually performed with prism bar or loose prisms, it is worth considering the synoptophore, given its characteristics.
Compared with measurement with cover test (CT) and prisms, in synoptophore we obtain: A. A higher value of deviation as it is more dissociating.

B.
More simplicity and comfort in the measurement of the 9 positions of gaze thanks to its mechanical structure.

C.
The values are more reproducible, making it easier for monitoring the case.
D. More accurate values in high-angle strabismus, since in these cases the measure is not performed in leaps.

E.
More objective measurement.

Materials and Methods
In a sample of 64 patients, with a mean age of 27.6 years, we consider two common clinical methods of measuring the angle of ocular deviation: synoptophore and Cover Test with prisms at far vision. We study which differences we find between both methods and what factors may influence the variability of the results. We studied the influence of factors such as the direction of the horizontal deviation (esotropia/ exotropia), if they had vertical deviation or some sort of alphabetical syndrome, if they had undergone strabismus surgery previously and finally the patient's gender.

Results
Comparing the measurement of the deviation angle in 64 patients with synoptophore and cover test with prisms at far vision we find: (Table 1):           We also assessed whether other factors such as vertical deviations or alphabetic syndromes could influence the comparison. Given the low percentage of patients who showed vertical strabismus or alphabetical syndrome, comparative under these factors was dismissed [3][4][5][6][7][8].

Discussion
Despite significant statistical differences in convergent strabismus, in regular motor examination we always include the measurement with synoptophore as well as the measurement with prism at far vision.
We consider that the greater accommodative capability that convergent strabismus present, both at far and near vision, allows the synoptophore, as being more dissociating, to find greater deviation angles. We believe that although with synoptophore there is a risk of causing greater dissociation in some type of strabismus, for us its use for justified the measure, because it is more accurate, objective and reliable. Anyway we still complete the study with the measurement with prisms. In our protocol for measuring ocular alignment we consider the measurement with synoptophore, objectively, a complementary study to consider.

Conclusion
In esotropia we found that there was statistically significant differences respect to the measurement at distance with prisms at far vision. In exotropia we didn't find statistically significant differences. Both in patients with previous strabismus surgery and non-surgery there are statistically significant differences. The fact that we find a greater deflection angle with the synoptophore at convergent strabismus confirms the ability of dissociation that we initially suspected occurs with Synoptophore [9][10][11][12].