Transactions from the Xth International Orthoptics Congress 2004

Table 3. Mean fusional convergence amplitude at baseline and varying levels of SE.
SE Mean fusional conv. amp. SE (cont.) Mean fusional conv. amp. (cont.)
Baseline (0 o ) 41.42 (+/- 24.52) 6 o 25.71 (+/- 18.75)
2 o 33.38 (+/- 19.53) 8 o 21.00 (+/- 14.57)
4 o 28.38 (+/- 15.63) 10 o 13.81 (+/- 10.65)
Table 4. Pairwise comparison between the baseline mean amplitude of fusional convergence and means at each
level of SE.
Orthophoria (O) Excyclotropia (E) Mean difference (O-E) Significance
0 o 2 o 8.04 0.806
4 o 13.04 0.063
6 o 15.71 0.056
8 o 20.42 0.002
10 o 27.61 0.000
All subjects achieved stereoacuity of 90” without any SE, as was the case at 2 o of SE.
At 4 o of SE, the first effects of the torsional disparity on stereoacuity became evident with 3
subjects (14.3%) no longer being able to appreciate the random dot slide images representing
90”. Of these 3 subjects, 2 were still able to appreciate stereoacuity of 180” and the remaining
subject was only able to appreciate gross stereopsis representing 360” or greater.
At 6 o of SE, 9 subjects (42.9%) were able to appreciate stereoacuity of 90”. Of the
remaining 12 subjects; 3 achieved stereoacuity of 180”; 3 identified the images representing
360”; 1 had stereoacuity of 720”; and 5 (23.8%) were not able to achieve any stereopsis.
At 8 o of SE, 4 subjects (19%) were still able to identify images representing 90”. Of
the remaining subjects, 1 (4.8%) had stereoacuity of 180” and 16 (76.2%) were not able to
achieve any stereopsis. At 10 o of SE, no stereopsis was achieved by any but 1 subject, who
unexpectedly maintained stereoacuity of 90”. This result was obviously an outlier.
Discussion
Two papers published in the last decade are particularly important as far as the present study
is concerned. The first is by Sharma et al (1999), who reported that baseline horizontal
fusional vergences decreased once levels of simulated torsion reached approximately 4 o .(9)
Their findings were indicative of a gradual decrease in horizontal fusional vergences across
the range of torsional fusion, the mean torsional fusion amplitudes being about 13 o . The
present study supports the findings of Sharma et al (1999) and expanded on them to provide a
more definitive explanation of the effect of SE on horizontal fusion.
The second study is by Yagasaki et al (1994), who explored stereoacuity in the
background of torsional disparity and found that a random dot stereoscopic image of 125”
could not be appreciated beyond 4 o of SE.(10) However, a coarse disparity of 805” was able
to be appreciated with up to 9 o of SE. The present study bridges the knowledge gap between
such fine and coarse stereoacuity levels.
The findings of the present study can be summarised as follows. With respect to
fusional divergence, the baseline measurements became statistically significantly affected at
6 o of SE, though even at 4 o of SE the significance level was 0.087. Similarly with fusional
convergence; the baseline measurements came statistically significantly affected at 8 o of SE,
though at 6 o and even 4 o of SE the significance levels were 0.056 and 0.063, respectively,
indeed close to the set alpha of 0.05. Certainly, these mean measurements relative to the
baseline were conspicuous from a clinical perspective.
With respect to stereopsis, there was no deterioration of stereoacuity with 2 o of SE.
Nor did 4 o of SE have a significant effect - 18 subjects (86%) appreciated stereoacuity of 90”,
and 20 subjects (95%) appreciated stereoacuity of 180”. Beyond 4 o of SE, there was a marked
decrease in the number of subjects able to appreciate the finest (90”) level of stereoacuity. At
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