We congratulate Brandner and associates for their article, “Biomechanical Analysis of X-Pattern Exotropia,” which made for an interesting read. We have the following comments regarding the methodology and interpretation of the results.
Fifty percent (5/10) of the patients included in the study had already undergone a strabismus surgery before their inclusion. It would have been better if the authors had given the details of the surgeries that were performed on these 5 patients. One of the patients underwent bilateral surgery. The authors also mention that the treatment goal was to reduce the deviations to a level acceptable to the patients. However, after surgery, a significant residual exotropia was noted in many, particularly in downgaze. The above-mentioned factors can affect the result of this simulation study greatly and also can skew the final outcome.
We are also concerned about the method of investigation. Without independent in vivo evidence of the actual mechanism of X pattern, it is very hard to know whether the simulation is producing the effect by the same mechanism. Multiple parameters like horizontal or oblique muscle tightness, or both; pulley heterotopia; and up shift and down shift of the horizontal muscles and their pulleys in isolation or in combination can lead to X pattern. These parameters can be manipulated in isolation or in combination on the computer-based simulator to produce a given pathologic feature. However, it is difficult to fit all these factors in one model. In addition, high-resolution magnetic resonance imaging could be a useful additional tool to test this. Hence, to extrapolate and conclude the exact mechanism(s) causing a particular pathologic feature in vivo may not be possible from this study.
The article tries to compare 2 theories, indicating that either of these 2 are the only causes of X-pattern exotropia, thereby failing to consider that there may be other causal associations. In addition, there may be a change in the innervational pattern to the horizontal rectus or oblique muscles or to the horizontal rectus muscle pulleys that may account for some postoperative changes in the eye position in different gazes.
Although the clinical data in this study seem to prove that only horizontal rectus muscle surgery is enough to treat X-pattern exotropia, the simulation model does not prove the same. Hence, inclusion of the above additional parameters along with a multicenter study (as described by the authors), may help in better understanding of this complex form of strabismus in future studies.