Previous reports on strabismus changes following orbital decompression were investigated mostly in the patients who underwent bilateral surgeries. The current study provided an evaluation on the postoperative changes in unilateral ocular deviation and duction, which might aid in the understanding of the biomechanics of ocular imbalance.
Simon et al. reported that the number of patients with esotropia increased after deep lateral decompression7, new-onset esotropia was reported to range from 10 to 18 PD8. Goldberg et al. suggested that esotropia could worsen after lateral decompression due to LR weakness9. However, in another of their studies the patients displayed a minor exotropia after deep lateral decompression with an average value of 3.7 PD3. Additionally, Shani Golan et al. found that the changes in strabismus were varied after inferolateral decompression10. These discrepancies resulted from different disease severity, preoperative strabismus and surgery technique. In our study, most of the patients with 1-wall decompression had only a minor change in horizontal deviation except for two patients with obvious enlarged MR.
New-onset esotropia has been reported to occur after balanced decompression with a range from 4 to 20 PD8. In our study, esotropia increased in nearly all patients with 2-wall decompression. Esotropia had an increasing tendency after 3-wall decompression but the difference didn’t reach statistical significance. The volume of bone removed from the lateral wall was expanded in most 3-wall decompression, including the basin region9. We supposed that the expansion of lateral wall decompression could further compensate the imbalanced shift of orbital tissue and reduce the potential esotropia. Additionally, a small but significant increase of hypotropia was observed in nearly all patients who underwent 3-wall decompression. Although the bone removed from the orbital floor was limited to the most posteromedial portion adjacent to ethmoid bone and the strut was preserved in our study11.
Although slight differences existed between the results of synoptophore and prism test due to binocular fusion, accommodation and the kappa angle, synoptophore may be more consistent with the subjective symptom of diplopia than prism test, and could be considered an important component of strabismus evaluation in TED. The change in torsional deviation following orbital decompression has not been discussed before. Our study showed that torsional deviation had no significant change after orbital decompression.
The development of oculomotor imbalance after orbital decompression is a multifactorial problem. Michael et al. reported a significant decrease in abduction after 3-wall decompression12. Inna et al. reported that adduction and abduction decreased after coronal 3-wall decompression, whereas infraduction decreased after swinging eyelid 3-wall decompression, possibly due to the difference in decompression extent13. Rootman et al. reported that abduction worsened after orbital decompression14. In our study, adduction increased but abduction decreased following 2-wall and 3-wall decompression. The infraduction increased following 3-wall decompression. The centrifugal displacement of the rectus muscle path might be a main reason for the changes.
The relationship between ocular duction and deviation following decompression was studied in our study. The increase of esotropia had a significant correlation with the increase of adduction and the decrease of abduction. It has been proved that the force and elasticity of extraocular rectus changed in TED patients, the limitation of abduction is not only due to the fibrosis of MR, but also due to the increased contractile force of MR15, 16. An enlarged, stiff MR could generate a powerful force pulling the globe when the orbital cavity is expanded12, and induced the increase of adduction, decrease of abduction and worsening of esotropia. Additionally, we found significant correlation existed between MR diameter and the increase of adduction and the decrease of abduction. In previous studies, a strong trend towards increased motility restriction with increased muscle diameter was observed in TED patients17. MR was more frequently involved than LR in TED, it may be why esotropia increased more frequently after decompression. No significant difference was found between the change of vertical deviation and ocular duction, it possibly due to the small sample and the limited extent of inferior wall decompression.
The main limitations of this study were the sample size and the relatively short follow-up, as many patients needed bilateral decompression surgeries simultaneously or in a short period of time. However, the change of ocular deviation during the postoperative recovery period was demonstrated to be typically small14.
In conclusion, the changes of ocular deviation and duction were different after 1-wall, 2-wall and 3-wall decompression. The expansion of orbital cavity and increased contractile force of rectus might be important reasons for strabismus change after orbital decompression.