The gold-standard strabismus angle evaluation consists in prism alternating cover test (PCT). This is a dissociative and subjective method requiring experienced user and cooperant children, with a significative inter-examiner variability, increasing with large angle deviations. Objective automated deviation measurement methods have been developed from clinical Hirschberg method and mathematical models.
Strabocheck (SK) is a free access website, consisting in a new deviation measurement tool based on the corneal reflects, developed by the ophthalmology department of the university hospital of Nantes, France.
The purpose of this study is to evaluate SK in comitant horizontal strabismus in children undergoing surgery.
We prospectively included children under 16 years old presenting a comitant horizontal strabismus undergoing surgery between October 2020 and May 2021, in Robert Debré Hospital, Paris, France. We separated our patients in 3 groups : Intermittent exotropia (XT), infantile esotropia (IET), and partially accommodative esotropia (PET).
All patients included had a complete clinical examination before and 6 months after surgery. Data collected were age, type of strabismus, angle measurement in diopter (D) with distance PCT (DPCT), near PCT (NPCT) and SK before surgery, after surgery and under general anesthesia (GA), best corrected visual acuity (BCVA), Lang stereotest, Newcastle score for exotropia, full refraction measurement after cycloplegia, spherical equivalent calculation and presence or absence of amblyopia.
We included 44 children undergoing horizontal strabismus surgery. There was 17 IET, 14 XT and 13 PET. Correlation (R) between DPCT and SK was 0.87 (p < 0.001 ), R NPCT-SK was 0,86. In XT, IET and PET group, the R DPCT-SK was respectively 0.13, 0.77 and 0.77. R DPCT-SK in XT group was not found significant. R DPCT-SK and NPCT-SK in postoperative patients were respectively 0.74 and 0.69 (p < 0.001 ). Mean difference in preoperative angle measurement between DPCT and SK, NPCT and SK, were respectively 11.9D +/- 9.8 and 14.8D +/- 9.9. Bland-Altman plots represents the disagreement between DPCT and SK measurement, preoperative 95% limit interval was -30.0 [-34.4 ; -25.6] to 31.0D [26.7 ; 35.4]. Mean difference in postoperative angle measurement between DPCT and SK, NPCT and SK, were respectively 7.8D +/- 5.7 and 8.5D +/- 7.1. Postoperative 95% limit interval was -21.8 [-24.9;-18.7] to 16.1D [13.1;19.2].
We found a high correlation between SK and PCT in preoperative and postoperative patients, except for XT group. This finding shows the limit of a non-dissociative method measurement in intermittent exotropia, as children can partially or completely compensate their angle deviation. Mean difference between PCT and SK, and Bland Altman plots show a non negligeable bias values, with wide intervals of limit of agreement. This indicates that SK cannot replace PCT angle measurement in children undergoing horizontal strabismus surgery.
In conclusion, we can recommend SK for preoperative and postoperative monitoring of esotropias in children, as its measurement is reproducible and quite reliable. However the discordance with PCT measurement (particularly in exotropias) does not allow it to be used for surgical strategy.
