Impact of strabismus management on the retinal microstructure

Aim: To examine whether change in retinal structure can improve vision and stereoacuity following strabismus management. Methods: We recruited patients who presented with strabismus from 2017 to 2019. Information on demographics, posterior and anterior segment ndings, and strabismus surgery was collected using a pretested online form. Patients’ stereoacuity was examined by employing the Titmus test. The central macular thickness and retinal nerve ber layer (RNFL) thickness of all the eyes were evaluated through SD-OCT. Stereoacuity examination and OCT were performed again 3 months postoperatively. Result: We recruited 54 patients (mean age: 19.74 ± 9.2 years). Stereoacuity and vision exhibited signicant improvements after treatment. Both eyes showed improvement in RNFL thickness but not CMT. Conclusion: Functional stereoacuity changes and structural RNFL changes were noted following appropriate treatment for strabismus.

This study explored whether strabismus management can improve alignment. The aligned eye may exhibit improved stereopsis and thus binocular vision (ref).
We explored the association of functional improvement with retina structure changes. We evaluated whether functional improvement leads neuroanabolism.

Materials And Methods
Both the study protocol and informed consent form were approved by the Hospital Ethical Committee of Drashti Netralaya and were according to the guidelines of the Helsinki Declaration. We obtained written informed consent from the legal guardian or parents of each child, and each patient provided consent before study participation. This prospective cohort study recruited patients visiting the motility department who were diagnosed as having strabismus requiring surgical correction from 2018 to 2020. We excluded those with other pathology or neurological diseases that can affect the retinal nerve ber layer (RNFL), optic nerve head (ONH), or central macular thickness (CMT).
All patients received comprehensive eye assessments, which included slit lamp biomicroscope or handheld slit lamp evaluation for ocular alignment, A-scan ultrasound biometry, and tests for refraction and visual acuity (VA). Assessments were conducted under anesthesia in younger children who were unable to cooperate. We examined intraocular pressure by employing Perkins applanation tonometer.
For children aged < 3 years, monocular distance VA was tested. For nonverbal children, VA was examined based on a child's ability to x and follow objects. Fixation was determined by examining each eye's ability to xate on an object, maintain the xation, and subsequently follow the object through varying gaze positions. Children aged 3-6 years were shown wall charts containing Snellen letters and numbers and subjected to the tumbling E test and HOTV as per the standard VA assessment.
For children, single optotypes of ETDRS acuity charts with surrounding bars were presented. We examined cycloplegic refraction for all children.
An indirect ophthalmoscope with + 20 D lens was employed to examine the posterior segment.
We assessed near stereopsis using the Titmus circle (Titmus, Optical Co, Inc., Chicago, IL, USA) and the Motor and sensory adoption were assessed using various tests for all patients. All patients underwent surgery.
We measured distance stereopsis after strabismus and refractive error correction. An arc of 240 s was used with patients wearing liquid-crystal shutter glasses at a 6-m distance. A correct result indicated a successful test. However, for an incorrect result, patients were again shown the circle in different directions more than two consecutive times. If the patients obtained a correct result, they were administered next tests. However, in case of an incorrect result, patients were considered to have high stereopsis. If stereopsis was undetermined, previous steps were repeated again. When correct results were obtained for more than two consecutive times, we used the obtained result as the nal nding. If 240 s were not perceived, patients were not included in statistical analysis. Moreover, we performed intraocular pressure and slit lamp, alternate prism cover, and fundus and refraction examinations .
All OCT measurements (Cirrus Spectral Domain OCT 4000; Carl Zeiss Meditec, Dublin, CA) were performed after dilating patients' pupils to at least 5-mm diameter. A single skilled ophthalmologist conducted all measurements. SD-OCT was employed to measure ONH parameters, central macular thickness (CMT), and RNFL.
CMT is the average macular thickness in the 1-mm diameter in the center. The signal strength for all scans was set to six.
Patients were followed on the postoperative third day, rst month, second month, and third month and then every 6 months.
Patients with strabismus were followed on the third day postoperatively, rst month, third month, and then every six months. Patients underwent comprehensive examination during each follow-up.
All patients received stereoacuity tests and SD-OCT during all follow-up visits as a standard protocol.
Details regarding patient characteristics, strabismus surgery, and HD-OCT were collected from hospital records.
During follow-up visits, data were entered online using a pretested format and exported to an Excel spreadsheet (Microsoft Corp.). Data were audited periodically to ensure complete data collection.
Statistical analyses were conducted using SPSS (version 22.0; SPSS Inc., Chicago, IL, USA). Cross tabulation and descriptive statistics were employed to compare cause and effect among different variables.
Differences in mean MCT values were observed using Student's t test and one-way ANOVA. The Pearson correlation was used to evaluate agreements between the variables. P < 0.05 indicated statistical signi cance.
Of the 54 patients, 25 (46.3%) were children. No signi cant differences were determined between the adult and pediatric populations for both eyes (p = 0.069 and p = 0.303, respectively).
Many other variables were comparable with the nal postoperative RNFL values.

Discussion
The results revealed improvements in stereopsis and vision following strabismus treatment. Moreover, mean RNFL but not CMT exhibited signi cant improvement after treatment.
Here, we observed increases in RNFL following improved stereoacuity and vision. No study has demonstrated this improvement. This nding suggests neuroanabolism, which refers to functional improvements in affected retinal tissue structures. Studies have examined stereoacuity and retinal cellular structures (38, 309).
Araki et al observed the reversal of macular changes after amblyopia management but could not nd any difference (45).

Okamoto et al found improved stereoacuity and retinal microstructure after macular hole surgery (46).
A study limitation is the inclusion of a small sample with a short follow-up.
Multicenter studies including individuals of different races and ethnicities should be conducted to establish this nding. Early improvement in strabismus can improve stereoacuity, thus resulting in structural improvement.

Conclusion
Functional improvement may be associated with structural improvement following stereoacuity correction and strabismus surgery.

Declarations
No nancial support was received from any company or institution.
This study has not been presented at any conference or meeting.
The authors have no nancial interest in any aspect of this study.
Con icting interests: None to declare. Tables   Table-1