This study is a prospective randomized study aiming to investigate the effect of prism adaptation before surgery for symptomatic esophoria on the number of reoperations, the occurrence of over- and under-correction after the first surgery, and symptom resolution. Additionally, we aim to define the optimal duration of prism adaptation.

SUBSTUDY 1 The Optimal Duration of Prism Adaptation in Symptomatic Esophoria Study Design: Prospective, randomized controlled study Patient Group: 100 participants with symptomatic esophoria, randomized into two groups of 50 participants each to undergo either one hour or four hours of prism adaptation.

The purpose of this study is to determine the optimal duration of prism adaptation.

The hypothesis is that the measured deviation angle is expected to increase after one hour of prism adaptation compared to before prism adaptation. Furthermore, it is expected that the measured deviation angle will not differ significantly between the two randomization groups. The primary endpoint is the measured deviation angle at the conclusion of prism adaptation.

The results are highly relevant for accurate surgical dosing (and thus postoperative symptoms and reoperation rates), efficient use of hospital resources, and reduced patient waiting times in the outpatient clinic.

SUBSTUDY 2 Strabismus Surgery for Symptomatic Esophoria Based on Prism Adaptation Study Design: Prospective, randomized controlled study Patient Group: 100 participants with symptomatic esophoria, using the same patients and randomization groups as in Substudy 1.

The purpose of this study is to investigate the effectiveness of strabismus surgery for symptomatic esophoria based on prism adaptation. Patients randomized in Substudy 2 to either one or four hours of prism adaptation will undergo surgery based on the full prism-adapted angle.

The hypothesis for this substudy is that surgical dosing based on one hour of prism adaptation will be equivalent to dosing based on four hours of prism adaptation. The primary endpoint will be the reoperation rate. Secondary endpoints will include comparisons of preoperative surgical planning metrics (target deviation and number of eye muscles operated), symptom resolution after the first surgery, and prism measurements (including rates of over- and undercorrections).

Inclusion Criteria:

Patients with symptomatic esophoria who meet one or more of the following criteria:

• Double vision
• Asthenopia (eye strain)
• Headaches related to esophoria
• Worsening of symptoms during fatigue, sleep deprivation, reading, illness, or alcohol consumption.

Exclusion Criteria:

• Age between 18 and 65 years at date of inclusion.
• Accommodative esotropia or esophoria
• Myopia exceeding 10 diopters in the most myopic meridian
• Treatment with botulinum toxin in an eye muscle within four months prior to inclusion
• Inability to undergo reliable orthoptic measurements or prism adaptation
• Previous strabismus surgery
• Pregnancy or breastfeeding within 12 months prior to inclusion
• Myogenic disorders (including Myasthenia Gravis)
• Restrictive strabismus (including previous eye muscle trauma or Graves' orbitopathy)
• Eye muscle paresis
• Maximum prism-adapted deviation angle >40 prism diopters (PD) in any gaze direction for distance
• Maximum prism-adapted deviation angle with ≥20 PD difference between near and distance measurements