Near Vergence Training for Distance Control in Intermittent Exotropia

Evaluating Near Vergence Training for Distance Control in Basic and Divergence Excess Intermittent Exotropia

Intermittent Exotropia (IXT) is a common type of eye misalignment where one eye periodically turns outward, and control is typically much worse when looking at objects far away than close up. However, most currently available training programme are focused on near-viewing tasks. The goal of this clinical trial is to investigate if a new, structured, home-based near eye training program works to improve distance eye alignment control in children and young adults with IXT.

The main questions the study aims to answer are:

• Does a 12-week near eye training program improve a participant's ability to control their eye alignment when looking at a distance?
• Does using a special reading glasses during near training help to control distance eye alignment better than doing the training without the reading glasses?
• Do the benefits of this eye training last after the training has stopped?

This is a two- phase, single- masked (examiner masked), randomized clinical trial with crossover design. The entire study duration is about 24 weeks. A total sample size of 42 participants, aged 6 to 25 years with diagnosed Basic (distance deviation is within 10 prism diopters of the near deviation) or Divergence excess (distance deviation is at least 10 prism diopters greater than the near deviation) IXT, will be recruited.

In Phase I (Weeks 0 to 12), participants will be randomly allocated into one of three groups, with 14 individuals in each group:

• Group 1 does eye training while wearing special plus-power reading glasses that remove the need to focus up close, mimicking how the eyes look at a distance.
• Group 2 does the exact same eye training but wears regular distance glasses without the special reading addition.
• Group 3 serves as a control group that does not do training during the first 12 weeks, but will receive the training in the second phase.

In Phase II (Weeks 13 to 24), which is a crossover phase, groups 1 and 2 will stop training to see if the visual improvements last, while group 3 will be re-randomized into two subgroups (group 3A, with special plus-power reading glasses, and group 3B with regular glasses with distance correction) that will undertake daily eye training for 12 weeks to ensure ethical treatment access. All participants will have a final eye exam at 24 weeks.

During home training, participants will:

• Wear standard glasses provided by the study.
• Practice daily, home-based eye exercises for 12 weeks using a standardized software program on a computer or tablet.
• Complete 3 training sessions every day, lasting 7 minutes per session.
• Wear red-blue 3D glasses during training and press arrow keys or swipe the screen to play a visual game that dynamically changes in difficulty.

Preliminary test and binocular vision assessment with fixed and uniform sequence will be performed at baseline visit, 6th week interim visit, 12th and 24th week evaluation. All eye examinations will be carried out in The Optometry Centre (A034), the School of Optometry, The Hong Kong Polytechnic University.

The following data will be statistically analyzed. The primary outcome data is distance Office Control Score (OCS) at 12-week visit. Secondary outcomes data include change in deviation angle, near and distance stereopsis and patient- / parent- reported health-related quality-of-life questions at 12-week.

Inclusion Criteria

• Age and Diagnosis: Participants must be aged 6 to ≤ 25 years at the time of screening, with a confirmed clinical diagnosis of either Basic or Divergence Excess (DE) Intermittent Exotropia (IXT).

• Ocular Alignment Magnitude: Distance exodeviation measuring between 10 and 40 prism diopters (pd) inclusive, determined by the prism and alternate cover test (PACT).

• Distance-Near Disparity Limits: Near exodeviation must not exceed the distance exodeviation by more than 10 pd (to exclude Convergence Insufficiency type exotropia).

• Baseline Distance Control: A baseline distance IXT Office Control Score (OCS) between Grade 2 and Grade 4 inclusive, based on the mean of 3 standardized assessments.

• Baseline Near Control: A baseline near IXT control score less than or equal to the distance IXT control score, based on the mean of 3 standardized assessments.

• Visual Acuity: Best-corrected visual acuity (BCVA) of Logarithm of the Minimum Angle of Resolution (logMAR) 0.1 or better in each eye at both distance and near fixation.

• Sensory Status: Measurable near stereoacuity of 400 arc seconds or better on the Preschool Randot stereoacuity test.

• Refractive Correction Stability: Compliance with wearing a stable, appropriate spectacle correction for ≥ 4 months prior to formal baseline assessment if the participant meets any of the following refractive thresholds:

  • Myopia < -0.50D spherical equivalent (SE) in either eye.
  • Hyperopia > +2.50D SE in either eye.
  • Anisometropia > 1.00 SE.
  • Astigmatism > 1.50D cylinder in either eye.

• Treatment History: No history of prior non-surgical interventions for IXT, including patching, vision therapy/orthoptics, or overminus lens therapy (≥ 0.50 D).

• Surgical History: No prior history of strabismus surgery, extraocular muscle botulinum toxin injections, intraocular surgery, or refractive surgery.

• Trial Commitment: No planned strabismus surgery or conflicting ocular therapies during the 24-week study period.

Exclusion Criteria

• Amblyopia and Co-morbid Strabismus: Presence of amblyopia, nystagmus, restrictive strabismus, or paretic strabismus.
• Vertical Misalignment: Presence of an associated vertical ocular deviation > 5 pd.
• Significant Pathologies: Co-existing significant ocular disease or neurological disorders (e.g., cerebral palsy) that could confound oculomotor or sensory evaluations.
• Confounding Medications: Regular use of systemic or topical ocular medications known to alter accommodative or vergence function within the 3 months prior to screening (including, but not limited to, atropine, pirenzepine, or systemic anti-epileptic agents).