Case Report: Novel Use of a Variable Neutral-Density Filter in Vision Therapy for Exotropia with Deep Suppression

Introduction Suppression is a common sensory adaptation in exotropia and may limit the recovery of fusion during vision therapy. However, simple clinical methods for grading suppression continuously and modifying interocular suppression during office-based vision therapy are limited. This case report describes the novel use of a variable neutral-density filter as both a diagnostic and therapeutic adjunct in the management of exotropia. Case Report An 8-year-old girl presented with worsening left exotropia. Examination showed 20/20 visual acuity in each eye, constant left exotropia measuring 50∆ at distance and 55∆ at near, and left eye suppression on Worth 4-dot testing across all distances and lighting conditions. A variable neutral-density filter was placed in front of the dominant right eye to reduce luminance in a graded manner. At an optical density of 0.6 log units, pathological diplopia was elicited at near with Worth 4-dot testing, and the patient achieved a near point of convergence of 20 cm. The filter was then incorporated into office-based vision therapy to facilitate sensory rebalancing, reduce suppression, and make earlier convergence training possible. Over 12 sessions, binocular stability was achieved without luminance attenuation. The final findings show well-controlled exodeviation, elimination of suppression on Worth 4-dot testing, recovery of global and local stereopsis, and improved fusional vergence reserves. Conclusion A variable neutral-density filter may be a practical, low-cost adjunct for treating deep suppression in exotropia. It allows continuous suppression grading, reduces reliance on multiple therapy tools, and enables earlier convergence training in patients with significant suppression.