Field expansion for homonymous hemianopia by optically induced peripheral exotropia.

PURPOSE To describe a novel method for prism correction of hemianopia that provides field-of-view expansion in a convenient and functional format and to evaluate initial clinical application. METHOD To expand the upper quadrant of the field, a high power prism segment (30-40delta) is placed base-out across the upper part of the spectacle lens, on the side of the loss, at about the level of the limbus. A similar prism segment at the lower part of the lens is used to treat the lower field. The peripheral location of the prisms causes peripheral exotropia. As a result a scene segment as high as the vertical span of the prism is shifted laterally by 15 to 20 degrees relative to the view of the other eye. At the edge of the hemianopic field loss, objects that would fall in the scotoma of one eye are seen through the prism in the other eye, providing a simultaneous awareness of details within the otherwise absent field-of-view. An approach for fitting the system to patients with abnormal binocular vision (strabismus and amblyopia, with or without diplopia) is discussed as well. The effect of the prisms was evaluated in a noncomparative case series (12 patients). RESULTS The field expansion is provided at any position of lateral gaze, including gaze away from the side of the scotoma. The effect of this technique on field expansion was demonstrated using standard binocular perimetry. Most patients reported substantial improvement in function and in obstacle avoidance. CONCLUSION A novel method for the optical treatment of hemianopia was developed and tested. It was found to be effective in expanding the field and helping patients' mobility.