title: Tunnel Vision Prismatic Field Expansion: Challenges and Requirements.
abstract: OBJECTIVE
  No prismatic solution for peripheral field loss (PFL) has gained widespread acceptance. Field extended by prisms has a corresponding optical scotoma at the prism apices. True expansion can be achieved when each eye is given a different view (through visual confusion). We analyze the effects of apical scotomas and binocular visual confusion in different designs to identify constraints on any solution that is likely to meet acceptance.
  
  
  METHODS
  Calculated perimetry diagrams were compared to perimetry with PFL patients wearing InWave channel prisms and Trifield spectacles. Percept diagrams illustrate the binocular visual confusion.
  
  
  RESULTS
  Channel prisms provide no benefit at primary gaze. Inconsequential extension was provided by InWave prisms, although accessible with moderate gaze shifts. Higher-power prisms provide greater extension, with greater paracentral scotoma loss, but require uncomfortable gaze shifts. Head turns, not eye scans, are needed to see regions lost to the apical scotomas. Trifield prisms provide field expansion at all gaze positions, but acceptance was limited by disturbing effects of central binocular visual confusion.
  
  
  CONCLUSIONS
  Field expansion when at primary gaze (where most time is spent) is needed while still providing unobstructed central vision. Paracentral multiplexing prisms we are developing that superimpose shifted and see-through views may accomplish that.
  
  
  CONCLUSIONS
  Use of the analyses and diagramming techniques presented here will be of value when considering prismatic aids for PFL, and could have prevented many unsuccessful designs and the improbable reports we cited from the literature. New designs must likely address the challenges identified here.
authors.affiliation: Schepens Eye Research Institute Massachusetts Eye and Ear, Harvard Medical School, Boston, MA, USA.
authors.forename: Henry
authors.initials: H
authors.lastname: Apfelbaum
authors.affiliation: Schepens Eye Research Institute Massachusetts Eye and Ear, Harvard Medical School, Boston, MA, USA.
authors.forename: Eli
authors.initials: E
authors.lastname: Peli
keywords: low vision rehabilitation; perimetry; prism treatment; retinitis pigmentosa; tunnel vision; visual field loss