When using prism for a visual field defect, should the apex or base be oriented toward the defect?

• A. Apex toward the defect
• B. Base toward the defect
• C. Apex away from the defect
• D. Base away from the defect

Answer: (B)

Prisms displace an image toward their base. To help reveal a visual field defect, you want stimuli from the seeing field to be shifted into the defective (blind) field. Placing the base toward the defect accomplishes this: the image moves into the defective side, making it detectable there. If the apex were toward the defect (or the base away), the image would shift away from the defective field, reducing the chance of detection. Therefore, the prism should be oriented with its base toward the defect.