prism diopter
Also written as: PD — prism diopter
A unit of measurement for ocular deviation, where one prism diopter deviates light by 1 cm at a distance of 1 meter.
Full Definition
The prism diopter (PD or Δ) is the standard unit for measuring strabismic deviations and prescribing prism correction. One prism diopter creates 1 centimeter of deviation at a 1-meter distance. Prism measurements are obtained using prism bars or the prism and alternate cover test. The direction of deviation is specified using standard notation: esotropia (ET), exotropia (XT), hypertropia (RHT/LHT), or hypotropia. Prism measurements are essential for surgical planning, with most horizontal muscle procedures correcting approximately 2-4 prism diopters per millimeter of recession or resection.
Usage
Usage note: Use the delta symbol (Δ) in clinical documentation and always specify direction of deviation.
In Context
- "The patient exhibited a 25 prism diopter esotropia at distance fixation." — Strabismus measurement documentation
- "Bilateral medial rectus recession was planned to correct the 40Δ exotropia." — Surgical planning notes