During Hess-Lancaster testing, which eye should wear the red lens when evaluating the secondary deviation?

• A. Fixating eye
• B. Paretic eye
• C. Both eyes
• D. Neither eye

Answer: (B)

In Hess-Lancaster testing you dissociate the eyes with a red-green filter system so you can map each eye’s movements separately. For evaluating the secondary deviation, you want to isolate the eye that is deviating—the paretic eye—so you place the red lens over that eye. The other eye fixes through the green lens, and the red tracing on the chart shows how far the paretic eye deviates when the non-paretic eye is fixing. This setup reveals the amount of secondary deviation accurately. If the red lens were over the fixing eye, you’d be measuring the movement of the eye that’s not deviating, which wouldn’t reflect the secondary deviation of interest.