A patient presents complaining about vertical diplopia. His spectacles are read as -15 D for both lenses. You measure a 3 prism diopter right hypertropia. Which of the following options could correct his vertical diplopia?
This is a very common question on any board exam, and tests your knowledge of Prentice's rule:
PD = h * F
(where PD is the induced prism; "h" is the distance from the optical center in cm; and "F" is the power of the lens in diopters)
In this question, PD = 3 prism diopters and "F" is 15. We are solving for "h":
h = PD / F
h = 3 / 15
h = 0.2 cm
h = 2 mm
Now we have to determine if we are looking above or below the optical center. A minus lens is basically two prisms placed apex-to-apex with a base up prism superiorly and a base down prism inferiorly. Also remember that the eye wants to move toward the base with induced prism.
Since we want the patient to compensate for a right hypertropia, we want to induce a base-down prism effect by having his right eye look 2 mm BELOW the optical center of the lens. An equivalent way of saying this is we want to move the optical center 2 mm UP in the right lens.
However, this is not an answer choice! Another way of achieving this is to move the optical center up by 1 mm for the right lens and down by 1 mm for the left lens (i.e. choice "D").