What is a relative afferent pupillary defect (RAPD) and how is it tested?

• A. RAPD indicates reduced afferent input from an affected optic nerve or retina; tested with swinging flashlight test showing reduced constriction when light is swung to the affected eye.
• B. RAPD indicates increased efferent output from CN III; tested with pupillary dilation test.
• C. RAPD is a defect in the efferent limb; tested with anisocoria measurement.
• D. RAPD indicates a problem in the sympathetic pathway; tested with pupil dilation test.

Answer: (A)

RAPD reflects reduced afferent input from the retina or optic nerve, so the brain gets a weaker signal about light when it enters the affected eye. The swinging flashlight test is used to detect this: you move a light back and forth between the eyes and watch how the pupils constrict. When the light is shone into the healthy eye, both pupils constrict normally. But when you swing the light to the eye with the defective afferent pathway, the signal is weaker, so both pupils constrict less. This relative reduction in constriction when light is directed at the affected eye is the telltale sign of RAPD.

If the issue were in the efferent pathway or in the sympathetic system, the pattern would be different and the swinging flashlight test would not reveal the same reduced constriction with the affected eye. The test specifically targets the afferent limb, making the described response the best way to identify RAPD.