RAPD stands for "relative afferent pupillary defect." This is another aspect of the pupillary exam whereby you are testing the relative ability of the two optic nerves to receive light signals. A RAPD will exist if one of the optic nerves is not functioning as well relative to the other one. A RAPD also can exist in conditions that severely affect the retinal tissue of one of the eyes (e.g. retinal detachment). This occurs since the retina is actually a type of neural tissue whose ganglion cell layer ultimately connects to the optic nerve. Since a RAPD indicates either a dysfunction of the optic nerve or retina, an eye with a RAPD will not see as well as the other eye. This decrease in vision takes the form of decreased brightness sense (i.e. things seem dimmer in the affected eye), a visual field defect, or less commonly, decreased visual acuity. A patient with a RAPD does not necessarily also complain about photopsias, eye redness, or eye pain.
More notes:
A relative afferent pupillary defect (rAPD) (aka "Marcus Gunn pupil") is detected by using the swinging flashlight test.
During this test, the penlight is first directed into one eye. This activates the "direct pupillary response" in the eye the light is first shone into and *also* the "consensual pupillary response" in the opposite eye. In a normal individual, both pupils will constrict to the same degree as light is shone into only one eye. If one of the optic nerves is damaged, then when the light is "swung over" to the damaged eye, it will appear to dilate paradoxically. The reason for this is because the direct response in the damaged eye is not as strong as the consensual response (i.e. when the light was shone into the opposite, non-damaged eye). When you see that paradoxical dilation, then you know that a rAPD exists in that eye.
