Relative Afferent Pupillary Defect (RAPD)

Relative Afferent Pupillary Defect (RAPD)

An afferent pupillary defect (APD), or relative afferent pupillary defect (RAPD), is an abnormal pupillary response in which the normally equal direct and consensual pupillary reflexes are asymmetric, the direct response being less than the consensual. This may be particularly evident using the "swinging flashlight" test, in which the two pupils are alternately illuminated every 2-3 seconds in a darkened room. Quickly moving the light to the diseased side may produce pupillary dilatation (Marcus Gunn pupil). Subjectively patients may note that the light stimulus seems less bright in the affected eye.
RAPD suggests an asymmetric optic nerve pathology, such as optic neuritis or tumor, causing a conduction defect; indeed this is the most sensitive sign of optic nerve pathology. Although visual acuity may also be impaired in the affected eye, and the disc appear abnormal on funduscopy, this is not necessarily the case. Since RAPD depends on asymmetry of optic nerve conduction, no defect may be observed if both optic nerves are affected.

RAPD has also been described with lesions of the retina, optic chiasm, optic tract (contralateral), brachium of the superior colliculus and pretectal nucleus (in the latter two situations without visual impairment).

References

Chen CJ, Scheufele M, Sheth M, Torabi A, Hogan N, Frohman EM. Isolated relative afferent pupillary defect secondary to contralateral midbrain compression. Archives of Neurology 2004; 61: 1451-1453 Kawasaki A. Approach to the patient with abnormal pupils. In: Biller J (ed.). Practical neurology (2nd edition). Philadelphia: Lippincott Williams & Wilkins, 2002: 135-146

Cross References

Amblyopia; Marcus Gunn pupil, Marcus Gunn sign; Pupillary reflexes; Swinging flashlight sign