During the relative afferent pupil defect test (RAPD-swinging flashlight) when one eye is illuminated, the fellow eye begins to dark-adapt, its light sensitivity increases, and the pupils will constrict when the light comes back, even if the changeover is done very quickly. This initial constriction is a very sensitive sign for subtle interocular differences in the pupillary light reaction. If a marked RAPD is present, for example in a case of optic neuritis, the pupil of the involved eye will dilate when the light returns, and the initial constriction has been lost. This behaviour is called pupillary escape. Pupillary escape is a very clear and specific sign of RAPD. Patients vary in the period of stimulation required of each eye to demonstrate the phenomenon, and the practitioner needs to be careful not to change the observation time or the direction or distance of the examination light from one eye to the other.

A disruption in the afferent pathway will affect direct and consensual responses. For example, in the presence of a disruption in the right afferent pathway, a light directed into the right eye will cause a poor response in the right and the left eyes, although both responses would be normal if the light were directed into the left eye. If the damage to the afferent pathway is complete (i.e., all the fibres from one eye are affected), there would be no direct and no consensual response when light is directed into the affected eye. When only some fibres are damaged, such that the abnormal pupillary responses might be recognised only when compared with the normal pupillary responses; thus the term relative afferent pupillary defect is applied.

When an RAPD is detected, the practitioner needs to find the reason for it. Disruption can occur anywhere in the afferent pathway: retina, optic nerve, chiasm, optic tract, or brain. Damage posterior to the crossing in the chiasm might not be evident with the swinging-flashlight test unless the damage affects a great number of fibres from one eye and significantly fewer fibres from the other eye. There are more crossed (contralateral) fibres in the optic tract than uncrossed (ipsilateral); therefore, with a complete optic tract lesion, the pupillary constrictions may be greater with light into the ipsilateral eye than with light into the contralateral eye.

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