The Swinging Light Test

An afferent pupillary defect is detected using the swinging light test.

The swinging light test is performed as its name suggests by 'swinging' a light from one eye to the other, shining into each eye for around 3 seconds.  Note the whole torch should be moved to avoid accidentally eliciting accommodation response. 

Normally shining the light into an eye provokes direct and consensual constriction of the pupil.

However, in a pupil that has an impaired direct response, when a light is being swung the recovery from the consensual constriction will be faster than the direct response, meaning the pupil will paradoxically appear to dilate when the light is shone into it.

This is also known as a 'Marcus Gunn' pupil and an 'afferent pupillary defect'

An afferent pupillary defect can be caused by:

• damage to the optic nerve before the optic chiasm eg compression by tumour
• optic neuritis
• asymmetrical glaucoma
• trauma

References
Broadway, D. How to test for a relative afferent pupillary defect. Community Eye Health. 2012; 25(79-89): 58-59.
Pearce, J. The Marcus Gunn pupil. J Neurol Neurosurg Psychiatry. 1996; 61(5): 520.