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Can be tested by the swinging torch test to compare the afferent pathway of the pupillary reflex. This can be used as a marker of the integrity of the afferent limb of the reflex and tests both retinal and nerve pathways.

Imagine a defect with the right eye - the swinging torch experiment

• Step 1 :A torch is shone in the right eye - due to a defect there is no pupillary constriction either direct or consensual
• Step 2 : Torch is shone in the left eye. The left pupil constricts as a direct response and the right pupil constricts too as a consensual response
• Step 3 : Torch then shone back in the right eye which had just been constricting and so is seen to dilate when the light shines
• This is called a Marcus Gunn pupil and is a sign of a relative afferent pupillary defect
• Brief oscillations of the pupillary size called hippus occur normally in response to light which should not be confused with an afferent pupillary defect.

Causes

• Optic neuritis e.g. MS
• Retrobulbar neuritis e.g. MS
• Toxic
• Ischaemic damage to Optic nerve
• Retinal causes