A poor response to light with normal constriction on accommodation is expected with severe visual impairment. Of greater value in detecting more subtle damage of the optic nerves or chiasm is the demonstration of a relative afferent papillary defect which is found with unilateral or asymmetric involvement. The simplest way to demonstrate such a defect is as follows: the direct and consensual light reactions are examined on each side in the usual way. Pupillary constriction is observed. The direct reaction on one side is then retested, and the light swung quickly to the other side. If the optic nerve on the second side is more abnormal, the pupil will now dilate because the neural input to the superior colliculus from the ipsilateral side is less powerful (as a result of damage to the nerve fibers) than the consensual input. The test is best performed with low ambient illumination. It is important to exclude hippus as a cause of papillary dilatation.

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