The swinging-flashlight test is used to help a practitioner decide whether reduced vision is due to ocular disease.

Process

For an adequate test, vision must not be entirely lost. In dim room light, the examiner notes the size of the pupils. The patient is asked to gaze into the distance, and the examiner swings the beam of a penlight

Flashlight

A flashlight is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode...

 back and forth from one pupil to the other, and observes the size of pupils and reaction in the eye that is lit.

Interpretation

• Normally, each illuminated eye looks or promptly becomes constricted. The opposite eye also constricts consensually.

• When ocular disease, such as cataract
  Cataract
  A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...
  
  , impairs vision, the pupils respond normally.

• When the optic nerve is damaged, the sensory (afferent) stimulus sent to the midbrain is reduced. The pupil, responding less vigorously, dilates from its prior constricted state. This response is an afferent pupillary defect (Marcus Gunn Pupil
  Marcus Gunn pupil
  Marcus Gunn pupil is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils constrict less when a bright light is swung from the unaffected eye to the affected eye...
  
  ).

External links

• http://library.med.utah.edu/neurologicexam/html/cranialnerve_normal.html#05