The swinging light test, also known as the swinging flashlight test, is used in medical examinations to identify a relative afferent pupillary defect (RAPD).

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

Normally, each illuminated pupil promptly becomes constricted. The opposite pupil also constricts consensually. 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 when the light is moved away from the unaffected eye and towards the affected eye. This response is a relative afferent pupillary defect.

Depending on severity, different symptoms may appear during the swinging flash light test:

  • Mild RAPD will presents as a weak pupil constriction initially, after which dilation continues to happen.
  • When RAPD is moderate, pupil size will remain, after which it dilates
  • When RAPD is severe, the pupil will dilate quickly

Four key metrics are highly correlated (p=0.003 or less) to traumatic brain injury (TBI) vs. non-TBI (source):

  • Constriction Latency
  • 75% Recovery Time
  • Average Constriction Velocity
  • Dilation Velocity

These and other metrics are what BRAD-BIT measure and track.