title: Origin of the relative afferent pupillary defect in optic tract lesions.
abstract: OBJECTIVE
  To determine the percent decussation of pupil input fibers in humans and to explain the size and range of the log unit relative afferent pupillary defect (RAPD) in patients with optic tract lesions.
  
  
  METHODS
  Experimental study.
  
  
  METHODS
  Five patients with a unilateral optic tract lesion.
  
  
  METHODS
  The pupil response from light stimulation of the nasal hemifield, temporal hemifield, and full field of each eye of 5 patients with a unilateral optic tract lesion was recorded using computerized binocular infrared pupillography. Six stimulus light intensities, separated by 0.5-log unit steps, were used; 12 stimulus repetitions were given for each stimulus condition.
  
  
  METHODS
  For each stimulus condition, the pupil response of each eye was characterized by plotting the mean pupil contraction amplitude as a function of stimulus light intensity. The percentage of decussating afferent pupillomotor input fibers was calculated from the ratio of the maximal pupil contractions elicited from each eye. The RAPD was determined pupillographically from full-field stimulation to each eye.
  
  
  RESULTS
  In all patients, the pupil response from the functioning temporal hemifield ipsilateral to the tract lesion was greater than that from the functioning contralateral nasal hemifield. This temporal-nasal asymmetry increased with increasing stimulus intensity and was similar in hemifield and full-field stimuli, eventually saturating at maximal light intensity. The log unit RAPD did not correlate with the estimated percentage of decussating pupil fibers, which ranged from 54% to 67%.
  
  
  CONCLUSIONS
  In patients with a unilateral optic tract lesion, the pupillary responses from full-field stimulation to each eye are the same as comparing the functioning temporal field with the functioning nasal field. The percentage of decussating fibers is reflected in the ratio of the maximal pupil contraction amplitudes resulting from stimulus input between the two eyes. The RAPD that occurs in this setting reflects the difference in light sensitivity between the intact temporal and nasal hemifields. Its magnitude does not correlate with the difference in the number of crossed and uncrossed axons, but its sidedness contralateral to the side of the optic tract lesion is consistent with the greater percentage of decussating pupillomotor input.
authors.affiliation: Department of Ophthalmology and Visual Science, University of Iowa and Veterans Administration Hospitals, Iowa City, Iowa 52242, USA. randy-kardon@uiowa.edu
authors.forename: Randy
authors.initials: R
authors.lastname: Kardon
authors.forename: Aki
authors.initials: A
authors.lastname: Kawasaki
authors.forename: Neil R
authors.initials: NR
authors.lastname: Miller
mesh_terms: D000293:Adolescent; D000328:Adult; D001927:Brain Diseases / Q000150:complications*; D006801:Humans; D008027:Light; D008875:Middle Aged; D010775:Photic Stimulation / Q000379:methods; D011680:Pupil / Q000528:radiation effects*; D011681:Pupil Disorders / Q000209:etiology* / Q000503:physiopathology; D014794:Visual Fields / Q000528:radiation effects; D014795:Visual Pathways