title: Mechanism of pupillary reflex dilation in awake volunteers and in organ donors.
abstract: BACKGROUND
  The mechanism of reflex pupillary dilation was investigated in eight patients who were declared brain dead after rupture of intracranial vascular malformations and in eight awake volunteers. The authors hypothesized that the reflex was primarily a spinal sympathetic reflex that would be blocked by topical application of the alpha1-adrenergic blocking agent dapiprazole and that it would be present in organ donors with intact spinal reflexes and no history of hypoxia.
  
  
  METHODS
  In volunteers, pupil size was measured with an infrared pupillometer while brief painful electric stimuli were delivered to the finger. Pain was assessed with a visual analog scale and adjusted with each volunteer to equal 3 on a visual analog scale of 0-10. Subjects were studied before and after topical application of the alpha1-adrenergic antagonist dapiprazole. In organ donors, the authors measured pupil size after high-intensity tetanic electric stimulation and in dapiprazole-blocked and -unblocked pupils after surgically induced nociception.
  
  
  RESULTS
  In volunteers, the pupil dilated 0.43 +/- 0.23 mm after nociceptive stimuli. Dapiprazole eyedrops blocked this dilation, confirming that the reflex in awake humans is primarily a sympathetic reflex. Baseline diameters were 5.7 +/- 0.5 mm before dapiprazole and 4.1 +/- 0.9 mm after dapiprazole. In organ donors, a tetanic electric current failed to dilate the pupil, whereas the skin incision dilated the pupil 0.4 +/- 0.4 mm, but this dilation was not blocked by dapiprazole.
  
  
  CONCLUSIONS
  The authors conclude that pupillary reflex dilation, as it is clinically performed in awake subjects by stimulating somatic nociceptors, is a sympathetic reflex. Because it is not present in organ donors, the neural pathway must require a supraspinal component for completion.
authors.affiliation: Department of Anesthesia and Perioperative Care, University of California, San Francisco 94143-0648, USA.
authors.forename: Laura L
authors.initials: LL
authors.lastname: Yang
authors.forename: Claus U
authors.initials: CU
authors.lastname: Niemann
authors.forename: Merlin D
authors.initials: MD
authors.lastname: Larson
mesh_terms: D000328:Adult; D000368:Aged; D004558:Electric Stimulation; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010879:Piperazines; D011680:Pupil / Q000502:physiology*; D012018:Reflex / Q000187:drug effects / Q000502:physiology*; D013564:Sympathetic Nervous System / Q000502:physiology; D014019:Tissue Donors; D014230:Triazoles / Q000494:pharmacology; D014851:Wakefulness / Q000502:physiology*
chemical_list: D010879:Piperazines; D014230:Triazoles; C035289:dapiprazole