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Cranial Nerves

Pupillary Light Reflexes

February 15th, 2021


The pupillary light reflexes rely on a reflex pathway with the optic nerve as the sensory nerve, the oculomotor nerve as the motor nerve and the midbrain as the processing centre.
  • How to Elicit

  • Shine a light into each pupil. Watch to see that both the ipsilateral (direct response) and contralateral (consensual response) pupils constrict as a result of the stimulus.
  • Causes of Absent Pupillary Reflexes

  • Intraocular conditions - glaucoma, retinal detachment
  • Optic (II) nerve lesion - trauma, compression, optic neuritis
  • Midbrain lesion - encephalitis, tumour, trauma, haemorrhage, MS, midbrain infarct
  • Oculomotor (III) nerve lesion - trauma, compression, small vessel ischaemia, Guillain barré
  • Medications - sympathomimetic, parasympatholytic (anticholinergics), barbiturates
  • Interpretation

  • No direct response - absent reflex in the tested eyeIpsilateral oculomotor nerve (III) lesion
  • No consensual response - absent reflex in opposite eyeContralateral oculomotor nerve (III) lesion
  • No afferent response - absent reflex in one eye when testing either eyeIpsilateral optic nerve (II) lesion
  • No response - absent reflex in both eyes when testing either eyeMidbrain lesion, medications
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