Pupillary Defects
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There are two nerves involved in pupillary responses:
  • Optic Nerve (II) – this is the afferent nerve – it detects the light, and sends this signal to the brain. HOWEVER – a defect in the afferent nerve does not necessarily have to be in the optic nerve itself, it could be at the optic chiasm (likely to be bilateral), or in the optic tract. Anywhere along the afferent pathway.
  • Occulomotor nerve (III) – this is the efferent nerve – when the brain receives a signal about strong light from EITHER EYE – it will send a signal down both occulomotor nerves, telling the pupil to constrict
Ime by Khadija Janoowala

Afferent pathway (red line)

  1. Action potential generated in optic nerve
  2. Action potential travels through right and left lateral geniculate bodies
  3. Axons synapse at right and left pre-tectal nuclei

Efferent pathway (green line)

  1. Action potential passes to right and left Edinger-westphal nuclei of CN III (oculomotor nerve) via interneuron’s (black lines with arrows)
  2. Then to the right and left ciliary ganglions via oculomotor nerve
  3. Constriction of
    • Pupil being illuminated (A) = direct reflex
    • Contra lateral pupil (B) = consensual reflex

Normal Response

Shining a light in either eye – will cause both pupils to constrict simultaneously.
Relative Afferent Pupillary defect (RAPD)
Marked – shining the light in the affected eye will result in no constriction of either pupil.
  • E.g – a RAPD in the left eye – shining the light in the left eye will result in no constriction of either pupil. Shining the light in the right eye will result in constriction of both pupils.
  • This is due to damage to the left afferent pathway
  • Bilateral presentation – neither pupil will contract in response to light in either eye.   

Mild – shining the light in the affected eye will result in a slow constriction of both pupils, but then a gradual dilation of both pupils.

Horner’s Syndrome

The responses are actually NORMAL – however, on the affected side:
  • There is ptosis
  • The pupil is naturally quite constricted. It will still respond relatively to light (ie. It will constrict further, and will dilate somewhat in the dark), but its generally ‘resting’ state is constricted to a greater degree than a normal pupil.

Third Nerve (Efferent) Palsy

The affected side will have:
  • An eye that points down and out
  • A dilated pupil that does not respond to light in either eye.
However! – as the afferent is still functioning normally:
  • The contralateral eye will respond to light in the affect eye. And the contralateral eye will respond normally to its own light.


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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University, and is studying for a Masters of Sports Medicine at the University of Queensland. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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