Pupil Examination

When referring a patient urgently to ophthalmology you are likely to be asked about pupil reactions, it is very valuable information in deciding the likely diagnosis and therefore urgency of review.

There are 3 parts to properly examining pupils

  1. Check for pupil size in light and dark (looking for difference in size: anisocoria)
  2. Check for a reaction to light in each eye
  3. Check for a Relative Afferent Pupil Defect (RAPD)

Normal pupils are therefore PERL and no RAPD. Pupils Equal and Reactive to Light and no Relative Afferent Pupil Defect.

You can take the pupil examination quiz as part of my pupil examination book. Now available to buy on itunes or using the ibook app on the iPad. Get to http://itun.es/i6xT3Yf

Watch my following short videos:

  • the first shows normal pupil examination,
  • the second shows more detail regarding RAPD
  • the third looks at assessing anisocoria, each includes the full transcript below the video.

normal pupil examination


A normal pupil examination can be documented as being PERL and NO RAPD. This shorthand states that the pupils are equal and reactive to light, and that there is no Relative Afferent Pupil Defect.

So how do I actually test these properly?

I ask to patient to fix in the distance, then i check the pupils are equal in size, and again with the lights off. Then, with the lights still off I check each eye has a direct response to light. Finally I do a swinging light test to check there is no RAPD.

Now lets recap with a little more explanation.

Firstly To avoid the near reflex where the eyes converge, accommodate and the pupils constrict, ask the patient to fix on an object in the distance.

You should check the pupils are equal in both light and dark or you may miss an abnormally small pupil such as is seen in horner’s syndrome. For more on unequal pupils watch the video titled anisocoria.

When examining the pupil reactions, having the patient in the dark with distance fixation makes the pupils as large as possible and makes the pupil reactions easier to see.

When you shine the light at the pupil, watch the same pupil for a quick constriction, followed by a slight relaxation.

There is no need to look for a consensual reaction here as both eyes have demonstrated a direct reaction indicating an intact afferent pathway from eye to brain, and efferent pathway from brain to pupil. There is also no need to check for a response to accommodation unless the pupils fail to respond to light.

Finally the swinging light test to check for a relative afferent pupil defect or RAPD. This is a comparative test of the two optic nerves, and may detect conditions such as optic neuritis or optic nerve compression where the nerve is functioning, but poorly when compared to the other side.

Again this should be in the dark with distance fixation. First shine the light at the first eye, the pupil will constrict and then relax a little, now swing the light source, quickly and directly, to the other eye. The pupil will have just started to dilate when the light hits it, causing a small constriction, followed by a relaxation. Make sure you hold the light on each eye for 2 to 3 seconds to allow each pupil to first constrict then relax before you swing the light to the other eye.

Your light source must be bright to reliably detect an RAPD, a standard direct ophthalmoscope or pen torch may not be bright enough.

OK, you have now examined these normal pupils.



Today we are looking at Relative Afferent Pupil Defects, or RAPD. This is also sometimes referred to as Marcus-Gunn pupil.

First we will compare its clinical appearance with that of normal pupils and also that of a complete afferent pupil defect.

To avoid pupil constriction while accommodating, ask to the patient to fix on a distant object throughout your examination.

Look for equal pupil sizes, and check again with the lights off. Anisocoria is not a feature of an afferent defect.

Now check for a reaction to light in each eye, again with the lights off. Here the normal pupils constrict briskly, then relax a little. They dilate again after the light is removed.

now swing the light from eye to eye, quickly – but pausing on each eye for around 2 seconds. In the normal patient the pupils will constrict then relax a little each time the light is swung to them.

Now a patient with a relative afferent defect. The pupils will be equal size in both light and dark. Both pupils will react to light, although sometimes a slower response is noted when light is shone on the affected side.

With the swinging light test the RAPD now becomes obvious. On the affected side, both pupils dilate when the light is swung across. Here the left side is affected.

You will miss an RAPD if you do not do the swinging light test, as it is only by comparing the relative strengths of the signals reaching the brain from the eyes that the abnormality is detected.

Finally with a complete afferent pupil defect, there is no pupil reaction to light shone on the affected side.

Due to crossing of nerve fibres at the optic chiasm, an RAPD localizes pathology to the visual pathway before the chiasm, that is the optic nerve or retina.

Some examples of pathologies causing an RAPD are Large Retinal Detachment, Central Retinal Artery or Ischaemic central retinal Vein Occlusion, Optic Nerve Ischaemia, Optic Neuritis, asymmetric glaucoma

It should be noted that an RAPD is not caused by either cataract or vitreous haemorrhage, and when associated with amblyopia is at most a mild RAPD. A Definite RAPD in these cases should prompt a look for another cause of visual loss.



Difference in pupil sizes is termed anisocoria.

Based on clinical findings, it can be divided into 3 groupings.

First is an abnormally large pupil. This is obvious in normal lighting but less so with the lights off, because the other normal pupil dilates.

Next is an abnormally small pupil. This may not be visible in normal lighting, but with the lights off becomes obvious due to dilation of the normal pupil.

Finally is pupil aysmmetry up to 2mm that doesn’t change in light and dark. Both pupils change size, but the relative difference remains the same. This is present in up to 20% of normal people and termed physiological anisocoria. Both eyes respond normally to light.

Back to the abnormally large pupil termed a mydriasis. The autonomic nervous system controls pupil movement, with constriction supplied by the parasympathetic fibers which travel with the 3rd cranial nerve. Loss of the parasympathetic signal causes the pupil to dilate.

Look, therefore, for diplopia or ptosis to suggest a 3rd nerve palsy. This can be caused by berry aneurysm compressing the 3rd nerve, which can accompany and occasionally precede subarachnoid haemorrhage. Here the affected right eye is dilated, down and out, with a ptosis.

A dilated pupil without ptosis or diplopia is unlikely to arise from a 3rd nerve palsy. See the video on 3rd nerve palsy.

Another cause may be Adies tonic pupil. This is characterized by a dilated pupil, with little response to light, but which may slowly constrict to accommodative effort and relax slowly as well. Adies pupil is presumed to be a postviral denervation of the pupil sphincter and is common in young women. Slit lamp examination may reveal segmental paralysis and flattening of the pupil border, giving rise to a pupil with an irregular shape. There may also be a vermiform movement of the non-paralyzed sections of the iris, literally a worm like constriction effort.

Adie’s pupil is confirmed by testing with dilute pilocarpine 0.125% eyedrops which shows constriction within 20 minutes, but this denervation supersensitivity usually takes some weeks to develop after onset of the adies pupil.

Althought a tonic pupil is typically idiopathic, they may arise in diabetes, giant cell arteritis and syphilis where they are usually bilateral, small and termed argyll-robertson pupils.

Blunt trauma to the eye may tear the pupil sphincter and cause a permanently dilated pupil, clinically similar in appearance to an adie’s pupil. Diplopia after trauma is suggestive of a blowout fracture. Acutely look for an associated hyphaema and later for angle recession or retinal dialysis. Previous eye surgery may also have damaged the pupil.

Acute glaucoma features a fixed mid-dilated pupil with brow ache, blurred vision and nausea or vomitting. The cornea is hazy on slit-lamp examination, with a very high intraocular pressure.

Finally the commonest cause of a dilated pupil is exposure to dilating drugs. Examples include the eydrops atropine, cyclopentolate and tropicamide. Atropine may dilate a pupil for up to 2 weeks. Gardeners may inadvertently expose themselves to atropine when cutting back the deadly nightshade or bella donna plant. They present with a dilated pupil, blurred vision and slight photophobia. The pupil is widely dilated, and doesn’t respond to pilocarpine 1%, but resolves over several days.

Now to the abnormally small pupil. Autonomic control of pupil dilation is by the oculosympathetic pathway. This arises in the hypothalamus, descends the brainstem and cervical spinal cord, ascends the cervical sympathetic chain, the carotid plexus and passes through the cavernous venous sinus with the ophthalmic branch of the trigeminal nerve. Damage along this pathway is termed a horner’s syndrome and features a small pupil or meiosis, slight ptosis and loss of sweating or anhidrosis on one side of the face. Confirmatory testing with Apraclonidine drops reverses the anisocoria and often the ptosis too. See the video on horner’s syndrome for more details. Causes of a horner’s syndrome include carotid artery dissection, which is both life threatening and treatable with anticoagulation.

Other causes of a small pupil are current or previous iritis and current or previous use of pilocarpine eye drops.

Some key points once more.

Anisocoria may arise due to a lesion impairing the efferent sympathetic or parasympathetic pathway to the eye, or due to factors within the eye itself.

The pupils should be examined in both light and dark, with distance fixation.

Ask about eye trauma or surgery, use of eye drops, and gardening.

With a dilated pupil, check for ptosis, diplopia, and response to dilute and 1% pilocarpine.

With a small pupil, confirm horners syndrome with apraclonidine and investigate further urgently

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