Pupillary Abnormalities

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PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.

The pupil is the space delineated by the internal margin of the iris and is important both physiologically and in identifying pathology. Its function is to control the amount of light entering the eye and it does this via contraction (miosis) and dilation (mydriasis) under the influence of the autonomic nervous system.

Components of the pupillary reflex pathways
  • Afferent pathway - optic disc/nerve, optic chiasm, optic tract and the pretectal nucleus (lying in the dorsal midbrain).
  • Efferent pathway - this passes through the Edinger-Westphal nucleus before travelling along with the third cranial nerve out to the iris (with a synapse at the ciliary ganglion which sits in the orbit).
There are several reflexes subserved by this pathway and each should be examined in turn. Damage at any level of the pathway or abnormalities of the iris itself can cause abnormalities of pupil shape or function.

See also the separate article on Examination of the Eye.

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General examination of the patient

This often-missed component of the examination may provide helpful clues as to what is going on, particularly where there is an underlying neurological cause. There may be a telltale neck scar and associated ptosis in patients with Horner's syndrome or a neurosurgical scar in patients with a third nerve palsy. It may be necessary to go back to the general observation and carry out a further examination depending on the pupillary findings (eg, check for the absent ankle or knee jerk in Adie's pupil).

Pupillary observation

Start by a general observation, noting the shape and size of the pupil in ambient bright light. Size is measured in millimetres and the normal pupil ranges from 1-8 mm. Next, dim the light and have the patient fixate on the far wall. You can then observe the pupils closely by shining a bright light on the patient's face from below (minimise the shadow cast by the nose by placing the light in the midline). If you think there is size asymmetry, a good trick is to stand back and observe the red reflex of both eyes simultaneously with the ophthalmoscope. A slight difference will then become more apparent. If you have access to a slit lamp, use it as a lot of more detailed information can be gauged about the abnormally shaped pupil.

Assessing pupillary reflexes

There are essentially three reflexes to specifically test for:[1]

Light reflex test

  • What it assesses - the integrity of the pupillary light reflex pathway.
  • How to perform it - dim the ambient light and ask the patient to fixate a distant target. Illuminate the right eye from the right side and the left from the left side. (Make sure you do not stand in front of the patient, as their pupils will accommodate to focus on you.) Record whether there is a direct pupillary response (the pupil constricts when the light is shone on it) and a consensual response (the fellow pupil constricts too).
  • Normal test - there should be a brisk, simultaneous, equal response of both pupils in response to light shone in one or the other eye.

Swinging flashlight test

  • What it assesses - compares direct and consensual responses of each eye (as opposed to seeing whether they are there or not).
  • How to perform it - use the same conditions as for the light reflex test and check this reflex first. Then, move the beam swiftly and rhythmically from one eye to the other, making sure that you allow the same amount of light exposure on each eye and that each is illuminated from the same angle. You should note the pupillary constriction of both eyes when the beam is maintained. However, when it is swung, look at what happens to the pupil of the eye you are concerned about and compare this with what is happening to the fellow eye.
  • Normal test - the pupil should constrict or stay the same size. If it dilates when light is shone on it, then this means that the light reflex is weaker than the consensual reflex (produced by withdrawing light from the unaffected eye), suggesting optic nerve pathology. This abnormal response is known as a relative afferent pupillary defect (RAPD) and is a very important sign. Note that this is a comparative test: you cannot have a bilateral RAPD.

Near reflex test

  • What it assesses - this assesses the miosis component of near fixation, otherwise known as accommodation. (The other two components of accommodation are increased lens thickness and curvature, and convergence of the eyes.)
  • How to perform it - in a normally lit room, instruct the patient to look at a distant target. Bring an object (a toy, the patient's thumb) into their near point (about an arm's length away) and observe the pupillary reflex when their fixation shifts to the near target.
  • Normal test - there should be a brisk constriction. A near-light dissociation describes the situation where the patient has a significantly better pupillary near reflex than light reflex.


This refers to unequal pupils. This is physiological in about 20% of people.[3] However, if this is a new complaint, the steps to the underlying diagnosis lie in determining which of the pupils is abnormal and then looking for associated signs. The first step is to compare the pupils in light and dim conditions:

  • If there is a poor reaction to light in one eye and the anisocoria is more evident in a well lit room, the affected pupil is abnormally large.
  • If there is a good reaction to light in both eyes but a poor dilation in the dark (ie the anisocoria is enhanced), the affected pupil is abnormally small.

The large pupil[4]

  • Features - there is poor constriction in a well lit room.
  • Differential diagnosis - traumatic iris damage, third cranial nerve palsy, pharmacological dilation (ie dilating drops), Adie's pupil, iris rubeosis.

The small pupil[4]

  • Features - there is poor dilation in a dim room.
  • Differential diagnosis - physiologically small pupil, pilocarpine drops, uveitis with synaechiae, Horner's syndrome.

The abnormally shaped pupil

  • Features - a pupil should be round. Deviation from this suggests abnormalities.
  • Differential diagnosis - congenital defects (eg, coloboma), iris inflammation or trauma, Argyll Robertson pupils. A fixed oval pupil in association with severe pain, a red eye, a cloudy cornea and systemic malaise suggests acute angle-closure glaucoma which warrants immediate referral.

The abnormally reacting pupil

  • Light reflex test - abnormalities arise as a result of severe optic nerve damage (eg, transection) - the patient will be blind in that eye, neither pupil reacts when the affected side is stimulated but both pupils react normally when the fellow eye is stimulated.[5]
  • Swinging flashlight test[2] - when the pupil exhibits an RAPD, it is described as a Marcus Gunn pupil. It suggests optic nerve disease, central retinal artery or vein occlusions (see the separate article on Non-diabetic Retinal Vascular Disease). A mild RAPD may also occur in amblyopia, with vitreous haemorrhage, retinal detachment or advanced macular degeneration.
  • Near reflex test - there are several causes of light-near dissociation which can be grouped according to whether the problem is unilateral or bilateral:[5]
    • Unilateral light-near dissociation - afferent conduction defect, Adie pupil, herpes zoster ophthalmicus, aberrant regeneration of the third cranial nerve.
    • Bilateral - neurosyphilis, diabetes, myotonic dystrophy, Parinaud's dorsal midbrain syndrome, familial amyloidosis, encephalitis, chronic alcoholism.

Congenital abnormalities[5]

  • Aniridia - this is a bilateral condition arising from the abnormal neuroectodermal development secondary to genetic mutation. It is associated with glaucoma and a number of serious, systemic abnormalities.
  • Coloboma - this is an uncommon, congenital condition characterised by a unilateral or bilateral partial iris defect. There may be other associated defects both within the eye (eg, the retina) and in the adnexial structures (eg, the lids).
  • Leukocoria - this refers to a white pupil and may be due to a number of conditions.[6] Congenital cataracts are generally easily identified but all patients must be assessed for the possibility of retinoblastoma. Other conditions causing a white pupil include persistent fetal vasculature syndrome, Coats' disease and retinopathy of prematurity.

Acquired structural abnormalities

  • Pseudoexfoliation syndrome - this is a condition characterised by the presence of a grey-white fibrogranular extracellular matrix material deposited on the anterior lens. It is seen, on slit-lamp examination, as a fine grey dusting around the pupil. Pupil shape and function are not affected - it is clinically significant due to its association with glaucoma and its potential to make cataract surgery more tricky.
  • Sphincter tear - iris tear can occur as a result of blunt or penetrating trauma and can also occur during intraocular surgery. Tears may be associated with glaucoma and, if large, visual problems. All tears, however small, need ophthalmological assessment.
  • Synechiae - this refers to adhesions between the lens and the iris (posterior synechiae) or the iris and the peripheral cornea (peripheral anterior synechiae). These adhesions will give rise to an abnormally shaped pupil; treatment depends on the underlying cause. Uveitic posterior synechiae are broken with mydriatics whereas glaucomatous anterior synechiae may be managed with miotics.

Neurological abnormalities

  • Horner's syndrome - see link for separate article.

    Causes of Horner's syndrome[7]

    Central (first-order) nerve lesionsPreganglionic (second-order) nerve lesionsPostganglionic (third-order) nerve lesions
    Cerebrovascular accidents.Apical lung tumours (eg, Pancoast's tumour).Cluster headaches or migraine.
    Multiple sclerosis.Lymphadenopathy (lymphoma, leukaemia, tuberculosis, mediastinal tumours).Herpes zoster infection.
    Pituitary or basal skull tumours.Lower brachial plexus trauma or cervical rib.Internal carotid artery dissection, may be traumatic.[8]
    Basal meningitis (eg, syphilis).Aneurysms of the aorta, subclavian or common carotid arteries.Raeder's syndrome (paratrigeminal syndrome).
    Neck trauma (eg, cervical vertebral dislocation or dissection of the vertebral artery).Trauma or surgical injury (neck or chest).[9]Carotid-cavernous fistula.
    Syringomyelia.Neuroblastoma. Temporal arteritis[10] 
    Arnold-Chiari malformation.Mandibular dental abscess. 
    Spinal cord tumours.  
    Confirmation of Horner's syndrome is with instillation of a drop of 4% cocaine: in physiological anisocoria, this results in dilation whereas it doesn't where there is a Horner's syndrome.

    Further localisation of the problem is carried out with 1% hydroxyamfetamine. Instillation is done >48 hours after the cocaine test as cocaine affects the hydroxyamfetamine. Pupillary dilation suggests a central or preganglionic Horner's syndrome whereas if dilation does not occur, the lesion is likely to be postganglionic.
  • Midbrain pupils - this refers to the bilateral mid-dilated pupils associated with dorsal midbrain lesions. There is a light-near dissociation but a good response to miotics and mydriatics.
  • Pupil-involving third cranial nerve (CN III) palsy:[2]
    • When the pupil is involved in a CN III palsy, it is fixed and dilated (or minimally reactive). A partially dilated pupil which reacts sluggishly to light suggests a relative pupil-sparing CN III palsy:
      • CN III palsies with involvement of pupil: requires urgent neuro-imaging with MRI (with magnetic resonance angiography - MRA) or high resolution CT angiograpy (CTA), with or without a lumbar puncture if MRA or CTA are normal.
      • Relative pupil-sparing CN III palsy: tends to be ischaemic in nature and does not need such urgent investigations unless there is progression.
    • Other signs of a CN III palsy include features of external ophthalmoplegia: limitation of extraocular movements in all fields of gaze except temporally and a ptosis.
    • There may also be evidence of aberrant regeneration where there has been previous CN III damage: as the patient looks down, their lid goes up.
    • The differential diagnosis of a CN III palsy includes:Parinaud's syndrome and giant cell arteritis can also cause CN III palsies. An extradural haematoma causes a progressively dilating pupil, due to gradual compression of the third nerve and similarly, a unilateral dilated pupil in a comatose patient should prompt the thought of a subarachnoid haemorrhage.
  • (Adie's) tonic pupil:[1]
    • This describes a unilateral (80% of cases), mydriatic pupil in otherwise healthy patients (typically, young adults, especially women).
    • Over months to years, the pupil diminishes in size to eventually become miotic. There is sluggish, sectoral or no reaction to light but a normal near reflex. Re-dilation after the near response is slow.
    • Slit-lamp examination may reveal slow, vermiform contractions of the iris but ultimately, the diagnosis is confirmed by the pupil's hypersensitivity to weak miotic drops (eg, 0.05-0.125% pilocarpine) which causes the abnormal pupil to contract vigorously and the normal pupil minimally.
    • Occasionally, it is associated with diminished deep tendon reflexes (Holmes-Adie syndrome) ± autonomic nerve dysfunction.
    • The exact cause is not known but it often occurs after a viral illness (eg, herpes zoster ophthalmicus) and denervation of the postganglionic supply to the sphincter pupillae is described.
    • This tends to be a benign condition and the patient is simply observed. However, infants <1 year old should be referred to a paediatric neurologist to rule out familial dystonias (Riley-Day syndrome).[2] 
  • Argyll Robertson pupils - these are caused by neurosyphilis. Although usually asymptomatic, they have characteristic features on examination. These include bilateral (usually asymmetrical) small, irregular pupils showing a light-near dissociation. They are difficult to dilate. Management is related to the underlying disease.

Topical drugs

  • Dilating - sympathomimetics (eg, phenylephrine, adrenaline) and antimuscarinics (eg, cyclopentolate, tropicamide, atropine).
  • Constricting - muscarinic agonists (eg, pilocarpine).

Systemic drugs

  • Dilating - sympathomimetics (eg, adrenaline (epinephrine)) and antimuscarinics (eg, atropine). Think also of tricyclic antidepressants, amfetamines and ecstasy.
  • Constricting - opiates (eg, morphine and organophosphates).

Further reading & references

  1. Wilson FM; Practical Ophthalmology: A Manual for Beginning Residents, American Academy of Ophthalmology, 2005
  2. The Wills Eye Manual (6th ed), 2012
  3. James B, Chew C, Bron A. Lecture notes on Ophthalmology, 9th ed. Blackwell Publishing (2003)
  4. Moorfields Manual of Ophthalmology (2nd Ed), 2014
  5. Clinical Ophthalmology, A Systematic Approach (8th ed); 2011.
  6. Denniston AKO, Murray PI; Oxford Handbook of Ophthalmology, Oxford University Press, 2009
  7. Bucci T, Califano L; Bernard-Horner's syndrome: unusual complication after neck dissection. J Oral Maxillofac Surg. 2008 Apr;66(4):833.
  8. Costopoulos C, Patel RS, Mistry CD; Painful Horner's syndrome. Emerg Med J. 2008 May;25(5):295.
  9. Allen AY, Meyer DR; Neck procedures resulting in Horner syndrome. Ophthal Plast Reconstr Surg. 2009 Jan-Feb;25(1):16-8.
  10. Pupillary abnormalities; Atlas of Ophthalmology - good pictures and simple descriptive notes
  11. Makhlouf F, Scolan V, Detante O, et al; Post-traumatic dissection of the internal carotid artery associated with ipsilateral facial nerve paralysis: diagnostic and forensic issues. J Forensic Leg Med. 2013 Oct;20(7):867-9. doi: 10.1016/j.jflm.2013.06.018. Epub 2013 Aug 13.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Olivia Scott
Current Version:
Peer Reviewer:
Dr Olivia Scott
Document ID:
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