Background

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.

Examining the pupils

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 tell-tale 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 1-8mm. 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. Shine 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. (The other two components are accommodation: 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 (toy, 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.

Abnormal pupils

Anisocoria^[2]

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 look 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, then 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), then 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 our dedicated article: 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 dorsal midbrain syndrome, familial amyloidosis, encephalitis, chronic alcoholism.

Diseases affecting the pupils

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 retina) and in the adnexial structures (eg lids).
• Leucocoria^[6] - this refers to a white pupil and may be due to a number of conditions. 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, Coat's disease and retinopathy of prematurity.

Acquired structural abnormalities

• Pseudoexfoliation syndrome - this is a condition characterised by deposition 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 - this syndrome is characterised by miosis with a pupil that is slow to dilate, a mild (1-2mm) ptosis, ipsilateral anhydrosis and apparent enophthalmos (affected eye appears to be sunken) as a result of a combination of the ptosis and a slight elevation of the inferior eyelid. The irides may be of different colours if the lesion is congenital or long-standing.^[5] Depending on the location of the lesion, some or all of these features will be present. The causes are summarised below.

  Causes of Horner's syndrome[8][9]
  Central (first-order) nerve lesions	Preganglionic (second-order) nerve lesions	Postganglionic (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.[10]
  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).[11]	Carotid-cavernous fistula.
  Syringomyelia.	Neuroblastoma.
  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% hydroxyamphetamine (Paredrine). Instillation is done >48 hours after the cocaine test as cocaine affects the Paredrine. 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. 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:
  • Myasthenia gravis
  • Thyroid eye disease
  • Chronic progressive external ophthalmoplegia
  • Orbital inflammatory pseudotumour
  • Internuclear ophthalmoplegia
  Parinaud 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][7] - 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 a sluggish, sectoral or no reaction to light but a normal near reflex. Redilation 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 a bilateral (usually asymmetrical) small, irregular pupils showing a light-near dissociation. They are difficult to dilate. Management is related to the underlying disease.

Drugs affecting the pupils^[3]

Topical drugs

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

Systemic drugs

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