Hi,
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.