Anisocoria (Unequal pupils)

The pupil is the hole in the center of the iris, that allows light to enter the retina. The iris is a thin strip of smooth muscle which regulates the amount of light entering the eye by controlling the size of the pupil.
The iris actually consists of two smooth muscles:  There is a circular group called the sphincter pupillae, which is innervated by the parasympathetic fibers that travel with the oculomotor nerve, and when activated constricts the pupil. There is also a radial muscle group called the dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, which when activated causes the pupil to dilate.


Anisocoria (unequal pupils) can be physiologic, or can result from a variety of disorders of the nervous system.


Anisocoria – the pupils are unequal, is the the pupil too large, or is the left pupil too small?

The first step in evaluating anisocoria is to determine which is the abnormal side – this can be accomplished by comparing the pupil sizes in response to bright light and dull ambient light:

horners, dark

If the smaller of the pupils is abnormal, a miosis, then the anisocoria will be more apparent (the affected pupil will be smaller) in dull ambient light conditions (a dark room), and less apparent (the pupils will be more equal) in bright light.







Alternatively, if the large pupil is the abnormal side, mydriasis, it will fail to contract in response to intense light.







Miosis (small pupil)

A unilateral miosis can be physiologic or can indicates a lesion affecting the sympathetic nerve fibers to the eye known as Horner’s syndrome.

Physiologic anisocoria is mild (usually less than 0.5 mm difference in size between the pupils, and the amplitude of the difference does not vary greatly under dim or bright light conditions.

Horner’s syndrome causes unilateral miosis, and the affected abnormal pupil either does not dilate or has a delayed dilation in the dark. Furthermore, there is usually an associated lid ptosis and facial hypohidrosis. Horner’s syndrome is important to diagnosis, because it can indicate a lesion in the brain stem (stroke), neck (carotid artery dissection) or lung apex (Pancoast’s tumor).

Horner pancoast

Right Horner’s syndrome (ptosis and miosis), from R apical lung tumor (Pancoast tumor).


Mydriasis (Dilated Pupil)

This is most commonly caused by an anticholinergic drug, such as atropine or hyoscyamine, hyoscyamine, either from a medication, eye drop or misplaced nebulizer:

nebulizer, anisocoria

Unilateral mydriasis, caused by direct anticholinergic drug effect on iris because of a badly fitting face mask.

Damage or compression of the oculomotor nerve buy an aneurysm or brain herniation can also cause mydriasis, typically associated with ptosis and ophthalmoparesis:

R ptosis, occulomotor palsy (eye is deviated down and outwards), with a dilated pupil, caused by nerve compression from aneurysm (red arrow)

R ptosis, occulomotor palsy (eye is deviated down and outwards), with a dilated pupil, caused by nerve compression from aneurysm (red arrow)

Mydriasis from Adie’s Syndrome

Adie’s syndrome presents with abrupt onset mydriasis often associated with loss of deep tendon reflexes, thought to be the result of a viral infection that causes damage to neurons in the ciliary ganglion (the area of the brain that provides parasympathetic control of eye constriction). Clinical exam may reveal sectoral paresis of the iris sphincter or vermiform iris movements:

tonic pupil

56 thoughts on “Anisocoria (Unequal pupils)

  1. Hello,

    I woke up and looked in the mirror to find my right pupil looked much larger than the left.
    I have a pineal region cyst which i am told sits close to the optic nerve in the brain. I am wondering if this could be an indication of growth and maybe it is now pressing on the optic nerve?
    Any advice you have would be greatly appreciated!

  2. Hi Dr I have a couple questions. I’m a healthy 24 year old female. I have been suffering from migraines since I had my first child in 2010. They recently diagnosed me with occipital neuralgia & prescribed me amitriptyline 10mg for the pain. Once I started taking that medication I noticed one of my pupils began to dilate. I continued to take it for a month or so & finally it bothered me so much that I called the doctor. She took me off the amitriptyline immediately and prescribed me topamax. I’ve now been on topamax for a week but my pupils are still uneven. I will say every now and then they will be slightly equal but majority of the time they are uneven within 1mm or so. I have had a CT Scan and it was completely normal. Should I be worried about something else? After I’ve started the topamax I’ve notice that the vision in my right eye is slightly blurry and I have pain above the eye (this is the pupil that is bigger).

    • Hard to know without seeing you Ashleigh, but I suspect that the uneven pupils would be more likely related to the underlying headache disorder than the meds. You should talk it over with your doctor.

  3. Hi, I recently had a eye injury and tore my sphincter muscle which led to anisocoria in my left eye. I was on 2 types of eye drops for a week and wore a patch. I Have had vision problems from the fourth grade and started wearing contacts in the sixth grade. I had always planned to have laser eye surgery but I don’t know if I can since my injury. Do you know if it is possible to have laser eye surgery with a torn sphincter muscle?

    • I do not know the answer to that question Nick – I think it would depend on the size of the pupil, but definitely something to ask the ophtho about in detail during an initial consult.

  4. Hello dr. My wife has been dealing with this issue for 10 years. She will develop uneven pupils and will have slowed speech and dissyness. This will last normally a day or so. She will have these episodes 3 to 4 times a year. Each time we go to the doctor the symptoms seem to go away and the doctor does not do anything. She has had cat scans and it does not show a bleed. This week the symptoms came back but this time with pain and iching in the left eye. The other symptoms have went away but the pain and iching has stayed. She went to an eye doctor and she thinks it is a slow bleed. I dont understand how you can have a slow bleed for 10 years, but my wife is upset about this possibility. Any help you can give would be great.

    • Really hard to say what this is Mike – sounds like you need to find a good neurologist, get a thorough baseline evaluation, and then make an arrangement that you can bring her in right away during an episode. Sometimes that’s the only way to know for sure.

  5. Hi Dr Holland, just wanted some reassurance. I used to suffer from benign rolandic epilepsy and after a seizure one time I hit my head, leading to concussion (I was around 10 at the time). I ended up in hospital where a doctor noticed my anisocoria and told my to get my eyes checked out. I didn’t get my eyes checked until I was 17 (I had no symptoms other than visual snow) after an evaluation I was told I had no problems and great vision. I am now 21 and my uneven pupils remain, the difference is not massive but still it causes anxiety. Could I of been born with this? the anisocoria is consistent, it never changes or goes away. Could my concussion that day of caused it? could a seizure of caused it and it just stayed that way? after a seizure one time I suffered complete loss of sight for up to 30 minutes afterwards. Could this episode of caused the damage?

  6. Hi Dr Holland,

    I noticed two weeks ago that my left pupil is noticeably bigger than my right. I’ve performed light tests with a penlight at home and found that in bright light both pupils constrict to a pinpoint yet the left is still bigger, if only fractionally in that light. From what I can see both pupils dilate in the dark and there is still slight a difference. But the asymmetry is most noticeable in dim lighting. I have an appointment with an ophthalmologist in a couple of weeks but am quite frightened. So many internet searches turn up results of brain tumours, aneurysms etc. my question is does this sound like anything very serious? Would the abnormal eye dilate and constrict in different lights if tumours were involved or would they remain fixed? (I don’t have any other symptoms by the way, no headaches, sickness, dizziness. I’m sure I’m being worked up over nothing but it is quite worrying I’ve only just noticed this!

    Many thanks

    • It’s really hard to say without examining you Rachel. Do you have any drooping of the right eye lid? I think you are doing the right thing getting it checked out – let us know what happens.

      • No, there’s no drooping or any other symptom at all. I just hope it’s not anything too serious

  7. Hello! It’s wonderful you take the time for all this. I am a healthy 26 year old new epileptic. Had some serious exposure to mold. Neuro says mri is fine and mold doesn’t affect the brain/eyes. After some cluster seizures i noticed my vision very blurry in right eye. Rubbed it for 6 hours before noticing it was pinpoint whereas the other regular. 2 days later grand mal in clusters now my left is dilating randomly on it’s own. Both eyes are doing their own things, reacting to light however very slowly. I have links to all ct,mri. Any thoughts?

  8. Dear Dr. Holland
    I really do need your help
    I have noticed recently that I have anisocoria, but it keeps alternating, both pupils react to light a dark, but sometimes the right pupil is bigger than the left one and sometimes the left one is bigger then the right one , I am so scared it could be sign of Horner syndrome, I don’t think I have myosis, because both pupils can dilate in the dark, no ptosis either. I really want to know if it is a physiological Anisocoria or it’s something else, and also if the physiological Anisocoria can appear later in life.
    Ps:( I went to an ophthalmologist he said everything is fine, he referred me to a neurologist appointment after 2 month)
    Thank you Dr.

    • I think it’s a great idea to see a neurologist. However, assuming the difference in pupil sizes is most obvious in dim light, you probably have benign physiologic anisocoria. This can switch sides, sometimes called alternating or see-saw anisocoria. Keep your neurology appointment and let us know what happens. Thanks for your interest and question.

      • Hello Dr. Holland thanks the response,
        I have managed to see a neurologist 3 days ago I told him about my anisocoria, he examined my eyes, he said it’s not Horner syndrome because there is no sign of it, and because I’ve told him that I do suffer from light headaches and dizziness from time to time he suggested a brain MRI, came out clear. Then he told me that you were probably born with this but you didn’t realize, I told him this happened recently I showed him a Trent picture of mine, no signs of anisocoria, he didn’t know about something called physiologic anisocoria or alternating or seesaw, he is from Iraq, he told u have nothing no Horner syndrome just go. I just want to know Dr. If physiologic Anisocoria can happen later in life? And if it can be more than one mm? I red on the internet than it can but no one is confirming it to me. So I wish I can get an answer from you. Sorry for asking too many questions and thanks a lot.

  9. Dear Dr Holland,

    I need your advice,

    I noticed that my left pupil is larger than the right one. I’ve had it probably for a year already.Recently I decided to do a PRK vision correction surgery. I brought up the size difference in my pupils during my preop exam. The doctor said both eyes looked healthy and they constrict and dilate symmetrically in low and bright light condition.(to clarify : changing % of pupil surface is the same, even though they are different in size). The doctor suggested I should see a neuro-ophthalmologist, even though he thought it is probably nothing serious and he said it is likely could not be fixed. I would like to go ahead with the laser surgery and decided to go with a different doctor after all. Do you think I can see the specialist regarding my pupils after the surgery?? When I called for an appointment the next available was in several month. I would prefer to do the surgery sooner due to my schedule of time-off availability at work. My question is, if I go ahead with the surgery and go to the specialist later, could it affect my surgery results, meaning is my prescription currently different with larger pupil vs if I had a normal size pupil?

    Thank you so much in advance…

    • Rita, thanks for your post. Obviously, I cannot give you medical advice over the internet without examining you. However, that being said: You probably have physiologic anisocoria. Although the neuro-ophtho will evaluate for other (rarer, but potentially more serious) causes of anisocoria, it is doubtful that they will “fix” your pupil inequality. The PHK procedure itself should not affect your pupil size. Hope that helps. One final word of caution – I do not claim to be an expert on PHK, but I do know that it is more difficult for people with larger pupils, so you might want to clarify that before doing the procedure.

  10. Hi, I have recently noticed that one of my pupils is larger than the other and my optician noticed this too. She did various tests and said my eyes were healthy.

    The thing is that it tends to come and go. Since first noticing it I was rather panicked, so would check the sizes any chance I got. For hours and hours they could be uneven and then suddenly they just got back to normal and some days they are just normal.

    Would that be indicative of physiologic anisocoria or something more serious?

    I should also say that I do suffer from migraine, have done since I was a child, about 11 years old. Sometimes when I being to feel one coming on, even if it doesn’t turn into a fully fledged migraine it can cause the pupil size to change,

    • Interesting question. In order to fulfill brain death criteria, both pupils must be nonreactive. They are typically both the same size (mid-position). However, I guess if there were some pre-existing gross traumatic anisocoria the pupils could be unequal.

  11. hi, thank you for your website. My question is… can an iridotomy be the cause of uneven pupil size? I had an iridotomy for narrow angles in both eyes and a month after the procedure, I have noticed that on and off my pupils are slightly unequal size. I have been back to the eye doc and he saw no problems with my pupils, since when I went to him, the pupils were equal size. thank you very much for your input.

  12. Hello, my right pupil is always larger than my left, first noticed ten years ago . The only time they are almost equal is in direct bright sunlight when both are tiny. I’ve seen a neuro ophth, and because I also have unexplained chronic head pain for years on the right I saw a neurologist. I’ve had a 3tesla MRI, Mra, MRV, and CTA all normal. No one can give me an answer for the pupil or the head pain. It is quite worrisome having the pupil issue especially on the same side as my pain and no explanation. I’m otherwise healthy, 36 year old female. When I was 18 I hit my head on and smashed a windshield in a car accident, my one doctor says maybe my pupil was knocked out of whack then, but I didn’t notice it until until ten years later. Do you think my testing is adequate? I have read that cerebral angiogram are the only gold standard testing? Thanks in advance, I actually tried to schedule an appointment with you but was told you do not not see neuro patients anymore.

  13. Hello. A few weeks ago I noticed that my pupils are not equal in size. especially in normal to dark lighting. I have been to the ophtalmologist and been told that it is physiological anisocoria. I have also been tested under infra-red light in complete darkness before the diagnosis was made. However my problem is that I keep noticing this difference and it makes me extremely nervous, even though I have no other symptom and I would say that my eyesight is impeccable. Is it possible for the diagnosis to be wrong? Is it necessary to check another doctor? And also, is it easy for any problem that can be causing the anisocoria to go unnoticed during the eye examintation?
    Thank you for your help.

    • Also, the examination was done by two young doctors, then the infra-red examination was done by a professor at the neuroophtalmology department at the local eye hospital. I know I am being silly, but can all three doctors have really missed something?

  14. Would vision be impaired by this? I am asking because I’ve always had one iris/eye larger than the other. I can’t say I’ve noticed difference in the pupil, but I can see a different in the shape and size of my eyes and have done since I was a chid (I’m now 30). Recently (in the past year or so) I have noticed that sometimes when I switch out a light at night, and we’re plunged into darkness, my left eye has a delay in reacting to the change in light? It can take anything up to 15 seconds for it to fully adjust and is most off-putting. My optician noticed a slight delay last year but he said that an anti-depressant I was on (citalopram) may have caused it? However, I’ve not been on this for some time (nearly 10 months) and it’s gotten worse – not to hinder everyday life but I’m worried about things like driving at night. Should I see my Dr or my optician again?

    • I suspect that you might have a tonic or Adie’s pupil – this most often affects one eye, the affected pupil is often larger in ambient light, but can also be slow to dilate in the dark resulting in blurry vision. However, you should follow-up with your eye doctor for a proper exam to be sure.

  15. Hello, I have a question regarding my 6month old daughter. I recently noticed that her left pupil is slightly larger than the right and also her left eyelid droops a little bit. They both react equally to light/dark. Since then, (about 1 month ago) I noticed that the left eye seems lazy. I’ve done my research, and understand how and why anisocoria can cause amblyopia. I also know that most cases of amblyopia can be rectified by patching the stronger eye. How is it corrected in the long run with anisocoria? It seems to me that if the patch corrects the problem as soon as it is taken off the lazy eye would return. Will she need to wear a patch forever? She has a well check up in 2weeks where I will be getting a referral to a pediatric ophthalmologist. Until then, I’m not sleeping well thinking about it 😦 Thank you for any feedback!

    • Thanks for you comment. Obviously, it is hard to make a diagnosis without examining your child, and that is why you are (very appropriately) going to be seeing a pediatric ophthalmologist. From your description, this sounds like a partial third nerve defect, and most cases in childhood are “congenital”. You are right, if there is an associated “squint” (srabismus) which is left untreated, this can lead to a “lazy eye” (amblyopia), but this is usually easily prevented. It is the weak eye muscle, not the anisocoria, that causes the strabismus (and potentially the amblyopia). Click here for a pretty good article with more information.

  16. Hi. Fisrt i am sorry for my bad english cause am not a native speaker. Second i really need your help, i hope u will benefit me!  I live in an arab country so the health care is not that good ! My problem is that i have one eye that is bigger than the other Since i was born ! It looks like that o_0 i am not kidding unfortunately! it is very noticeable so it makes me uncomfortable and unable to have an eye contact with people. Am wondering is there any treatment for this? Like implantable contact lens? A also have a lazy eye which is the same smaller eye so i can not look with it , i only see depending on the bigger eye which lower my sight every year:( i also have Strabismus but it is not that big problem. I am saying all these to let you know. Any Help please! 

    • I am not sure I understand the problem – it the whole eye that is bigger, or is it just the pupil. It might be helpful to send a photo of the eyes so that I can see for myself. However, if it is just the pupils that are unequal, and it is truly a congenial problem (something you were born with) this type of problem can sometimes be disguised by wearing color tinted contact lenses.

  17. Hi, I was just wondering could you help me out. I’ve thralled over the internet and have found nothing on possible remedies or treatments for Anisocoria. In dark lighting or when i’m run down one pupil is considerably larger than the other. I think it could be hereditary as my mother sometimes gets it as well. It can be quite obvious and has completely lowered my self confidence. I have asked my doctor about treatment but he just said it was natural and not to worry. Could you please tell me if there are any eye drops or medications I could possibly take that might help. Any information you could provide would be massively appreciated (its impossible to find out treatment online!).

  18. It would have been better if the video regarding Aide’s pupil had shown Light near dissociation and tonic response which are important features of the condition.

      • Hi i am ajinkya 24 yrs old. I had the eye injury on 15 th july 2013.From that onwards my right side pupil is became larger.please help me is there any tratment to reduce the size of pupil.

      • Is other permanent treatment is there ??Can it natural heal it self and reduce size by naturally after some days..

      • It’s true that your trauma was less then 1 month ago, and some nerve injuries are partial and will ultimately improve on their own. If not, this type of problem doesn’t usually improve with surgery, and your best option might be the cosmetic contact lens.

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