Bright Sight

Oliver Backhouse, Consultant Eye Surgeon

Unequal pupils (Anisocoria)

1] What is Anisocoria?

Anisocoria means unequal pupils. A small difference in the pupil size is a normal finding in approximately 20% of people (1 in every 5). Should the difference between the pupils change in different lighting conditions, this may indicate a problem and so it is sensible to consult a Neuro-Ophthalmologist who has a particular interest in both Neurology and Ophthalmology.

2] What is the Pupil?

The pupil is the black circular area in the middle of the Iris (the coloured part of the eye). The pupil is controlled by muscles that can make it bigger (dilation) or smaller (constriction) and so let in different amount of light to the back of the eye. In dim light the pupil normally dilates and in bright light the pupil normally constricts. A problem with the muscle that dilates the pupil in the dark will cause a small pupil. A problem with the nerve that constricts the pupil produces an abnormally large pupil.

3] Causes of a Small Pupil?

The oculosympathetic nerve controls the muscle that dilates the pupil and also that which helps hold the eyelid open. When this nerve is not working than the pupil on that side looks small in dim light and the upper eyelid can droop a little. This is called Horner Syndrome.

Horner Syndrome: Having Horner syndrome does not damage the vision but it can indicate damage to the nerve supply that travels from the brain, down the spinal cord, up over the lungs into the neck and then over the carotid artery to eventually reach the eye. Damage to the nerve anywhere along this track will produce a Horner Syndrome and so it is important to find out the cause. In many cases no cause is found but it can be due to something pressing on the nerve or following trauma. It is important to tell your doctor if there is any associated pain in the neck or face as this may indicate a dissection of the carotid artery that needs urgent attention. Eye drops are sometimes used to help identify where the problem in the nerve lies along with a Magnetic Resonance Imaging (MRI) scan.

Some other cause of a small pupil include: Eye drops / Medication, Iritis, Old Age, Old Adie pupil (see below).

4] Cause of a Large Pupil?

Part of the 3rd cranial nerve controls the pupil muscle that makes it constrict along with some of the muscles that move the eye and open the eyelid. A problem with the 3rd cranial nerve can therefore cause a droopy eyelid, double vision and / or an enlarged pupil. Especially if the event is sudden and painful, this may indicate an aneurysm and so an urgent CT or MRI scan is needed.

If only the pupil is large then this can be due to some medicine or chemical that may have accidentally gone into the eye, sometimes unknown to the patient. This will return to normal after a few hours or days depending on what it was that went into the eye.

A large pupil associated with pain, reduced vision, red eye and a cloudy cornea (window of the eye) can indicate Acute Glaucoma (high pressure in the eye) and immediate Ophthalmic attention is needed to save the sight in the eye.

Another cause of a large pupil is an Adie Pupil where the pupil is large in all lighting conditions. Anyone can develop an Adie pupil but it is most common in young adult women who are otherwise healthy. There can be difficulty in seeing close objects (eg reading) and increased glare from the pupil staying large in bright lights. In most cases the cause is unknown and detailed investigation is normal. Near focus returns a few weeks to months later and usually no treatment is needed. Over years this large pupil becomes smaller but still never reacts well to light. Approximately 4% of people will become affected in the other eye per year.

Some other cause of a large pupil include: Iris trauma, Parinauds syndrome, Migraine.

If uneven pupils are found what can I do to help the doctor?

Knowing how long the pupils have not been equal for is very helpful. Sometimes it has been present for a long time but not noticed until attention has brought to the eye for a reason such as grit getting into the eye. Bringing old photographs that shows your pupils well may help show that the pupil change is longstanding.

If I have a Horner Syndrome how will I know what has caused it?

Detailed imaging of the brain and blood vessels may show the cause such as a stroke or carotid dissection. Your doctor will evaluate all your history and examination to arrange appropriate investigations. A carotid dissection is a tear in the wall of the artery and can be caused by minor trauma such as turning the neck while swimming or parking a car. They usually heal on their own under the cover of medication to thin the blood so as to prevent any circulating blood clots.

If I have an Adie pupil what can I do to help?

Most people with an Adie pupil have no symptoms. If you do, some drops or reading glasses can be given until it begins to spontaneously get better. Usually no cause is found but many are thought to follow a viral inflammation of the nerve. Approximately 4% of eyes on the opposite side will become affected each year but there is nothing known that can be done to prevent this.

Disclaimer: Adapted from Nanos.