Your doctor has noted that you
have anisocoria, which means that the pupil in the right eye
and left eye are not the
same size. A small amount of anisocoria or difference in pupil
size is normal. On some days, a person’s right pupil might
be larger than the left and on other days, the pupils might be
the same size or the left might be larger than the right. But if
more than a small difference in pupil size is present and persists,
you may have a neurologic problem.
Anisocoria
The iris is the colored (brown, blue, etc.) portion of the eye
and the pupil is the black, circular area in the center of the
iris. The pupil is actually a hole in the iris which controls the
amount of light that gets into the eye. In dim light, the hole,
or pupil, gets larger (dilates) to let in more light, while in
bright light, the pupil gets smaller (constricts) in order to protect
the eye from too much light.
The size of the pupil is controlled by muscles
in the iris – the
iris dilator and the iris constrictor muscles. The muscles are
controlled by nerves from the brain to the eye. Problems with these
nerves cause the size of the pupil to be abnormal. A problem with
the nerve that normally dilates the pupil in the dark causes a
small pupil. A problem with the nerve that normally constricts
the pupil produces an abnormally large pupil.
An Abnormally Small Pupil
The nerve that dilates the pupil in the dark, the oculosympathetic
nerve, also controls a little muscle that helps to hold the eyelid
open. When the oculosympathetic nerve is not working, the pupil
on that side is abnormally small and the upper eyelid droops a
bit. This is called Horner Syndrome.
Horner Syndrome
Although having Horner Syndrome by itself will not damage the eye
or cause loss of vision, it may be a signal of damage to one
of the structures along the nerve. The nerves that dilate the
pupils in dark come in contact with many structures. They begin
in the brain and pass down to the spinal cord. They exit from
the spinal cord and run up over the lung and into the neck, where
they travel with the carotid artery. They follow the carotid
artery back into the brain, from where they enter the eye and
go to the dilator muscle of the iris. Damage anywhere along this
path will cause Horner Syndrome. It is important to find out
where the damage is and what is causing it because sometimes
it can be serious. Examples of serious conditions causing Horner
syndrome are a stroke in the brain or a tumor in the lung or
a break in the wall of the carotid artery called a carotid
dissection.
In other cases, the Horner syndrome is due to birth trauma. Sometimes
no cause can be found and the patient remains well.
Your doctor may do one or two eye drop tests to confirm that a
Horner Syndrome is present and to determine the location of the
nerve damage. Detailed radiologic studies, usually magnetic resonance
imaging (MRI), are done to look at the places the nerves pass through
for possible causes of the damage.
An Abnormally Large Pupil
The nerve that goes to the constrictor muscle is part of the IIIrd
cranial (oculomotor) nerve. The IIIrd cranial nerve controls several
of the muscles that move the eye up, down and in. It also controls
the muscle that opens the eyelid and the muscle that constricts
the pupil. A problem with the IIIrd nerve can result in a droopy
eyelid, double vision and/or enlarged pupil.
IIIrd Nerve Damage – lid droop and
pupil enlargement
Pupil enlargement in addition to lid droop and eye muscle weakness
may signal an aneurysm and is a medical emergency. The patient
should immediately have neuro-imaging (CT, MRI, MRA, and/or angiogram)
to look for the aneurysm.
When only the pupil portion of the IIIrd nerve is not working,
the pupil is large and does not constrict in bright light. This
might be caused by a medicine or chemical that dilates the pupil.
Examples include motion-sickness patches, chemicals used in the
garden, and certain eye drops. The patient often is unaware of
what she touched that caused the problem but careful questioning
by the doctor may reveal the cause. A pupil that is chemically
dilated will return to normal size as the chemical wears off and
eventually will react normally to light. Depending on the specific
chemical, this can take hours or even days.
Adie Pupil is another type of pupil that is large and constricts
poorly in light.
Adie Pupil
An eye with an Adie pupil initially has difficulty focusing on
objects held close, for example, for reading. Adie pupil is commonly
seen in young adult women but men can develop it, too. In most
cases, the cause of the damage is unknown and radiology studies
rarely show any abnormality. A person can have Adie pupils in one
or both eyes. Adie syndrome is the combination of Adie pupils and
reduced reflexes such as knee jerks. The cause of Adie syndrome,
like Adie pupil, is unknown. There is no treatment for Adie pupils.
With time, the ability to focus up close usually returns. The Adie
pupil may remain enlarged but more often shrinks in size gradually
over several years, even becoming smaller than the normal pupil
in the other eye. The Adie pupil will never react well to a light
shined in the eye. It is important that you remember this for future
eye exams.
Recovered ability to focus up close
Frequently Asked Questions
If uneven pupils are found, what can I do to help my doctor determine
the cause?
Knowing how long the pupils have been unequal
is very helpful. Bring old photographs to your appointment
that show your pupils
well (without camera “red eye”), such as childhood
school pictures.
If I have a Horner Syndrome, how can I tell what is wrong?
A stroke has other neurologic symptoms, and can be seen by MRI
imaging of the brain. A lung tumor can be seen on imaging of the
chest. A carotid dissection can be seen by MRA imaging of the blood
vessels in the neck. Your doctor will evaluate your history and
all of your symptoms together to determine the appropriate imaging
study to do.
My doctor has told me I had a carotid dissection. How did that
happen? What can I expect?
A carotid dissection is a tear in the wall of one of the large
arteries going to the brain. A dissection can be caused even by
minor trauma such as turning the neck while swimming or parking
a car. Carotid dissections usually heal on their own, but you will
probably be given a blood thinner to reduce the chances of blockage
of the artery or blood clots circulating to the brain or lungs.
I am a nurse working in the surgery department, and my doctor
told me my enlarged pupil was caused by the atropine I administer
to patients in pre-op. How can I prevent this from happening in
the future?
Wash your hands, not only before and after
touching the patient, but also after drawing up medications
and touching the tubing used
for IV administration of medications and after removing your
gloves. Don’t rub your eyes unless you are sure your hands
are free of contamination.
What can I do to help focus up close if I have an Adie pupil?
Most patients with Adie pupil don’t have
any problem with this, but if you do, your doctor may be
able to prescribe reading
glasses that will help.
I have an Adie pupil. What can I do to prevent the other eye from
being affected?
We often don’t know what causes Adie
pupil. Sometimes it follows a virus, and the other eye will
probably not be affected.
Sometimes it has no recognizable cause, but the other eye is
affected, though sometimes it may be years later. We do not know
of any way
to prevent this.
