Read more about Pupils that Differ in Size

Anisocoria is a medical term used when the pupils are different sizes. Physiologic anisocoria is the most common cause of unequal pupil sizes, affecting up to 20% of the population. Your pupil is the black center of your eye and its size varies depending on the environment that it’s in. When you're in a dark room, your pupil will expand in size and dilate to let as much light into the room as possible, but when you're outside on a bright sunny day, your pupil will contract in size.

What causes a difference in pupil size between both eyes?

You can have different pupil sizes between your two eyes for many different reasons.

There may be a physiological reason for this and it's normal to have one pupil that's just a bit larger or smaller than another. Typically, the difference is not significant and does not affect your eyesight.

Can different sized pupils be a symptom of a more serious condition?

In some cases, there can be serious eye conditions that cause a difference in pupil size. The most common eye conditions associated with pupils of different sizes is third nerve palsy and horner syndrome. These occur when there is damage to the nerve that relays information between your eye and brain. This damage can be due to a viral infection, a tumor, an aneurysm, a stroke, a neurological eye condition, or complications during eye surgery.

How is anisocoria diagnosed?

An eye doctor will measure the diameter of your pupils both in light and in darkness. Slit lamps, which are special microscopes that allow eye doctors to examine your eyes in greater detail, can also be used to examine the pupil in more detail.

If one's pupils vary in size, what are the common symptoms one might experience?

It can be difficult to notice a difference in the size of your pupils, especially if your pupil blends in with your iris, as opposed to someone with light blue eyes. Sometimes, especially if the reason for the different sized pupils is an underlying medical condition, anisocoria occurs with other symptoms which are more noticeable, such as:

  • A droopy eyelid
  • Pain when moving their eyes
  • Fever
  • Swelling
  • A stiff neck
  • Reduced sweating on one side of your face

How is anisocoria treated?

Most of the time, anisocoria does not need treatment since it does not affect your vision or your eyes' health. In the event it affects your vision, your eye doctor can prescribe appropriate glasses that may help. If anisocoria is caused by an underlying cause, treatment ultimately depends on what that cause is.

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