Anisocoria: one pupil bigger than the other

What causes anisocoria?

Anisocoria is a condition where one of the pupils in your eye is a different size to the other. Your pupils are the black circles in the centre of your eyes, and your eye pupil size should be the same.

Anisocoria is temporary in some cases but can also be experienced on a regular basis. It can even leave you with one eye appearing smaller than the other on a permanent basis.

In most cases of anisocoria, unequal pupils are not usually significant enough for anyone to notice.

What are the symptoms of anisocoria?

Symptoms of Anisocoria are not limited to different pupil size. The following symptoms may occur depending on the cause of anisocoria:

• Double vision
• Blurry vision
• Fever
• Nausea
• Vision loss
• Headache
• Stiff neck

What is anisocoria a symptom of?

Many cases of mild anisocoria don't have underlying causes and are classed as normal. More significant anisocoria can be a symptom of many different factors. Anisocoria causes include the following:

• Eye trauma
• Eye medications such as pilocarpine eye drops used to treat glaucoma
• Inflammation of the iris, also known as anterior uveitis
• Adie's tonic pupil-in this case, the pupil affected does not respond to light. While the cause is unknown, it is associated with eye trauma including trauma caused by cataract surgery

Damaged nerves in the brain or spinal can cause neurological disorders, which may result in anisocoria.

An example of this is Horner's Syndrome. Anisocoria caused by nervous system disorders are often accompanied by double vision, strabismus or a drooping eyelid.

Brain disorders causing anisocoria include strokes, tumours and haemorrhage (head injury).

David Bowie Eyes

David Bowie's eyes, which are considered to be iconic, are the result of anisocoria. Bowie’s eyes were blue and not different colours. (Having two different coloured eyes is known as heterochromia). Bowie did not have different coloured eyes, but his left pupil was much larger and so appeared much darker. This is why Bowie is often mistaken for having heterochromia, (a condition which causes each iris to be a different colour.)

Bowie's anisocoria was permanent and was the result of being punched in the eye by a school friend. The punch scratched his eyeball and caused a paralysis of the muscles which contract the iris.

How common is anisocoria?

Mild anisocoria is relatively common and is when one pupil is bigger than the other by less than 1.0mm. Mild anisocoria is referred to as simple anisocoria, benign anisocoria or physiologic anisocoria. There is no underlying medical issue with this type of anisocoria.

Can Anisocoria be normal?

Many cases of mild anisocoria are without any underlying causes. These are classed as physiologic anisocoria and considered to be normal.

Physiologic anisocoria is the most common form, with between 15-30% of the population experiencing it. It can be temporary or permanent.

Types of anisocoria

There are three types of anisocoria, including physiologic anisocoria. The other types of anisocoria are known as mechanical anisocoria and pathological anisocoria.

Mechanical anisocoria

Mechanical anisocoria is the result of physical damage to the eye. Damage could be an eye injury or a condition which causes inflammation to the eye, such as anterior uveitis.

Pathological anisocoria

Pathological anisocoria is when uneven pupils are caused by the following:

• A disease affecting the coloured part of the eye, the iris
• A disease which affects the pupil
• A disease with affects the information pathways to the pupil

Does anisocoria affect vision?

Anisocoria can cause a range of vision problems, including blurry vision, double vision and vision loss.

Diagnosis of anisocoria

Pathological anisocoria is diagnosed by medical professionals who will examine how the pupils react to light.

The irregular pupil is bigger in bright light. In dark conditions, the smaller pupil is the abnormal one and may indicate Horner’s syndrome.

Can you fix anisocoria?

Depending on the type of anisocoria, in some cases, it is possible to fix.

Treatment plan

The treatment plan for anisocoria depends on the type of anisocoria, and it's underlying causes.

Physiologic anisocoria is usually harmless and therefore, typically doesn't require any treatment.

Mechanical anisocoria, on the other hand, may require surgery to correct any damage caused by the trauma. If the trauma is caused by an underlying medical condition, the doctor will create a plan of treatment based around this.

If there are symptoms of pathological anisocoria, the doctor will look at any underlying causes before proceeding with treatment. These can include a brain aneurysm, tumour or stroke. CT and MRI scans will be used as soon as possible to determine these causes.

Anisocoria resulting in uneven pupils, which then causes unequal accommodation, can be treated with bifocals or reading glasses. A doctor may even recommend photochromic lenses to reduce light sensitivity.

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