Exophthalmos is a medical term for a bulging or protruding eyeball or eyeballs.

It is often used interchangeably with the term "proptosis", which means the same thing.

What causes exophthalmos?

Exophthalmos can be caused by many different conditions. It's important that the underlying cause is identified so appropriate treatment can be given.

Thyroid eye disease

One of the main causes of exophthalmos in the UK is thyroid eye disease, also known as Graves' ophthalmopathy.

This is an autoimmune condition that affects around one in every three people with an overactive thyroid gland (hyperthyroidism) caused by Graves' disease. It is particularly common in women who are 30-50 years of age and people who smoke.

An autoimmune condition is where the immune system (the body's defence against illness and infection) mistakenly attacks healthy tissue.

In the case of thyroid eye disease, the immune system attacks the muscles and fatty tissues around and behind the eye, causing them to become inflamed (swollen).

Graves' disease is an autoimmune condition in which the thyroid is attacked, leading to an increase in the level of thyroid hormones in your blood. It's not clear exactly why thyroid eye disease sometimes occurs along with this condition.

Thyroid eye disease can also occasionally affect people with an underactive thyroid gland (hypothyroidism) and, in rare cases, people with seemingly normal thyroid function.

Other causes

Exophthalmos can also have a number of other causes, but these are generally less common than thyroid eye disease.

Other cause of exophthalmos can include:

• an injury to the eyes
• bleeding behind the eyes
• abnormally shaped blood vessels behind the eyes
• an infection of the tissue in the eye socket
• cancerous tumours – such as those caused by neuroblastoma and some soft tissue sarcomas

Exophthalmos can also affect newborn babies if they are born with eye sockets that are shallower than normal.

Other problems associated with exophthalmos

Depending on what is causing your bulging eyes, you may also have other associated symptoms. For example, if exophthalmos is caused by a thyroid eye disease, your eyes may also be:

• inflamed, red and painful
• dry and "gritty"
• tearful
• sensitive to light (photophobia)

You may also experience some double vision.

In severe cases of exophthalmos, you may not be able to close your eyes properly. This can damage your corneas (the transparent tissue that covers the front of your eye) by causing them to dry out.

If your corneas become very dry, an infection or ulcers (open sores) may develop. These could damage your vision if left untreated.

There is also a small risk of the optic nerve (which transmits signals from the eye to the brain) becoming compressed if you have exophthalmos, which may affect your sight permanently if it is not treated quickly.

Seeking medical advice

It's important to see your GP or an optometrist (optician) if you notice that one or both of your eyes are protruding, as treatment is often more effective if it's started as soon as possible.

If necessary, your GP or optometrist can refer you to an ophthalmologist (a specialist in diagnosing and treating eye conditions) for further assessment.

The ophthalmologist will check how well you are able to move your eyes. They may also use an instrument called an exophthalmometer to measure how far your eyeball protrudes.

If the ophthalmologist wants to examine your eye socket in more detail, a computerised tomography (CT) scan or magnetic resonance imaging (MRI) scan may be carried out.

You may also have a blood test to assess how well your thyroid gland is working, and you may be referred for an assessment by an endocrinologist (a specialist in conditions affecting glands and hormones) if you are found to have abnormal thyroid hormone levels.

Treating exophthalmos

If exophthalmos is being caused by thyroid eye disease, the following treatments are often helpful:

• medication to correct the level of thyroid hormones in your blood – this will not necessarily improve the problems with your eyes, but may stop them getting worse
• corticosteroid medication given directly into a vein (intravenously) – this can help reduce the inflammation associated with the condition
• corrective surgery – this may be carried out to improve the appearance of your eyes once the inflammation is under control

Other useful measures include stopping smoking, using artificial tears to reduce eye dryness and irritation, and wearing special lenses to correct double vision.

In other cases, treatment will vary depending on the underlying cause of the condition. For example, treatments such as radiotherapy, chemotherapy and/or surgery may be recommended if exophthalmos is caused by a tumour.

Read more about treating exophthalmos.

Outlook

The outlook for exophthalmos can vary considerably.

Many of the symptoms of thyroid eye disease tend to improve over time, although this can take a number of years, and there is a chance your eyes will continue to protrude if corrective surgery is not carried out.

Some people with exophthalmos are left with long-term vision problems, such as double vision, but permanent visual impairment is rare if the condition is identified and treated promptly.