Taken from the Greek (literally meaning 'with prominent eyes'), exophthalmos is a condition in which either one or both eyeballs bulge out or protrude from the eye socket. Many different conditions can lead to exophthalmos, which is also known as proptosis.
How does exophthalmos occur?
The eye socket forms a solid protective space for the eyeball, but there is little room to accommodate anything other than the eyeball and related soft tissues such as the muscles supporting the eyeball, fat and the optic nerve - a bundle of nerves that transmits light signals to the brain. Any inflammation or introduction of a foreign material inside the eye socket cavity can force the eyeball forwards and potentially make it bulge out.
If only one eye is involved, the condition may be referred to as unilateral exophthalmos; bilateral exophthalmos is the term used if both eyes bulge out. The term "exophthalmos" is sometimes reserved for bulging eyes linked to the endocrine system, with "proptosis" being used for bulging eyes resulting from other causes.
What causes exophthalmos?
Exophthalmos can be linked to a disease or it can occur because of an isolated condition that affects only the eye. It can occur if:
- Muscle or other soft tissues inside the eye socket become inflamed or swollen
- A foreign object gets into the eye socket
- There is a problem with a vein supplying the eyeball
- There is a tumour inside the eye socket such as on the optic nerve or lacrimal gland.
The most common cause for exophthalmos is thyroid eye disease. This is an autoimmune condition in which one or both eyes can be affected. It is often associated with a thyroid problem, in particular Graves' disease - it is also known as Graves' ophthalmopathy. The thyroid is one of the glands in the endocrine system that produces hormones. An overactive thyroid gland is the most likely cause of this condition, but thyroid eye disease can also occur if the gland is underactive and sometimes when the thyroid gland itself is not affected. Thyroid eye disease can become active after a thyroid condition has already been treated.
Thyroid eye disease occurs most often in adults between 30 and 50 years old, especially women, and smoking increases the risk. In thyroid eye disease, the fatty tissues and muscles around the eyes are attacked by the immune system (which normally fights off infections), making them inflamed. There is usually an active stage, when the symptoms are particularly noticeable for several months but occasionally up to 2 years; it is followed by an inactive stage, when the symptoms have diminished but there can still be long-term problems. In general, the disease is usually mild, but in 10% of cases the eyes become worse within a few months from when the problem started; if the eyes stay the same for a 6-month period, it is unlikely they will become worse.