The thyroid is one of the body's endocrine glands; these glands produce hormones, which carry messages around the bloodstream between the different systems in our bodies. The thyroid is located in the neck and produces hormones which regulate our body chemistry (metabolism).
What can go wrong with the thyroid?
There are two main problems which can occur: the thyroid: can work too much (hyperthyroidism) or it can not work enough (hypothyroidism). Generally when the thyroid is overactive, our metabolism speeds up and we become anxious, hyperactive, tremulous, sweaty. Our hearts speed up, and we lose weight despite increased hunger. When our thyroid becomes underactive, we become lethargic and sluggish; our skin and hair become dry and coarse, and we gain weight.
How does hyperthyroidism affect the eyes?
When someone has an overactive thyroid, the eyes can be affected in a number of ways. Most often the eyes feel dry and gritty, they can be painful, look red and may water more than usual. The eyelids may become swollen and look larger than normal. Sometimes the muscles and fat at the back of the eye swell, this can push the eyes forward so that they can look like they are staring or bulging out of the sockets (exophthalmos).
If the eyes are affected by exophthalmos, then this can make them even more prone to feeling dry and painful. In more severe cases the swelling behind the eye can cause double vision, and in some cases the swelling can become so severe that it can affect the optic nerve and cause sight problems.
What causes thyroid problems?
Hyperthyroidism is an autoimmune problem. The body's immune system, which usually fights off infection starts to attack the body's own tissues.
When the thyroid is attacked, it may respond by producing more of its hormones, or it may become so damaged that it produces less hormone.
How does this manage to affect the eyes?
When the immune system attacks the thyroid, there is often a parallel attack on the tissues around the eyes, particularly the muscles which move the eyes and the eyelids (the extraocular muscles). In addition, if the thyroid hormone is in excess it exaggerates the affects of adrenaline on the muscles to the eyelids, which contract to raise the upper lid and pull down on the lower lid, producing the staring appearance.
How is thyroid eye disease treated?
The first priority is to correct the thyroid problem with thyroid hormone or antithyroid drugs followed by radioactive Iodine or thyroid surgery. In the meantime the eyes can be protected by the use of lubricating eye drops (artificial tears). These can help to make the eyes feel more comfortable and reduce the risk of the eyes being damaged by being exposed and dry.
This may be all that is needed. Once the thyroid is stable, however, there are surgical procedures which address specific problems.
How can thyroid eye disease affect sight?
When the extraocular muscles and fat behind the eye becomes swollen, this can affect how well the muscles are able to control the movement of the eyes together, which can cause double vision. When the eyes do not quite point in the same direction they send two messages to the brain which don't quite match up. The brain perceives this as a double image.
What can be done about double vision?
Prisms in glasses can help double vision. These can be added to a lens by using 'stick-on' Fresnel prisms, or incorporated into a spectacle lens when it is made. Prisms work by bending light, and can make it appear as if both eyes are looking from the correct angle.
Prisms are usually fitted by an orthoptist or an optometrist. In general 'stick-on' Fresnel prisms will be used first, as there is a period of trial and error until the right strength of prism is found, at which point it is possible to have them incorporated into the actual lens of the spectacles.
What about surgery?
If thetissues behind the eye swell and start to put pressure on the opticnerve then there can be permanent damage to vision. This may require treatemtn with high dose steroids, orbital radiotherapy or emergency orbital decompression.
Orbital decompression can also be performed in a planned procedure to replace they eyes where they belong in the bony sockets, reducing the amount of exophthalmos.
Sometimes surgery can be used to help with double vision. In very basic terms this surgery lengthens one or more of the extraocular muscles so that the eyes are brought back into alignment.
The upper eyelids can be lowered and the lower eyelids raised to reduce the staring appearance and the risk of exposure of the eyes.
John Pitts is an oculoplastic surgeon in full-time private practice in London and Barbados following a substantive NHS consultant position. He graduated MB ChB from Glasgow University in 1983 and, in 1987, after working in pathology and neurology, he trained in ophthalmology in Glasgow, Nottingham and London. He has travelled extensively, working in centres of excellence in Los Angeles, New Orleans, Melbourne, Barbados, Brunei and Vancouver. He has undertaken Fellowship training in oculoplastics at Moorfields Eye Hospital.
