Also termed PROPTOSIS, this refers to forward displacement of the eyeball distinguishable from retraction of the eyelids, which causes an illusion of exophthalmos. It is caused by inflammatory and infiltrative changes in the retro-orbital tissues and is essentially a feature of Graves’ disease, though it has also been described in chronic thyroiditis (see THYROID GLAND, DISEASES OF). Exophthalmos commonly starts shortly after the development of thyrotoxicosis but may occur months or even years after it has been successfully treated. The degree of exophthalmos is not correlated with the severity of hyperthyroidism even when their onset is simultaneous. Some of the worst examples of endocrine exophthalmos occur in the euthyroid state and may appear in patients who have never had thyrotoxicosis; this disorder is named ophthalmic Graves’ disease. The exophthalmos of Graves’ disease is due to autoimmunity (see IMMUNITY). Antibodies to surface antigens on the eye muscles are produced and this causes an inflammatory reaction in the muscle and retro-orbital tissues.

Exophthalmos may also occur as a result of OEDEMA, injury, cavernous venous THROMBOSIS or a tumour at the back of the eye, pushing the eyeball forwards. In this situation it is always unilateral.