Possible Thyroid ophthalmopathy-CT

Left axial proptosis for evaluation.

CT reveals:  Diffuse fusiform enlargement of the inferior rectus with relative sparing of the tendinous insertion. Isolated inferior rectus enlargement- with relative sparing of tendinous insertion leads to possibility of  thyroid ophthalmopathy versus pseudotumour. Although, this is usually bilateral but can be unilateral.   The characteristic finding on CT scan is enlargement of the extraocular muscle bellies. The muscle tendon is typically spared. The muscles most commonly enlarged are the inferior rectus followed by the medial rectus, although any of the muscles may be involved. 

The most common cause of bilateral and unilateral exophthalmus among adults is Graves’ disease. Unilateral exophthalmus, although frequently seen in connection with thyroid diseases, has a much larger differential diagnosis than bilateral exophthalmus. With unilateral presentation one should think of orbital pseudotumour as differential.