Orbital diseases mimicking graves' orbitopathy: a long-standing challenge in differential diagnosis.

Graves' orbitopathy (GO) is the most common cause of orbital tissue inflammation, accounting for ~ 60% of all orbital inflammatory conditions in the population aged 21-60 years, and for ~ 40% in the population aged > 60 year. GO is observed in 25-30% of patients with Graves' hyperthyroidism and more rarely in association with hypothyroid autoimmune thyroiditis. In addition, a small proportion of GO patients (1-2%) do not have a clinically overt thyroid dysfunction. Clinically, GO is characterized by proptosis, inflammation involving the eyelids and the conjunctiva, extraocular muscle hypertrophy, with consequent reduction of ocular motility and diplopia, and in the most severe cases, compression of the optic nerves at the orbital apex, with reduction of visual acuity. At CT scan or MRI, a muscle increase involving the superior, medial and inferior rectus is quite typical. In the most severe forms, compression of the optic nerves at the orbital apex can be observed. Euthyroid GO is usually an early sign of a full-blown Graves' disease; however, in some cases, the orbital disease can remain isolated. Moreover, euthyroid GO can rarely be unilateral, which makes the picture even more confusing. Under those circumstances, the diagnostic process becomes obviously quite difficult, having other conditions mimicking GO been excluded. A number of inflammatory conditions affecting orbital tissue can mimic GO, thereby requiring an accurate evaluation for a proper differential diagnosis. The majority of these conditions are immune mediated. Most of them are benign, but they can be rather aggressive and some can cause visual loss. The most common inflammatory condition affecting orbital tissues and mimicking GO is idiopathic orbital inflammation. Other, more rare, orbital diseases that should be considered in the differential diagnosis are infections, orbital manifestations of systemic diseases, primitive and secondary orbital neoplasms, and orbital vascular alterations. In most instances, when an orbitopathy occurs in the absence of hyperthyroidism, the diagnosis of the disease underlying the ocular symptoms and signs is based on exclusion of the other conditions. Here we review the conditions that can mimic GO and how to distinguish them from this obnoxious eye disease.