Pathogenesis of Graves’ ophthalmopathy

Graves’ ophthalmopathy (GO) is a medically incurable, chronic autoimmune process that affects the orbital tissues and is tightly linked with autoimmune thyroid diseases, most notably Graves’ disease (GD). The close clinical association between immunogenic hyperthyroidism, ophthalmopathy, and pretibial dermopathy suggests that the antigen responsible for these diverse conditions is common to thyroid gland, orbital tissues and pretibial skin.1,2 Gross examination of orbital tissues in patients with severe, active GO reveals edematous, enlarged extraocular muscle bodies in conjunction with an increased volume of orbital connective and fatty tissue (Chapter 1). Microscopically, two characteristic abnormalities are apparent: the presence of excessive amounts of hydrophilic glycosaminoglycans (GAGs), predominantly hyaluronic acid and chondroitin sulfate, and marked infiltration of the orbital connective tissue and extraocular muscles by immunocompetent cells (macrophages and T lymphocytes, few B cells). The orbital inflammatory process is likely to be driven by T-cells which, in response to an uncertain antigen, access and infiltrate the orbital space via certain adhesion molecules.1,3 Once recruited, these T cells release numerous cytokines capable of stimulating cell proliferation, GAG synthesis, and generation of new fat cells from orbital adipose precursor cells responding to adipogenic stimuli. Moreover, many of these autocrine and paracrine factors act to alter the expression of an array of immunomodulatory molecules in orbital preadipocyte fibroblasts.