With enlargement of the extraocular muscle at the orbital apex, the optic nerve is at risk of compression. The orbital fat or the stretching of the nerve due to increased orbital volume may also lead to optic nerve damage. The patient experiences a loss of visual acuity, visual field defect, afferent pupillary defect, and loss of color vision. This is an emergency and requires immediate surgery to prevent permanent blindness. Pathophysiology
TAO is an orbital autoimmune disease. The thyroid-stimulating hormone receptor (TSH-R) is an antigen found in orbital fat and connective tissue, and is a target for autoimmune assault.On histological examination, there is an infiltration of the orbital connective tissue by lymphocytes, plasmocytes, and mastocytes. The inflammation results  in a deposition of collagen and glycosaminoglycans in the muscles, which leads to subsequent  enlargement and fibrosis. There is also an induction of the lipogenesis by fibroblasts and preadipocytes, which causes enlargement of the orbital fat and extra-ocular muscle compartments. This increase in volume of the intraorbital contents within the confines of the bony orbit may lead to dysthyroid optic neuropathy (DON), increased intraocular pressures, proptosis, and venous congestion leading to chemosis and periorbital oedema. In addition, the expansion of the intraorbital soft tissue volume may also remodel the bony orbit and enlarge it, which may be a form of auto-decompression.
A study demonstrated that at the time of diagnosis, 90% of the patients with clinical orbitopathy were hyperthyroid according to thyroid function tests, while 3% had Hashimotos thyroiditis, 1% were hypothyroid and 6% did not have any thyroid function tests abnormality. Of patients with Graves hyperthyroidism, 20 to 25 percent have clinically obvious Graves ophthalmopathy, while only 3–5% will develop severe ophthalmopathy. History
In medical literature, Robert James Graves, in 1835, was the first to describe the association of a thyroid goitre with exophthalmos (proptosis) of the eye. Graves ophthalmopathy may occur before, with, or after the onset of overt thyroid disease and usually has a slow onset over many months. See also
Infiltrative ophthalmopathy
Exophthalmos

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