NeuroradiologyThyroid orbitopathy
a common complication of Graves' thyroidopathy. Orbitopathy in Graves' disease may occur in both hyperthyroid and euthyroid individuals. Clinically it is characterized by upper lid retraction, exophthalmos, periorbital oedema and decreased motility of the globe, often ensuing in diplopia. Ophthalmoplegia is usually absent. Symptoms are bilateral in more than 90% of cases. Corneal exposure due to lid retraction and exophthalmos, and compressive optic neuropathy due to increased extraocular muscle size and orbital fat may result in loss of vision. In case of compressive optic neuropathy surgical decompression (i.e., bony wall removal near the orbital apex) or
radiotherapy must be considered in the event of failure of medical therapy.
Upper lid retraction may be seen independently of exophthalmos resulting from toxic mechanisms. Exophthalmos is usually secondary to inflammatory congestive retrobulbar orbitopathy with involvement of fat and extraocular muscles. Fat and extraocular muscles are involved by a lymphocytic inflammatory infiltrate associated with interstitial oedema resulting respectively in increased fatty mass and muscle enlargement. In the chronic stages of the disease fatty infiltration of atrophic muscles as well as fibrosis may occur.
Enlargement of extraocular muscles may be well documented on both CT and MR tomographic imaging. Typically, the inferior and medial recti are the earliest and most severely involved. Muscle belly involvement usually predominates and tendinous insertion points are characteristically spared. The differential diagnosis includes orbital pseudotumour that shares with thyroid orbitopathy a similar histopathology. The MR signal pattern of involved muscles is not specific and is usually as follows: isointense to normal muscle on short TR/TE and low intensity or mildly hyperintense on long TR/TE images. Occasionally definite T2 hyperintensity has been described which is believed to be the result of a predominating oedematous component in inflammatory tissue infiltration. In chronic stages hyperintense signal on T1-weighted images may be found as expression of fatty infiltration.
Optic nerve compression at the apex by enlarged muscles may also be well depicted on tomographic imaging (particularly MR) and provides useful information for eventual surgical treatment planning.
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