mercoledì 4 giugno 2008
Thyroid ophthalmopathy (Graves disease)
Bilateral proptosis. Diffuse, uniform thickening of all the bilateral extraocular muscles. Tapering of muscles near insertion on the globe.
- Graves' orbitopathy
- Orbital pseudotumor
- Infectious myositis
Diagnosis: Thyroid ophthalmopathy (Graves disease)
Graves' orbitopathy or thyroid associated orbitopathy refers to autoimmune inflammatory disease of the orbits. Most affected individuals are between 30 and 50 years of age. It also has a predilection for females (80%). The majority of individuals at the time of diagnosis demonstrate laboratory values consistent with hyperthyroidism. There are two common theories explaining an offending antigen. One theory focuses on the presence of thyroid stimulating hormone receptors within the orbital tissues (similar to the thyroid). Another theory focuses on an orbital autoantigen.
The most common signs/symptoms include orbital edema, gaze restriction and proptosis. Other clinical findings include lid lag, diplopia, corneal ulceration and dryness of the eyes. In severe cases, optic nerve entrapment can ensue resulting in loss of vision. The first step in treatment involves controlling thyroid hormone dysfunction. Corticosteroids can be beneficial in limiting extraocular muscle enlargement. Lubricating agents are used to prevent corneal complications. For visual symptoms such as diplopia, orbital decompression surgery can be performed. In some cases, emergent surgery is warranted if optic nerve entrapment is suspected. Other non-emergent surgical procedures focus on eyelid lengthening to address proptosis. Cigarette smoking, male gender, uncontrolled hyperthyroidism, and diabetes are considered poor prognostic factors.
Radiologic overview of diagnosis
If the diagnosis is clinically established, imaging may not be necessary. However, imaging is used for confirmation and followup. Additionally, imaging is useful for surgical planning. CT and MRI are the imaging modalities of choice. The best diagnostic clue is exopthalmos with bilateral extraocular muscle enlargement.
On noncontrasted CT, findings to look for include isodense extraocular muscle enlargement and increased orbital fat. After contrast administration, greater than usual enhancement of the extraocular muscles is visible. Similarly, findings to look for on MRI include isointense enlargement of the extraocular muscles (T1 weighted sequences). On T2 weighted images, hyperintensity is seen with edema and hypointensity is seen with fibrosis (late stage finding). Increased signal on STIR reflects disease activity. Post contrast T1 weighted images demonstrate increased extraocular muscle enhancement. Compression of the optic nerve is more readily identified on MRI rather than CT.
The bilateral orbits are involved in over 90% of cases (even if symptoms are unilateral). The most commonly involved muscles (in descending frequency of involvement) are the inferior, medial and superior rectus muscles. The lateral rectus and oblique muscles are much less likely to be involved. Typically, isolated muscle involvement is unlikely. In the five percent of cases demonstrating isolated muscle involvement, the superior rectus is usually involved. Isolated involvement of the lateral rectus muscle should prompt the search for another inciting etiology.