Extraocular muscle enlargement with tendon involvement in thyroid-associated orbitopathy.

Purpose: To evaluate the configuration of extraocular muscle and tendon enlargement in patients with thyroid-associated orbitopathy (TAO).
Design: Retrospective, noncomparative case series.
Methods: We retrospectively evaluated the preoperative computed tomography (CT) or magnetic resonance imaging (MRI) scans, or both, of 125 consecutive patients with previously diagnosed TAO. Axial CT (n = 88) or MRI (n = 37) slices of the orbits were examined for tendon involvement of enlarged medial or lateral recti muscles. A ratio of tendon to muscle width greater than 0.5 was considered as tendon involvement.
Results: Eight patients (6.4%) with TAO demonstrated tendon involvement on axial CT or MRI. These patients had significantly greater diplopia in primary gaze than patients with no tendon involvement or patients with no muscle enlargement.
Conclusion: The configuration of extraocular muscle enlargement on imaging studies has traditionally been used to differentiate TAO from other inflammatory processes because the tendon is typically spared (fusiform configuration) in TAO. However, we found that the configuration of tendon involvement (cylindrical configuration) can occasionally be noted in TAO and may be more frequently associated with primary gaze diplopia. Tendon involvement does not eliminate the diagnostic possibility of TAO.