ABSTRACT

Background: the sequence of orbit anatomical structure involvement in patients with thyroid
associated orbitopathy (TAO) is not completely understood. In particular, the chronology of
extraocular muscle (EOM) and orbital fat involvement is not clear, probably because of the
different imaging methodologies used and the difficulty to establish TAO clinical stage. To explain
the different observations, two major TAO subtypes have been proposed, one with predominant
EOM enlargement and the other one with preminent soft retrobulbar tissue involvement.
Design: in order to identify the first orbital structure involved in TAO, we looked for a possible
correlation between tissue involvement chronology in the orbit, Clinical Activity Score (CAS) and
disease duration. 64 orbits from 32 patients with TAO were studied both at presentation and 18
months later. Clinical Activity Score (CAS) at presentation and CAS relative change 18 months
later were also evaluated. Orbital EOM volume/orbital fat ratio ( OMV/OFVr) was calculated and
compared with 46 orbit measurements from 23 euthyroid controls. OMV/OFV ratio change in
respect to presentation was evaluated 18 months later.
Main outcome and Conclusion: we observed a progressive OMV/OFV ratio decrease due to initial
EOM involvement and subsequent fat prevalence. OMV/OFV ratio was directly correlated to CAS.
These data suggest that EOM is the first target for the autoimmune process in TAO.