8
30
were thicker than those in the controls at all gaze posi- However, in absolute values, the increment of OFT vol-
tions, especially for IR and MR. ume was larger than that of all EOM together, implying
By calculating the ratios between muscle volume of that OFT volume change is an important contributor to
dysthyroid and normal subjects, we found that the most exophthalmos.
enlarged muscles were IR (ratio: 3.3) and MR (ratio:
In summary, MRI technique will make it possible to
2
.6), which is in accordance with what we found in the observe and evaluate more quantitatively and more cir-
muscle thickness study. All patients in our study had se- cumstantially than does CT the volume and thickness of
vere or moderate restriction of elevation, and three also EOM in dysthyroid ophthalmopathy. Of the six extraocu-
had limitation of abduction, which would suggest that lar muscles, IR and MR showed the largest changes in
swelling of IR and MR may cause the clinical findings of muscle volume, and IO the smallest. Muscle thickness
eye movement limitations. Similar findings were report- changed with eye deviation in the same way as in normal
ed in a previous CT study where muscle cross-sectional muscles, which did support the notion that the restriction
area and muscle width were measured [13], and the limi- of eye movement in dysthyroid ophthalmopathy may be
tation of elevation and abduction seemed to be in rela- due to mechanical restrictions by the enlarged antagonist
tion to the enlargement of IR and MR.
muscles and not to paresis of the muscles. The orbital
The histopathological changes in dysthyroid ophthal- fatty tissue would also seem to be involved in the exoph-
mopathy consist primarily of inflammation of the extra- thalmos, and the change in volume was larger than in
ocular muscles and the orbital fat [13]. However, a previ- muscles alone.
ous study with CT [7, 25] did not show any change in
the absolute amount of orbital fat. In the present study, Acknowledgement This study was supported by grants from the
Swedish Medical Research Council (no. 4751), the Karolinska
we found a significant increase of fatty tissue volume in
patients with dysthyroid ophthalmopathy, but the relative
change in volume was smaller than that of muscle vol-
Institutet, the Wenner-Gren Center Foundations, the Sigvard &
Marianne Bernadotte Research Foundation for Children’s Eye-
care, and Stiftelsen Kronprinsessan Margaretas Arbetsnämnd för
ume, which was doubled in the patients on an average. Synskadade.
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