[New quantitative parameters for CT evaluation of orbitopathy in dysthyroid ophthalmopathy]

The new index (c/o ratio) is defined as the content of the orbit divided by the capacity of the bony orbit on the mid-orbital axial CT slice showing the medial and the lateral rectus muscles and optic nerve. Other quantitative parameters such as for exophthalmos and the capacity of bony orbit were also obtained on CT films. These parameters were measured in 474 eyes of patients with dysthyroid ophthalmopathy. The capacity of bony orbit showed large variations among all patients. The average for males was 17.8cm3 and that for females was 16.1cm3. Exophthalmos measurement on CT is more reliable than that by the Hertel exophthalmometer. The new index (c/o ratio) is a very reliable parameter for evaluating orbitopathy in dysthyroid ophthalmopathy. A large c/o ratio (greater than 1.6) was of clinical significance, because approximately 70% of the enlargement of the extraocular muscle observed in older patients (greater than 40 yrs old) was associated with a large c/o ratio (greater than 1.6).