Orbital tissue expansion expressed as a number: usefulness in grading the severity of thyroid-associated ophthalmopathy (TAO)
GM Vannucchi1, I Campi1, S Avignone2, AM Minetti3, C Guastella3, F Sbrozzi4, P Beck-Peccoz1 & M Salvi1
In assessing TAO, orbital CT scan allows measurement of extraocular muscle and optic nerve dimensions but it is useful mainly for a qualitative analysis and cannot be used in the clinical score of the disease. Aim of the present study was to seek a number which could express the expansion of the orbital tissue and be used in grading the severity of TAO. 39 TAO patients and 24 control subjects were studied. We have measured the orbital area (OA) on the central axial section of the CT scan image, approximated to a triangle, and the area of the portion of the ocular globe within the orbit (ocular globe chord area), defined as ocular cord (OC). The areas of ROIs (in mm2) were obtained by manually contouring the image on the computer consolle. We have then calculated the OC/OA ratio as an additional parameter, in order to reduce the variability of the obtained measurements. The areas and the OC/OA ratio of patients and controls were compared. In addition, the correlation between the orbital parameters and ocular proptosis (mm), palpebral fissure (mm) and intraocular pressure (IOP, mmHg) were studied. We did not observe significant differences of the mean orbital area of TAO patients (783.6±12.1 mm2) and normals (758.5±20.4 mm2; ANOVA, P=NS); the value of OC in TAO patients was 130.2±11.5 mm2, significantly lower than that of controls (281.8±9.7 mm2; ANOVA, P<0.0001). OC/OA ratio was also significantly lower in TAO patients compared to controls (0.16±0.01 vs 0.38±0.01; ANOVA, P<0.0001). A significant correlation was found between OC/OA ratio and proptosis (r=0.65, P<0.0001), palpebral fissure (r=0.35, P<0.02) and IOP (r=0.36, P<0.03). Manual measurement on CT scan images yielded a very low error (<2%). In conclusion, by measuring OC/OA ratio, we have obtained a number which represents the degree of orbital involvement in TAO patients and can be used as a score of the severity of the disease. The significant correlation of OC/OA with proptosis confirms that this parameter is associated to TAO.