EVALUATION OF IMMUNOLOGICAL MECHANISMS MEDIATING THYROID-ASSOCIATED OPHTHALMOPATHY BY RADIONUCLIDE IMAGING USING THE SOMATOSTATIN ANALOG IN-111-OCTREOTIDE

Thyroid associated ophthalmopathy (TAO) is a disorder involving the so ft tissues and extraocular muscles of the orbit seen mainly in cases o f Graves' disease. Although an immunogenic pathogenesis has been propo sed, the actual mechanisms of the in vivo retrobulbar involvement are not well defined. The recent introduction of the In-111-labeled somato statin analog, octreotide, which can bind in vivo to the cell membrane of activated lymphocytes expressing somatostatin receptors, has provi ded a new investigational tool for diseases with a presumed immunologi cal background. Based on this property, octreotide scans can be expect ed to be positive in cases of immunological disease showing tracer acc umulation within affected sites. The aim of this study was to evaluate the utility of scintigraphic imaging with octreotide of the retrobulb ar space in cases of TAO, including sequential studies of patients und ergoing immunosuppressive therapy. We studied a series of 51 patients who had Graves' disease with varying degrees of TAO. Nine patients had received immunosuppressive therapy. The degree of orbital inflammatio n was classified according to the clinical activity score of Mourits. Both planar and tomographic imaging of the orbit were carried out usin g 111 MBq of the (111)ln-labeled octreotide (OctreoScan(R)) 2 h after tracer injection. A significant tracer accumulation in the retrobulbar space was seen in all 20 patients with a high activity score, in 8 of 16 cases with a negative score, and in 11 of 20 cases with an interme diate Mourits' score. In cases of persistent eye disease in spite of i mmunosuppressive therapy, the octreotide scan remained positive. Succe ssful therapy either with prednisolone, external radiation, or iv immu noglobulins showed a significant diminution of tracer uptake after fin ishing the therapeutic regime. Three-dimensional reconstruction of the images also revealed a significant tracer accumulation in the areas o f the lacrimal gland, the nasal region, and the pituitary. Controls ca ses (n = 30) showed no uptake in the orbital region. We conclude that In-111 octreotide scintigraphy is an objective method that identifies patients with active inflammatory eye disease, i.e., having significan t tracer uptake in the retrobulbar space. This uptake appears to refle ct an immunological process, since immunosuppressive therapy will sign ificantly decrease tracer accumulation.