We thank Dr. Balink et al for their interest in our recent article and for their valuable comments. They address several aspects of the similarities and differences between TA and GCA, as well as the limitations of the ACR 1990 criteria in regard to the specific diagnosis of these vasculitis syndromes. The authors presume that some of the patients in our study, who were diagnosed as having TA in accordance with the ACR 1990 criteria, may instead have GCA based on the epidemiologic features of TA and GCA and also on our reported data that the median standard uptake value intensity in 24 patients with clinically active disease was 1.01 (range 0.71–2.36).
We agree that TA and GCA have similarities in the histopathology of arterial lesions and angiographic lesions and that the 1990 ACR criteria have limitations. However, these 2 diseases are different in terms of ethnicity, age at onset, and arterial territories involved. The characteristics of the patients in our study supported the diagnosis of TA rather than GCA.
First, the patients did not have any symptoms suggestive of GCA, such as new-onset headaches, jaw claudication, vision symptoms, and PMR. In addition, we found that FDG uptake was not present in shoulders, hips, spinous process, trochanteric bursa, and ischial bursa of the TA patients, whereas this is found in GCA patients with PMR (1–4).
Second, 33 of 38 patients were <50 years of age at the time of diagnosis. GCA rarely develops before the age of 50 (5).
Third, all of the patients in our study were Korean. Ethnicity is an important risk factor for GCA, with the highest incidence found among people of Scandinavian origin (6). GCA is rare in people of Asian descent, and to date only a few cases have been reported in South Korea (7–10). In contrast, TA seems to be more prevalent in Asian countries compared to Western countries. The prevalence of TA in Japan has been estimated to be 40 cases per million persons, whereas prevalence of TA in the UK was only 4.7 per million persons (11, 12). We previously reported the clinical characteristics of and outcomes in 108 Korean patients who were diagnosed as having TA from 1991 to 2003 in a single hospital (13).
Fourth, all of the patients in our recent study underwent angiographic assessment at the time of diagnosis, and they were judged by the radiologists at our institution to have angiographic features compatible with TA. The diagnosis of TA in our patients was made by the rheumatologists at our institution and was based upon relevant clinical findings, as well as fulfillment of the ACR 1990 criteria.
Finally, the parameter used for quantitative assessment of FDG uptake in our study, namely, standard uptake value intensity, is different from maximal standard uptake value in that the maximal standard uptake value of a vascular region of interest is adjusted by the maximal standard uptake value of the liver. The methods used to assess FDG uptake differ among studies, and currently there is no consensus about the most appropriate method to measure FDG uptake. To the best of our knowledge, only one study used the same method as ours to measure FDG uptake in patients with GCA (14).
GCA and TA have similar angiographic and histopathologic features, and both of them respond well to corticosteroids. It may be reasonable to speculate that these 2 vasculitides are part of the spectrum of a single disease, but several differences certainly exist between them, as we have described above. Further study is needed to address these issues.