Extracardiac Tracer Uptake on Technetium-99m–Labeled Pyrophosphate Scintigraphy in Patients With Non-transthyretin Cardiac Amyloidosis

Background: In patients with wild-type transthyretin (ATTRwt) amyloidosis, formerly known as senile systemic amyloidosis, ATTRwt deposition in soft tissues causes orthopedic diseases several years before cardiac manifestations. However, few studies have demonstrated extracardiac ATTRwt deposition before myocardial deposition through imaging modalities. This retrospective study aimed to determine whether technetium-99m–labeled pyrophosphate ( 99m Tc-PYP) is taken up by extracardiac tissue, particularly skeletal muscle, in patients with non-ATTR cardiac amyloidosis (CA), defined as no myocardial tracer uptake. Patients and Methods: Chest-centered and abdomen-centered 99m Tc-PYP scintigraphy images were obtained 2 hours after the intravenous injection of the tracer (20 mCi). First, the frequency of 99m Tc-PYP uptake in the internal oblique muscle (IOM)—where an association between tracer uptake and histologic ATTRwt deposition has been reported in patients with ATTRwt-CA—was investigated in patients with non–ATTR-CA. Second, the study participants were divided into 2 subgroups according to the presence (positive group) or absence (negative group) of 99m Tc-PYP uptake in the muscle, and the differences in clinical characteristics were evaluated between the two subgroups. Results: Overall, 125 patients (median age, 85 y; interquartile range, 77–89 y; 51.2% male individuals) were included. The frequency and multivariate predictors of 99m Tc-PYP uptake in the IOM were 35.2% and older age (odds ratio, 2.27; 95% CI, 1.04–4.92; P = 0.039), respectively. Conclusions: In patients with non–ATTR-CA, the greatest predictor of 99m Tc-PYP uptake in the IOM, suspected to show ATTR deposition, was advanced age. Follow-up of these patients may lead to early detection of ATTR-CA.