Cardiac amyloidosis, sarcoidosis, and cardiovascular infections are major causes of morbidity and mortality. Early diagnosis and timely application of specific therapy are critical to improve clinical outcomes.1,2 Echocardiography, cardiac magnetic resonance imaging (CMR), and cardiac CT can detect increased wall thickness, expanded extracellular volume, diffuse late gadolinium enhancement, wall motion abnormalities, fibrosis, vegetations, and abscesses resulting from cardiac amyloidosis, sarcoidosis, or infection. However, by the time the above cardiac structural changes are evident, the disease is at a fairly advanced stage. Moreover, some of these changes are nonspecific and may represent sequelae of other forms of heart diseases and are not disease specific. Radionuclide imaging methods are evolving as specific disease markers in amyloidosis, sarcoidosis, and infection imaging.
RADIONUCLIDE IMAGING OF CARDIAC AMYLOIDOSIS
Cardiac amyloidosis is a subset of systemic amyloidosis, which is caused by deposition of insoluble nonbranching protein aggregates of amyloid fibrils in the extracellular space of the myocardium. This results in left ventricular thickening and heart failure. Heart failure is a frequent cause of death in these patients. Two major forms of systemic amyloidosis have cardiac involvement3,4: (1) Light chain amyloidosis (AL) where amyloid fibrils are formed from immunoglobulin light chains produced by clonal population of plasma cells and (2) Transthyretin amyloidosis (ATTR) where misfolded monomers or dimers from either mutant or wild type TTR deposit as fibrils in the myocardium. The mutant type (ATTRm) is an autosomal dominant disorder while the wild type disease (ATTRwt) is associated with aging (senile systemic amyloidosis). Some of the ATTRm diseases have predominant cardiomyopathic manifestations while others have coexistent or predominant neuropathy, including carpal tunnel syndrome and/or autonomic neuropathy along with cardiomyopathy. ATTRwt may be significantly underdiagnosed clinically, as a recent autopsy study showed a prevalence of up to 30% of myocardial amyloid deposits in individuals over 75 years with heart failure and preserved ejection fraction.5
Distinction between the two types of cardiac amyloidosis is critical as the treatment and prognosis are vastly different. For AL amyloidosis, chemotherapy is used to inhibit the plasma cell dyscrasia which results in the abnormal light-chain production. Overall mortality as well as treatment-related mortality for AL patients is markedly increased in individuals with cardiac involvement, therefore demonstrating cardiac involvement can guide treatment in these patients. AL cardiac amyloidosis can be rapidly progressive (median survival, if untreated, is <12 months after heart failure onset).3 On the other hand, median survival after heart failure onset is better for ATTR patients (median survival 75 months)4 and within this subset, individuals with ATTRwt do better than those with ATTRm. In recent years, new therapies for ATTR have emerged including drugs that inhibit TTR synthesis, stabilize TTR, or degrade proteins.6 Hence, there is an urgent need to image and quantify myocardial amyloid burden for early diagnosis and for assessment of response to therapy with these new therapies.