Cardiac Amyloidosis and Endomyocardial Biopsy: Correlation of Extent and pattern of Deposition with Amyloid Immunophenotype by immunogold in a single institution

Background: Amyloidosis is an infiltrative systemic disease characterized by the extracellular deposition of fibrillar proteins in different organs leading to tissue damage and dysfunction. Cardiac involvement by amyloidosis is common. The identification of the type of amyloid deposits is critical to the correct diagnosis and treatment. Several methods are currently used to characterize amyloid deposits. Aim of the study: The aims of the present study were to (1) investigate the accuracy and reliability of light microscopy compared to immune-electron microscopy (IEM) study in diagnosis and characterization of cardiac amyloidosis on fixed paraffin-embedded tissues; (2) to study at light microscopy the association in endomyocardial biopsies (EMB) between the amyloid burden and its patterns of deposition (perimisial, interstitial nodular-like, perivascular) and the type of amyloid characterized by means of IEM; (3) to evaluate the diagnostic value of abdominal fat pad excisional biopsies (FPEB) vs endomyocardial biopsy (EMB) when both are performed, (4) and to find clinico-pathological correlations in cases of cardiac amyloidosis. Methods: Consecutive EMBs, of patients with clinical evidence of hypertrophic-restrictive cardiomyopathy, suspected to be due to amyloidosis, submitted to our Pathology Unit over the last 5 years, were re-examined. Cases with FPEB besides EMB were also reviewed. All EMBs underwent to H&E, Congo Red and Thioflavin stains. IEM, performed with gold-labelled antibodies against light chains (AL, kappa light chains, lambda light chains), and transthyretin (TTR), was used to identify specific amyloid fibrils. Forty cases of the Hospital of Padua were re-examined to find clinical-pathological correlations. Results: In the time interval 2017-2022, a series of 171 EMBs were collected (118 males, 69%; average age at diagnosis 66.6±11.5 years). A subgroup of 36 patients (21%) underwent also FPEB; in 27 (75%) of them, EMB was positive, 21 of which negative at FPEB. In 45 cases (26%) EMB amyloid deposits were not identified by means of H&E, Congo Red, and Thioflavin stains nor by IEM. In the remaining 126 EMB cases (74%), IEM was diagnostic, revealing even small amounts of amyloid fibrils, positive for AL in 82/126 (65%) and TTR in 44/126 (35%). Both the interstitium and the vessels were affected, with different burden based on the type of amyloid. Cardiac structural damage and dysfunction is more relevant in ATTR amyloidosis. Conclusion: IEM represents a sensitive, reliable, and low-cost method for amyloid typing. It can be used to test the reliability of light microscopy. EMB with IEM amyloid typing is crucial to establish diagnosis, prognosis, and appropriate treatment. Localized amyloidosis can only be diagnosed by biopsy of the affected organ or tissue. Choosing the correct tissue/organ to biopsy is essential to avoid false negatives and delays to diagnosis. The choice of a surrogate tissue should be discouraged.