[Differential diagnosis of amyloidosis]

Amyloid is defined as a proteinaceous tissue deposit that shows a typical green birefringence in polarized light after staining with Congo red, the presence of non-branching linear fibrils of indefinite length with an approximate diameter of 10-12 nm, and a distinct X-ray diffraction pattern consistent with Pauling's model of a cross-beta fibril. Amyloid may deposit locally, organ-limited, tumor-like, or may present as a generalized disease. The origin of amyloid is diverse: twenty different fibril proteins have been described so far. The precursor proteins differ from each other in their primary structures and functions. The only common denominator is the propensity to form anti-parallel cross beta-fibrils under certain circumstances. Early diagnosis of amyloid is still a major challenge in surgical pathology. Histological proof can be obtained using Congo red staining and polarisation microscopy. However, small deposits may be difficult to discern and sensitivity can be improved by using fluorescence microscopy. Classification of amyloid is mandatory, since amyloid is treatable and different treatment regimens are applied to different amyloid diseases. AA amyloidosis requires intensified treatment of the underlying disease. Patients with generalized AL amyloidosis receive high dose chemotherapy and stem cell transplantation. Familial amyloidoses (AApoAI, AFib, ATTR) may undergo liver transplantation.