Systemic amyloidosis can be classified as follows: (1) primary systemic amyloidosis (PSA), usually with no evidence of preceding or coexisting disease, paraproteinemia, or plasma-cell dyscrasia; (2) amyloidosis associated with multiple myeloma; or (3) secondary systemic amyloidosis with evidence of coexisting previous chronic inflammatory or infectious conditions.
The current nomenclature refers to amyloidoses based on a capital A (for amyloid), with an abbreviation for the fibril protein following. Primary systemic amyloidosis is referred to as AL amyloidosis, with the A signifying amyloid and the L designating it as light-chain amyloidosis. Terms such as AL describe the protein (light chain), but do not necessarily describe the clinical phenotype.
Primary systemic amyloidosis involves mainly mesenchymal elements, and cutaneous findings are observed in 30-40% of patients. Secondary systemic amyloidosis does not involve the skin, whereas localized amyloidosis does.
Primary systemic amyloidosis involves the deposition of insoluble monoclonal immunoglobulin (Ig) light (L) chains or L-chain fragments in various tissues, including smooth and striated muscles, connective tissues, blood vessel walls, and peripheral nerves.
The amyloid of primary systemic amyloidosis is made by plasma cells in the bone marrow. These L-chains are secreted into the serum. Unlike the normal L-chain and the usual form seen in patients with myeloma, these L-chains are unique in that they undergo partial lysosomal proteolysis within macrophages, and they are extracellularly deposited as insoluble amyloid filaments attached to a polysaccharide. Sometimes, instead of an intact L-chain, this amyloid has the amino-terminal fragment of an L-chain.
In 1838, Mathias Schleiden (a German botanist) coined the term amyloid to describe the normal amylaceous constituent of plants. In 1854, Rudolf Virchow used the term amyloid. Virchow described its reaction with iodine and sulfuric acid, which, at the time, was a marker for starch; thus, the term amyloid or starchlike is used. Virchow adopted the term to describe abnormal extracellular material that is seen in the liver during autopsy.
Some 70 years after Virchow’s description, Divry and associates recognized that the amyloid deposits showed apple-green birefringence when specimens stained with Congo red were viewed under polarized light. This observation remains the sine qua non of the diagnosis of amyloidosis.
In 1959, with the use of electron microscopy, Cohen and Calkins first recognized that all forms of amyloidosis demonstrated a nonbranching fibrillar structure. Electron microscopy remains the most sensitive method for recognizing the disorder.
Also see Amyloidosis.