Chapter 72: Amyloidosis

DEFINITION

• Amyloidosis is a heterogeneous group of diseases characterized by tissue infiltration with misfolded protein precursors.

• The term amyloid is used to describe a substance with a homogeneous eosinophilic appearance by light microscopy when stained with hematoxylin and eosin, a green birefringence on polarizing light microscopy, and a characteristic β-pleated sheet appearance by x-ray diffraction.

• Nomenclature for the type of amyloidosis is with an “A” followed by the abbreviation of the name of the subunit protein.

• Some types of amyloidosis along with their sites of organ involvement are listed in Table 72–1. Clinicians are more likely to see patients with immunoglobulin light chain (AL), although other types are clinically important.

EPIDEMIOLOGY

• AL amyloidosis has an incidence of 12 per million persons per year, making it one-sixth as common as myeloma, but it is frequently unrecognized. The median age at diagnosis is 76 years.

• Serum amyloid A (AA) amyloidosis occurs in less than 1% of persons with chronic inflammatory diseases (eg, rheumatoid arthritis, inflammatory bowel disease, and chronic infections) in the United States and Europe. AA amyloidosis is more common in Turkey and the Middle East, where it occurs in association with familial Mediterranean fever. It is the only type of amyloidosis that occurs in children.

• Transthyretin (ATTR) wild-type amyloidosis is not caused by any known genetic mutations and is increasingly recognized. It is a systemic disease, but the most common sites of involvement are the heart and carpal tunnel.

• The inherited amyloidoses are rare in the United States, with an estimated incidence of less than approximately 1 per 100,000 persons. Inherited forms of amyloidosis are generally composed of mutant transthyretin (TTR) but can also be a consequence of point mutations in apolipoprotein, fibrinogen (AFib amyloidosis), and gelsolin.

• Leukocyte chemotactic factor 2 (ALect 2) amyloidosis manifests as nephropathy.

• Insulin (A Ins) amyloidosis is associated with deposits of amyloid at sites of subcutaneous insulin injection in diabetics. Crystalline insulin can form amyloid, which can cause discolored firm deposits that, when biopsied, will be Congo red positive but by mass spectroscopic analysis are confirmed to be insulin in origin.

• Amyloid β₂-microglobulin (Aβ₂M) amyloidosis usually manifests as deposits in the joint synovial and occurs in patients on long-term dialysis. The incidence is declining with changes in dialysis techniques.