ESSENTIALS OF DIAGNOSIS
Congo red positive amyloid protein on tissue biopsy.
Primary amyloid protein is kappa or lambda immunoglobulin light chain.
Serum or urine (or both) light chain paraprotein.
Amyloidosis is a rare condition whereby a protein abnormally deposits in tissue resulting in organ dysfunction. The propensity of a protein to be amyloidogenic is a consequence of disturbed translational or posttranslational protein folding and lack of consequential water solubility. The input of amyloid protein into tissues far exceeds its output, so amyloid build up inexorably proceeds to organ dysfunction and ultimately organ failure and premature death.
Amyloidosis is classified according to the type of amyloid protein deposited. The six main categories are primary (immunoglobulin light chain [AL]), secondary (serum protein A, produced in inflammatory conditions [AA]), hereditary (mutated transthyretin [TTR]; many others), senile (wild-type TTR; atrial natriuretic peptide; others), dialysis-related (beta-2-microglobulin, not filtered out by dialysis membranes [Abeta-2M]), and LECT2 (associated with Latinx ethnicity). Amyloidosis is further classified as localized (amyloid deposits only in a single tissue type or organ) or, most common, systemic (widespread amyloid deposition).
Patients with localized amyloidosis have symptoms and signs related to the affected single organ, such as hoarseness (vocal cords) or proptosis and visual disturbance (orbits). Patients with systemic amyloidosis have symptoms and signs of unexplained medical syndromes, including heart failure (infiltrative/restrictive cardiomyopathy), nephrotic syndrome, malabsorption and weight loss, hepatic dysfunction, autonomic insufficiency, carpal tunnel syndrome (often bilateral), and sensorimotor peripheral neuropathy. Other symptoms and signs include an enlarged tongue; waxy, rough plaques on skin; contusions (including the periorbital areas); cough or dyspnea; and disturbed deglutition. These symptoms and signs arise insidiously, and the diagnosis of amyloidosis is generally made late in the disease process.
The diagnosis of amyloid protein requires a tissue biopsy that demonstrates deposition of a pink interstitial substance in the tissue with the hematoxylin and eosin stain. This protein stains red with Congo red and becomes an apple-green color when the light is polarized. Amyloid is a triple-stranded fibril composed of the amyloid protein, amyloid protein P, and glycosaminoglycan. The amyloid fibrils form beta-pleated sheets as demonstrated by electron microscopy. In primary amyloidosis, the amyloid protein is either the kappa or lambda immunoglobulin light chain.
When systemic amyloidosis is suspected, a blind aspiration of the abdominal fat pad will reveal amyloid two-thirds of the time. If the fat pad aspiration is unrevealing, then the affected organ needs biopsy. In 90% of patients with primary amyloidosis, analysis of the serum and urine will reveal a kappa or lambda light chain paraprotein by PEP, IFE, or free light chain assay; in the remainder, mass spectroscopy demonstrates light chain in the tissue biopsy. Lambda amyloid is ...