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Amyloidosis is any of a group of disparate conditions characterized by extracellular deposition of insoluble fibrils composed of misaggregated proteins. These proteins may accumulate locally, causing relatively few symptoms, or widely, involving multiple organs and causing severe multiorgan failure. Amyloidosis can occur de novo or be secondary to various infectious, inflammatory, or malignant conditions. Diagnosis is by biopsy of affected tissue; the amyloidogenic protein is typed using a variety of immunohistologic and biochemical techniques. Treatment varies with the type of amyloidosis.
Amyloid deposits are composed of small (about 10 nm diameter), insoluble fibrils that form β-pleated sheets that can be identified by x-ray diffraction. In addition to the fibrillar amyloid protein, the deposits also contain serum amyloid P component and glycosaminoglycans. Amyloid fibrils are made of misfolded proteins that aggregate into oligomers and then fibrils. A number of normal (wild-type) and mutant proteins are susceptible to such misfolding and aggregation (amyloidogenic proteins), thus accounting for the wide variety of causes and types of amyloidosis. For amyloidosis to develop, in addition to production of amyloidogenic proteins, there is probably also a failure of the normal clearance mechanisms for such misfolded proteins. The amyloid deposits themselves are metabolically inert but interfere physically with organ structure and function. However, some prefibrillar oligomers of amyloidogenic proteins have direct cellular toxicity, an important component of disease pathogenesis.
Amyloid deposits stain pink with hematoxylin and eosin, contain carbohydrate constituents that stain with periodic acid-Schiff dye or with Alcian blue, but most characteristically have apple-green birefringence under polarized light microscopy after Congo red staining. On autopsy inspection, affected organs may appear waxy.
In systemic amyloidosis, circulating amyloidogenic proteins form deposits in a variety of organs. Major systemic types include
AL (primary amyloidosis): Caused by acquired overexpression of clonal immunoglobulin light chains
AF (familial amyloidosis): Caused by inheritance of a mutant gene encoding a protein prone to misfolding, most commonly transthyretin (TTR)
SSA (senile systemic amyloidosis): Caused by misfolding and aggregation of wild-type TTR (thus also termed ATTRwt)
AA (secondary amyloidosis): Caused by aggregation of an acute phase reactant, serum amyloid A
Amyloidosis caused by aggregation of β2-microglobulin (Aβ2) can occur in patients on long-term hemodialysis, but the incidence has declined with use of modern high-flow dialysis membranes.
Localized forms of amyloidosis appear to be caused by local production and deposition of an amyloidogenic protein (including immunoglobulin light chains) within the affected organ rather than by deposition of circulating proteins. Frequently involved sites include the CNS (eg, in Alzheimer disease), skin, upper or lower airways, bladder, and other sites.
AL is caused by overproduction of an amyloidogenic immunoglobulin light chain in patients with a monoclonal plasma cell or other B cell lymphoproliferative disorder. Light chains can also form nonfibrillar tissue deposits (ie, light chain deposition disease). Rarely, immunoglobulin heavy chains form amyloid fibrils (AH). Common sites for amyloid deposition include the skin, nerves, heart, GI tract (including tongue), kidneys, liver, spleen, and blood vessels. Usually, a low-grade plasmacytosis is present in the bone marrow, which is similar to that in multiple myeloma, although most patients do not have true multiple myeloma (with lytic bone lesions, hypercalcemia, renal tubular casts, and anemia). However, about 10 to 20% of patients with multiple myeloma develop AL amyloidosis.
This form can occur secondary to several infectious, inflammatory, and malignant conditions and is caused by aggregation of isoforms of the acute-phase reactant serum amyloid A. Common causative infections include TB, bronchiectasis, osteomyelitis, and leprosy. Inflammatory conditions include RA, juvenile idiopathic arthritis (formerly juvenile RA), Crohn disease, inherited periodic fever syndromes such as familial Mediterranean fever, and Castleman disease. Inflammatory cytokines (eg, IL-1, tumor necrosis factor, IL-6) that are produced in these disorders or ectopically by tumor cells cause increased hepatic synthesis of serum amyloid A.
AA shows a predilection for the spleen, liver, kidneys, adrenal glands, and lymph nodes. Involvement of the heart and peripheral or autonomic nerves is rare.
AF is caused by inheritance of a gene encoding a mutated aggregation-prone serum protein, usually a protein abundantly produced by the liver. Serum proteins that can cause AF include transthyretin (TTR), apolipoprotein A-1, lysozyme, fibrinogen, gelsolin, and cystatin C. A recently identified form that is speculated to be familial is caused by the serum protein leukocyte chemotactic factor 2 (LECT2); however, a specific inherited gene mutation for this latter type has not been clearly demonstrated.
Amyloidosis caused by TTR (ATTR) is the most common type of AF. More than 100 mutations of the TTR gene have been associated with amyloidosis. The most prevalent mutation, V30M, is common in Portugal, Sweden, and Japan, and a V122I mutation is present in about 4% of American blacks. Disease penetrance and age of onset are highly variable but are consistent within families and ethnic groups. ATTR causes peripheral sensory and autonomic neuropathy and cardiomyopathy. Carpal tunnel syndrome is common. Vitreous deposits or cerebrovascular amyloid angiopathy may also develop due to production of mutant TTR by the retinal epithelium or choroid plexus, respectively.
SSA is caused by aggregation and deposition of wild-type TTR, mainly in the heart. SSA is increasingly recognized as a cause of infiltrative cardiomyopathy in older men. The genetic and epigenetic factors leading to SSA are unknown.
Because SSA and AL amyloidosis both can cause cardiomyopathy, and because monoclonal gammopathies not associated with amyloidosis may be present in patients in this age group, it is essential to accurately type the amyloid so that patients with SSA are not inappropriately treated with chemotherapy (which is used for AL).
Localized amyloidosis outside the brain is most frequently caused by deposits of clonal immunoglobulin light chains and within the brain by Aβ protein. Localized amyloid deposits typically involve the airways and lung tissue, bladder and ureters, skin, breasts, and eyes. Rarely, other locally produced proteins cause amyloidosis, such as keratin isoforms that can form deposits locally in the skin.
Aβ protein deposits in the brain contribute to Alzheimer disease or cerebrovascular amyloid angiopathy. Other proteins produced in the CNS can misfold, aggregate, and damage neurons, leading to neurodegenerative diseases (eg, Parkinson disease, Huntington disease). Clonal immunoglobulin light chains produced by mucosal-associated lymphoid tissue in the GI tract, airways, and bladder can lead to localized AL in those organs.
Symptoms and signs of systemic amyloidosis are nonspecific, often resulting in delays in diagnosis. Suspicion of amyloidosis should be increased in patients with a progressive multisystem disease process.
Renal amyloid deposits typically occur in the glomerular membrane leading to proteinuria, but in about 15% of cases the tubules are affected, causing azotemia with minimal proteinuria. These processes can progress to nephrotic syndrome with marked hypoalbuminemia, edema, and anasarca or to end-stage renal disease.
Hepatic involvement causes painless hepatomegaly, which may be massive. Liver function tests typically suggest intrahepatic cholestasis with elevation of alkaline phosphatase and later bilirubin, although jaundice is rare. Occasionally, portal hypertension develops, with resulting esophageal varices and ascites.
Airway involvement leads to dyspnea, wheezing, hemoptysis, or airway obstruction.
Infiltration of the myocardium causes a restrictive cardiomyopathy, eventually leading to diastolic dysfunction and heart failure; heart block or arrhythmia may occur. Hypotension is common.
Peripheral neuropathy with paresthesias of the toes and fingers is a common presenting manifestation in AL and ATTR amyloidoses. Autonomic neuropathy may cause orthostatic hypotension, erectile dysfunction, sweating abnormalities, and GI motility disturbances.
Cerebrovascular amyloid angiopathy most often causes spontaneous lobar hemorrhage but some patients have brief, transient neurologic symptoms.
GI amyloid may cause motility abnormalities of the esophagus and small and large intestines. Gastric atony, malabsorption, bleeding, or pseudo-obstruction may also occur. Macroglossia is common in AL amyloidosis.
A firm, symmetric, nontender goiter resembling that found in Hashimoto thyroiditis may result from amyloidosis of the thyroid gland; other endocrinopathies can also occur. Lung involvement (mostly in AL amyloidosis) can be characterized by focal pulmonary nodules, tracheobronchial lesions, or diffuse alveolar deposits. In several hereditary amyloidoses, amyloid vitreous opacities and bilateral scalloped pupillary margins develop.
Diagnosis of amyloidosis is made by demonstration of fibrillar deposits in an involved organ. Aspiration of subcutaneous abdominal fat is positive in about 80% of patients with AL or ATTR, but only about 50% of patients with SSA. If the fat biopsy result is negative, a clinically involved organ should be biopsied. Tissue sections are stained with Congo red dye and examined with a polarizing microscope for characteristic birefringence. Nonbranching 10-nm fibrils can also be recognized by electron microscopy on biopsy specimens from heart or kidney.
After amyloidosis has been confirmed by biopsy, the type is determined using a variety of techniques. For some types of amyloidosis, immunohistochemistry or immunofluorescence may be diagnostic, but false-positive typing results occur. Other useful techniques include gene sequencing for AF, and biochemical identification by mass spectrometry.
If AL is suspected, patients should be evaluated for an underlying plasma cell disorder using quantitative measurement of serum free immunoglobulin light chains, qualitative detection of serum or urine monoclonal light chains using immunofixation electrophoresis (serum protein electrophoresis and urine protein electrophoresis are insensitive in patients with AL), and a bone marrow biopsy with flow cytometry or immunohistochemistry to establish plasma cell clonality. Patients with > 10% clonal plasma cells should be tested to see if they meet criteria for multiple myeloma, including screening for lytic bone lesions, anemia, renal insufficiency, and hypercalcemia (see also Multiple Myeloma : Diagnosis).
Patients are screened for organ involvement beginning with noninvasive testing of kidney, liver, GI, nervous system, and cardiac function, particularly when symptoms suggest organ involvement. Patients should have urinalysis and measurement of serum BUN and creatinine to screen for renal involvement, liver function tests to screen for hepatic involvement, and ECG and measurement of brain (B-type) natriuretic peptide (BNP) or N-terminal-pro-BNP (NT-proBNP) and troponin to screen for cardiac involvement. Cardiac involvement can be suggested by low voltage on ECG (caused by a thickened ventricle), and/or dysrhythmias. If cardiac involvement is suspected because of symptoms, ECG, or cardiac biomarkers, echocardiography is done to measure diastolic relaxation and systolic function and to screen for biventricular hypertrophy. In ambiguous cases, cardiac MRI can be done to detect delayed subendocardial gadolinium enhancement, a characteristic finding. Lung involvement can be detected by chest x-ray, CT, and/or pulmonary function testing.
Prognosis depends on the type of amyloidosis and the organ system involved, but with appropriate disease-specific and supportive care, many patients have an excellent life expectancy.
AL complicated by severe cardiomyopathy still has the poorest prognosis, with median survival of < 1 yr. Untreated ATTR amyloidosis usually progresses to end-stage cardiac or neurologic disease within 10 to 15 yr. SSA typically has the slowest progression of any systemic amyloidosis involving the heart, but some patients do progress to symptomatic heart failure and death within a few years of diagnosis.
Prognosis in AA amyloidosis depends largely upon the effectiveness of treatment of the underlying infectious, inflammatory, or malignant disorder.
Currently, there are specific treatments for most forms of amyloidosis, although some therapies are investigational. For all forms of systemic amyloidosis, supportive care measures can help relieve symptoms and improve quality of life.
Supportive care measures are directed at the affected organ system:
Renal: Patients with nephrotic syndrome and edema should be treated with salt and fluid restriction, and loop diuretics; because of the ongoing protein loss, protein intake should not be restricted. Kidney transplantation is an option when the underlying disease process is controlled, and can provide long-term survival comparable to that in other renal diseases.
Cardiac: Patients with cardiomyopathy should be treated with salt and fluid restriction and loop diuretics. Other drugs for heart failure, including digoxin, ACE inhibitors, Ca channel blockers, and β-blockers are poorly tolerated and contraindicated. Heart transplantation has been successful in carefully selected patients with AL amyloidosis and severe cardiac involvement. To prevent recurrence in the transplanted heart, patients must be given aggressive antiplasma cell therapy (usually IV melphalan) followed by autologous stem cell transplantation.
GI: Patients with diarrhea may benefit from loperamide. Those with early satiety and gastric retention may benefit from metoclopramide.
Nerves: In patients with peripheral neuropathy, gabapentin or pregabalin may relieve pain.
Orthostatic hypotension often improves with high doses of midodrine; this drug can cause urinary retention in older males, but supine hypertension is rarely a problem in this population. Support stockings can also help, and fludrocortisone can be used in patients without peripheral edema, anasarca, or heart failure.
For AL amyloidosis, prompt initiation of antiplasma cell therapy is essential to preserve organ function and prolong life. Most drugs used for multiple myeloma (see Multiple Myeloma : Treatment) have been used in AL amyloidosis; choice of drug, dose, and schedule often must be modified when organ function is impaired. Chemotherapy using an alkylating agent (eg, melphalan, cyclophosphamide) combined with corticosteroids was the first regimen to show any benefit. High-dose IV melphalan, combined with autologous stem cell transplantation can be highly effective in selected patients. Proteasome inhibitors (eg, bortezomib) and immunomodulators (eg, lenalidomide) also can be effective. Combination and sequential regimens are being investigated. Localized AL can be treated with low-dose external beam radiation therapy because plasma cells are highly radiosensitive.
For ATTR amyloidosis, liver transplantation—which removes the site of synthesis of the mutant protein—can be effective in certain TTRmutations with early neuropathy and no heart involvement. Recently, certain drugs have been shown to stabilize TTR in the plasma, preventing misfolding and fibril formation and inhibiting neurologic disease progression while preserving quality of life. These TTR stabilizers include diflunisal, which is widely available, and tafamidis, which is available in Europe and Japan. TTR gene silencing using anti-sense RNA or RNA interference to block translation of mRNA into protein effectively reduces serum levels of TTR and is in clinical trials. These approaches should also be effective in SSA but have not been tested; liver transplantation is not effective for SSA patients because the amyloidogenic protein is wild-type TTR.
For AA amyloidosis caused by familial Mediterranean fever, colchicine 0.6 mg po once/day or bid is effective. Colchicine is not effective in other disorders predisposing to AA amyloidosis. For other AA types, treatment is directed at the underlying infection, inflammatory disease, or malignancy. Eprodisate, a sulfonated molecule that alters the stability of AA amyloid deposits, is a promising drug now under study.
Amyloidosis is a group of disorders in which certain misfolded proteins aggregate into insoluble fibrils that are deposited within organs, causing dysfunction.
Many different proteins are prone to misfold; some of these proteins are produced by a genetic defect or by certain disease states, while others are immunoglobulin light chains produced by monoclonal plasma cell or other B-cell lymphoproliferative disorders.
The amyloidogenic protein determines the amyloid type and clinical course of disease, although the clinical manifestations of the different types may overlap.
Many organs can be affected, but cardiac involvement carries a particularly poor prognosis; amyloid cardiomyopathy typically leads to diastolic dysfunction, heart failure, and heart block and/or arrhythmia.
Diagnosis is by biopsy; type of amyloidosis is determined by a variety of immunologic, genetic, and biochemical tests.
Appropriate supportive care will help relieve symptoms and improve quality of life; organ transplantation can help selected patients.
Treat the underlying process; for AL amyloidosis due to plasma cell or lymphoproliferative disorders, chemotherapy can be highly effective; for secondary AA amyloidosis, anti-infectious and anti-inflammatory therapies can help; for some hereditary AF amyloidosis, small molecule therapies and gene-targeting therapies hold great promise.
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