What is the prevalence of AA (inflammatory) amyloidosis?

Updated: Jan 08, 2021
  • Author: Jefferson R Roberts, MD; Chief Editor: Herbert S Diamond, MD  more...
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Answer

The absolute prevalence of AA amyloidosis is difficult to ascertain because it depends on both the occurrence of predisposing inflammatory disorders and the proportion of individuals with those conditions who develop tissue amyloid deposition. The diseases in which AA amyloidosis has been reported are noted below, as are the frequencies (when such data are available).

AA amyloidosis is far less common in the United States than in other countries, even in the setting of the same inflammatory disease. The variation in the occurrence of amyloid in a particular disease in different geographic locales may reflect genetic background, differences in treatment of the primary disease, or factors that are not currently understood.

As in the United States, the frequency of AA amyloidosis is determined by the prevalence of the associated diseases, as well as the incidence of amyloid deposition in those conditions. For instance, in some Middle Eastern countries, the prevalence of familial Mediterranean fever (FMF) is higher than anywhere else in the world. The frequency of renal amyloidosis in some populations with untreated FMF is almost 100%. In those countries, amyloidosis represents a significant proportion of all renal disease.

Most available data to approximate the epidemiology of AA amyloidosis are derived from autopsies. The overall autopsy incidence of AA amyloidosis in western nations ranges from 0.50-0.86%. [23]

Currently, rheumatic diseases such as rheumatoid arthritis (RA), ankylosing spondylitis (AS), psoriatic arthritis, and juvenile idiopathic arthritis are the most frequent causes (70%) of AA amyloidosis. The reported prevalence of amyloidosis in RA ranges from 7%-26%. [24]  The rates vary by the diagnostic procedure used (that is, autopsy, kidney biopsy or subcutaneous fat aspiration), the clinical status (preclinical or symptomatic disease), and the type of study (case series or population-based study).

The most common cause of renal involvement in ankylosing spondylitis (AS) is AA amyloidosis (62%), followed by IgA nephropathy (30%). [25] Although its prevalence might be in decline, renal AA amyloidosis is a serious complication of AS, with a median survival time after onset of dialysis of 2.37 years, and with a 5-year survival rate of only 30%.

In Japanese people, in whom the SAA 1.5 allele is far more common than in whites (37.4% vs 5.3%), the 1.5 allele is enriched among patients with RA and amyloidosis. Individuals with RA and a single 1.5 gene have twice the risk for developing amyloid as those with no 1.5 alleles. People who are homozygous for the 1.5 allele have a relative risk of 4.48 compared with those with RA who lack any 1.5 alleles. The mechanism of the association may reside in the fact that the SAA 1.5 allele is associated with higher SAA levels in Japanese patients. The duration of the inflammatory disease prior to the development of amyloidosis appeared to be inversely related to the dose of the allele. [26]

In the United States, AA amyloidosis is more common in females, reflecting the fact that the major predisposing disease, RA, is predominantly a disorder of younger women and middle-aged men; hence, women are apt to have the disease for a longer period than men.

Despite the statistical female predominance in terms of overall numbers of AA amyloidosis cases, males seem to have an earlier average age of onset. FMF is more common in males than in females (male-to-female ratio, 60:40), but the frequency of renal amyloidosis in people who are affected appears to be similar.

The age of onset of amyloidosis is related to the age of onset of the inflammatory disease, its severity, and the duration of the disease within the constraints imposed by the alleles of SAA carried by the patient. Thus, in the course of juvenile rheumatoid arthritis (JRA), amyloidosis occurs in teenagers. When it is a consequence of adult RA, it develops in late middle age. In the course of inadequately treated FMF, the renal amyloidosis is also of relatively early onset.


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