A 52-year-old woman with a 15-year history of rheumatoid arthritis presents to the clinic with progressive fatigue, ankle edema, and foamy urine. Laboratory investigations show: serum creatinine 2.1 mg/dL, 24-hour urine protein 3.8 g, and normal serum and urine electrophoresis. Abdominal fat pad biopsy shows deposits that are Congo red positive with apple-green birefringence. Immunohistochemistry is negative for light chains (κ and λ) but positive for serum amyloid A. What is the most likely diagnosis?

Explanation

## Diagnosis: AA Amyloidosis ### Pathological Confirmation **Key Point:** Immunohistochemistry positive for serum amyloid A (AA) with negative light chain staining definitively identifies AA amyloidosis. This is the diagnostic gold standard. ### Clinical Context: Secondary Amyloidosis **High-Yield:** AA amyloidosis is *secondary* amyloidosis that develops as a complication of chronic inflammatory diseases. The patient's 15-year history of rheumatoid arthritis is the predisposing factor. ### Pathogenesis of AA Amyloidosis ```mermaid flowchart TD A[Chronic inflammatory disease<br/>RA, TB, chronic osteomyelitis]:::outcome A --> B[Persistent elevation of<br/>serum amyloid A<br/>acute phase reactant]:::action B --> C[Misfolding and<br/>polymerization of SAA]:::action C --> D[Amyloid fibril deposition<br/>in organs]:::action D --> E[Target organs:<br/>kidney, liver, spleen]:::outcome E --> F{Clinical manifestation}:::decision F -->|Kidney| G[Nephrotic syndrome<br/>progressive renal failure]:::outcome F -->|Liver| H[Hepatomegaly]:::outcome F -->|Spleen| I[Splenomegaly]:::outcome ``` ### Distinguishing AA from AL Amyloidosis | Feature | AA Amyloidosis | AL Amyloidosis | |---------|---|---| | **Precursor protein** | Serum amyloid A (SAA) | Misfolded light chains | | **Etiology** | *Secondary* to chronic inflammation | *Primary* (plasma cell dyscrasia) | | **Associated condition** | RA, TB, chronic osteomyelitis, IBD | Multiple myeloma, plasma cell dyscrasias | | **Serum/urine electrophoresis** | Normal | May show monoclonal spike | | **Immunohistochemistry** | Positive for AA, negative for light chains | Positive for κ or λ light chains | | **Cardiac involvement** | Rare | Common | | **Renal involvement** | Common (nephrotic syndrome) | Common (nephrotic syndrome) | | **Organ distribution** | Kidney > liver > spleen | Kidney, heart, nervous system | ### Clinical Pearl **Clinical Pearl:** The normal serum and urine electrophoresis in this patient is a critical clue *against* AL amyloidosis. AL typically shows a monoclonal spike or free light chain elevation. The negative light chain immunohistochemistry further excludes AL. ### Management Strategy **Key Point:** Unlike AL amyloidosis (which requires chemotherapy to suppress the plasma cell clone), AA amyloidosis management focuses on: 1. **Aggressive treatment of the underlying inflammatory disease** (e.g., DMARDs, biologics for RA) 2. Monitoring renal function and proteinuria 3. Symptomatic management of renal failure Controlling the chronic inflammation reduces SAA levels and may halt amyloid progression. ### Mnemonic **Mnemonic:** **AA = Arthritis & Acute phase** — AA amyloidosis is secondary to chronic arthritis and elevated acute phase reactants (SAA).