Learning Objectives
Define the structural characteristics of Amyloidosis and its $\beta$-pleated sheet configuration. Master the diagnostic staining techniques using Congo red and polarized light. Distinguish between systemic forms like AL and AA amyloid, and identify localized versions associated with Alzheimer’s disease and Type 2 Diabetes.
1. Structure and Diagnosis
Amyloid is characterized by the extracellular deposition of misfolded proteins. These proteins arrange themselves into $\beta$-pleated sheets, making them insoluble and resistant to proteolysis, eventually leading to pressure atrophy and cell death.
| Diagnostic Method | Appearance / Observation | Clinical Significance |
|---|---|---|
| Congo Red (Non-polarized) | Red/Orange deposits. | Primary screening stain for amyloid. |
| Polarized Light | Apple-green birefringence. | Confirms  -pleated sheet configuration. |
| H&E Stain | Amorphous, acellular pink material. | Often, the first hint of deposition is on a routine biopsy. |
Activity:
2. Systemic Amyloidosis
In systemic amyloidosis, the precursor protein is typically produced at a distant site and deposited in multiple organs, most commonly the kidneys (nephrotic syndrome) and heart (restrictive cardiomyopathy).
| Type | Protein (Fibril) | Etiology & Notes |
|---|---|---|
| Primary (AL) | Ig Light chains | Plasma cell dyscrasias (e.g., Multiple Myeloma). |
| Secondary (AA) | Serum Amyloid A | Chronic inflammation (RA, IBD, FMF, protracted infection). |
| Dialysis-Related |  -microglobulin |
Long-term dialysis presents as carpal tunnel syndrome. |
| Transthyretin (TTR) | Transthyretin | Sporadic (wild-type) in aging or Hereditary (mutated) polyneuropathy. |
Activity:
3. Localized Amyloidosis
Localized amyloidosis involves specific proteins depositing within a single organ, often linked to distinct metabolic or neoplastic disease processes.
| Condition | Protein (Fibril) | Location / Mechanism |
|---|---|---|
| Alzheimer Disease | $\beta$-amyloid | Cerebral plaques; cleaved from APP. |
| Type 2 Diabetes | Amylin (IAPP) | Pancreatic islets; contributes to $\beta$-cell failure. |
| Medullary Thyroid Ca | Calcitonin | Thyroid stroma; tumor cells secrete excess calcitonin. |
| Isolated Atrial | ANP | Cardiac atria are associated with aging and AFib risk. |
Activity
High-Yield Mnemonics & Tips:
- AL vs AA: Remember AL (Light chain) for Multiple Myeloma and AA (Acute phase reactant) for Chronic Inflammation.
- Macroglossia: If you see an elderly patient with an enlarged tongue and nephrotic syndrome, amyloidosis should be at the top of your differential.
- Restrictive Cardiomyopathy: Amyloid “stiffens” the heart wall, leading to diastolic heart failure.