Amyloidosis is one of those “classic boards” diagnoses that hides in plain sight: nephrotic syndrome with massive proteinuria, waxy casts, and a biopsy that glows apple-green under polarized light. For Step 1, the key is to recognize what amyloid is, why it deposits in the glomerulus, and how to connect the renal presentation to systemic clues (plasma cell dyscrasia, chronic inflammation, dialysis, Alzheimer disease, etc.).
Where Amyloidosis Fits in Renal Pathology (Big Picture)
In the glomerular disease universe, amyloidosis is a cause of nephrotic syndrome due to protein-leaking damage at the glomerular capillary wall and mesangium.
Nephrotic syndrome pattern (Step 1 anchor):
- Proteinuria > 3.5 g/day
- Hypoalbuminemia
- Edema
- Hyperlipidemia and lipiduria
- Increased risk of thrombosis (loss of antithrombin III) and infection (loss of Ig)
Common nephrotic causes to keep straight:
- Minimal change disease
- FSGS
- Membranous nephropathy
- Diabetic nephropathy
- Amyloidosis
First Aid cross-reference: Renal—Nephrotic syndrome causes; Pathology—Amyloid; Hematology—Multiple myeloma; Endocrine/Neuro—Alzheimer disease (Aβ).
Definition: What Is Amyloid?
Amyloid is a misfolded protein that adopts a β-pleated sheet configuration and deposits extracellularly, disrupting normal tissue architecture and function.
Core Step 1 phrase: Extracellular deposition of misfolded proteins in β-pleated sheet conformation.
Key Structural/Diagnostic Properties
| Property | High-yield detail |
|---|---|
| Conformation | β-pleated sheet (explains staining behavior) |
| Location | Extracellular deposition |
| Congo red stain | Positive |
| Polarized light | Apple-green birefringence |
| EM | Non-branching fibrils (often described as randomly arranged fibrils) |
Pathophysiology: How Amyloid Causes Proteinuria
In the kidney, amyloid classically deposits in:
- Mesangium
- Glomerular basement membrane (GBM)
- Vessel walls (can worsen ischemic injury)
This causes:
- Disruption of the filtration barrier → albumin leakage → nephrotic syndrome
- Progressive scarring and loss of function → chronic kidney disease over time
Why the β-pleated sheet matters
The β-pleated sheet configuration creates a repetitive structure that binds Congo red dye and produces birefringence under polarized light.
Types of Amyloidosis You Must Know (Step 1 Map)
Think of amyloid as a category—the exam tests which protein is depositing and what condition it’s associated with.
Major Amyloid Proteins & Associations
| Amyloid type | Protein | Classic associations | High-yield clue |
|---|---|---|---|
| AL (Primary) | Ig light chain (often λ) | Plasma cell dyscrasias (e.g., multiple myeloma) | Nephrotic + M spike, Bence Jones proteins |
| AA (Secondary) | Serum amyloid A (acute phase reactant) | Chronic inflammatory states (RA, IBD), chronic infections (TB, osteomyelitis) | Long-standing inflammation → renal amyloid |
| Aβ (Beta-amyloid) | Aβ peptide | Alzheimer disease | Neurodegeneration, plaques |
| Aβ2M | β2-microglobulin | Hemodialysis-associated amyloid | MSK symptoms (shoulder pain/carpal tunnel) ± renal issues |
| ATTR | Transthyretin | Familial amyloid polyneuropathy; senile systemic amyloid | Cardiomyopathy + neuropathy |
| AIAPP (Amylin) | Islet amyloid polypeptide | Type 2 diabetes mellitus | Islet deposition |
First Aid cross-reference:
- AL: Multiple myeloma section (Heme/Onc)
- AA: Chronic inflammatory disease (Immunology/Rheum tie-in)
- Aβ: Alzheimer (Neuro)
- Aβ2M: Dialysis complications (Renal)
Clinical Presentation (Renal + Systemic Clues)
Renal presentation (most testable)
- Nephrotic syndrome
- Generalized edema (periorbital → dependent)
- Foamy urine
- Proteinuria (often heavy)
- Progressive CKD over time
- Urinalysis may show fatty casts/oval fat bodies (nephrotic in general)
Systemic clues that point to amyloid
Because amyloid is often systemic, exam questions frequently sprinkle in nonrenal hints:
- Cardiac: restrictive cardiomyopathy, diastolic dysfunction, arrhythmias
- GI: hepatosplenomegaly, malabsorption
- Neuro: peripheral neuropathy, autonomic dysfunction
- Heme/Onc (AL): multiple myeloma symptoms (bone pain, recurrent infections)
- Tongue: macroglossia (classically AL)
- Skin: easy bruising/purpura (classically periorbital in AL)
Step 1 classic combo:
Nephrotic syndrome + multiple myeloma findings (M spike, lytic lesions, recurrent infections) → think AL amyloidosis.
Diagnosis: The Step 1 Workflow
1) Suspect it clinically
- Nephrotic syndrome plus systemic features (cardiac, neuropathy, macroglossia)
- Or nephrotic syndrome in a patient with chronic inflammatory disease / plasma cell disorder / dialysis history
2) Confirm with tissue biopsy
Gold standard: Biopsy (often kidney, abdominal fat pad, rectal mucosa, or involved organ)
Histology must-know:
- Congo red positive
- Apple-green birefringence under polarized light
3) Identify the amyloid type (important for treatment)
- Serum and urine protein electrophoresis (SPEP/UPEP) ± immunofixation for AL
- Serum free light chain assay
- Work-up for chronic inflammation/infection for AA
- Consider dialysis history for Aβ2M
- Genetic testing in suspected hereditary ATTR
Renal Pathology: What You’d See (Boards Style)
Light microscopy
- Amorphous, eosinophilic extracellular material in glomeruli/mesangium
Special stains
- Congo red: salmon-pink/red deposits
- Polarized light: apple-green birefringence
Electron microscopy
- Randomly arranged, nonbranching fibrils (often described as straight fibrils)
Treatment (What Step 1 Wants You to Say)
Treatment depends on the amyloid type—don’t give one-size-fits-all answers.
AL (Primary) amyloidosis
Goal: treat the plasma cell clone producing light chains.
- Chemotherapy targeting plasma cells (e.g., bortezomib-based regimens)
- Consider autologous stem cell transplant in selected patients
- Supportive renal management (diuretics for edema, ACEi/ARB for proteinuria when appropriate)
AA (Secondary) amyloidosis
Goal: reduce chronic inflammation → reduce serum amyloid A production.
- Treat underlying disease (RA control, IBD therapy, treat chronic infection)
Dialysis-related (Aβ2M)
- Optimize dialysis modality (high-flux membranes can help)
- Kidney transplant may reduce β2-microglobulin levels long-term
Supportive renal care (all types)
- Control edema (salt restriction, diuretics)
- Reduce proteinuria (often ACEi/ARB)
- Manage CKD complications
- Evaluate thrombotic risk in severe nephrotic syndrome
For Step 1, it’s enough to link AL → plasma cell dyscrasia treatment and AA → treat chronic inflammation, plus supportive nephrotic care.
High-Yield Associations & Board Triggers
Rapid-fire “If you see this, think amyloid”
- Nephrotic syndrome + apple-green birefringence → amyloidosis until proven otherwise
- Multiple myeloma (M spike, Bence Jones, lytic lesions) + renal proteinuria → AL amyloid
- Rheumatoid arthritis or chronic osteomyelitis/TB + nephrotic syndrome → AA amyloid
- Long-term hemodialysis + carpal tunnel/shoulder pain → Aβ2M amyloid
- Alzheimer → Aβ amyloid in brain plaques
- Restrictive cardiomyopathy + nephrotic syndrome → consider systemic amyloid
Common testable pitfalls
- Amyloid is extracellular (vs many intracellular inclusion diseases)
- Congo red is the stain; apple-green birefringence is the polarized-light finding
- Don’t confuse nephrotic syndrome etiologies: amyloid deposits in glomeruli → leaky filter → heavy protein loss
Mini Table: Amyloidosis vs Other Nephrotic Syndromes (Quick Contrast)
| Disease | Key mechanism | Hallmark clue |
|---|---|---|
| Minimal change | Podocyte effacement | Child; steroid responsive |
| FSGS | Podocyte injury/scarring | HIV, heroin, obesity; poor steroid response |
| Membranous nephropathy | Immune complex deposition | Spike and dome; PLA2R association |
| Diabetic nephropathy | GBM thickening, mesangial expansion | Kimmelstiel-Wilson nodules |
| Amyloidosis | Extracellular β-pleated deposits | Congo red+, apple-green birefringence |
Step 1-Style One-Liner Summary
Amyloidosis causes nephrotic syndrome due to extracellular deposition of misfolded β-pleated sheet proteins in the glomerulus; diagnose with Congo red and apple-green birefringence, and tie type to cause (AL = plasma cell dyscrasia, AA = chronic inflammation, Aβ2M = dialysis).