Everything You Need to Know About Fabry Disease for Step 1
Fabry disease is a high-yield lysosomal storage disorder that loves to show up on Step 1 via a classic triad: X-linked inheritance, α-galactosidase A deficiency, and angiokeratomas + acroparesthesias—with progressive renal and cardiac failure if untreated.
Where It Fits (Biochemistry → Lysosomal Storage Diseases)
Fabry disease is a lysosomal storage disease caused by failure to degrade specific sphingolipids, leading to toxic substrate accumulation in multiple tissues—especially vascular endothelium, kidney, heart, and nervous system.
Step 1 framing:
- Lysosomal storage diseases → enzyme deficiency → accumulation of substrate → organ dysfunction
- Fabry specifically involves a glycosphingolipid buildup.
Definition (Know This Cold)
Fabry disease is an X-linked recessive lysosomal storage disorder caused by deficiency of α-galactosidase A, leading to accumulation of ceramide trihexoside (aka globotriaosylceramide, Gb3) in lysosomes.
One-liner:
XLR α-galactosidase A deficiency → ↑ ceramide trihexoside (Gb3) → small-vessel disease + neuropathic pain + kidney/heart complications
Pathophysiology (Mechanism → Symptoms)
The Enzyme + Substrate
- Deficient enzyme: α-galactosidase A
- Accumulated substrate: ceramide trihexoside (Gb3)
- Main cells affected: vascular endothelial cells, smooth muscle, neurons, renal podocytes/tubules, cardiomyocytes
Why Symptoms Happen
Accumulation in endothelium and other tissues causes:
- Small vessel dysfunction → ischemia, pain crises, stroke risk
- Renal involvement → proteinuria → progressive CKD/ESRD
- Cardiac involvement → LVH, arrhythmias, cardiomyopathy
- Peripheral/autonomic nerve dysfunction → acroparesthesias, sweating abnormalities
High-yield link: Many manifestations are basically “small vessel disease everywhere.”
Clinical Presentation (Classic + Board-Relevant)
Early / Childhood–Adolescence Clues
- Acroparesthesias: episodic burning pain in hands/feet, often triggered by:
- exercise
- fever/illness
- heat
- Angiokeratomas: small, dark red/purple papules
- classically in “bathing trunk” distribution (hips/groin/periumbilical area)
- Hypohidrosis/anhidrosis → heat intolerance
- Corneal verticillata (whorl-like corneal opacities)
- often asymptomatic but very testable
- GI symptoms: abdominal pain, diarrhea (autonomic involvement)
Progressive / Adult Complications (High Yield)
- Renal failure: proteinuria → CKD → ESRD (major cause of mortality)
- Cardiac disease:
- LVH (can mimic hypertrophic cardiomyopathy)
- arrhythmias, conduction defects
- heart failure
- CNS vascular events:
- TIA/stroke, especially in younger patients
- vertigo, hearing loss can occur
Female Carriers Can Be Symptomatic
Even though it’s X-linked recessive, heterozygous females can have significant disease due to lyonization (random X-inactivation). This is a common Step trick.
Diagnosis (What the Test Writers Want)
Screening/Labs
- Low α-galactosidase A activity
- very reliable in males
- may be normal/near-normal in females → don’t rule out
- Elevated Gb3 (globotriaosylceramide) or related biomarkers (e.g., lyso-Gb3) may support diagnosis.
Genetics
- GLA gene mutation testing is confirmatory (especially useful in females).
Pathology Clues (Classic Vignette Material)
- Renal biopsy (or other affected tissue) can show:
- lipid-laden cells (“foamy” appearance)
- on EM: lamellated “zebra bodies” (myelin figures) in lysosomes
- very high-yield association for Fabry
Treatment (Step-Relevant Therapeutics)
Disease-Specific Therapy
- Enzyme replacement therapy (ERT)
- recombinant α-galactosidase A
- slows progression, especially when started early
- Pharmacologic chaperone therapy
- migalastat (for specific amenable mutations)
- stabilizes misfolded enzyme to improve trafficking/function
Supportive/Complication Management
- ACE inhibitors/ARBs for proteinuria/renal protection
- Dialysis/transplant for ESRD (note: transplant treats kidney failure but does not correct systemic enzyme deficiency)
- Cardiac management: arrhythmia control, HF therapy as indicated
- Neuropathic pain management (e.g., gabapentinoids, carbamazepine—often discussed conceptually)
High-yield: Fabry is one of the lysosomal storage diseases with a specific enzyme replacement therapy—a common test point.
High-Yield Associations & “Buzzwords” (USMLE Favorites)
Must-Memorize Bundle
- Inheritance: X-linked recessive
- Enzyme: α-galactosidase A
- Substrate: ceramide trihexoside (Gb3)
- Key symptoms:
- angiokeratomas
- acroparesthesias (burning pain hands/feet)
- hypohidrosis
- corneal verticillata
- Complications: renal failure, cardiomyopathy/LVH, stroke
- Histology clue: zebra bodies on EM
Common Differential Traps
- Fabry vs Gaucher
- Gaucher: AR, hepatosplenomegaly, bone crises, “crumpled tissue paper” macrophages
- Fabry: XLR, angiokeratomas + neuropathic pain + renal/cardiac disease, zebra bodies
- Fabry vs Pompe (a glycogen storage disease)
- Pompe: AR, acid α-glucosidase deficiency → cardiomegaly, hypotonia, lysosomal glycogen
- Fabry: sphingolipid accumulation, neuropathic pain/skin findings, renal failure
- Fabry vs Niemann-Pick
- Niemann-Pick: sphingomyelinase deficiency → neurodegeneration, hepatosplenomegaly, cherry-red spot, foam cells
First Aid Cross-References (Conceptual Map)
In First Aid for the USMLE Step 1, Fabry is covered under:
- Biochemistry → Lysosomal storage diseases
- Often summarized with:
- X-linked recessive
- α-galactosidase A deficiency
- ↑ ceramide trihexoside
- angiokeratomas, acroparesthesias, hypohidrosis
- progression to renal failure and cardiovascular disease
Study tip: Learn Fabry in a “triangle”:
- X-linked inheritance clue
- Angiokeratomas + pain clue
- Renal/cardiac long-term complication clue
Rapid Review (Exam-Day Checklist)
If you see: young male + burning pain in extremities + angiokeratomas + decreased sweating
Think: Fabry disease
Confirm with: low α-galactosidase A (esp. in males) ± genetic testing
Treat with: enzyme replacement therapy (and/or migalastat for amenable variants)
Watch for: renal failure, LVH/cardiomyopathy, stroke