DiGeorge syndrome is one of those Step 1 immunology topics that shows up everywhere—immunodeficiency, embryology, cardiology, ENT, endocrinology—because it’s really a developmental problem with immune consequences. If you can link 3rd/4th pharyngeal pouch failure to absent thymus/parathyroids, the rest becomes a predictable set of clinical clues and testable associations.
Quick Definition (Step 1 framing)
DiGeorge syndrome (22q11.2 deletion) is a congenital disorder caused by abnormal development of the 3rd and 4th pharyngeal pouches, leading to:
- Thymic aplasia/hypoplasia → T-cell deficiency
- Parathyroid aplasia/hypoplasia → hypocalcemia (→ tetany/seizures)
- Conotruncal cardiac defects (e.g., truncus arteriosus, Tetralogy of Fallot, interrupted aortic arch)
Classic triad to memorize:
Cardiac defects + Abnormal facies + Thymic aplasia + Hypocalcemia = CATCH-22 (because of 22q11 deletion)
First Aid cross-reference: Immunodeficiencies (DiGeorge), Embryology (pharyngeal pouches), Cardio (conotruncal defects), Endocrine (hypocalcemia).
Pathophysiology: what’s actually going wrong?
1) Genetic basis
- Most commonly 22q11.2 microdeletion
- Typically affects genes involved in neural crest migration (classically linked to TBX1 in many teaching resources)
2) Embryology: pharyngeal pouches (HY)
3rd pharyngeal pouch gives rise to:
- Inferior parathyroids
- Thymus
4th pharyngeal pouch gives rise to:
- Superior parathyroids
- Ultimobranchial body → C cells of thyroid (calcitonin)
In DiGeorge, defective pouch development → thymic and parathyroid hypoplasia/aplasia.
3) Immunology: why T-cells matter
- Thymus is where T-cells mature → low/absent CD3+ T cells
- Downstream effects:
- Decreased cell-mediated immunity (viral, fungal, protozoal infections)
- Reduced T-cell help to B cells → can see low class-switched antibodies (variable), but the headline is T-cell deficiency
4) Calcium: why hypocalcemia happens
Parathyroid hypoplasia → low PTH → hypocalcemia:
- perioral numbness/tingling
- tetany
- seizures
- Chvostek and Trousseau signs (HY)
Clinical Presentation: what you’ll see in stems
Classic “board-style” features
- Recurrent viral and fungal infections (especially early in life)
- Hypocalcemic tetany or seizures in a newborn
- Congenital heart disease (especially conotruncal)
- Abnormal facies
- low-set ears
- micrognathia
- short philtrum / cleft palate (may appear as feeding difficulty or nasal speech)
- Possible developmental delay / learning difficulties (more Step 2 flavor, but fair game)
Infection pattern (HY)
Because this is primarily a T-cell problem, think:
- Candida, Pneumocystis jirovecii, severe viral infections
- Opportunistic infections, especially if thymic aplasia is profound
Cardiac defects to name-drop (HY)
DiGeorge is strongly tied to conotruncal abnormalities (neural crest-related), especially:
- Truncus arteriosus
- Tetralogy of Fallot
- Interrupted aortic arch
- (Also sometimes VSD)
Diagnosis: how questions test it
The “one-liner diagnosis”
A baby with hypocalcemia + conotruncal defect + recurrent infections → think DiGeorge (22q11 deletion).
Labs & studies (what’s high yield)
- Low T cells (low CD3)
- Low PTH → low calcium
- Immunology testing:
- Decreased T-cell zones in lymph nodes/spleen (paracortex)
- Absent thymic shadow on CXR (classic image clue)
- Genetic confirmation:
- FISH or chromosomal microarray for 22q11.2 deletion
High-yield “anatomy/path” correlations
| Finding | Why it happens | HY takeaway |
|---|---|---|
| Absent thymic shadow on CXR | Thymic aplasia | Classic imaging clue |
| ↓ T-cell zones (paracortex) | T-cell deficiency | “Where are T-cells?” |
| Hypocalcemia + tetany | ↓ PTH | Think perioral tingling/tetany |
| Conotruncal defects | Neural crest migration issue | Links to embryology/cardiology |
Treatment: what Step 1 expects you to know
Management depends on severity (partial vs complete DiGeorge), but the board-relevant interventions include:
Immune support
- Thymic transplant (or hematopoietic stem cell transplant in select contexts) for severe T-cell immunodeficiency
- Aggressive treatment/prophylaxis for opportunistic infections when severe
Correct hypocalcemia
- Calcium supplementation
- Calcitriol (active vitamin D) as needed to maintain calcium
Cardiac and ENT management
- Surgical correction of congenital heart defects
- Address cleft palate/feeding issues when present
Vaccines (HY test point)
- Avoid live vaccines in patients with significant T-cell deficiency (risk of disseminated infection)
- Examples: MMR, varicella, intranasal influenza, rotavirus
High-Yield Associations & Differentiation (don’t mix these up)
DiGeorge vs Bruton's vs Hyper-IgM vs SCID (fast compare)
| Disorder | Key defect | Key labs | Classic clue |
|---|---|---|---|
| DiGeorge (22q11) | T-cell maturation failure (absent thymus) | ↓ T cells, ↓ PTH, ↓ Ca | Conotruncal defects + hypocalcemic tetany |
| XLA (Bruton) | No B-cell maturation (BTK) | ↓ B cells, ↓ Ig | No tonsils/lymph nodes, recurrent bacterial/enteroviral |
| Hyper-IgM | Class switching defect (CD40L often) | ↑ IgM, ↓ IgG/IgA/IgE | Severe pyogenic/opportunistic infections |
| SCID | T-cell failure ± B-cell dysfunction | ↓ T cells, nonfunctional B | Chronic diarrhea, thrush, severe infections, absent thymic shadow |
Board tip: Absent thymic shadow can be DiGeorge or SCID—use hypocalcemia and cardiac defects to favor DiGeorge.
First Aid-Style “Buzzwords” You Should Be Ready For
- “CATCH-22”: Cardiac abnormality, Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia; 22q11 deletion
- “Absent thymic shadow”
- “Conotruncal defects” (TOF, truncus arteriosus)
- “Hypocalcemic tetany” (Chvostek/Trousseau)
- “Decreased T-cell zones (paracortex)”
- “FISH positive for 22q11 deletion”
Common USMLE Stem Patterns (how it’s asked)
Pattern 1: Newborn tetany + heart defect
A newborn with seizures/tetany, low calcium, and a conotruncal murmur → DiGeorge, due to low PTH from parathyroid aplasia.
Pattern 2: Opportunistic infections + absent thymic shadow
Infant with recurrent candidiasis/viral infections + absent thymic shadow → think T-cell deficiency. Add hypocalcemia or heart defects → DiGeorge.
Pattern 3: Embryology question disguised as immunology
They ask: “Failure of development of which pharyngeal pouch?”
Answer: 3rd and 4th pharyngeal pouches (thymus + parathyroids).
Ultra-Condensed Step 1 Summary
- Cause: 22q11 deletion → abnormal 3rd/4th pharyngeal pouch development
- Immune: absent/hypoplastic thymus → ↓ T cells → viral/fungal/protozoal infections
- Endocrine: absent/hypoplastic parathyroids → ↓ PTH → ↓ Ca → tetany/seizures
- Cardiac: conotruncal defects (TOF, truncus arteriosus)
- Dx clues: absent thymic shadow; FISH/microarray
- Tx: calcium/calcitriol, manage infections, thymic transplant if severe; avoid live vaccines if T-cell deficiency is significant