What are the differential diagnoses for an adolescent with frontal bossing, down‑slanting palpebral fissures, seizures, and mild intellectual disability?

Differential Diagnoses for Frontal Bossing, Down-Slanting Palpebral Fissures, Seizures, and Mild Intellectual Disability in an Adolescent

The most critical differential diagnosis to evaluate first is 22q11.2 deletion syndrome (DiGeorge syndrome), which should be tested via chromosomal microarray or MLPA, as this diagnosis has significant implications for multisystem management, cardiac surveillance, and family genetic counseling 1.

Primary Differential Diagnoses

1. 22q11.2 Deletion Syndrome (DiGeorge Syndrome)

This is the highest priority diagnosis given the clinical presentation. Key features include:

• Facial dysmorphism: While the guideline describes "upslanting palpebral fissures" as typical 1, the syndrome has variable expressivity and antimongoloid (down-slanting) features can occur
• Seizures: Can be idiopathic or secondary to hypocalcemia (check calcium, PTH, magnesium immediately) 1
• Intellectual disability: Mild to moderate cognitive deficits are characteristic, particularly evident in adolescence 1
• Critical associated features to screen for:
  • Congenital heart disease (perform echocardiogram and EKG urgently) 1
  • Hypocalcemia/hypoparathyroidism (check PTH, calcium, magnesium, creatinine) 1
  • Immunodeficiency (T and B cell phenotyping) 1
  • Renal anomalies (renal ultrasound) 1

Testing: Order chromosomal microarray or MLPA as first-line genetic test 1. FISH is acceptable if other methods unavailable but less comprehensive 1.

2. 1p36 Deletion Syndrome

This microdeletion syndrome presents with overlapping features 2:

• Characteristic facies: Frontal bossing, down-slanting palpebral fissures, flat nasal root, midface hypoplasia
• Developmental delay/intellectual disability: Variable severity
• Seizures: Common feature
• Additional features: Congenital heart defects, hearing loss, hypotonia

Testing: Chromosomal microarray (karyotyping misses most cases) 2.

3. FOXP1-Related Intellectual Disability

A recognizable monogenic syndrome 3, 4:

• Facial features: Broad forehead, down-slanting palpebral fissures, frontal hair upsweep (distinctive pointer)
• Speech/language: Moderate to severe delay with expressive speech most affected
• Intellectual disability: Mild to severe
• Behavioral issues: Autistic traits common
• Macrocephaly: Relative or true (contrasts with microcephaly in other syndromes)

Testing: Exome sequencing or targeted FOXP1 gene analysis 3, 4.

4. Baraitser-Winter Syndrome or Related MCA/MR Syndromes

Consider when features include 5:

• Down-slanting palpebral fissures with ptosis
• Hypertelorism
• Seizures and intellectual disability
• Microcephaly (if present)

Testing: Exome sequencing for ACTB or ACTG1 mutations.

5. Chromosomal Abnormalities (General)

Given that chromosomal abnormalities account for 15-40% of severe intellectual disability cases 6:

• Sex chromosome aneuploidies (47,XXY; 47,XXX; 47,XYY): Can present with learning disabilities and mild dysmorphism
• Subtelomeric rearrangements: Account for ~5% of intellectual disability cases 6

Testing: Chromosomal microarray is superior to karyotyping for detecting submicroscopic abnormalities 6.

Diagnostic Algorithm

1. Immediate laboratory evaluation:

   • Serum calcium, PTH, magnesium (to evaluate seizure etiology and screen for 22q11.2DS) 1
   • Complete blood count with differential 1

2. First-line genetic testing:

   • Chromosomal microarray (detects 22q11.2 deletion, 1p36 deletion, and other copy number variants) 1, 6, 2

3. Cardiac evaluation:

   • Echocardiogram and EKG (essential for 22q11.2DS and 1p36 deletion) 1

4. If microarray is negative:

   • Exome sequencing to evaluate for FOXP1 mutations and other monogenic causes 3, 4

5. Additional subspecialty evaluations based on initial findings:

   • Immunology (T and B cell phenotyping if 22q11.2DS suspected) 1
   • Endocrinology (thyroid function, growth hormone if indicated) 1
   • Nephrology (renal ultrasound) 1
   • Audiology (hearing assessment) 1
   • Ophthalmology (vision screening) 1

Critical Pitfalls to Avoid

• Do not assume seizures are idiopathic without checking calcium levels 1—hypocalcemia from hypoparathyroidism in 22q11.2DS is treatable
• Do not rely on standard karyotyping alone—it misses most microdeletions including 1p36 deletion 2 and many 22q11.2 deletions
• Do not overlook cardiac evaluation—congenital heart disease is life-threatening and present in multiple syndromes in this differential 1, 2
• Do not dismiss parental testing—if 22q11.2 deletion is confirmed, test both parents as ~10% are inherited and recurrence risk is 50% if parent affected 1

Key Distinguishing Features

Feature	22q11.2DS	1p36 Deletion	FOXP1
Palpebral fissures	Usually upslanting (variable)	Down-slanting	Down-slanting
Head size	Normal	Microcephaly common	Macrocephaly common
Cardiac defects	75% (conotruncal)	Common (VSD, ASD)	May be present
Hypocalcemia	Common	Rare	Rare
Frontal hair upsweep	No	No	Distinctive feature
Speech impairment	Present	Present	Severe, especially expressive

The chromosomal microarray should be ordered immediately as the single most informative first test, followed by targeted cardiac and metabolic evaluation to identify life-threatening complications, particularly in 22q11.2 deletion syndrome 1, 6.