= TRISOMY 16–18 GROUP SYNDROME

Etiology: additional chromosome at 18 or E group location

Sex: usually female

• Marked phenotypic variability!
• hypertonic spastic infant; mental + psychomotor retardation
• typical facies: micrognathia, high narrow palate with small buccal cavity, low-set deformed ears
• flexed ulnar-deviated fingers + short adducted thumb
• 2^nd finger overlapping 3^rd (CHARACTERISTIC)

Associated with: congenital heart disease in 100% (PDA, VSD); hernias; renal anomalies; eventration of diaphragm

• stippled epiphyses
• Skull
  • thin calvarium
  • persistent metopic suture
  • dolichocephaly with prominent occiput
  • micrognathia ← hypoplastic mandible (most constant feature) + maxilla
• Chest
  • increase in AP diameter of thorax
  • “shield deformity” ← hypoplastic short sternum
  • hypoplastic clavicles (DDx: cleidocranial dysostosis)
  • 11 rib pairs with slender hypoplastic + tapered ribs
  • diaphragmatic eventration (common)
• Pelvis
  • small pelvis with forward rotation of iliac wings
  • increased obliquity of acetabulum
  • acute iliac angle (DIAGNOSTIC)
• Hand & foot
  • adducted thumb = short 1^st metacarpal + phalanges (DIAGNOSTIC)
  • 2^nd finger overlapping 3^rd (DIAGNOSTIC)
  • flexed ulnar-deviated fingers
  • short 1^st toe
  • varus deformities of forefoot + dorsiflexion of toes
  • rocker bottom foot / extreme pes planus (frequent)

OB-US:

• hydrocephalus
• cystic hygroma
• diaphragmatic hernia
• clubfoot
• overlapping index finger
• choroid plexus cyst (30%)

Prognosis: child rarely survives beyond 6 months of age

DDx: osteogenesis imperfecta, trisomy 13 syndrome, Cockayne syndrome, Werdnig-Hoffmann disease