= SACRAL AGENESIS = CAUDAL DYSPLASIA SEQUENCE

= midline closure defect of neural tube with a spectrum of anomalies including complete / partial agenesis of sacrum + lumbar vertebrae and pelvic deformity

Etiology: disturbance of caudal mesoderm <4^th week of gestation from toxic / infectious / ischemic insult

Prevalence: 1÷60,000 births; 0.005–0.01% of population; in 0.1–1% of pregnancies in diabetic women; M÷F = 2.7÷1

Predisposed: infants of diabetic mothers; risk increases 200–400 times in women dependent on insulin

◊16–22% of children with sacral agenesis have mothers with diabetes mellitus

◊NOT associated with VATER syndrome!

1. Musculoskeletal anomalies
   • Lower extremity
     • symptoms from minor muscle weakness to complete sensorimotor paralysis of both lower extremities
     • hip dislocation
     • hypoplasia of lower extremities
     • flexion contractures of lower extremities
     • foot deformities
   • Lumbosacral spine = SACRAL AGENESIS
     Spectrum:
     • Type 1 = unilateral partial agenesis localized to sacrum / coccyx
     • Type 2 = bilateral partial symmetric defects of sacrum + iliosacral articulation
     • Type 3 = total sacral agenesis + iliolumbar articulation
     • Type 4 = total sacral agenesis + ilioilial fusion posteriorly
     • nonossification of lower spine
     • fusion of caudal-most 2 or 3 vertebrae
     • spina bifida (lipomyelomeningocele often not in combination with Arnold-Chiari malformation)
     • narrowing of spinal canal rostral to last intact vertebra
     • hypoplastic iliac wings
2. Spinal cord anomalies
   • characteristic club- / wedge-shaped configuration of conus medullaris (= hypoplasia of distal spinal cord)
   • ± tethered spinal cord
   • ± dural sac stenosis with high termination
   • ± spinal cord lipoma, teratoma, cauda equina cyst
   • ± syrinx
3. Genitourinary anomalies
   • neurogenic bladder (if >2 segments are missing)
   • malformed external genitalia
   • ± bilateral renal aplasia with pulmonary hypoplasia and Potter facies
4. Hindgut anomalies
   • lack of bowel control
   • anal atresia

OB-US:

• normal / imperforate anus
• short CRL in 1^st trimester ← diabetic embryopathy
• normal / mildly dilated urinary system
• normal / increased amniotic fluid
• 2 umbilical arteries
• 2 hypoplastic nonfused lower extremities in a CHARACTERISTIC froglike position
• fusion of pelvic bones
• sacral agenesis, absent vertebrae from lower thoracic / upper lumbar spine caudally

N.B.: brain, proximal spine, and spinal cord are notably spared!