What is the incidence of sacral anomalies in low-type (anorectal malformation)?

Incidence of Sacral Anomalies in Low Type Anorectal Malformation

The incidence of sacral anomalies in low-type anorectal malformations is approximately 11%, which is significantly lower than the 43% incidence seen in complex anorectal malformations. 1

Understanding Sacral Anomalies in Anorectal Malformations

• Anorectal malformations (ARMs) result from abnormal development during early embryogenesis involving the caudal spinal cord, anorectal, and urogenital systems due to their intimate temporospatial relationships 2
• ARMs are classified as low, intermediate, or high based on their relationship to the levator ani muscle 2
• Low-type ARMs comprise about half of all anorectal anomalies and are traditionally considered to have better outcomes than high anomalies 3

Incidence and Types of Sacral Anomalies

• Between 10% and 52% of children with all types of anorectal malformations have associated dysraphic spinal malformations 2
• Simple anorectal malformations (including most low-type ARMs) have an 11% association with sacral anomalies, compared to 43% in complex malformations 1
• Sacral anomalies commonly seen in ARM patients include:
  • Sacral agenesis (partial or complete absence of sacral vertebrae) 1
  • Hemisacral agenesis with "scimitar sacrum" (as seen in Currarino triad) 1
  • Tethered spinal cord 4
  • Filum terminale lipoma (present in 30% of ARM patients with spinal cord anomalies) 4
  • Low conus medullaris (present in 10% of ARM patients with spinal cord anomalies) 4

Clinical Significance and Evaluation

• Sacral ratio (SR) is a well-established tool to quantify sacral development in patients with ARM and can predict fecal continence 5
• Lower sacral ratios correlate significantly with the presence of urologic and renal malformations, including hydronephrosis, high-grade vesicoureteral reflux, and solitary kidney 5
• This association between sacral anomalies and urologic abnormalities is independent of the presence of tethered cord 5
• Secondary urinary incontinence, especially when combined with fecal incontinence and/or constipation, may indicate tethered cord syndrome 1

Specific Associations: Currarino Triad

• Currarino triad is a rare but important association that includes:
  • Anorectal malformation (often low-type)
  • Sacral bony defect (usually hemisacral agenesis with "scimitar sacrum")
  • Presacral mass (commonly teratoma or anterior sacral meningocele) 1, 6
• In one series, 38% of patients with anorectal stenosis (a low-type ARM) had Currarino triad 6
• Tethered cord was found in 18% of patients with Currarino triad 6

Management Implications

• MRI is the preferred diagnostic method for evaluating sacral anomalies in ARM patients due to the high incidence of associated tethered cord 6
• Urodynamic evaluation before surgical correction provides information about sacral spinal cord involvement and serves as a baseline for postoperative comparison 1
• A sacral ratio assessment should be part of the VACTERL screening to identify patients who need closer urologic follow-up 5
• Despite the presence of spinal cord anomalies, long-term functional outcomes for patients with low-type ARMs may not differ significantly from patients with normal spinal cords if there are no abnormal neurological findings in the lower limbs 4

Common Pitfalls and Caveats

• Not all sacral dimples require imaging; coccygeal dimples (below the gluteal cleft) are normal variants and not associated with spinal dysraphism 1
• In contrast, lumbosacral dermal sinus tracts (located cranial to the gluteal cleft) are abnormal and require surgical correction 1
• Maternal gestational diabetes is a well-described risk factor for sacral agenesis 1
• Even "low" ARMs can have significant histological wall abnormalities that may contribute to functional problems, with constipation reported in 42%-70% of cases 7