A Dimple Isn’t Always Simple

Definitions! 

To understand sacral dimples, we first need to review some definitions related to neural tube defects (NTD): 

  • Spina bifida = imprecise term that describes multiple congenital spinal anomalies
  • Spina bifida occulta = radiographic finding that describes the incomplete osseous fusion of the vertebrae’s posterior elements
  • Open NTD (aka open spinal dysraphism) = exposed brain, spinal cord, or spinal nerves; covered by a membrane or nothing at all
    • ex: meningomyelocele, meningocele, encephalocele, anencephaly
  • Closed NTD [aka occult spinal dysraphism (OSD) or closed spinal dysraphism] = skin-covered lesion in which underlying nervous system structure is malformed
    • ex: split cord malformation, tethered spinal cord, dermal sinus tract, spinal cord lipoma (aka lipomyelomeningocele)

Sacral dimples!

Source: Zywicke HA, Rozzelle CJ. Sacral dimples. Pediatr Rev. 2011 Mar;32(3):109-13

Sacral dimples are very common—they’re present in 2-4% of newborns overall! 

Almost all neurosurgical referrals for suspected OSD in children <1yo are for evaluation of a dimple. More than 50% of OSDs are diagnosed when a dimple is noted, but obviously not all dimples are associated with an OSD. In fact, the risk of significant spinal malformations in asymptomatic, healthy infants with an isolated simple sacral dimple is exceedingly low.

Benign features:

  • single midline dimple in sacrococcygeal region
  • visible intact base
  • <2.5 cm above anal verge
  • <0.5 cm in diameter

Concerning features:

  • multiple dimples (look along the entire spine!)
  • not in sacrococcygeal region
  • base not visible or not intact
  • >2.5 cm above anal verge
  • >0.5 cm in diameter
  • overlying skin findings (e.g., tuft of hair, hemangioma, caudal appendage)
  • gluteal cleft anomalies (e.g., deviated, split/duplicated)
  • drainage
  • abnormal neurological exam
  • orthopedic findings (e.g., scoliosis, clubfeet, joint contracture, hip dislocation)

Work-up!

The presence of concerning features leads to imaging—either US or MRI. 

US is generally only useful in infants <3-6mo because ossification of the vertebral arches has not yet occurred. If US is abnormal, then MRI and neurosurgery referral are indicated. 

Prior guidance suggested that MRI be performed in the presence of overlying cutaneous findings. More recent evidence suggests that US is generally a reasonable first step; however, MRI is preferred in the presence of >2 cutaneous markers, dermal sinus tract, appendage, or abnormal neurological/orthopedic exams. 


Take-Home-Points!

  • Sacral dimples are relatively common in infants and are usually benign, but may be indicative of underlying neural tube defect
  • Dimples in the presence of concerning findings (see above) require imaging and possibly a referral to neurosurgery
  • It’s often reasonable to start with US in infants <3mo who have an isolated atypical dimple. 

Blog post based on Med-Peds Forum talk by Cameron Ulmer, PGY1