Sacrococcygeal dimples (also called simple sacral dimples or sacrococcygeal pits) are common findings in newborns, with a prevalence of approximately 2 to 5%. They are not associated with any increased risk of occult spinal dysraphism (e.g., low lying conus, fatty filum, lipomyelomeningocele, split cord malformation, dermal sinus tract, etc.) compared with the general population of infants without sacrococcygeal dimples. There is therefore no need to investigate infants with this finding, with either ultrasound or MRI. Red flags for which investigating would be indicated include the presence of midline tuft of hair, sacral dimple or sinus tract above the gluteal cleft, hemangioma, dermal appendage, and/or a subcutaneous lump. The ideal choice for initial investigation (ultrasound or MRI) would depend on the specific cutaneous findings and clinical symptoms present.
Sources:
Albert GW. Spine ultrasounds should not be routinely performed for patients with simple sacral dimples. Acta Paediatr. 2016 Aug;105(8):890-4. PMID: 27059606.
Kucera JN, et al. The simple sacral dimple: diagnostic yield of ultrasound in neonates. Pediatr Radiol. 2015 Feb;45(2):211-6. PMID: 24996813.
Zywicke HA, et al. Sacral dimples. Pediatr Rev. 2011 Mar;32(3):109-13; quiz 114, 151. PMID: 21364014.