What features of a sacral dimple (sacral pit or spinal dimple) in a baby warrant further investigation?

Features of Sacral Dimples in Babies That Warrant Further Investigation

The most important feature of a sacral dimple requiring further investigation is its location - dimples located above an imaginary line connecting the tops of the gluteal cleft are abnormal and require imaging and surgical evaluation, while dimples at or below this line are typically benign and require no further workup. 1

Location Assessment

The location of the sacral dimple is the single most critical factor in determining the need for further investigation:

• High-risk location (requires investigation):

  • Dimple located on the flat part of the sacrum, cranial to the gluteal cleft 1
  • Dimple located above an imaginary line drawn between the tops of the two forks of the gluteal cleft 1
  • Dimple located well above the gluteal crease (Type 3 dimple) 2
  • Dimple at the upper edge of the gluteal crease with associated curving or deformity of that crease (Type 2 dimple) 2

• Low-risk location (typically benign):

  • Dimple located within the gluteal cleft, within about 1 cm of the coccyx 1
  • Dimple at or below an imaginary line connecting the tops of the gluteal cleft 1
  • Dimple within the gluteal crease (Type 1 dimple) 2

Associated Features That Warrant Investigation

The presence of any of these additional features with a sacral dimple significantly increases the risk of underlying spinal dysraphism:

1. Cutaneous markers surrounding the dimple:

   • Vascular anomalies (hemangiomas, capillary malformations) 1
   • Tufts of hair (hypertrichosis) 1
   • Skin tags 1
   • Subcutaneous masses 1

2. Multiple cutaneous stigmata - The presence of more than one skin marker significantly increases risk 3

3. Dermal sinus tract - A visible tract or opening within the dimple has the highest correlation with spinal cord lesions 3

High-Risk Cutaneous Anomalies

When evaluating a sacral dimple, be alert for these high-risk cutaneous anomalies that warrant immediate investigation:

• Hypertrichosis (focal tuft of hair in midline) 1
• Infantile hemangioma (raised vascular lesion with well-defined borders) 1
• Subcutaneous lipoma 1
• Caudal appendage 1
• Deviated or forked gluteal cleft 1

Common Misconceptions

• Misconception: A dimple is innocent if its base can be visualized and abnormal if the bottom cannot be seen
• Reality: The visibility of the bottom of the dimple has little correlation with its pathologic nature; location is far more important 1

Diagnostic Algorithm

1. Assess dimple location:

   • Above gluteal cleft line → Further investigation needed
   • At or below gluteal cleft line → Likely benign

2. Examine for associated features:

   • Presence of any high-risk cutaneous anomalies → Further investigation needed
   • Multiple cutaneous stigmata → Further investigation needed

3. For dimples requiring investigation:

   • Initial screening with spinal ultrasonography (for infants <3-6 months) 3, 4
   • Abnormal ultrasound findings requiring MRI include:
     • Low-lying conus medullaris (below L2-L3 disc space)
     • Decreased conus or nerve root motion
     • Abnormal filum terminale (especially if >2mm thick)
     • Intraspinal mass
     • Osseous dysraphism
     • Sinus tract leading to thecal sac 5

Clinical Implications

Pathologic dimples (dermal sinus tracts) can present clinically in concerning ways:

• CNS infection (meningitis, intraspinal abscess)
• Aseptic meningitis from desquamation of epithelial cells
• Spinal cord compression from dermoid/epidermoid cyst growth
• Neurologic deterioration from tethering 1, 6

Infection is the most feared complication as it can be highly morbid and creates intradural scarring that complicates surgical correction 1.

Key Points for Practitioners

• The traditional teaching that visibility of the base of the dimple determines risk is incorrect
• Location is the most important feature in risk assessment
• Simple isolated coccygeal dimples have an extremely low risk of significant spinal malformations (0.13%) 5
• Type 2 dimples (at upper edge of gluteal crease with deformity) have a 45% rate of spinal malformations and require investigation 2
• Type 3 dimples (well above gluteal crease) have a 55% rate of spinal malformations and require investigation 2