What Are Sacral Dimples?
Sacral dimples are midline skin depressions in the lower back region that can be either benign anatomical variants (innocent coccygeal dimples) or markers of underlying spinal dysraphism (pathologic lumbosacral dermal sinus tracts), with the critical distinguishing feature being their location along the craniocaudal axis rather than whether you can see the bottom of the dimple. 1
Two Distinct Types of Sacral Dimples
Innocent Coccygeal Dimples (Benign)
- Location is key: Found at or below an imaginary line drawn between the tops of the two forks of the gluteal cleft 1
- Typically lie within 1 cm of the coccyx, within the gluteal cleft itself
- Usually invisible unless buttocks are parted
- A finger placed over the dimple can be rolled over the tip of the underlying coccyx 1
- Found in approximately 4% of the population 1
- No imaging or further workup required 1
- Not connected to the spinal cord; tract extends only to the coccygeal tip, well below the thecal sac 1
Pathologic Lumbosacral Dermal Sinus Tracts (DSTs)
- Location: Above the gluteal cleft line, on the flat portion of the sacrum 1
- Occur with frequency of approximately 1 in 2,500 live births 1
- Most commonly occur at S2 level 1
- Contain an underlying tract of epithelial and fibrous tissue that pierces the skin 1
- May be associated with surrounding cutaneous markers: vascular anomalies, hair tufts, skin tags, or subcutaneous masses 1
- Always abnormal and require surgical correction 1
Critical Clinical Pitfall
The traditional teaching that a dimple is innocent if you can see its base is incorrect. 1 The presence or absence of a visible "bottom" has little to do with pathologic nature. Location along the craniocaudal axis is the most important distinguishing feature.
Clinical Consequences of Pathologic DSTs
When left undiagnosed, pathologic DSTs can present in five ways 1:
- Visible cutaneous tract
- CNS infection (meningitis or intraspinal abscess)
- Aseptic meningitis from desquamation of epithelial cells
- Spinal cord compression from dermoid/epidermoid cyst growth
- Neurologic deterioration from tethering
Infection is the most feared complication because it creates intradural scarring that makes surgical excision significantly more difficult and increases risk of additional neurologic deficits 1.
Use as Surface Landmarks for Lumbar Puncture
While the provided evidence discusses sacral dimples primarily in the context of spinal dysraphism screening, sacral dimples themselves are not reliable landmarks for lumbar puncture. The evidence shows that:
- The iliac crests (intercristal line) traditionally mark the L4-L5 interspace 2
- However, the intercristal line technique is inaccurate, correctly identifying L4-L5 only 8% of the time in pregnant patients and producing estimations above L3 in 31% of cases 3
- The sacral anatomical interspace landmark (SAIL) technique is more accurate (49% correct identification) and produces estimations above L3 in only 1% of cases 3
- Ultrasound guidance is recommended for difficult cases with spinal deformities or prior lumbar surgery 4
The key takeaway: sacral dimples are anatomical findings requiring evaluation for spinal dysraphism based on location, not procedural landmarks for lumbar puncture.