Is a normal ultrasound sufficient to rule out spinal issues in a sacral dimple?

Sacral Dimples and Spinal Ultrasound: When to Stop Worrying

A normal ultrasound is sufficient to rule out significant spinal issues in a patient with a sacral dimple, and no further imaging or intervention is needed in this scenario.

Understanding Sacral Dimples

Sacral dimples are common cutaneous findings in infants, occurring in approximately 2-4% of newborns. It's crucial to distinguish between two types:

1. Innocent Coccygeal Dimples:

   • Located at or below an imaginary line between the tops of the gluteal cleft
   • Usually within the gluteal cleft, close to the coccyx
   • No associated skin abnormalities
   • Do not require imaging or further workup 1

2. Pathologic Lumbosacral Dermal Sinus Tracts (DSTs):

   • Located above the gluteal cleft on the flat part of the sacrum
   • May have associated cutaneous findings (hair tufts, vascular anomalies, skin tags)
   • Require surgical correction due to risk of infection and neurological complications 1

Diagnostic Algorithm for Sacral Dimples

Step 1: Clinical Assessment

• Location: Determine if the dimple is above or below the gluteal cleft line
• Associated findings: Check for hair tufts, skin tags, vascular anomalies, or subcutaneous masses
• Rule of thumb: If you can draw an imaginary line between the tops of the gluteal cleft, a dimple at or below this line is normal, while one above is abnormal 1

Step 2: Imaging Decision

• For innocent coccygeal dimples: No imaging is necessary 1
• For dimples above the gluteal cleft or with associated skin findings: Proceed with spinal ultrasound

Step 3: Ultrasound Results Interpretation

• Normal ultrasound: No further imaging or intervention needed
• Abnormal ultrasound: Consider MRI for further evaluation

Evidence Supporting Normal Ultrasound as Sufficient

The evidence strongly supports that a normal ultrasound effectively rules out significant spinal pathology in patients with sacral dimples:

1. In a study of 230 infants with sacral dimples who underwent ultrasound, all but one had benign imaging findings, and those with minor abnormalities showed normalization or insignificant change on follow-up 2.

2. A large retrospective review of 592 spinal ultrasounds found that only 4% of ultrasounds performed for sacral dimples were abnormal. Of these, only 14% had abnormal findings on follow-up MRI 3.

3. Another study of 439 newborns with cutaneous stigmata found that simple sacral dimples alone denoted a very low risk of occult spinal dysraphism 4.

When to Consider Further Imaging

MRI should be considered only in specific circumstances:

1. Abnormal ultrasound findings, particularly:

   • Low-lying conus medullaris
   • Decreased cord pulsatility
   • Thickened filum terminale
   • Presence of intrathecal mass or lipoma 4

2. High suspicion despite normal ultrasound when there are:

   • Associated markers of dysraphism (skin appendage, tuft of hair, lipoma)
   • Neurological symptoms 1

3. Deep dimples have a higher association with fibrolipoma of the terminal filum (34.6%) compared to shallow dimples (8.6%) 5

Common Pitfalls to Avoid

1. Overreliance on dimple depth: The old teaching that a dimple is innocent if its base can be visualized is incorrect. Location along the craniocaudal axis is more important than depth 1.

2. Unnecessary imaging: Isolated coccygeal dimples do not require imaging, avoiding unnecessary radiation, sedation risks, and healthcare costs 1.

3. Misinterpreting normal variants: Echogenic filum terminale and filar cysts are common incidental findings on ultrasound that rarely have clinical significance 2.

4. Failure to recognize true risk factors: The presence of multiple cutaneous markers or congenital anomalies significantly increases the risk of spinal dysraphism and warrants more thorough evaluation 5, 4.

In summary, a normal ultrasound in a patient with a sacral dimple is reassuring and sufficient to rule out significant spinal pathology. Further imaging with MRI should be reserved for cases with abnormal ultrasound findings or high clinical suspicion despite normal ultrasound.