A sacral dimple is a small indentation or pit in the skin, typically located on the lower back just above the crease of the buttocks, over the sacrum bone. This common congenital finding is present at birth in approximately 2% to 4% of newborns. For most people, a sacral dimple is an isolated, harmless feature that requires no special treatment. While they can occasionally be associated with an underlying spinal condition, these cases are rare. A simple physical examination by a healthcare provider can usually confirm the dimple is benign. This article explores the biological reason for their appearance and explains the characteristics that determine when a dimple needs further medical attention.
The Developmental Origins
The presence of a sacral dimple is explained by a minor variation in the normal process of embryonic development. During the first few weeks of gestation, the primitive tissues that form the spine and spinal cord must close completely, a process known as neurulation. The sacral dimple arises from a slight, localized incomplete fusion of the skin layers during the formation of the caudal spine.
This developmental variation means the dimple is a congenital anomaly. The indentation represents a surface-level marker that occurs when the skin does not completely seal over the underlying structures. In the vast majority of instances, this is a minor, isolated event that does not involve the spinal cord or nerves, making the dimple a simple, benign feature.
Classifying Typical and Atypical Dimples
Sacral dimples are broadly categorized as either typical (simple) or atypical (complex), based on their physical characteristics. A typical dimple is considered a normal variant: it is small (less than five millimeters in diameter), shallow enough that its bottom is easily visible, and centrally located in the midline, usually within the gluteal crease.
An atypical dimple possesses features that suggest a potential connection to deeper underlying spinal structures, known as occult spinal dysraphism. These dimples are usually larger than five millimeters, deep enough that the base cannot be seen, or located high up on the back, more than 2.5 centimeters from the anus. A dimple positioned above the imaginary line connecting the tops of the gluteal folds is considered atypical and warrants closer inspection.
Red Flags and Signs That Warrant a Doctor’s Visit
While a simple sacral dimple is harmless, the presence of certain associated skin markers alongside the indentation may indicate an underlying spinal cord issue. These “red flags” are physical signs that necessitate a medical evaluation, though their presence is rare. Any dimple that shows drainage or appears to have an open sinus is particularly concerning because it suggests a direct pathway to the spinal canal.
Associated Skin Markers
Other concerning features include:
- A tuft of hair (hypertrichosis) appearing near or emerging from the dimple.
- A skin tag.
- A fatty lump called a lipoma.
- A vascular lesion like a hemangioma in the surrounding area.
- Discoloration of the skin, such as a pigmented patch or a birthmark.
Medical Evaluation and Follow-up Procedures
If a sacral dimple is classified as atypical or is accompanied by any concerning skin markers, a healthcare provider will recommend imaging to check for occult spinal dysraphism.
Imaging Procedures
For infants younger than about six months, a spinal ultrasound is the preferred initial screening method because the tissues have not yet fully hardened. This non-invasive test uses sound waves to visualize the lower end of the spinal cord and check for abnormalities like a tethered cord. A tethered spinal cord occurs when tissue attaches the cord abnormally to the spinal canal, limiting its movement and causing tension as the child grows. If the ultrasound results are inconclusive, or for older children and adults, a Magnetic Resonance Imaging (MRI) scan may be ordered for a more detailed view of the spinal canal. Early detection of an underlying issue allows for timely intervention, such as neurosurgical consultation, which can prevent potential neurological or urological problems later in life.