A sacral dimple is a small indentation or pit in the skin, located just above the crease of the buttocks near the base of the spine. This congenital feature is common, appearing in about 2% to 8% of all newborns. The vast majority of these dimples are isolated, harmless findings that represent a minor variation in development. They require no treatment or specialized follow-up because they are not connected to the underlying spinal cord. However, in rare instances, a sacral dimple can serve as a superficial marker for a deeper spinal abnormality. Understanding the difference between a simple and a complex dimple is the first step in knowing when to seek further medical evaluation.
Differentiating Simple and Complex Sacral Dimples
Physicians categorize sacral dimples based on specific physical characteristics to determine the need for further investigation. A “simple” sacral dimple is the most common and reassuring presentation. These dimples are usually shallow, small—often less than five millimeters in diameter—and are located directly in the midline, within the gluteal crease. The base of a simple dimple can be easily seen by the examining doctor.
A complex, or “atypical,” sacral dimple raises suspicion because its features suggest a potential connection to the deeper structures of the spinal canal. These dimples are often larger, measuring more than five millimeters across, or are noticeably deeper, making it difficult or impossible to see the bottom. A dimple situated significantly higher than the gluteal crease, or one that is not perfectly centered in the midline, is classified as atypical.
Specific Warning Signs That Require Medical Evaluation
The most important markers that signal a potential underlying spinal issue are the associated skin features nearby. These cutaneous stigmata suggest that the skin closure over the spine during development was incomplete. The presence of a tuft or patch of hair, medically known as hypertrichosis or a “Faun’s tail,” is a significant warning sign near the dimple.
Skin tags, fatty lumps (lipomas), or areas of unusual skin discoloration, such as pigmented lesions or hemangiomas (vascular birthmarks), also warrant immediate attention. These features indicate a disruption that may extend beneath the skin to the spinal cord. The presence of two or more of these skin markers, or an atypical dimple combined with any single marker, increases the risk of an occult spinal dysraphism.
Although rare, a dimple that is actively draining fluid or pus, or shows signs of infection like swelling and redness, requires urgent medical evaluation. The dimple may represent a dermal sinus tract, which is an abnormal channel connecting the skin surface to the spinal canal, posing a risk of severe infection, such as meningitis.
In older infants or toddlers, the focus shifts to neurological symptoms. Signs such as unexplained weakness or numbness in the legs, an asymmetrical appearance or movement of the legs, or difficulties with bladder or bowel control are indicators that require prompt investigation.
Diagnostic Procedures and Associated Spinal Conditions
If a complex sacral dimple or associated skin features are present, a physician will typically recommend imaging to screen for underlying spinal conditions. The standard initial screening tool for infants is a spinal ultrasound. This non-invasive procedure uses sound waves to visualize the lower end of the spinal cord and is highly effective because the infant’s vertebrae are still mostly cartilaginous, allowing the sound waves to pass through.
This ultrasound is most accurate when performed before the infant reaches about three to four months of age, before the bones begin to harden. If the ultrasound results are inconclusive, or if the child is older than six months, a Magnetic Resonance Imaging (MRI) scan may be necessary. An MRI provides more detailed, cross-sectional images of the neural structures but often requires sedation to ensure the child remains perfectly still.
The two main underlying concerns being screened for are occult spinal dysraphism, which includes conditions like tethered spinal cord and spina bifida occulta. Tethered spinal cord occurs when the lower end of the spinal cord is abnormally anchored to the surrounding tissues, restricting its natural movement. As the child grows, this tension can stretch the cord, potentially leading to neurological symptoms. Spina bifida occulta is a milder form of a neural tube defect where a small gap exists in one or more vertebrae, but the spinal cord and nerves remain in their proper place. While spina bifida occulta often causes no symptoms, a diagnosis allows for careful monitoring to catch any potential complications early.