A sacral dimple is a small, shallow indentation in the skin of an infant’s lower back, present from birth. This common congenital feature appears in approximately 2% to 4% of newborns and is usually a harmless anomaly that requires no treatment. While most sacral dimples are simple variations in development, recognizing specific physical characteristics is important. This article explores the role of genetics in sacral dimples and outlines the signs that may indicate a need for further medical evaluation.
Defining the Sacral Dimple
A sacral dimple is located in the midline of the body, just above the crease of the buttocks, overlying the sacrum (the triangular bone at the base of the spine). A typical, or simple, sacral dimple is small (less than 5 millimeters in diameter) and positioned near the anal verge, usually within 2.5 centimeters. Simple dimples are shallow, and the base of the indentation is clearly visible, often appearing as a blind-ending pit. They are considered a normal variant of development with no other symptoms.
Genetics and Fetal Development
The formation of a simple sacral dimple is linked to a minor variation during early fetal development concerning the neural tube and surrounding skin layers. The neural tube, which forms the brain and spinal cord, typically closes completely around 28 days post-conception. A sacral dimple results from an incomplete or minor separation of the skin from underlying structures as the lower part of the neural tube closes, leaving a small depression in the sacral region.
The role of inheritance is not fully understood, but simple dimples often run in families. While some reports suggest a dominant pattern, the specific genetic mechanism for an isolated dimple is not clearly established. Genetic predisposition likely involves a complex, multifactorial, or polygenic pattern, meaning multiple genes and environmental factors contribute to the trait. The simple sacral dimple represents a minor developmental variation rather than a sign of a specific inherited disease.
Identifying Complex Sacral Dimples
While most dimples are benign, certain features suggest an underlying connection to the spinal canal, necessitating medical attention. These are called complex or atypical sacral dimples and are associated with a higher risk of occult spinal dysraphism (OSD). A dimple is considered atypical if it is large (greater than 5 millimeters in diameter) or deep, with an invisible base. Location is also a concern if the dimple is situated far above the gluteal crease, specifically more than 2.5 centimeters from the anus.
The most concerning signs are other skin markers appearing near the dimple. These associated features can include:
- A tuft of hair (hypertrichosis).
- A small skin tag.
- A fatty lump (lipoma).
- Discoloration, such as a hemangioma or birthmark.
These external markers can indicate internal conditions like tethered cord syndrome (where the spinal cord is restricted) or spina bifida occulta (a mild neural tube defect with a gap in the spinal bones). These underlying conditions can affect nerve function and may require monitoring or intervention.
Medical Screening and Monitoring
When a complex sacral dimple or one with associated cutaneous markers is identified, medical screening is recommended to visualize the end of the spinal cord. For infants under four to six months of age, spinal ultrasound is the preferred initial imaging tool. This is because bone structures have not fully hardened, allowing sound waves to provide clear images of the spinal cord and determine the position of the conus medullaris (the tapered end of the spinal cord).
If ultrasound results are inconclusive, or if the child is older than six months and the vertebral arches have ossified, Magnetic Resonance Imaging (MRI) is used for a more detailed image. For a simple, isolated dimple that is small and located within the gluteal crease, no imaging is typically needed, and the prognosis is excellent. Early diagnosis of complex dimples is important for appropriate monitoring or intervention to prevent potential long-term neurological complications.