Sacral Agenesis (SA) is a rare birth defect characterized by the incomplete development or total absence of the lower spine, specifically the sacrum and often parts of the lumbar vertebrae. This condition is part of a broader group of congenital disorders known as caudal regression syndromes (CRS). SA prevalence is estimated to be between 1 in 60,000 to 1 in 100,000 live births. It results from a failure in the normal development of the caudal portion of the embryo during the first few weeks of gestation.
Anatomy and Physical Effects
Sacral agenesis primarily involves the partial or complete absence of the five fused vertebrae that make up the sacrum, which forms the back wall of the pelvis. In more severe cases, the defect may extend upward to include the coccyx and several lower lumbar vertebrae. This skeletal discontinuity directly impacts the attachment of the pelvis and the stability of the entire lower trunk.
The physical manifestations often include a noticeable flattening or dimpling of the buttocks due to missing bone and muscle bulk. Neurological deficits are a common consequence because the lower spinal cord and the nerves that control the lower body are malformed or abruptly terminated at the level of the skeletal defect. These nerve impairments frequently lead to reduced or absent sensation and motor function in the lower extremities.
Lower limb deformities are common, sometimes presenting as a characteristic “frog-leg” posture, clubfeet, or flexed and webbed knees. The extent of the motor deficit generally corresponds to the highest level of the missing vertebra. The neurological disruption also affects the function of organs innervated by the sacral nerves, most significantly the bladder and bowel.
Etiology and Associated Risk Factors
Most cases of sacral agenesis occur sporadically without a clear genetic link. The strongest and most consistently identified risk factor is poorly controlled maternal diabetes mellitus, particularly Type 1 or Type 2, during the first trimester of pregnancy. Infants born to mothers with diabetes have a significantly increased risk of developing SA, estimated to be 200 to 300 times higher than the general population.
The prevailing hypothesis suggests that high blood sugar levels (hyperglycemia) in the mother during the critical period of embryonic development interfere with the formation of the caudal mesoderm. This disruption of early cellular development prevents the proper formation of the lower spine. While maternal diabetes is a significant risk factor, it is not the cause in the majority of cases.
Other factors, though less common, include certain genetic mutations, such as those in the HLXB9 gene, which are associated with familial forms of the condition. Environmental factors, such as exposure to toxins or vascular disruptions affecting the blood supply to the developing caudal region, are also suspected to play a role.
Categorizing the Severity of Agenesis
Medical professionals use classification systems to categorize the severity of sacral agenesis, which helps guide treatment and predict functional outcomes. The most widely recognized system is the Renshaw classification, which grades the defect based on the extent of the vertebral agenesis and the stability of the pelvic articulation. This classification ranges from Type I, the mildest form, to Type IV, the most severe.
Type I involves a partial or total unilateral absence of the sacrum. Type II is characterized by a partial but symmetrical sacral defect with a stable articulation between the remaining sacral segment and the iliac bones. Individuals classified as Type I or II generally have a more favorable prognosis, often retaining the ability to walk and having less severe neurological deficits.
More severe forms include Type III, which involves a total absence of the sacrum, with the ilia articulating with the sides of the lowest remaining lumbar vertebra. Type IV is the most severe, featuring a total sacral absence where the lowest remaining vertebra rests above either fused ilia or an iliac amphiarthrosis, leading to significant spinopelvic instability. The severity of the skeletal defect in Types III and IV correlates with a higher likelihood of severe motor impairment and requires intensive orthopedic management.
Treatment Approaches and Long-Term Outlook
The management of sacral agenesis requires a coordinated, multidisciplinary approach involving orthopedists, urologists, neurologists, and physical therapists. Orthopedic interventions focus on stabilizing the spine, correcting lower limb deformities, and maximizing mobility. For severe cases with spinopelvic instability (Renshaw Type IV), surgical fusion may be necessary to facilitate sitting balance and improve functional outcomes.
Management of neurogenic bladder and bowel dysfunction affects the majority of individuals with SA. This dysfunction leads to urinary incontinence, frequent urinary tract infections, and chronic constipation. Urological care often involves clean intermittent catheterization and medications to maintain kidney health and achieve continence.
Physical therapy and the use of assistive devices, such as braces, walkers, or wheelchairs, are fundamental for maximizing independence. While the condition is lifelong, the long-term outlook depends on the severity of the spinal defect and the effectiveness of urological management. With comprehensive medical care, individuals with sacral agenesis can achieve significant independence and a good quality of life, though many may still require mobility aids.