Last Updated on March 16, 2025
Sacral agenesis is a congenital condition characterized by the absence of the variable portion of the caudal portion [lower end] of the spine. It is a very rare deformity. Sometimes, the lumbar spine is variably absent, called lumbosacral agenesis.
Sacral agenesis can occur as an isolated condition or as part of caudal regression syndrome in which part of the spine and spinal cord fail to develop. Caudal regression syndrome may range from absent coccyx as an isolated finding without neurological deficit, to sacral or lumbosacral agenesis.
Patients with this deformity lack motor function at the affected vertebral level and sensory functions below the affected level. Sacral agenesis is highly associated with maternal diabetes. About 15-20 percent of these patients have insulin-dependent diabetes.
There is a development failure of variable degrees. Associated anomalies such as anorectal, vertebral, urological, genital, and lower limb anomalies could be present.
Relevant Anatomy
The spine consists of vertebrae stacked together from the neck to the upper part of the gluteal cleft. The spine is divided into the following regions
- Cervical
- Thoracic
- Lumbar
- Sacal
- Coccygeal
The sacral region is in the lower part of the back below the lumbar. The sacrum is a triangular bony structure formed by a fusion of sacral vertebrae. It also connects to the pelvis by sacroiliac joints [joints between the sacrum bone and iliac bones of the pelvis]. The sacrum connects to the coccyx below.
Types
Renshaw classification divides the condition into four groups depending on the amount of sacrum remaining and the characteristics of the articulation between the spine and the pelvis
Type I – Partial or total unilateral sacral agenesis (least common)
Type II – Partial sacral agenesis with a bilaterally symmetrical defect, a normal or hypoplastic sacral vertebra, and a stable articulation between the ilia and the first sacral vertebra. (most common)
Type III – Variable lumbar and total sacral agenesis, with the ilia articulating with the sides of the lowest vertebra present.
Type IV – Variable lumbar and total sacral agenesis, with the caudal endplate of the lowest vertebra resting above either fused ilia or an iliac amphiarthrosis.
Types I and II usually have stable vertebral-pelvic articulation. Type III and IV lead to instability and possibly progressive kyphosis.
Causes of Sacral Agenesis
The exact cause is not known. Both environmental and genetic factors have been thought to play a role and multifactorial causation has been suggested. The condition has been found to have the following associations
- Maternal Diabetes: Sacral agenesis and caudal regression syndrome is seen in greater frequency in women with diabetes when compared to the general population. Maternal diabetes is the risk factor.
- Exposure to Organic Solvents: Exposure to organic solvents in early pregnancy may increase the incidence.
- Genetic predisposition: Some children have a genetic predisposition to developing the disorder (A person who is genetically predisposed carries the gene for the disease and may require the environmental triggers to express it)
- Environmental Factors: The exact environmental factors are not known. Alcohol, retinoic acid, lack of oxygen (hypoxia), and amino acid imbalances have been suggested to be potential factors
Associated Anomalies
- Congenital hip or knee dislocation
- Coxa vara
- Foot deformities like talipes equinovarus and calcaneovalgus
- Absent or atrophic extremities
- Myelomeningocele
- Vertebral anomalies – Absent, dysplastic or fused vertebra
- Anorectal malformations
- Hernia
- Duodenal atresia
- Vesico-ureteral reflux
- Congenital hydronephrosis
- Hypospadias
- Cryptorchidism (failure of testes to descend)
- Renal agenesis (fatal if bilateral)
- Tracheo-oesophageal fistula
- Hydrocephalus
- Strabismus
- Congenital heart disease
Clinical Presentation
The symptoms and severity can vary from one person to another. The spectrum ranges from mild or no symptoms to cases with severe, disabling or potentially life-threatening complications. I
Patients with partial sacral or coccygeal agenesis may have no symptoms.
The following signs and symptoms are generally associated with sacral agenesis
- Narrowing of the hips
- Flattening of buttocks with underdeveloped muscles
- Skin indentation on back ( sacral dimple)
- Abnormalities of vertebrae
- Neural deficits
- Bladder and bowel control affected
- increased urinary frequency
- Neurogenic bladder (bladder does not empty completely)
- Flexion contractures of the knee and hip (due to asymmetry of power of flexor and extensor groups)
- Reduced muscle mass in lower limbs
- Clubfeet
- Walking difficulties
- Other anomalies as listed in the associated anomalies section
The posture of the lower extremities has been compared with Sitting Buddha (see image below).
Examination of the back reveals a bone prominence representing the last vertebral segment and may reveal gross motion between this vertebral prominence and the pelvis. Flexion and extension may be found to occur at the junction of the spine and pelvis rather than at the hips.
Neurological examination usually reveals intact motor power down to the level of the lowest vertebral body that has pedicles. Sensation, however, is present down to more distal levels. Bladder and bowel control often is impaired.
Often there is no correlation between bony abnormality and neurological deficit. The motor deficit is more pronounced as compared to the sensory.
Differential Diagnoses
- Sirenomelia: Also called mermaid syndrome. sirenomelia is a rare congenital disorder that leads to abnormalities of the lower spine and lower limbs. There is partial or complete fusion of the legs. In addition, there could be abnormalities of the genitourinary system, gastrointestinal system, lumbar spine, sacrum, and pelvis.
The affected infants may not have one foot or no feet. The coccyx is usually absent and the sacrum is partially absent as well. Associated abnormalities like anal atresia, spina bifida, and heart malformations may be present. The cause is not known. - VACTERL: It is an acronym that is short for multiple abnormal associations that affect multiple organ systems. The term VACTERL stands for – Vertebral abnormalities, Anal atresia, Cardiac defects, Tracheal anomalies, Esophageal atresia, Renal (kidney) and Radial abnormalities, and Limb abnormalities. Failure to thrive is seen in affected children. The cause is not known.
- Currarino Syndrome: There is incomplete development or absence of the sacrum. A mass in the sacral region is present. There could be malformation of anus and/or rectum.
Abdominal distention, gastrointestinal obstruction, and renal abnormalities are seen. Urinary incontinence and recurrent urinary tract infections are seen. The disorder is inherited in an autosomal dominant pattern.
Antenatal Diagnosis of Sacral Agenesis
This can be done in first trimester by noting the short crown–rump length on ultrasound. Ultrasound later in the second or third trimester ay demonstrate the absence of sacrum and shortened femurs. Other things that may suggest are
- Flexion and abduction of hips
- Clubfeet
- Urinary abnormalities – Renal agenesis, cystic dysplasia
- Gastro-intestinal anomalies such as duodenal atresia [5].
Treatment of Sacral Agenesis
Type I and II patients [partial or complete absence of the sacrum] have an excellent chance of becoming community ambulators. Foot and knee deformities in these patients should be corrected.
Management of types III and IV type deformities is more controversial.
Nonprogressive kyphosis or scoliosis do not require any treatment whereas progressive scoliosis or kyphosis requires operative stabilization.
The treatment is directed toward the specific symptoms and thus needs to be individualized and require a multispecialty approach.
The outcome varies with the severity of the disease. Multiple surgical procedures may be required to address the associated abnormalities.
Early intervention is important to achieve the best possible outcome.
Type I and II patients would require physical therapy. Foot and knee deformities need to be addressed.
Type 3 and 4 may require spinal surgeries for stabilization followed by gait training.
Nonfunctional limbs may need to be amputated.
If the progressive deformity is present, lumbopelvic arthrodesis [Fusion of the lumbar spine with pelvis] is recommended.
Reconstruction of the lower limb in type IV sacral agenesis has not been successful, because of the absence of muscle fibers and major motor nerves. Bilateral subtrochanteric amputation followed by fitting a pelvic-thoracic bucket and a hip disarticulation prosthesis can provide partial ambulation.
Soft-tissue releases, supracondylar femoral extension osteotomies, and serial casting or amputation may be chosen for limb deformities.
Image Sources
- Classification images adapted from Renshaw TS: Sacral agenesis: classification in the review of 23 cases, J Bone Joint Surg 60A:373, 1978
- Clinical photograph from of Sacral Agenesis Phillips WA, Cooperman DR, Lindquist TC, et al: Orthopaedic management of lumbosacral agenesis: long-term follow-up, J Bone Joint Surg 64A:1282, 1982.
References
- Andrish J, Kalamchi A, MacEwen GD. Sacral agenesis: a clinical evaluation of its management, heredity, and associated anomalies. Clinical Orthopaedics and Related Research. 1979 Mar-Apr(139):52-57. [Link]
- 5. Caudal agenesis: understanding the base of the wide clinical spectrum. Lee JY, Shim Y, Wang KC. J Korean Neurosurg Soc. 2021;64:380–385. [Link]
