Spina Bifida Occulta at S1 Level: X-ray Radiographic Features and Clinical Implications

Spina bifida occulta represents one of the mildest forms of spinal dysraphism, characterized by incomplete fusion of the posterior vertebral elements without protrusion of the spinal cord or meninges. The radiographic image demonstrates a classic presentation of spina bifida occulta affecting the first sacral vertebra (S1), highlighted by the red circle. This congenital anomaly results from a failure in the normal embryological development of the neural tube, specifically inadequate formation of the vertebral arch components. Unlike more severe forms of spina bifida (such as myelomeningocele or meningocele), the occulta variant is typically covered by skin and often remains asymptomatic throughout life. The prevalence of spina bifida occulta at the S1 level is relatively common, affecting approximately 10-20% of the general population, making it one of the most frequently encountered vertebral anomalies in clinical practice. Understanding its radiographic features, clinical significance, and potential associations is essential for healthcare professionals interpreting spinal imaging studies.

Labeled Structure in the Image

Unfused posterior elements of S1 (red circle): The red circle highlights the incomplete fusion of the posterior elements of the first sacral vertebra (S1). This defect is visible as a midline gap where the laminae and spinous process failed to unite during embryological development. The posterior neural arch normally forms when two halves grow from the vertebral body and fuse in the midline, a process that failed to complete in this case, resulting in the characteristic appearance of spina bifida occulta.

Understanding Spina Bifida Occulta: Embryology and Classification

Developmental Origins

Spina bifida occulta develops during the critical period of embryonic neurulation and vertebral formation. The intricate processes that lead to this condition involve specific developmental timeframes and cellular mechanisms.

• Primary neurulation occurs between the 3rd and 4th weeks of embryonic development, during which the neural plate folds to form the neural tube.
• Concurrently, the somites differentiate into sclerotomes that migrate to surround the neural tube and form the vertebral bodies and arches.

The posterior elements of each vertebra normally develop from paired chondrification centers that extend dorsally and eventually fuse in the midline. In spina bifida occulta, this fusion fails to occur, leaving a gap in the neural arch.

• The sacral vertebrae are among the last to complete the fusion process, which partially explains the higher prevalence of spina bifida occulta at these levels.
• Genetic factors, folate deficiency, and environmental influences may all contribute to disruption of these precisely timed developmental events.

Classification of Spinal Dysraphisms

Spinal dysraphisms encompass a spectrum of congenital anomalies affecting the spine and spinal cord. Understanding where spina bifida occulta fits within this spectrum helps practitioners appreciate its relative clinical significance.

• Closed dysraphisms, including spina bifida occulta, have intact skin covering and typically involve minimal or no neural tissue abnormalities.
• Open dysraphisms, such as myelomeningocele, involve exposure of neural tissue and are associated with more significant neurological impairment.

Spina bifida occulta specifically refers to the bony defect without herniation of neural elements. This distinguishes it from more complex dysraphic states that may require surgical intervention.

• The defect may affect a single level (as in this case at S1) or multiple vertebral levels.
• When limited to the S1 level, it often represents a normal variant rather than a true pathological condition.

Radiographic Features and Diagnostic Imaging

X-ray Characteristics

Plain radiography remains the most common initial imaging modality for identifying spina bifida occulta. The anteroposterior view of the lumbosacral spine shown in this image demonstrates typical radiographic features.

• The defect appears as a V-shaped or U-shaped gap in the posterior arch of the affected vertebra (S1 in this case).
• The edges of the unfused laminae typically show smooth cortication, reflecting their developmental rather than traumatic origin.

Additional radiographic findings may sometimes accompany spina bifida occulta, though they are not apparent in this particular image. These associated features may provide clues to underlying complexity.

• Widening of the interpedicular distance may suggest associated dural ectasia or other intraspinal abnormalities.
• Transitional vertebrae or other segmentation anomalies may coexist, particularly at the lumbosacral junction.

Advanced Imaging Considerations

While plain radiography is usually sufficient for identifying the bony defect of spina bifida occulta, advanced imaging may be warranted in certain clinical scenarios. These modalities provide additional information about potential soft tissue or neural components.

• Computed tomography (CT) offers superior bone detail and can better delineate complex vertebral anomalies.
• Magnetic resonance imaging (MRI) is essential when evaluating for associated tethered cord, intraspinal lipoma, or other neural axis abnormalities.

The decision to pursue advanced imaging depends on clinical presentation and the presence of cutaneous markers or neurological symptoms. Asymptomatic spina bifida occulta at S1, as likely represented in this image, rarely requires further imaging evaluation.

• MRI is recommended for patients with cutaneous stigmata (dimples, hair tufts, or hemangiomas) over the affected area.
• Neurological symptoms such as foot deformities, bladder dysfunction, or progressive scoliosis also warrant advanced imaging.

Clinical Significance and Management

Symptomatology and Natural History

Spina bifida occulta at the S1 level, as depicted in this radiograph, is typically asymptomatic and discovered incidentally during imaging performed for unrelated reasons. Its clinical significance must be interpreted in context.

• The vast majority of individuals with isolated S1 spina bifida occulta remain asymptomatic throughout their lives.
• No specific limitations or activity restrictions are necessary for asymptomatic individuals.

When symptoms do occur, they are typically related to associated abnormalities rather than the bony defect itself. Understanding these potential associations helps guide appropriate management.

• Back pain is rarely attributable directly to spina bifida occulta unless there are associated abnormalities such as spondylolysis or spondylolisthesis.
• Neurological symptoms, if present, suggest the possibility of tethered cord syndrome or other neural complications requiring further evaluation.

Management Approaches

The management of spina bifida occulta depends entirely on the clinical presentation and associated findings. For the typical asymptomatic case, as likely represented in this image, management is straightforward.

• Patient education and reassurance about the benign nature of isolated S1 spina bifida occulta is usually sufficient.
• No specific treatment or follow-up is necessary for asymptomatic cases without cutaneous markers or neurological findings.

When spina bifida occulta is associated with clinically significant tethered cord or other neural complications, neurosurgical consultation and potential intervention may be warranted. These scenarios are the exception rather than the rule.

• Surgical detethering may be indicated for progressive neurological deficits associated with tethered cord syndrome.
• Postoperative rehabilitation focuses on optimizing recovery and preventing retethering.

Associated Findings and Clinical Correlations

Cutaneous Manifestations

External cutaneous markers sometimes provide visible clues to underlying spinal dysraphisms, including spina bifida occulta. These markers warrant particular attention during physical examination.

• Lumbar hypertrichosis (hair patch) is reported in approximately 50% of patients with underlying spinal dysraphism.
• Other cutaneous stigmata include dimples, subcutaneous lipomas, hemangiomas, and abnormal pigmentation over the affected area.

The absence of cutaneous markers does not rule out spina bifida occulta, but their presence increases the likelihood of more complex underlying pathology. The relationship between external markers and internal dysraphism informs clinical decision-making.

• Simple dimples less than 5 mm in diameter and within 2.5 cm of the anus are rarely associated with significant dysraphism.
• Atypical dimples (deep, large, or high on the back) may indicate more complex underlying abnormalities warranting MRI evaluation.

Urological Considerations

The relationship between sacral spina bifida occulta and urological dysfunction remains controversial. While isolated S1 defects (as in this image) rarely cause urological issues, awareness of potential associations is important.

• Occult neurogenic bladder has been reported in some patients with spina bifida occulta, though this association is much stronger with higher-level or more extensive defects.
• Urodynamic testing may be considered in patients with unexplained lower urinary tract symptoms and spinal dysraphism.

The sacral nerves controlling bladder and bowel function emerge primarily from S2-S4 levels, which may explain why isolated S1 defects typically spare these functions. Clinical correlation is essential for appropriate management.

• Routine urological screening is not recommended for asymptomatic patients with isolated S1 spina bifida occulta.
• New-onset urinary symptoms in patients with known spinal dysraphism should prompt evaluation for tethered cord or other progressive neurological complications.

Epidemiology and Public Health Perspectives

Spina bifida occulta, particularly at the S1 level, is remarkably common in the general population. Understanding its epidemiology helps place the finding in proper context for both clinicians and patients.

• Prevalence estimates range from 10-20% for S1 spina bifida occulta, making it one of the most common congenital vertebral variations.
• Higher rates are often reported in studies using CT or high-resolution radiography compared to conventional radiography.

The relationship between spina bifida occulta and more severe forms of neural tube defects reflects shared embryological origins but different severities. This understanding informs preventive strategies.

• Periconceptional folic acid supplementation, which dramatically reduces the risk of open neural tube defects, may also reduce the incidence of spina bifida occulta.
• A family history of neural tube defects slightly increases the risk of spina bifida occulta, suggesting some shared genetic factors.

Conclusion

Spina bifida occulta at the S1 level, as demonstrated in this radiographic image, represents a common variation in vertebral development that rarely causes clinical symptoms. The incomplete fusion of the posterior elements of S1, highlighted by the red circle, reflects a minor disruption in the embryological development of the neural arch. While this finding may raise concerns when discovered incidentally, it is important for healthcare professionals to recognize its typically benign nature, particularly when limited to the S1 level. The absence of associated cutaneous markers or neurological symptoms further supports a conservative approach without the need for additional imaging or intervention. For medical students and practitioners, understanding the spectrum of spinal dysraphisms helps place spina bifida occulta in proper context, distinguishing this common variant from more significant neural tube defects that require specialized management. This knowledge facilitates appropriate patient education and clinical decision-making when interpreting spinal imaging studies.

1. S1 Spina Bifida Occulta: Radiographic Identification and Clinical Significance
2. Understanding Sacral Spina Bifida Occulta: Radiological Features and Management Approaches
3. Spinal Dysraphism Spectrum: Radiographic Analysis of S1 Spina Bifida Occulta
4. Incidental Finding of S1 Spina Bifida Occulta: Diagnostic Imaging and Clinical Implications
5. Posterior Arch Defects: Recognizing and Interpreting S1 Spina Bifida Occulta on Radiographs