Hemivertebrae is a congenital spinal malformation, present at birth, resulting from a structural error during the earliest stages of embryonic development. This anomaly is classified as a failure of vertebral formation, meaning the bony structure of one or more spinal segments did not fully develop. The presence of a hemivertebra disrupts the spine’s normal alignment, which can lead to a progressive curvature of the back.
Defining Hemivertebrae and Its Origin
A hemivertebra is an incomplete, wedge-shaped vertebral body, where only one side of the segment forms completely. This defect means the affected vertebra acts as an asymmetrical wedge within the spinal column. The presence of this wedge introduces an imbalance, which pushes the spine away from the side where the partial formation occurred.
The root cause of this defect lies in the embryonic period, specifically during the first trimester of pregnancy. Between the fifth and eighth weeks of gestation, the mesenchymal tissue that forms the spine undergoes segmentation and chondrification. A normal vertebral body develops from two lateral chondrification centers that are meant to unite.
Hemivertebrae results from the failure of one of these two lateral centers to develop or properly fuse. This developmental failure prevents the formation of the full, symmetrical vertebral body. The resulting half-vertebra retains an unbalanced growth plate, acting like a perpetual wedge, causing the spine to curve as the child grows.
Classification and Clinical Impact
The clinical significance of a hemivertebra is tied to its classification, which is determined by the relationship of the abnormal segment to the adjacent vertebrae. This classification system focuses on the presence or absence of a functional intervertebral disc space above and below the hemivertebra, as the disc space signifies a potential for continued, unbalanced growth.
The most aggressive type is the fully segmented hemivertebra, which possesses a functional disc space both above and below the anomalous segment. These open disc spaces mean the hemivertebra has two active growth plates, allowing it to grow disproportionately and rapidly push the spine into a severe curve. This type has the highest risk of progression and is most likely to cause significant congenital scoliosis.
A semi-segmented hemivertebra is fused to one adjacent vertebra but has a functional disc space on the other side. Its growth potential is partially constrained, making it less aggressive. Conversely, the non-segmented or incarcerated hemivertebra is fused to both the vertebra above and the vertebra below, lacking any functional disc spaces. Since this type is fused, it has minimal growth potential and is often stable, posing the lowest risk for progressive curvature.
The clinical manifestation is typically congenital scoliosis (sideways curvature) or kyphosis (excessive forward curvature). The location of the hemivertebra also dictates the rate of progression, with those in the thoracolumbar region progressing more rapidly than those in the upper thoracic or lumbar spine.
Diagnosis and Management Approaches
The identification of a hemivertebra often begins either prenatally or during early childhood. Prenatal diagnosis can occur through a detailed fetal ultrasound, which may identify the asymmetrical vertebral body in the second trimester of pregnancy. Postnatally, a physical examination may reveal an abnormal spinal curvature, which prompts further investigation.
Imaging studies are necessary to confirm the diagnosis and define the anomaly. Standard X-rays are typically the first step, providing clear, two-dimensional images that characterize the wedge shape and determine the degree of spinal curvature. Computed tomography (CT) scans offer detailed, three-dimensional bone visualization, which is helpful for surgical planning. Magnetic resonance imaging (MRI) is also used to assess the spinal cord and surrounding soft tissues for associated neurological anomalies, which may be present in up to 20% of cases.
The management approach is individualized and depends primarily on the hemivertebra’s classification and the rate of curve progression. For non-progressive or mild curves, particularly those caused by non-segmented or incarcerated hemivertebrae, the approach is observation and monitoring. This involves regular clinical check-ups and repeated imaging to ensure the curve remains stable over time, especially during periods of rapid growth.
Surgical intervention is recommended for progressive or severe curves, which are most often associated with fully segmented hemivertebrae. The goal of surgery is to prevent further progression of the spinal deformity and correct the existing curvature. A common procedure is hemivertebra excision and spinal fusion, where the wedge-shaped segment is surgically removed. The vertebrae above and below the excised segment are then fused together, creating a straight, stable column that eliminates the source of the unbalanced growth. This correction is often performed early in life to maximize correction potential and prevent severe deformity before the spine fully matures.