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Congenital anomalies of the pediatric cervical spine arise from a failure of normal development occurring early in the embryonic process. These anomalies can lead to problems for treating physicians and surgeons. Because the immature spine is largely cartilaginous, recognition and definition of pathologic features can be difficult even for experienced clinicians. Failure to recognize these pathologic processes risks overlooking segmental instability, developing progressive spinal deformity, encroachment on the space available for the spinal cord (SAC), and the risk for myelopathy. To recognize congenital anomalies and best manage these patients, it is helpful to understand the embryology and biomechanics of the normal immature cervical spine. Once these fundamental issues are understood, then anomalous development can be better appreciated.
Embryology
Segmentation
By the end of the fifth week of development, mesodermal cells that surround the notochord segment into epithelial spheres called somites ( Fig. 6-1 ). This process is known as segmentation and produces 42 to 44 pairs of somites. The somites develop in a craniocaudal fashion, and each somite has three parts: the sclerotome, the dermatome, and the myotome. The sclerotomes are responsible for the formation of the vertebrae, whereas the dermatomes and myotomes are responsible for the formation of the overlying dermis and muscles, respectively.
The paired aggregation of cells within the somites is patterned so that the cells in the caudal half are densely packed and the cells in the cranial half are loosely packed (see Fig. 6-1 ). Separation or segmentation of the stacked somites occurs through the loosely packed cells in the cranial half of each somite. The cranial half becomes the disk space and the annulus fibrosus, whereas the caudal, tightly packed half of the somite becomes the vertebral body. Finally, the notochord slowly regresses to become the nucleus pulposus within the annulus fibrosus.
For one complete vertebra to form properly, a tight interaction between a pair of somites is necessary. Failure of the proper segmentation process may result in congenital abnormalities. Two families of regulatory genes have been implicated in the control of the processes of somitogenesis and segmentation: Pax and Hox . The Pax family of genes contributes to the development of the central nervous system and also controls the establishment of boundaries for each sclerotome. The Hox family of genes regulates the sequential craniocaudal development of the midline axial structures. Mutations in these genes may contribute to the development of congenital anomalies and are a topic of ongoing investigation.
Chondrification and Ossification
During the sixth week of development, chondrification occurs and ultimately leads to ossification of relevant structures and regression of the notochord. Defects in these two processes, which are controlled by signals from the notochord, can lead to congenital abnormalities. The vertebrae of the lower cervical spine (C3 to C7) have a similar pattern of development. Each vertebra has three primary ossification centers: one on either side of the neural arch and one in the vertebral centrum. The ossification centers are separated anteriorly by the neurocentral synchondroses, which lie parallel to each other on either side of the centrum ( Fig. 6-2 ). To develop normal vertebral growth, it is important for the neurocentral synchondroses to have paired growth that is symmetric and equal. Asymmetric growth leads to deformity. Normally, the synchondroses close between 6 and 8 years of age, at which time the spinal canal diameters have reached adult size. Premature closure of the neurocentral synchondroses may lead to reduced spinal cord diameters with an increased risk of spinal stenosis and deformity.
Additional organ systems are derived from the same primitive areas of the mesoderm. Any process that can affect the normal development of the mesoderm can lead to spinal defects, as well as anomalies in other areas. The estimated incidence of additional abnormalities in children with congenital spinal anomalies is up to 60%. The genitourinary system is most commonly associated with congenital spine abnormalities.
Upper Cervical Spine
The development of the occipitoatlantoaxial complex is a unique variation of the foregoing process ( Fig. 6-3 ).
Atlas
Cells from the fourth occipital somite, also called the proatlas, combine with cells from the first cervical somite to form the atlas. During the segmentation period, failure to segment or separate can occur, leading to synostosis between the atlas and the occiput. This condition is known as occipitalization of the atlas ( Fig. 6-4 ). Another developmental failure can occur in the condylar joints. The result is a loss of the normal rounded shape of the joints, thus causing a loss of the required smooth motion with flexion and extension and leading to relative instability. In addition, cells from the first and second somites that contribute to the atlas centrum descend to form part of the odontoid process or dens. The caudal migration of cells from the centrum of the proatlas results in a central space and the ring shape of the atlas.
Axis
Cells from the first and second somites contribute to formation of the axis (C2). The migration of cells that form the centrum of the proatlas also aids formation of the odontoid or dens and the supporting ligaments (see Fig. 6-3 ). Failure of this process may cause odontoid aplasia or hypoplasia. Finally, the basilar synchondrosis of the dens is a weak spot that can become the site of failure in the event of injury; this sequence of events is believed to be the cause of os odontoideum (OO).
Biomechanics of the Immature Cervical Spine with Congenital Osseous Anomalies
Spinal biomechanics of the normal cervical spine in children differs significantly from that of healthy adults. First, the relative ligamentous laxity in most children can have an adverse effect on stability of the facet joints, as well as on the competence of both the atlanto-occipital and the atlantoaxial joints. Second, the facet joints and the condylar development of the atlanto-occipital articulation are relatively shallow compared with those in the mature spine. Third, the relatively larger heads of children subject the immature spine to increased acceleration-deceleration forces and a higher risk of instability.
With the addition of congenital anomalies, the risk of cervical instability is considerably greater. For example, in patients with vertebral fusion, excess motion can develop, and vertebral translation may occur at the adjacent motion segment. This translation of a vertebra can result in dynamic encroachment on the SAC and an increased risk for developing spinal cord injury (see Fig. 6-4 ). An example of this is observed in a patient with occipitalization of the atlas combined with congenital fusion of C2 and C3. This situation leaves the atlantoaxial joint as the only motion segment in the upper cervical spine and increases the risk of encroachment on the SAC. This type of phenomenon is also observed in Klippel-Feil syndrome (KFS), in which a single mobile disk space exists between two fusion masses.
Another type of encroachment is static. This disorder occurs when, for example, an intraspinal mass, such as protrusion of the dens in basilar invagination (BI) or the Chiari type I malformation, encroaches on the SAC. A combination of static and dynamic encroachment may also exist. Thus, encroachment and instability are the underlying mechanisms that can ultimately lead to neurologic signs and symptoms in the setting of congenital osseous anomalies.
Congenital Osseous Anomalies of the Cervical Spine
Menezes provided a practical classification of craniocervical anomalies. He divided these anomalies into a congenital group and a developmental group, with abnormal embryology and abnormalities that develop later in childhood ( Table 6-1 ). The two groups are not mutually exclusive because some abnormalities may be present at birth but more frequently develop as the patient matures.
| Classification | Anomalies | Associated Conditions |
|---|---|---|
| 1. Congenital | Occipitalization | KFS, Goldenhar syndrome, 22q11.2 deletion, Morquio syndrome, basilar invagination, atlas defects, os odontoideum, occipital vertebra, syringomyelia |
| Condylar hypoplasia | Down syndrome, Goldenhar syndrome, skeletal dysplasias | |
| Atlas defects | Down syndrome, Goldenhar syndrome, Morquio syndrome | |
| Proatlas segmentation failure | Chiari type I malformation | |
| Basilar invagination | Chiari type I malformation, KFS, syringomyelia, syringobulbia, atlas defects, hydrocephalus | |
| 2. Developmental | Os odontoideum | Skeletal dysplasia, Down syndrome, KFS, Larsen syndrome, occipitalization |
| Ossiculum terminale persistens | Down syndrome |
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