Chapter 7, Craniocervical Developmental Anatomy and Its Implications (continued)
IMPLICATIONS OF CRANIOVERTEBRAL ABNORMALITIES
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Table 7.2 provides a practical classification of the most frequently encountered congenital cervical anomalies, which are divided into those that are present at birth (congenital) and those that are developmental, which have an abnormal embryology leading to symptomatic abnormalities during early childhood and into adulthood (26). The immediate relevance of this classification is the understanding of the basis of abnormalities that occur with atlas assimilation, remnants of the occipital sclerotomes, fusion abnormalities, os odontoideum, basilar invagination, segmentation failures at the occiput, atlas and axis vertebra, and, more importantly, the natural history. Thus, a wide variety of abnormalities exist, which can occur singularly or in multiples in the same individual; involving both the osseous and neural structures. An insult to both of the structures may occur between the fourth and seventh week of intrauterine life, resulting in a combination of abnormalities consisting of failures of segmentation, failure of fusion of different components of each bone, hypoplasia, and ankylosis.
There is a high incidence of both anterior and posterior spina bifida of C1 as well as os odontoideum in connective tissue diseases, such as mucopolysaccharidosis, Down’s syndrome, and Morquio’s syndrome. This subsequently results in the atlantoaxial subluxation. It is possible that, because of the abnormal excessive head movements in the embryo between the 50th and 53rd day, the process of chondrification is impaired, resulting in anterior and posterior spina bifida of C1 (27). This has previously been alluded to in the development of the atlas. Spinal trauma in children younger than 8 years of age is mainly centered at the craniovertebral border because of the high fulcrum of neck motion (25). This results in ligamentous injuries more than fractures. However, odontoid fractures in this age group are usually observed as avulsion injuries with separation of the neural central synchondrosis (25).
Growth of the posterior fossa and especially the clivus continues past late adolescence and certainly provides a rationale for the need to continue observing children who have undergone an occipitocervical stabilization or craniovertebral decompression. The downward growth of the brain and the elongation of the posterior fossa and clivus may recreate a ventral bony abnormality later in life, despite a satisfactory previous ventral decompression at the craniovertebral junction performed during the first two decades of life. We have observed this occur, although infrequently, in our patients with ventral or dorsal posterior fossa decompression.
Abnormalities of the craniovertebral junction must be suspect in infants with Goldenhar’s syndrome, skeletal dysplasias, and the Conradi syndrome. It should be suspected in infants who present with torticollis. Diseases such as Down’s syndrome have a 14 to 20% incidence of atlantoaxial dislocation (35). Once the stage is set by congenital craniovertebral anomalies, the developmental and acquired phenomenon may supervene, producing atlantoaxial instability and, subsequently, basilar invagination. This is more common in developing countries where heavy loads are carried on the head from childhood. An erroneous diagnosis of “congenital dislocations” thus appears in their literature. Likewise, upper respiratory infections can cause stiff neck, torticollis, and ligamentous instability, and may come to attention later in developing countries than in places where medical attention is readily available. For this reason, it seems that abnormalities of the craniovertebral junction are more frequently encountered in the populous and less advantaged countries.
In 1981, Marin-Padilla demonstrated that the basichondrocranium of fetuses with hindbrain malformations, such as the Chiari syndrome, is shorter than normal and elevated in relation to the axis of the vertebral column (22). The shortness of the basichondrocranium of these fetuses is attributed to underdevelopment of the occipital bone, especially noticeable in its basal component. The defect results in a short and small posterior fossa, inadequate to contain the developing nervous structures of that region. The elongation of the odontoid process, referred to as the “dolicho-odontoid process,” is explained by the depression of the basiocciput, resulting in the basilar impression often observed in the clinical Chiari malformations. These changes have been experimentally reproduced in pregnant hamsters by a single dose of vitamin A early in the morning of the eighth day of gestation, thus, inducing a typical Chiari II malformation as well as various types of axial skeletal dysraphism (21).
Proatlas Segmentation Failures or Manifestations of Occipital Vertebrae
Malformations and anomalies of the most caudal of the occipital sclerotomes are caused by proatlas segmentation failures. These abnormalities surround the foramen magnum and usually involve the posterior arches of C1 (9, 12, 27, 32, 36). A hindbrain herniation is associated in 33% of individuals (28). At times, the proatlas component of the dens may fail to separate from that portion that forms the basiocciput of the clivus. Thus, the anterior arch of the atlas comes to rest above the axis body. At times, the proatlas abnormality is united with the clivus, grossly distorting the cervicomedullary junction ventrally. Variations of this may be observed in the midline ventrally, laterally, and, at times, dorsally. Thus, one may see paramesial invagination.
In our series of 70 patients, 85 to 90% presented between the first and second decade of life. The earliest presentation was 3 years of age and the oldest was 23 years. A spastic quadriparesis was a presenting symptom in 80% of the patients and lower cranial nerve palsies in 33% of the patients. Vascular presentation of vertebrobasilar system dysfunction was observed in 40%, and trauma presentation in 60% of individuals.
The best definition of the abnormality was on three-dimensional (3-D) CT combined with magnetic resonance imaging. Earlier in the series, pleuridirectional tomography was used in the frontal and lateral projections (25). A hindbrain herniation was present when the posterior fossa volume was reduced, especially by distortion of the vertical height of the posterior fossa. Thus, the treatment of this condition rests with precise definition of the anatomic and pathological abnormality, relief of neurovascular compression, and prevention of recurrence by stabilization. The surgical approach depends on the manner of encroachment on the neurovascular structures. The patient shown in Figure 7.3A–C required a transpalatopharyngeal decompression of the ventral cervicomedullary junction with a dorsal occipitocervical fixation. The patient shown in Figure 7.4A–E required a posterior approach for decompression of the dorsolateral cervicomedullary junction and a fusion procedure. However, the patient shown in Figure 7.5A–C was managed with a posterolateral decompression of the foramen of the occipital condyle on the left and a simultaneous midline approach on the right, decompressing the medial condyle, and a fusion procedure.