Atlantoaxial Instability

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► [Excessive mobility at the atlanto-axial articulation, which is prone to dislocation]

The primary motion at the C1-2 joint is rotation rather than flexion or extension. The dens is tightly bound to C-1 by the transverse ligament, which prevents forward subluxation of the atlas during neck flexion and extension. Incompetence of the transverse ligament due to rupture or ligamentous laxity results in excessive nonphysiologic movements of the atlas (instability, subluxation) or complete loss of joint congruity (dislocation). In general, atlantoaxial instability can result from any defect, whether congenital or acquired, in the anatomical or functional integrity of one or more components of the atlanto-axial articulation (odontoid, anterior arch of the atlas, ligamentous supports). Clinical symptoms are related to compression of structures such as the spinal cord, cerebellar tonsils, and cranial nerves and can range from none to sudden death. Variability in the degree of neurological compromise depends mainly on the size of the spinal canal. In normal individuals, the spinal canal is wide enough to compensate for sudden reduction down to one-third of its caliber before spinal cord compression takes place, thus explaining the relatively low incidence of neurological sequelae in subjects who have experienced cervical spine trauma (Steel 1968). On the other hand, the effects of atlanto-axial subluxation may be devastating in any subject with a constitutionally narrowed spinal canal (Martich et al. 1992). Possible complaints in atlanto-axial subluxation include persistent neck pain, torticollis, transient or persistent tetraparesis or tetraplegia, cranial nerve irritation, and cerebral or brain stem ischemia (Hensinger et al.

Odontoid Hypoplasia
Fig. 3.70 a,b. Dyggve-Melchior-Clausen syndrome. a In a 14-month-old girl. Note hypoplastic odontoid process causing atlantoaxial instability. b In a 17-year-old girl. Odontoid hypoplasia and subluxation at the atlanto-axial junction

1978; Phillips et al. 1988). Isolated posttraumatic atlanto-axial subluxation is a rare injury (Bohlman 1979), occurring most commonly with severe head trauma. Almost all cases of atlanto-axial dislocation manifest anterior displacement of the atlas relative to the axis. In turn, the abrupt incompetence of the transverse ligament secondary to the trauma makes the atlanto-axial articulation prone to further motion. Nontraumatic atlanto-axial subluxation is much more frequent, occurring in association with arthritides, including rheumatoid arthritis and the seronegative spondyloarthropathies (ankylosing spondylitis, psoriasis and Reiter's syndrome), and connective tissue diseases, including systemic lupus erythemato sus (Weissman et al. 1982; Koss and Dalinka 1980). In these disorders, either cartilaginous and osseous tissue destruction, or capsular and ligamentous laxity account for joint instability.

Atlanto-axial instability can also be related to congenital vertebral defects involving the atlas and/or the odontoid, or occur in association with inherited disorders (Dawson and Smith 1979; Greenberg 1968; Sinh 1983). Odontoid hypoplasia/aplasia, either isolated or as part of a malformation spectrum, is associated with atlanto-axial instability in many conditions, including mucopolysaccharidoses (Hurler, OMIM 607014; Morquio, OMIM 253010) (Thomas et al. 1985), hypochondrogenesis (OMIM 200610), spondyloepiphyseal dysplasia congenita (OMIM 183900), spondylo-epi-metaphyseal dysplasia (Strudwick type, OMIM 184250) and pseudoachondroplasia (OMIM 177170). Another condition with odontoid hypoplasia and atlanto-axial instability is metaphyseal chon-drodysplasia, McKusick type (cartilage hair hypopla-sia, OMIM 250250). Additional spinal findings in this disorder include mild end-plate abnormalities resembling Schmorl's nodes, mild scoliosis, and accentuated lumbar lordosis (Ray and Dorst 1973). Similarly, odontoid hypoplasia occurring in the context of disorders with deficient bone ossification, including chondrodysplasia punctata (OMIM 215100, 302960), hypophosphatasia (OMIM 241500), and metatropic dysplasia (OMIM 250600) (Shohat et al. 1989), can be associated with C1-2 subluxation. By contrast, spondylo-epiphyseal dysplasia tarda and brachyo-lmia usually do not show problems at C1-2 despite the presence of odontoid hypoplasia. Regardless of the presence of hypoplasia or hyperplasia of the odontoid process, patients with diastrophic dysplasia (OMIM 22600) and Dyggve-Melchior-Clausen syndrome (OMIM 223800) are often affected by atlantoaxial subluxation (Naffah and Taleb 1974) (Fig. 3.70 a,b). As mentioned elsewhere, os odontoideum (nonfusion of the dens with the body) is often associated with atlanto-axial instability and secondary spinal cord impingement between the os and the lamina of the axis (Fig. 3.71 a,b). The development of cervical myelopathy has been found to be related to the distance from the os to the spinous process of the axis (Shirasaki et al. 1991). Similarly, nonfusion of the apical apophysis of the dens (ossiculum terminalis persistens) has been described as a conceivable cause of atlanto-axial dislocation (Bret et al. 1980). An association of short stature, radiologic abnormalities involving predominantly the vertebrae and femurs, and atlanto-axial instability has been designated spondy-lo-epiphyseal dysplasia with atlanto-axial instability

Persistent Apophysis Cervical

Fig. 3.71 a,b. Os odontoideum. a In a young adult. Note anterior displacement of the atlas over the axis. Also note amputation of the axis, with a separate ossicle located superoanteriorly and representing the dislocated os odontoideum. Cortical thickening of the anterior arch of the atlas suggests chronic altered stress in this region. b In an adult. There is posterior dislocation of the atlas over the axis in this case.Note the hypoplastic odontoid lying just below the anterior arch of the atlas, and a small ossicle representing os odontoideum at its normal location between the anterior and posterior arch of the atlas

Fig. 3.71 a,b. Os odontoideum. a In a young adult. Note anterior displacement of the atlas over the axis. Also note amputation of the axis, with a separate ossicle located superoanteriorly and representing the dislocated os odontoideum. Cortical thickening of the anterior arch of the atlas suggests chronic altered stress in this region. b In an adult. There is posterior dislocation of the atlas over the axis in this case.Note the hypoplastic odontoid lying just below the anterior arch of the atlas, and a small ossicle representing os odontoideum at its normal location between the anterior and posterior arch of the atlas

(OMIM 600561). Os odontoideum was found in both members of the same pedigree who presented with neurological symptoms of acute atlanto-axial dislocation following a fall. End-plate irregularity, especially in the dorsal spine, spondylolisthesis of L-5, small femoral epiphyses, hip subluxation, and delayed bone age were additional features (Reardon et al. 1994).

Atlanto-axial subluxation is a typical occurrence in Down syndrome (OMIM 190685), with a prevalence of 10-20% in different series (Miller et al. 1986; Pueschel and Scola 1987; White et al. 1993). There is some uncertainty about the mechanism of atlanto-axial instability in these patients, but ligamentous laxity, odontoid abnormalities, repeated trauma, and pharyngeal infection have all been implicated. Down children in whom atlanto-axial instability is associated with the presence of odontoid hypoplasia or accessory ossicles are at risk of spinal cord damage (Elliott et al. 1988). The risk is further amplified by the presence of hypoplasia of the C-1 posterior arch, which narrows the vertebral canal posteriorly, thus exposing the spinal cord to hazards during dorsal movement of the odontoid process (Martich et al. 1992). Syringomyelia and neurological deficits, including hyperreflexia, clonus, abnormal gait, ataxia, and tetraparesis have been described as potential sequelae of atlanto-axial dislocation in Down syndrome (Semine et al. 1978). In patients with Marfan syndrome (OMIM 154700), atlanto-axial subluxation due to laxity of the ligamentous structures about the C1-2 joint is a rare but severe complication (Levan-der et al. 1981). On the other hand, scoliosis is common, occurring in about half of the affected individuals. Posterior scalloping of the vertebral bodies, Scheuermann disease, thoracic kyphosis, widening of the central canal in the sacral region, and increased interpediculate distance are typical skeletal features in these patients. Ligamentous laxity is likely to be the causative mechanism of C1-2 dislocation in pseudodiastrophic dysplasia (OMIM 264180), as well.

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