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Nontraumatic Upper Cervical Spine Instability in Children Abstract The upper cervical spine begins at the base of the occiput, continues caudally to the C2-C3 disk space, and includes the occipitoatlantal and atlantoaxial joints. Nontraumatic upper cervical spine instability can result from abnormal development of osseous or ligamentous structures or from gradually increasing ligamentous laxity associated with connective tissue disorders. Such instability can lead to compression of the spinal cord during movement of the cervical spine. Establishing a correct diagnosis includes performing a thorough physical examination as well as evaluating radiographic relationships and measurements. Appropriate management of syndromes associated with instability of the upper cervical spine includes preventive care and recommendations for sports participation. Surgical treatment for the upper cervical spine includes a posterior surgical approach, used for instability, and the use of rigid plate implants, wiring, and bone graft materials to achieve a solid spinal fusion. T he upper cervical spine runs from the occiput to the C2-C3 disk space and includes the occipitoatlantal and atlantoaxial joints. Nontraumatic instability of this seg- ment is relatively rare in the pediatric population. However, familiarity with the effective evaluation and treatment of upper cervical spine instability is important because per- manent neurologic compromise can result from this condition. Addition- ally, orthopaedic surgeons who un- derstand the unique aspects of the developing upper cervical spine are better able to make sports participation recommendations for children with conditions such as Down syndrome. Nontraumatic upper cervical spine instability can result from the abnormal development of osseous or ligamentous structures. Alternatively, instability can develop as a result of the gradually increasing ligamentous laxity associated with connective tissue disorders. Instability resulting from either cause can lead to compression of the spinal cord during movement of the cervical spine. Such compression may be present at the occipitoatlantal joint, atlantoaxial joint, or both. Instability of the upper cervical spine in a child presenting clinically is largely variable and can range from a complete absence of signs and symptoms to frank quadriparesis. For example, in Down syndrome, radiographic evidence of instability in an asymptomatic patient is a common finding; by contrast, in Morquio’s syndrome, myelopathy frequently ac- companies radiographic evidence of upper cervical instability. Brian P. D. Wills, MD John P. Dormans, MD Dr. Wills is Resident, Department of Orthopedics and Rehabilitation, University of Wisconsin, Madison, WI. Dr. Dormans is Chief of Orthopaedic Surgery, The Children’s Hospital of Philadelphia, Philadelphia, PA, and Professor of Orthopaedic Surgery, University of Pennsylvania School of Medicine, Philadelphia. None of the following authors or the departments with which they are affiliated has received anything of value from or owns stock in a commercial company or institution related directly or indirectly to the subject of this article: Dr. Wills and Dr. Dormans. Reprint requests: Dr. Dormans, The Children’s Hospital of Philadelphia, Second Floor, Wood Building, 34th and Civic Center Boulevard, Philadelphia, PA 19104. J Am Acad Orthop Surg 2006;14:233- 245 Copyright 2006 by the American Academy of Orthopaedic Surgeons. Volume 14, Number 4, April 2006 233 Instability of the upper cervical spine often is accompanied by other pathology involving the structures in this anatomic region, including spinal stenosis, basilar impression, occipitalization of the atlas, Klippel- Feil syndrome, and central nervous system abnormalities (eg, Arnold- Chiari syndrome malformation). In- stability of the upper cervical spine and stenosis often are two major factors in the development of myelopathy. Neurologic signs and symptoms can result from any of a constellation of anomalies that may be present in a child with instability of the upper cervical spine. Further, such instability also is associated with a number of syndromes and conditions, such as those related to ligamentous laxity or abnormal bone development. In such instances, these cervical anomalies may be the first indication of other organ abnormalities, which should be evaluated with appropriate screening strategies. Because orthopaedic surgeons often are the first to evaluate these patients, the surgeon should begin such assessment with a full history and clinical examination; fo- cusing only on the neck may delay accurate diagnosis of other conditions. Developmental and Functional Anatomy Much has been learned recently about the development of the mam- malian spine. An example is the dis- covery that homeobox (Hox) genes play a significant role in regulating the development of the axial and ap- pendicular skeletons. These genes di- rect the embryonic differentiation and segmentation along the craniocaudal axis by activating and repress- ing various DNA sequences and en- coding transcription factors and proteins. 1 Development of the base of the skull, the basiocciput, is similar to that of the atlas (C1) and axis (C2): all arise from medial and lateral com- ponents of sclerotomes and the perinotochord in a manner that dif- fers from the remainder of the vertebral column. The basiocciput 2 develops from somites 1 to 4, whereas the atlantoaxial column develops from somites 5 to 7 (Figure 1). Organogen- esis occurs simultaneously with development of the axial skeleton. This temporal relationship explains, in part, the frequent association reported between spinal and visceral anomalies. It is important to be aware of these potentially associated anomalies to ensure that they are identified and treated appropriately. The atlas develops from three ossification centers, one for each lateral mass (present at birth) and one for the body (developing by age 1 year). The posterior arches fuse at age 3 to 4 years; the lateral masses fuse to the body by age 7 years 3 (Figure 2). The axis is formed from five primary ossification centers: two lateral masses, two vertically oriented halves of the dens, and the body. Two secondary ossification centers include the tip of the odontoid (ossiculum ter- minale) and the inferior ring apophy- sis. The odontoid process is separated from the body by the dentocentral synchondrosis, which closes between the ages of 5 and 7 years 4 (Fig- ure 2). Orthopaedic surgeons should know these ossification centers and the approximate ages at which they fuse so that sites of bone growth are not mistaken for fractures during radiographic evaluation. Stability at the atlanto-occipital junction is provided by the cup- shaped joints between the occipital Figure 1 Embryologic development of the spine. Unsegmented presomitic mesoderm (PSM) matures into somites, pairs of segments on either side of the future spinal cord, in a process called somitogenesis. The somites further differentiate into sclerotome, which forms the adult vertebrae, and dermomyotome, which forms the axial musculature and also contributes to the adult dermis. This maturation occurs in a craniocaudal direction as shown by the coronal section on the right. The three axial views to the left demonstrate the stages of maturation. (Reproduced with permission from Tracy MR, Dormans JP, Kusumi K: Klippel-Feil syndrome. Clin Orthop 2004;424:187.) Nontraumatic Upper Cervical Spine Instability in Children 234 Journal of the American Academy of Orthopaedic Surgeons condyles and the superior articular facets of C1, as well as by the capsu- lar ligaments that surround and an- chor these joints. The tectorial membrane, a continuation of the posterior longitudinal ligament, also provides considerable support. At the atlantoaxial joint, the bony integrity of the odontoid process and the integrity of the transverse ligament provide most of the support. Paired alar ligaments connect the odontoid to the occipital condyles, and together with the apical ligament, which runs from the odontoid to the foramen magnum, act as secondary stabilizers and check ligaments during rotation 5 (Figure 2). The mobility of the cervical spine at the occipitoatlantoaxial complex can be separated into flexion- extension, lateral bending, and rotation. In the mature spine, range of motion between the occiput and the atlas is 15° in flexion-extension, 10° in lateral bending, and negligible in rotation. 5 Between the atlas and axis, range of motion is 10° in flexion and extension, negligible in lateral bending, and 50° in rotation. 5 The biome- chanics of the developing cervical spine, which are likely to change during maturation of the cervical spine, have not been fully studied. Clinical Presentation and Evaluation Children with instability of the upper cervical spine may present for any of a number of reasons. The orthopaedic surgeon often is consulted to evaluate children with syndromes or conditions known to have frequent involvement of the musculoskeletal system, as well as to assess children with incidental radiographic findings of cervical spine anomaly. In such cases, the surgeon should evaluate the cervical spine as part of the initial evaluation, including ordering flexion-extension radiographs. Occasionally, patients will present with a history of head or neck trauma, neck pain, torticollis, loss of neck range of motion, or other clear signs of upper spinal cord involvement. More often, however, the presentation of this involvement is less obvious, and the constellation of signs and symptoms may lead the surgeon to sites of spinal cord compression (Table 1). Frequently, multiple tracts in the spinal cord are involved along with associated vertebral artery and cerebellar signs and symptoms, which can make locating the site of compression difficult. Perovic et al 6 reported on a series of children with instability of the atlantoaxial joint, a condition in which the earliest sign of myelopathy is a gradual loss of physical endurance, which occurs before signs of pyramidal tract involvement. This development of progressive weakness with the absence of other neurologic findings is especially frequent with the instability of the upper cervical spine reported in Morquio’s syndrome. With posterior cord impingement, changes in proprioception and Figure 2 Anatomy and ossification centers of the atlas (A) and axis (B). C, The relationship of the apical, alar, and transverse ligaments to the odontoid. (Reproduced from Copley LA, Dormans JP: Cervical spine disorders in infants and children. JAmAcad Orthop Surg 199 8;6:204-214.) Brian P. D. Wills, MD, and John P. Dormans, MD Volume 14, Number 4, April 2006 235 pain perception, as well as vibratory sense, can occur as a result of the involvement of the posterior spinal columns. When the cerebellum is involved, ataxia, incoordination, and nystagmus also may be observed. Posterior cord compression can be caused by the posterior rim of the foramen magnum or the posterior ring of C1. In addition to spinal cord involvement, vertebral artery compression, which can occur without spinal cord involvement, 7,8 can lead to syncopal episodes, decreased mental acuity, dizziness, and seizures. Patients with concomitant occipitalization of the atlas or basilar impression accompanying instability of the upper cervical spine are more likely to have symptoms of anterior cord compression resulting from odontoid impingement. 7,9 Damage to the anterior pyramidal tracts can result in muscle weakness and atrophy, pathologic reflexes (eg, hyperreflexia, spasticity, clonus), and ataxia. 7,9 Indentation of the brainstem has been found at autopsy to result from the abnormal odontoid. 9 Cranial nerve involvement may result from instability of the upper cervical spine. Compression of the lower cranial nerves as they exit the medulla may occur from the instability itself or from associated anomalies, such as basilar impression or Arnold-Chiari malformation. 10 The cranial nerves involved most often are the trigeminal (V), glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII). However, involvement of other cranial nerves has been reported. 9,10 Given the wide range of neurologic signs and symptoms that may be seen in a patient with instability of the upper cervical spine, it is important to perform a complete and thorough neurologic examination and to clearly document results at each patient visit. Subtle changes between clinical visits may be the first sign of impending spinal cord compromise. Radiographic Assessment Initial imaging to evaluate for instability of the upper cervical spine should include lateral neutral, anteroposterior, and open-mouth odontoid views. Flexion-extension views should be obtained only when the spine is clearly stable and there is no recent history of trauma. The relationship of the foramen magnum to the atlas and odontoid can be measured by the McGregor, McRae, Chamberlain, Wackenheim, and Wiesel-Rothman lines as well as by the Power ratio (Figure 3). McRae’s line often is the easiest to discern for basilar invagination because the anterior and posterior rims of the foramen magnum usually are visible on radiographs, regardless of film qual- ity. McRae’s line connects the posterior rim of the foramen magnum to the anterior lip of the most caudal aspect of the foramen magnum (the basion). Chamberlain’s line is drawn from the posterior aspect of the hard Table 1 Possible Neurologic Findings of Upper Cervical Spine Instability Site of Compression Signs and Symptoms* Posterior spinal column involvement Changes in pain, proprioception, vibratory sense Anterior spinal column involvement Muscle weakness and atrophy, pathologic reflexes (hyperreflexia, spasticity, clonus), ataxia Cerebellar involvement Nystagmus, ataxia, incoordination Vertebral artery compression Syncopal episodes, decreased mental acuity, dizziness, seizures Cranial nerve involvement II (optic) Visual disturbances III (oculomotor) Ptosis, diplopia, strabismus IV (trochlear) Diplopia when looking downward V (trigeminal † ) Decreased facial sensation, weakness with mastication VI (abducens) Diplopia VII (facial) Paralysis of muscles of facial expression, loss of taste VIII (vestibulocochlear) Vertigo, nystagmus, hearing loss IX (glossopharyngeal † ) Dysphagia, absent gag reflex X (vagus † ) Hoarseness, dysphagia, dysphonia, decreased gag reflex, uvular deviation, cardiac and gastrointestinal abnormalities (parasympathetic input) XI (accessory † ) Paralysis of sternocleidomastoid and trapezius XII (hypoglossal) Asymmetrical tongue protrusion * It is common for children to present with combinations of findings. † Cranial nerves most commonly affected Nontraumatic Upper Cervical Spine Instability in Children 236 Journal of the American Academy of Orthopaedic Surgeons palate to the posterior rim of the foramen magnum. McGregor’s line is drawn from the most caudad point of the occipital curve of the skull to the posterior edge of the hard palate. Wackenheim’s line runs down the posterior surface of the clonus, with its inferior extension just touching the posterior tip of the odontoid. The atlantodens interval (ADI), the space between the posterior aspect of the anterior ring of C1 and the anterior border of the odontoid, should be <4 mm in children younger than age 8 years and become <3 mm in children age 8 years and older through adulthood 11,12 (Figure 3). The ADI measures maximally in flexion and can decrease in extension; therefore, measurements should be performed for both positions. Children with chronic instability at the atlantoaxial joint often have an ADI that is increased. In these instances, the space available for the spinal cord (SAC) should be measured. Steel’s rule of thirds should be used at C1, with the odontoid, the spinal cord, and additional space each occupying one third of the spinal canal. 11 In 2001, Wang et al 13 evaluated the development of the pediatric cervical spine radiographically, thus providing reference values to objec- tively assess the developing cervical spine, including the SAC. Their data show that the spinal canal markedly increases in diameter from birth to age 8 years; growth then slows but continues through adolescence. In contrast, the ratio of canal diameter to the corresponding vertebral body width linearly decreases from birth through adolescence. 13 Because canal diameters are correlated between adjacent levels, comparing the canal diameter above and below the suspected anomalous vertebrae is a highly sensitive approach to detecting spinal stenosis when it is suspected. Interpretation of plain posteroan- terior and lateral radiographs can be difficult in patients with conditions such as spondyloepiphyseal dyspla- Figure 3 Lateral craniometry. A, Lines used to determine basilar invagination and measurements of atlantoaxial instability. ADI = atlantodens interval, SAC = space available for the spinal cord. B, Method for calculating the Wiesel-Rothman line for atlanto-occipital instability. A line connecting the anterior and posterior arches of the atlas (points 1 and 2, respectively) is drawn. Two perpendicular lines to this line are then drawn, one through the basion (the line intersecting point 3) and the other through the posterior margin of the anterior arch of the atlas. The distance (x) between these lines should not change by more than 1 mm in flexion and extension. C, The Power ratio is calculated by drawing a line from the basion (B) to the posterior arch of the atlas (C) and a second line from the opisthion (O) to the anterior arch of the atlas (A). The length of line BC is divided by the length of line OA. A ratio ≥1.0 demonstrates anterior atlanto-occipital dislocation. (Reproduced from Copley LA, Dormans JP: Cervical spine disorders in infants and children. J Am Acad Orthop Surg 1998;6:204-214.) Brian P. D. Wills, MD, and John P. Dormans, MD Volume 14, Number 4, April 2006 237 sia because the mucopolysaccharidoses have abnormal bone. Radio- graphs of a child with multiple congenital anomalies of the upper cervical spine can be equally challenging to interpret; however, mag- netic resonance imaging (MRI) can be effective in diagnosing anomalies with instability of the upper cervical spine. MRI provides the additional benefit of allowing evaluation of the spinal cord and other soft tissues, including the spinal ligaments and disks, which can be only indirectly evaluated by computed tomography (CT). Dynamic MRI, in which imag- es are taken with the cervical spine in flexion and extension, can provide evidence of cord compression in patients who have signs and symptoms suggestive of cord compression but have normal plain radiographs. 14 CT also is useful to visualize osseous anomalies of the upper cervical spine that are difficult to interpret using plain radiographs. In addition, CT has been used dynamically to evaluate instability. 15 Occasionally, fluoroscopy and cineradiography also are indicated. When evaluating the pediatric cervical spine radiographically, it is important to keep in mind a number of features that are unique to the developing spine. Increased neck motion is seen in children younger than age 10 years for the following reasons: relative ligamentous laxity, relative muscle weakness, incomplete ossification of cartilaginous ele- ments, wedge-shaped vertebral bodies leading to decreased cervical lor- dosis (Figure 4), a more horizontal orientation of shallow facet joints, or decreased tensile strength of ligaments and facet capsules. 16 Apparent subluxation, termed pseudosubluxation, may be observed in radiographs of the cervical spine of healthy children. Pseudosublux- ation at C2-C3 (and less commonly at C3-C4) measuring up to 4 mm can be seen in 40% of children younger than age 8 years with normal cervical spines. 16 Also, when comparing flexion-extension radiographs, a pseudosubluxation should reduce in extension, whereas an actual subluxation will be maintained because of guarding and muscle spasm. Cattell and Filtzer 16 also noted that, during extension in young children, apparent overriding of anterior arch of the atlas relative to the odontoid may occur (Figure 4). This is a result of the nonossified ossiculum termina- le and also of the anterior body of C1, which may be only partially os- sified, depending on the child’s age. Syndromes and Conditions Associated With Instability Children with one or more of the syndromes and conditions frequently associated with anomalies and instability in the upper cervical spine should be routinely followed to pre- vent neurologic compromise (Table 2). Aside from the careful attention that must be given to the upper cervical spine, it also is important to maintain a high index of suspicion for serious underlying pathology in any child presenting with atraumatic neck pain and/or signs of myelopathy. The threshold for ordering cervical spine radiographs in these cases should be exceedingly low. Conditions Associated With Connective Tissue Abnormalities Down syndrome (trisomy 21) occurs in 1 in 700 to 1,000 live births and is associated with a number of medical conditions, including congenital heart disease and leuke- mia. 17 Instability of the cervical spine at both the atlanto-occipital and atlantoaxial levels, and hyper- mobility at one or both of these levels, is common. However, most of these patients remain asymptomatic. In a prospective study of 236 children with Down syndrome, instability at C1-C2 was noted in 17% of patients; however , only 18% of these patients were reported to be symptomatic. Thus, approximately 3% of children with Down syndrome, most of whom will present between the ages of 5 and 15 years, develop symptomatic atlantoaxial instabili- Figure 4 A, Lateral neutral radiograph of a normal cervical spine in a 3-year-old child. Note the wedge-shaped vertebral bodies and apparent high-riding atlas. B, Lateral neutral radiograph of a normal cervical spine in a 40-year-old patient for comparison. The vertebral bodies are rectangular in shape, and the anterior arch of the atlas no longer appears to override the odontoid. Nontraumatic Upper Cervical Spine Instability in Children 238 Journal of the American Academy of Orthopaedic Surgeons ty. 18 Orthopaedic surgeons generally agree that children with Down syndrome who have overt symptomatic instability of the upper cervical spine should undergo surgical stabilization. Preoperatively, all potential levels of instability should be evaluated. Before undertaking a stabilization procedure, we obtain flexion- extension MRI scans in all patients with suspected instability of the upper cervical spine in order to look for dural sac impingement. In the asymptomatic patient with upper cervical spine instability, indi- cations for surgical stabilization are less clear. At our institution, posterior arthrodesis is usually performed on asymptomatic Down syndrome patients with >8 to 10 mm of atlantoaxial instability and dural sac impingement on flexion-extension MRI. However, before proceeding with arthrodesis in Down syndrome patients with significant asymptomatic upper cervical spine instability, the importance of individualized patient assessment in deciding whether to perform occipital cervical arthrodesis cannot be overem- phasized. Postoperative complications such as incision and pin-site infection, and a reported 60% rate of pseudarthrosis, 19 are more common in patients with Down syndrome than in the general population. 19 The connective tissue defects reported in Marfan syndrome result from abnormalities in the protein fibrillin, predisposing patients to ligamentous and bony abnormalities in the cervical spine. These defects also predispose patients to increased risk of dissecting aortic aneurysm, ectop- ic lentis, and kyphoscoliosis. In a prospective series, atlantoaxial hy- permobility was noted in 18% of patients and basilar impression in 36%. 20 Similarly, patients with Ehlers-Danlos syndrome (EDS), par- ticularly type IV, may develop instability of the upper cervical spine because atlantoaxial subluxation has been reported in two of three patients with this type of Ehlers- Danlos syndrome. 21 Although Lar- sen syndrome is more commonly associated with cervical spine ky- phosis, which responds to early posterior spinal fusion, these patients also may develop instability of the upper cervical spine resulting from the underlying ligamentous laxity. 22 It also is important to evaluate for cervical stenosis when assessing a child with known ligamentous laxity because the space available for the spinal cord is affected by both Table 2 Conditions Associated With Pediatric Upper Cervical Spine Instability Syndromes Down syndrome (trisomy 21) Skeletal dysplasias Kniest dysplasia Chondrodysplasia punctata Metaphyseal chondrodysplasia Diastrophic dysplasia Kozlowski spondylometaphyseal dysplasia Metatropic dysplasia Spondyloepiphyseal dysplasia congenita Pseudoachondroplasia Campomelic dysplasia Mucopolysaccharidoses Morquio’s syndrome Maroteaux-Lamy mucopolysaccharidosis syndrome Hurler syndrome Mucopolysaccharidosis VII Klippel-Feil syndrome Marfan syndrome Hajdu-Cheney syndrome Goldenhar syndrome DiGeorge syndrome (22q11.2 deletion syndrome) Larsen syndrome Ehlers-Danlos syndrome Shprintzen-Goldberg craniosynostosis syndrome Dyggve-Melchoir-Clausen syndrome Marshall-Smith syndrome Weaver syndrome Spondylocarpotarsal synostosis syndrome Others Infectious/Inflammatory Conditions Pyogenic atlantoaxial rotatory subluxation (AARS; Grisel syndrome) Juvenile rheumatoid arthritis Juvenile ankylosing spondylitis Others Conditions With Acquired Instability Trauma Os odontoideum Cerebral palsy Others Brian P. D. Wills, MD, and John P. Dormans, MD Volume 14, Number 4, April 2006 239 stenosis and instability. In our experience, children with ligamentous laxity often have secondary cervical spine stenosis, a result of spinal cord compression both from the instability and from an underlying tight spinal canal. Skeletal Dysplasias The skeletal dysplasias are a col- lection of more than 200 conditions that have in common abnormalities in the development and remodeling of bone and cartilage. Dysplasias that commonly involve the cervical spine are spondyloepiphyseal dysplasia, diastrophic dysplasia, Kniest dysplasia, chondrodysplasia punctata, metatropic dysplasia, and metaphyseal chondrodysplasia. Patients with a skeletal dysplasia should undergo a skeletal survey and flexion-extension lateral cervical spine radiographic views during the initial visit to screen for the osseous anomalies. 23 The mucopolysaccharidoses are included in the International Classi- fication of Skeletal Dysplasias. 24 These include Morquio’s syndrome, in which odontoid aplasia or hypo- plasia causing C1-C2 instability is nearly universal; however, the instability can be effectively treated by posterior occipitocervical arthrodesis. 25 In our experience, patients with Morquio’s syndrome with C1- C2 instability nearly always require surgical fusion of C1 to C2 or of the occiput to C2 when the arch of C1 is incompetent, or when or there is occipitalization of C1. For children with instability of the upper cervical spine and an underlying diagnosis of skeletal dysplasia, the patient evaluation and treatment algorithm used is similar to that used for children with syndromes of ligamentous laxity. Before any surgical stabilization procedure, children with radiographic evidence of instability of the upper cervical spine should undergo flexion-extension MRI to assess any spinal cord impingement. Inflammatory and Infectious Conditions Instability of the upper cervical spine can result from the inflammatory reaction that follows adenoton- sillectomy and from other conditions that cause swelling of the soft tissues around the upper cervical spine. Oc- casionally, pyogenic atlantoaxial rotatory subluxation (AARS; Grisel’s syndrome) leading to atlantoaxial instability can result from adenotonsil- lectomy because of pathogens enter- ing the periodontoid vascular plexus after the procedure. W ith early recog- nition, isolation of the infectious or- ganism and treatment with appropriate antibiotics, and immobilization of the cervical spine, most patients fully recover. At our institution, patients with inflammatory AARS of less than 1 week’s duration are usually treated with nonsteroidal anti- inflammatory medication and fitted with a loose hard cervical collar until symptom resolution. When the AARS does not improve after 1 week, the patient is admitted for soft-halter traction. Patients with AARS that persists for >4 weeks are treated with traction until resolution followed by a cervicothoracic or thotic or halo ring and vest; skeletal traction may be needed to obtain resolution for these more resilient or for delayed presentation cases. At the occipitocervical junction, tuberculosis infection leading to instability also has been reported and should be considered in the differential diagnosis, especially in children with a history of international travel or with high- exposure risk. 26 Children with juvenile rheumatoid arthritis may present with an increased ADI as a result of inflammation of the transverse ligament and erosion of the odontoid because of synovial hypertrophy. As a result of chronic inflammation, lateral radiographs may show an apple core appearance of the odontoid in patients with long-standing juvenile rheumatoid arthritis. Actual instability is uncommon in this population, and neck pain and neurologic manifestations are infrequently associated with juvenile rheumatoid arthritis. The thinning of the odontoid does, however, make it more susceptible to fracture. 27 Juvenile ankylosing spondylitis most commonly presents with the sacroiliac joint and back pain or with peripheral arthritis. Atlantoaxial instability occurs infrequently, even in patients with chronic juvenile ankylosing spondylitis. However, atlantoaxial instability has been described as a presenting manifestation. 28 Thus, when patients with juvenile ankylosing spondylitis complain of neck pain or similar symptoms, instability of the upper cervical spine should be considered. Klippel-Feil Syndrome Klippel-Feil syndrome is charac- terized by congenital fusions and anomalies of the cervical spine. 29 Stenosis also is commonly seen in the cervical spine of these patients; the combination of stenosis and instability is the major factor in the development of myelopathy. Klippel- Feil syndrome often is associated with other musculoskeletal and organ anomalies, including scoliosis and renal and cardiac maldevelop- ment. Renal ultrasound and echocar- diogram should be performed on these children for further assessment. Auditory anomalies, neurologic abnormalities (synkinesis, or uncon- scious mirror movements), and skeletal anomalies (Sprengel’s deformity, cervical ribs) also may be present. The classic clinical presentation is a triad of low posterior hairline, short neck, and limited neck mobility; however, this triad occurs in less than half of patients with Klippel-Feil syndrome. 30 Patterns of malformation associated with a high risk for instability are those that limit cervical motion at one level; these include atlanto-occipital fusion with C2-C3 block vertebrae, abnormal atlanto- occipital junction with several distal block vertebrae, and a single open in- Nontraumatic Upper Cervical Spine Instability in Children 240 Journal of the American Academy of Orthopaedic Surgeons terspace between two block segments. 31 Children with these patterns of malformation should be monitored closely with annual physical examination and flexion- extension plain radiographs until age 10 years, then followed every 2 to 3 years through adulthood (Figure 5). Os Odontoideum Trauma is thought to be the most likely cause of os odontoideum. Damage to the basilar synchondrosis results in the separation of the odontoid from the body of the axis. 32 At- lantoaxial instability then develops because the odontoid is not a functional stabilizer. These patients often present in late adolescence with complaints of atraumatic local neck pain. Open-mouth odontoid views demonstrate an oval ossicle located in place of the normal odontoid tip. 32 CT is useful to confirm os odontoideum when plain radiographs are questionable. Management of Nontraumatic Upper Cervical Spine Instability Preventive Care, Injury Prevention, and Sports Participation Patients with syndromes associated with instability of the cervical spine (Table 2) should undergo screening studies consisting of lateral neutral, anteroposterior, and open- mouth odontoid views. Flexion and extension views should be obtained only when the patient is neurologi- cally stable, there is no history of recent significant trauma, and there are no findings in the history or physical examination to suggest gross cervical spine instability. These children should be routinely seen by an orthopaedic surgeon for a careful history, physical examination, and repeat radiographs, in addition to regular visits to the pediatrician. Although treatment should be individualized, children younger than age 10 years usually should be seen annually and then, from age 10 years through adulthood, every 2 to 3 years. By age 10 years, the cervical spine has largely taken on adult characteristics, which decreases the likelihood that stability will develop. Patients with congenital syndromes, such as Morquio’s syndrome, may benefit from multidisciplinar y care programs. The orthopaedic surgeon should educate patients and their families about the natural history of the condition and potential medical problems, emphasizing that, if any neurologic symptoms develop, the child should be seen immediately by a physician trained in the detec- tion of instability of the cervical spine (Table 1). Symptomatic patients should undergo additional workup, such as CT and MRI, in addition to plain radiographs. Flexion-extension MRI should be obtained in symptomatic patients who demonstrate instability on plain radiographs. When these imaging studies demonstrate dural sac compression, spinal fusion usually is indicated. As discussed, asymptomatic patients who initially present with evidence of instability are challenging Figure 5 A and B, Lateral flexion-extension preoperative radiographs of a 3-year-old with Klippel-Feil syndrome demonstrating a block vertebrae of C2-C3 and assimilation of C1 with occiput. The atlantodens interval is grossly widened, indicating instability of C1-C2. C and D, Postoperative lateral flexion-extension postoperative radiographs taken 16 months after occipitocervical arthrodesis demonstrating solid fusion of the occiput to C2-C3. Brian P. D. Wills, MD, and John P. Dormans, MD Volume 14, Number 4, April 2006 241 to treat. These children often have a baseline ADI greater than is accept- able for normal children, which makes establishing a guideline for prophylactic fusion difficult. Chil- dren with particular syndromes associated with instability of the upper cervical spine, such as Down syndrome, will remain unstable but asymptomatic throughout their life- times. Other conditions, such as Morquio’s syndrome, frequently have progressive instability; therefore, these patients should undergo preventive fusion before neurologic symptoms develop. However, most patients fall somewhere between these two ends of the spectrum in terms of risk for developing symptomatic instability. Thus, for those in whom upper cervical spine instability is suspected, determining the degree of instability at the initial visit is important in order to make baseline radiographic measurements, which are then repeated at each follow-up visit and compared with the baseline. Patients whose upper cervical spine instability is progressing are candidates for preventive surgical stabilization. Sports participation remains con- troversial for children with asymptomatic instability of the cervical spine. Children with congenital fusions resulting from Klippel-Feil syndrome are in this category. For such children, contact sports and sports that involve excessive bending; twisting; or axial loading of the neck, such as diving and gymnastics, could lead to catastrophic neurologic injury. We recommend that children with demonstrated instability of the upper cervical spine be dis- couraged from participating in these high-risk activities, although the de- cision to participate in these sports must be made on an individual basis. In addition, patients who have un- dergone surgical stabilization of the cervical spine should not participate in high-risk sports. In 1983, the Special Olympics mandated cervical spine screening with plain lateral and flexion- extension views in all Down syndrome patients participating in high- risk sports. However, the American Academy of Pediatrics Committee on Sports Medicine has concluded that “lateral plain radiographs of the cervical spine are of potential but un- proven value in detecting patients at risk for developing spinal cord injury during sports participation.” 33 In- stead, the committee recommended, as the greater priority, identifying patients with signs or symptoms con- sistent with symptomatic spinal cord injury . 33 In our opinion, children with possible upper cervical spine instability should be screened radiographically for several reasons. 34 Screening radiographs not only allow for assessment of cervical spine instability but also establish a baseline for future reference; thus, screening radiographs allow for evaluation for possible congenital bony anomalies. Further- more, they provide reassurance for families of patients with normal studies. They also provide helpful in- formation for patients with poor communication skills or those un- able to cooperate with a history and physical examination. 34 Surgery The posterior surgical approach, which allows for cord decompres- sion when that is indicated, is most commonly used for treatment of instability of the upper cervical spine. Several techniques of stabilization using rigid plate implants, wiring techniques, and bone graft materials have been described. For isolated atlantoaxial instability, the technique of Brooks-Jenkins is the most commonly used method of arthrodesis at our institution. 35 Recently, transarticular C1-C2 fixation with facet screws has been reported with good results in a large series predominant- ly made up of adults, but including some children. 36 However, this procedure historically has not been performed in children, and there are no reported pediatric series. The prox- imity of the vertebral artery and C2 spinal nerve makes transarticular C1-C2 fixation technically demand- ing, and the relatively small bone mass of C1 and C2 in young children greatly increases risk to these structures. In skeletally mature children, however, transarticular facet screws provide rigid fixation and in selected cases may eliminate the need for halo ring and vest immobilization after arthrodesis. The use of lateral mass plates and screws for rigid internal fixation may be appropriate, especially in older children and when the lower cervical spine will be incorporated into the fusion. Patients with occipitoatlantal instability and those in that group with atlantoaxial instability who require more extensive fusion (ie, because of a coexisting incompetent posterior atlantal arch or occipitalization of C1) are treated with occipitocervical arthrodesis. Two techniques of occipitocervical arthrodesis (Figures 5 and 6) have been developed, both of which can be adapted for abnormal osseous anatomy seen in some congenital conditions. 37,38 Instrumenta- tion using a Luque rectangle, as well as other methods of rigid internal fixation using screws with rods and/or plates, also have been described. 39 Al- though their use is usually indicated only in the setting of an intraspinal tumor or infectious process, techniques involving anterior or transoral approaches for upper cervical spine instability in children have been described. In children with a history of intraspinal tumor or with a condition in which future MRI is anticipated, the use of MRI-compatible titanium instrumentation is preferred to stainless steel because the ferromagnetic properties of stainless steel can make future MRI studies difficult to interpret. 40 For intraoperative positioning and prolonged postoperative cervical spine immobilization, the halo ring and vest offer better immobilization and positioning with fewer skin complications. They also allow for Nontraumatic Upper Cervical Spine Instability in Children 242 Journal of the American Academy of Orthopaedic Surgeons

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