Developmental Abnormalities of the Craniocervical Junction

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CHAPTER 218 Developmental Abnormalities of the Craniocervical Junction

The first anatomic description of “manifestations of occipital vertebrae” was attributed to Meckel in 1815 by Gladstone and Erickson-Powell.¹ Its clinical significance became appreciated after the classic radiologic studies on basilar invagination by Chamberlain in 1939.²

The term craniovertebral junction refers to the occipital bone that surrounds the foramen magnum and the atlas and axis vertebrae. Into the 1970s, surgical treatment of conditions affecting the craniovertebral junction consisted of posterior decompression by enlargement of the foramen magnum and removal of the posterior arch of the atlas and axis vertebrae. However, mortality and morbidity rates associated with this treatment were high in patients with irreducible lesions and cervicomedullary compression.³ A physiologic approach based on an understanding of craniovertebral dynamics, the site of encroachment, and stability of the craniovertebral junction was adopted at the University of Iowa Hospitals and Clinics in 1977. Since then, 6000 patients with neurological symptoms and signs secondary to an abnormality in the craniocervical region have been studied. Understanding of the pathology of these abnormalities and their treatment is simplified if one has knowledge of the bony anatomy, biomechanics, and embryology of the region.

Anatomy of the Craniovertebral Junction

Bone-Ligament Complex

The atlantoaxial complex is unique among the intervertebral joints in that it is horizontally oriented. The lateral facet joints are relatively flat and allow a pivoting motion at the atlantodental articulation, which is permitted by the special ligamentous support. The second cervical nerve exits from the cervical canal immediately adjacent and dorsal to the joint capsules. The transverse atlantal ligament is a band 3 to 5 mm thick that originates from the tubercles and the inner aspect of the lateral masses of the atlas vertebra and is in close apposition to the odontoid; this ligament allows axial rotation.

By itself, the geometry of the craniovertebral complex is meant to provide mobility at the cost of stability.⁴ However, the complex is maintained in a clamp-like vise by the craniovertebral musculature, which assists in stabilization of the spine, initiates movements within the joints, and maintains fluidity of the motion complex.

Blood Supply

The blood supply to the odontoid process is via anterior and posterior ascending vessels from the vertebral arteries, with a contribution from the carotid arteries, which form an apical arcade around the alar ligament.⁵ Thus, the blood supply of the odontoid process is vulnerable with a type II odontoid fracture.⁶

Parke and coworkers demonstrated pharyngovertebral veins with frequent lymphovenous anastomoses.⁷ This connection may provide an additional route for septic involvement of the craniovertebral complex, which can result in osteomyelitis of the bone, as well as joint effusions.

Lymphatic Drainage

Lymphatic drainage of the occipitoatlantoaxial joint complex is primarily into the retropharyngeal lymph nodes and then into the upper deep jugular cervical chain.³ These nodes also receive drainage from the nasopharynx, paranasal sinuses, and retropharyngeal area. A retrograde infection may affect the synovial lining of the craniovertebral joint complex and cause an inflammatory effusion, instability, and possible neurological deficit, thereby contributing to the so-called Grisel syndrome.⁸

Embryology and Development of Craniovertebral Junction Disorders

Congenital anomalies of the base of the skull and the atlanto-occipital region involve both the osseous structures and the nervous system. The frequent occurrence of patterns with various combinations suggests an interrelationship between if not a common cause of the origin and development of these structures.³

The bony cranial base is formed by the process of endochondral ossification in which a cartilaginous framework is first developed and subsequently resorbed, with further deposition of bone based on distorting forces such as brain and eye development.⁹^,¹⁰ The clivus is elongated by sutural growth of the spheno-occipital synchondrosis and by further sutural growth along the lateral portion of the base.

The majority of the skull and facial bones develop by intramembranous ossification. Such development bypasses the intermediate cartilaginous stage characteristic of development of the bony cranial base.¹⁰^,¹¹

The occipital sclerotomes correspond to the segmental nerves that group together to form the hypoglossal nerve, which passes through the bone via the individual foramina.⁶^,¹⁰ The first two occipital sclerotomes ultimately form the basiocciput. The third sclerotome is responsible for the exoccipital center as it forms the jugular tubercles.¹¹^,¹² The key to understanding the craniovertebral junction is the embryology of the fourth occipital sclerotome, which in lower animals is called the proatlas. The hypocentrum of the fourth occipital sclerotome forms the anterior tubercle of the clivus. The centrum of the proatlas itself forms the apical cap of the dens, as well as the apical ligament. The neural arch component of the proatlas divides into a ventral-rostral component and a caudal-dorsal portion. The ventral portion forms the U-shaped anterior margin of the foramen magnum, as well as the occipital condyles and the midline occipital condyle. The cruciate ligament and the alar ligaments are condensations of the lateral portion of the proatlas. The caudal division of the neural arch of the proatlas forms the lateral atlantal masses of C1, as well as the superior portion of the posterior arch of the atlas.

The atlas vertebra is formed by the first spinal sclerotome. It is modified from the remaining spinal vertebrae, and the centrum separates to fuse with the body of the axis and form the odontoid process. The neural arch of the first spinal sclerotome forms the posterior and inferior portions of the atlas arch. The hypochordal bow of the proatlas itself may survive and join with the anterior arch of the atlas to form a variant in which an abnormal articulation may exist between the clivus, the anterior arch of the atlas, and the apical segment of the odontoid process.

During embryogenesis, the hypocentrum of the second spinal sclerotome disappears. The centrum forms the body of the axis vertebra, and the neural arches develop into facets and the posterior arch of the axis. Thus, the body of the dens arises from the first sclerotome, whereas a terminal portion of the odontoid process arises from the proatlas. The most inferior portion of the body of the axis is formed by the second spinal sclerotome. At birth, the odontoid process is separated from the body of the axis vertebra by a cartilaginous band that represents a vestigial disk and is referred to as the neural central synchondrosis.⁶^,¹³ It lies below the level of the superior articular facets of the axis and does not represent the anatomic base of the dens. This synchondrosis is present in most children younger than 3 to 4 years and disappears by 8 years of age. In most instances, the odontoid process is seen at birth but does not fuse to the base of the axis.¹⁴ The tip of the odontoid is not ossified at birth and thus is not seen on lateral radiographs. It is represented by a separate ossification center, which is usually seen at 3 years of age and fuses with the remainder of the dens by the age of 12 years.

Expansion of the posterior fossa occurs as a result of a combination of endochondral resorption, sutural growth, and bony accretion.¹⁵ Growth of the basion elongates the basiocciput and lowers the frontal margins of the foramen magnum. There is a comparably matched resorptive drift downward and backward at the opisthion as a result of downward displacement of the cerebellum, together with rotation of the occipital and temporal lobes of the brain.¹⁶

Because of the forward inclination of the top-heavy cranial end of the fetus, the stability of the craniovertebral articulation must be maintained by the geometry of the articular surfaces of the craniovertebral junction, as well as by the ligamentous attachments and, more importantly, by the heavy development of the dorsal and lateral cervical musculature, which provides a clamping action on the craniovertebral region.³^,⁴

Significant advances in recent years have occurred with the discovery of developmental control genes.9–11,15–18 Two families of regulatory genes have been implicated in the subsequent development of the sclerotomal portions of the somites during their resegmentation to form the vertebral primordia and in specification of the identity of each vertebra. Common to all these genes are phylogenetically highly conserved DNA sequences termed homeovox (Hox genes) and paired vox (Pax genes) sequences. They promote the production of proteins that modulate morphogenesis by influencing the transcription of specific downstream genes.

Teratogen-induced disturbances in Hox gene expression and mutations in Hox genes can cause alterations in both the number and identity of the cervical vertebrae forming at or near the limit of their expression domain. For example, inactivation of the Hox-D3 gene results in mutant mice with assimilation of the atlas to the basiocciput.¹⁹ The sensitivity of the occipitocervical junction to disturbances in Hox gene expression might prove to be the underlying cause of malformations in this region. Pax genes are expressed in diverse cell types and contribute to development of the early nervous system. Control of resegmentation of the sclerotomes to establish the intervertebral boundaries seems to be independently regulated by two genes in the Pax family.¹⁷^,²⁰

Implications of Craniovertebral Abnormalities

A wide variety of congenital anomalies of the craniovertebral junction exist and can occur singularly or multiply in the same individual and involve both osseous and neural structures. An insult to both types of structures may occur between the fourth and seventh weeks of intrauterine life and result in a combination of anomalies consisting of failure of segmentation, failure of fusion of different components of each bone, hypoplasia, and ankylosis.

Congenital diseases involving connective tissue, such as Morquio’s syndrome and other mucopolysaccharidoses, as well as Down syndrome, can lead to severe atlantoaxial subluxation.^3,^14,²¹ There is a high incidence of anterior and posterior spina bifida of C1 and also os odontoideum in these instances. It is possible that because of abnormal, excessive head movements in the embryo between days 50 and 53, the process of chondrification is impaired, thereby resulting in anterior and later posterior spina bifida of C1.¹⁶

Os odontoideum was previously thought to be congenital and was described as a failure of fusion between the centrum component of C1 and that of C2. However, this radiographic abnormality always has a hypoplastic dens, and the neural central synchondrosis is a definite visible entity.¹³ To be congenital, the defect would have to be below the superior slopes of the C2 facets.⁶ Os odontoideum most likely originates as odontoid trauma that may predate birth but certainly occurs before the age of 4 years, with subsequent distraction and separation of the odontoid fragments.³^,²² The superior segment is distracted upward by the apical and alar ligaments and receives its blood supply from the descending branch of the occipital artery, which courses along the apical ligament. This subsequently leads to incompetence of the cruciate ligament and further abnormalities.

Spine trauma in children younger than 8 years is mainly centered at the craniovertebral border because of the high fulcrum of neck motion.³ It results in ligamentous injuries more often than fractures. However, odontoid fractures in this age group are usually seen as avulsion injuries with separation of the neural central synchondrosis.

Anomalies and malformations of the caudal occipital sclerotomes are collectively called “manifestations of occipital vertebrae” and result in abnormal bony ridges and outgrowths of the ventral aspect of the foramen magnum.¹ At times, segmentation abnormalities of the clivus are recognized and represent failure of the last occipital sclerotome to separate from the basiocciput and clivus. Consequently, bone growth in the anterior aspect of the foramen magnum indents the ventral cervicomedullary junction with age (as the clivus expands inferiorly and dorsally).

Ongoing growth of the posterior fossa from birth into late adolescence provides a rationale for the need to monitor children who have undergone dorsal occipitocervical stabilization or ventral craniovertebral decompression. The downward growth of the brain, as well as elongation of the posterior fossa and clivus, may re-create a ventral bony abnormality later in life despite previous performance of ventral decompression at the craniovertebral junction during the first 2 decades of life.

Biomechanics

The unique anatomic configuration of the craniovertebral junction creates a distinct biomechanical behavior that differs from that of the other spinal joints. The motion characteristics of the different levels of the craniovertebral junction are due to the geometry of the opposing bones of the vertebrae and the skull base, the shape of the joints, and the arrangements of the ligaments.^4,²³^–²⁵ An intervertebral disk is absent between the occiput, C1, and C2. The ball-and-socket–shaped occiput-C1 joint allows slightly more flexion-extension than the other levels of the cervical spine do, although it is quite rigid in axial rotation and lateral bending.

The biconvex articular surfaces of the C1 and C2 joints allow gliding and wide rotation of C1 around the dens. The atlantoaxial motion segment is the most flexible motion segment in the entire spine with respect to axial rotation in that it allows a bilateral range of motion of 90 degrees. More than half of all cervical axial rotation occurs at the C1-2 motion segment, a point that should be kept in mind when considering atlantoaxial arthrodesis. In a child, the amount of anterior-posterior translation that occurs between the dens and the anterior ring of the atlas is up to 5 mm. When the transverse component of the cruciate ligament has been disrupted, the alar ligaments are still intact.²⁵ Hence, the amount of displacement remains between 5 and 6 mm until the alar ligaments become incompetent. It is only when the alar ligaments and the transverse portion of the cruciate ligament are incompetent that a separation of more than 5 to 6 mm occurs. Beyond the age of 8 years, excursion of the predental space must be limited to 3 mm.²⁶

Failure of one alar ligament results in moderate rotary atlantoaxial instability.²⁷ Bilateral transection of the alar ligaments causes considerably more alterations in craniovertebral motion, and the coupling action is disturbed. The transverse atlantal ligament is the strongest and thickest ligament in the entire spine. It is the predominant stabilizer of the atlas and constrains C1 around the dens. When torn, the transverse ligament is incapable of repair. Because injury to this ligament renders C1 grossly unstable, C1-2 fusion is mandated. The largest extent of rotation occurs at the atlantoaxial joint. When rotation exceeds 40 to 50 degrees, an interlock of the lateral inferior facet of the atlas over the superior articular facet of the axis vertebra occurs. If the transverse ligament becomes deficient, the anterior arch will sublux forward and result in unilateral dislocation and an interlock at less than 40 degrees.¹³^,²⁶ Rotation of more than 30 to 35 degrees produces an angulation of the contralateral vertebral artery.²⁵^,²⁸ With greater rotation, the vertebral artery is stretched, and the ipsilateral vessel may demonstrate occlusion.²⁸^,²⁹ This phenomenon has an implication in wrestling and football injuries and in certain head rotation maneuvers performed under general anesthesia or forced chiropractic manipulations.³⁰^,³¹ There is a muscle-clamping action that takes place in vivo and produces compressive force across the cervical spine. This muscular action results in the “interlocking stiffening” that maintains alignment during rotation. When the protective muscles are relaxed or inadequately developed, as in the case of a young child under general anesthesia, the craniovertebral junction becomes inherently less stable than that in an adult.³ As muscular development occurs, there is less tendency for instability at the craniovertebral junction.

Biomechanical Comparison of Cervical Orthoses

An orthosis acts as a restraint to restrict spinal motion or immobilize the surrounding tissues. Orthoses are used as load-sharing devices to reduce the external loads applied to the spine while an injury or a fixation fusion is healing.

A halo brace is most effective at constraining cervical motion at all levels, and control of spinal motion is related to the distance of the spinal segment from the region of the rigid skull fixation.³^,²⁷ A halo brace provides the best control of motion at the craniovertebral junction, intermediate control of midcervical motion, and the least control of motion at the cervicothoracic junction. This pattern reflects snaking movements of the spine.

The soft cervical collar provides little immobilization and tends only to remind the patient to restrict motion. Molded collars, four-poster braces, sternal-occipital-mandibular immobilization, and cervicothoracic braces have intermediate effects in controlling flexion and extension, lateral bending, and axial rotation.

Classification of Craniovertebral Junction Abnormalities

A wide variety of congenital, developmental, and acquired abnormalities can occur at the craniovertebral junction, and there may be single or multiple abnormalities in the same individual. The pathology of these abnormalities is extensive.³ A working classification has been provided, but it must be appreciated that there are overlapping causes within this classification (Table 218-1).

TABLE 218-1 Classification of Craniocervical Junction Abnormalities

Epidemiology

Abnormalities of the craniovertebral junction must be suspected in infants with Goldenhar’s syndrome, skeletal dysplasias, and Conradi’s syndrome.³ It should also be suspected in infants with “torticollis.” Diseases such as Down syndrome are associated with a 14% to 20% incidence of atlantoaxial dislocation. In Morquio’s syndrome, a combination of atlantoaxial instability, os odontoideum, and cervicothoracic abnormalities occurs in 30% to 50% of individuals.^3,^14,³² The incidence of atlas assimilation is 0.25% in the general population. Many such patients have associated segmentation failure of the upper cervical spine.³³

We have reviewed syndromes cited as altering the craniovertebral junction in more than 6000 patients seen at our institution. A search of the scientific literature identified genetic syndromes that affect the craniovertebral junction. In addition, the Online Mendelian Inheritance in Man (OMIM) database was used to identify disorders of the craniovertebral junction encountered in various clinical populations. The information recorded includes systemic manifestations of the syndromes, the effect on the craniovertebral junction and cervical vertebrae, inheritance pattern, and genes or genetic loci associated with the disorder. A total of 84 syndromes have been identified that affect the craniovertebral junction.³⁴

Once the stage is set by congenital craniovertebral anomalies, the developmental and acquired phenomena may supervene and produce atlantoaxial instability and subsequently basilar invagination. This is much more common in developing countries where heavy loads are carried on the head from childhood. Thus, the erroneous diagnosis of “congenital dislocations” has been reported. Likewise, upper respiratory tract infections that lead to a stiff neck, torticollis, and ligamentous instability come to attention much later in developing countries than in places where medical attention is more readily available. For this reason, it appears that abnormalities of the craniovertebral junction are more frequently encountered in populous, less advantaged countries.

Clinical Findings

The most interesting feature of the clinical manifestation of craniovertebral abnormalities is their diversity as a result of compromise of the lower brainstem, cervical spinal cord, cranial nerves, cervical roots, and vascular supply.⁶ The symptoms of craniovertebral dysfunction may be insidious and be manifested as false localizing signs. Infrequently, rapid neurological progression is followed by sudden death. More often than not, an antecedent history of minor trauma induces a pattern of symptoms and signs that may progress at a galloping pace.

Congenital abnormalities of the craniovertebral junction are frequently associated with an abnormal general physical appearance. The head may be cocked to one side, as in patients with rotary luxation of the atlas on the axis, or the classic triad of Klippel-Feil syndrome (abnormally low hairline posteriorly, limitation of neck motion, and short neck) may be noted. Facial asymmetry and webbing of the neck may occur in conjunction with this syndrome. At times, scoliosis is present. It is not uncommon to see children with a small dysmorphic stature. There is an increased incidence of craniovertebral abnormalities with disease states such as achondroplasia, spondyloepiphyseal dysplasia, and related diseases of dwarfism. The most common neurological symptoms and signs are enumerated in Table 218-2.

TABLE 218-2 Signs and Symptoms of Craniovertebral Anomalies (Insidious or Rapid Onset of Symptoms and Signs)

The most frequent symptom is neck pain originating in the suboccipital region with radiation to the cranial vertex, which occurs in 85% of patients. False localizing signs associated with abnormalities of the foramen magnum are usually of a motor nature and include monoparesis, hemiparesis, paraparesis, and quadriparesis. Central cord syndrome is often seen in children with basilar invagination, and in such children the myelopathy mimics a lower cervical spinal cord disturbance.

Sensory abnormalities are usually manifested as neurological deficits related to posterior column dysfunction. Brainstem and cranial nerve deficits cause abnormalities such as dysphagia and sleep apnea. Not uncommonly, internuclear ophthalmoplegia is present and can lead to a misdiagnosis of mesencephalic and upper pontine disturbance. Downbeat nystagmus is seen with strictly compressive lesions of the craniovertebral border, with or without an associated Chiari malformation.

The phenomenon of basilar migraine affects about 25% of children with basilar invagination and compression of the medulla. It usually involves compression of the vertebrobasilar arterial system. The symptoms regress with decompression of the area and stabilization. The excessive mobility of an unstable craniovertebral junction may cause repeated trauma to the anterior spinal artery and the perforating vessels of the upper cervical cord and medulla oblongata, as well as the vertebral and basilar arteries. This may lead to spasm or occlusion and attendant neurological deficits.

The most common neurological deficit encountered in affected children is myelopathy, and the most common cranial nerve dysfunction is hearing loss, which occurs in 25% of cases. There has been an increased incidence of this finding in patients with Klippel-Feil syndrome. Unilateral or bilateral paralysis or dysfunction of the soft palate or pharynx may lead to repeated bouts of aspiration pneumonia, as well as poor feeding and inability to gain weight.

Vascular symptoms such as intermittent attacks of altered consciousness, transient loss of visual fields, confusion, and vertigo appear in 15% to 25% of patients with abnormalities of the craniovertebral junction. This may be provoked by extension or rotation of the head, as occurs during manipulation of the head and neck.

Neurodiagnostic Imaging

The diagnostic procedure of choice is magnetic resonance imaging (MRI), but plain cervical spine radiographs with lateral flexion and extension views are essential to gain a better understanding of the biomechanics. Numerous craniometric reference lines were described before the advent of computed tomography (CT) and MRI.^33,³⁵^–³⁸ They were used to locate the position of neural structures with the associated osseous abnormality (Fig. 218-1). Chamberlain’s palato-occipital line joins the hard palate to the posterior edge of the foramen magnum.² In normal individuals, the tip of the dens should lie below this line or at most 3 to 5 mm above it. Basilar invagination occurs when the dens projects above this line. Wackenheim’s clivus canal line is of greatest significance.³⁷^,³⁸ This line is drawn along the clivus and extrapolated into the cervical spinal canal. The odontoid process must be ventral to this line or tangential to it. The odontoid process transects this line in patients with basilar invagination, atlantoaxial dislocation, and anterior occipitoatlantal dislocation. McRae’s foramen magnum line joins the anterior and posterior edges of the foramen magnum.³³ The tip of the dens must be below this line. If sagittal space for the cervicomedullary junction is less than 20 mm in a child older than 8 years, a neurological deficit is usually present.

FIGURE 218-1 Lateral craniometry with points of reference.

MRI is used to identify the neural abnormalities and osseous compression, as well as to recognize any instability if present. With this examination, flexion and extension dynamic views are obtained in the parasagittal dimension in both the T1- and T2-weighted modes. Axial views are supplementary. The effects of cervical traction are also documented with MRI to visualize the neural-osseous relationships. Magnetic resonance angiography and CT angiography can provide a better understanding of the vasculature, as well as any vascular deformities that may occur with motion of the craniovertebral junction.

Three-dimensional CT is useful for defining the abnormality, as well as for facilitating coronal and sagittal sectioning.³⁸^,³⁹ We have found CT to be invaluable in recognizing the presence or absence of epiphyseal growth plates in toddlers and young children and for assessing the extent of fusion and segmentation defects, as well as “missing” osseous components. Spondylolisthesis and spondylolysis are best appreciated with three-dimensional studies.

Each of the aforementioned imaging modalities provides complementary information to define the craniovertebral deformity.

Treatment

Several factors have an impact on the specific treatment of abnormalities of the craniovertebral junction ⁶: (1) reducibility of the bony lesion (i.e., the ability to restore anatomic alignment and thereby relieve the compression on neural structures); (2) the mechanics of compression and the direction of encroachment; (3) the cause of the pathologic process, as well as the presence of hindbrain herniation, syrinx, and vascular abnormalities; and (4) the presence of abnormal ossification centers and epiphyseal growth plates.

The primary aim of treatment is to relieve compression at the cervicomedullary junction. Stabilization is paramount in reducible lesions to maintain the neural decompression. Irreducible lesions require decompression at the site at which the compression has occurred; these can be subdivided into ventral and dorsal compression states. In the former, the operative procedure is ventral decompression through a transpalatopharyngeal route, a Le Fort drop-down maxillotomy, or a lateral extrapharyngeal route. In dorsal compression states, posterolateral decompression is required. If instability is present after decompression, posterior fixation is mandated for stability. It is thus necessary to base selection of the operation or combination of operations for each individual patient on a clear understanding of the pathophysiology and the functional anatomy.

Disease states such as spondyloepiphyseal dysplasia, mucopolysaccharidosis, osteogenesis imperfecta, Goldenhar’s syndrome, and allied situations in very young children present an interesting and difficult problem of instability at the craniovertebral junction. In such circumstances, it is essential that the treating physician be able to identify the potential for osseous development by recognizing the epiphyseal growth plates, which can be seen only on thin-section CT. When there is an absence of growth plates, development can be allowed to take place by supporting the occipitocervical region with custom-built cervical orthoses that are revised every few months. The toddler or young child is then reevaluated periodically with diagnostic procedures aimed at identifying the status of the developing craniovertebral junction and the neural compromise. Once the child is 3 to 4 years of age (when all components of the craniovertebral junction are present), surgical therapy may be attempted if the situation remains unchanged.

Skeletal traction should be performed with an MRI-compatible halo device. We prefer a crown halo rather than a complete ring because distortion on MRI is minimized. In children younger than 2 years, 8- to 10-point skull fixation is used. Pin pressure is maintained with the thumb and forefinger. This generates no more than 1 to 2 lb of torque. At 5 years of age, the maximal pin pressure needed is 4 lb. Traction is initiated at a very low rate, starting at 3 to 4 lb in a 5-year-old child and not exceeding 7 lb. In an adult, traction should start at 7 lb and not exceed 15 lb for the craniovertebral region. It is imperative that the crown of the halo be placed below the equator of the cranium so that the pins do not migrate and slip.

Reducible lesions that are the result of inflammatory states or recent trauma will respond to conservative management with external immobilization once reduction is achieved. The healing is usually ligamentous and may include bony reconstitution. If healing does not occur or if the condition is not the result of trauma or infection, bony fixation is mandated.

Atlantoaxial arthrodesis requires a minimum of 3 months of immobilization for fusion. For immobilization we use a custom-built occipitocervical shell orthosis for 5 to 6 months with fusions that extend to the occiput. Rates of fusion failure reach 50% when the immobilization is inadequate. Wire fixation between the cervical vertebrae or between the skull and the cervical spine should be avoided.

Developmental Abnormalities Affecting the Craniovertebral Region

Grisel’s Syndrome

Grisel’s syndrome is defined as an inflammatory, spontaneous subluxation that affects the atlantoaxial joints after parapharyngeal infection.⁸ It may occur with tonsillitis, mastoiditis, retropharyngeal abscess, and otitis media. Most affected children are younger than 12 years. These children have torticollis, neck pain, or a neurological deficit.⁴⁰ The severe subluxation may produce symptoms and signs of cervical cord compression (Fig. 218-2).

FIGURE 218-2 Three-dimensional computed tomographic scan of the craniovertebral junction and upper cervical spine in a 7-year-old with significant rotational abnormality of the occipitoatlantoaxial joints. Note the perching of the occiput on the right lateral atlantal mass of C1. Rotational luxation of C1 on C2 is present and is best visualized on frontal (8A), lateral (8B), and upward view into the craniocervical region (8C). The patient had undergone tonsil and adenoid resection a few days previously. AR, posterior; F, caudal; H, cranial; LP, anterior.

In 1987, Wilson and coworkers reviewed 62 cases of Grisel’s syndrome,⁴¹ including 14 children in whom subluxation developed after tonsillectomy, adenoidectomy, mastoiditis, and resection of a pharyngeal rhabdomyosarcoma. Twelve children had symptoms of pharyngitis or cervical adenitis, 7 had tonsillitis, and 7 had cervical abscesses. There were 5 children with acute rheumatic fever and 4 with acute mastoiditis. Several patients were assigned a diagnosis of nonspecific parapharyngeal infection.

The erythrocyte sedimentation rate is invariably elevated. MRI identifies parapharyngeal soft tissue masses and the presence of dislocation and osteomyelitis or bone erosion. Needle biopsy of the prevertebral masses is essential to confirm the presence of a pyogenic focus and to obtain a specimen for bacterial culture. Appropriate antibiotics must be administered intravenously as quickly as possible for treatment of the primary infection. Reduction of the dislocation by manipulation with cervical traction is reserved for gross dislocations. Otherwise, immobilization with a sterno-occipital mandibular immobilizer is sufficient. However, occipitocervical dislocation requires halo immobilization. It is only rarely necessary to perform fusion.

Down Syndrome

Down syndrome is easily recognized by the characteristic facial features, hypotonia, mental retardation, ligamentous laxity, and transverse palmar creases.³ Almost every organ is involved. This syndrome is the most commonly recognized chromosomal abnormality in humans, with an incidence of 1 in 700 live births. Craniovertebral instability in Down syndrome has received increasing attention since the report by Spitzer and associates of occipitoatlantal dislocation in 9 of 26 patients investigated with Down syndrome. However, atlantoaxial instability in Down syndrome has been described most since the initial publication by Tisher and Martel in 1965.⁴² The incidence of atlantoaxial instability is 14% to 24% in patients with Down syndrome, although the incidence of symptomatic atlantoaxial instability is believed to be less than 1% (Fig. 218-3). The prevalence of bony abnormalities such as os odontoideum, hypoplastic odontoid process, and rotary atlantoaxial luxation in patients with Down syndrome has caused concern about their participation in the Special Olympics, and it has been recommended that cervical spine radiographs be required for such participants.⁴³ An atlantodental interval of greater than 4.5 mm was thought to require medical attention.⁴⁴

FIGURE 218-3 Midsagittal T2-weighted magnetic resonance image of the posterior fossa and upper cervical spine in a 13-year-old with Down syndrome. There is posterior displacement of the axis body with a marked constriction of the subarachnoid sac at the cervicomedullary junction. This patient was quadriparetic.

The lack of attendant neurological symptoms and signs has resulted in the unfortunate belief that these children do not require surgical attention. Minor trauma, as well as upper respiratory infection, has resulted in severe neurological deficits in children with previously recognized pathologic instability without any intervention.^4,^35,⁴⁵

There are pessimistic reports on the outcome of arthrodesis of the cervical spine in children with Down syndrome.⁴⁶^–⁴⁸ Fortunately, excellent surgical results have been reported by Menezes and Ryken,⁴⁹^,⁵⁰ as well as by Brockmeyer.⁵¹

Burke and coworkers studied 32 individuals with Down syndrome and found that only 1 had atlantoaxial instability at the onset of examination.³⁵ However, at the 13-year follow-up, a predental space of greater than 5 mm had developed in 7 children. Two patients underwent atlantoaxial arthrodesis, and 1 succumbed to the disease before treatment. In 3 of the 32 individuals in this study, an odontoid abnormality consistent with os odontoideum developed. Review of the initial radiographs showed that these abnormalities were not present then.

One hundred ten patients were evaluated by the senior author up until 2004.⁵⁰ Of these 110, 54 underwent surgical treatment. There were 34 males and 20 females ranging in age from 2 to 56 years. A follow-up of 54 patients showed that 36 underwent operative stabilization with a follow-up period of 26 to 118 months. Eighteen patients were treated conservatively.

Of the 36 patients who underwent surgery, 7 had an irreducible pathology requiring ventral decompression of the medulla and dorsal occipitocervical fusion. A reducible dislocation of C1-2 (atlantodental space >8 mm between flexion and extension) was encountered in 11 who underwent C1-2 fusion (4 with transarticular and 7 with C1-2 interlaminar fusion). Twenty-five patients required occipitocervical stabilization for occipitoatlantoaxial instability. We encountered 11 previously failed atlantoaxial arthrodeses.

Of the 54 patients, cervical pain was present in 22, occipital pain in 4, torticollis in 16, upper respiratory infection in 8, acute injury after minor trauma in 7, quadriparesis in 17, gait ataxia in 11, and atrophy in 7. Eighteen patients had os odontoideum, 12 had a bifid anterior and posterior atlantal arch, 3 exhibited occipitoatlantal assimilation, and 1 patient had condylar hypoplasia. The clinical outcomes in these 54 patients consisted of complete resolution of neurological symptoms in 19 of 54 and good results with significant improvement in 16 of 54. The 18 patients treated conservatively had a stable outcome. The fusion rate was 95% at the first attempt. Two patients younger than 5 years at the initial operative procedure who were not instrumented had to undergo a repeat operation.

Worley and colleagues from Duke Medical Center reviewed patients with Down syndrome and new onset of focal weakness.⁵² Ten cases of a new onset of focal weakness in patients with Down syndrome were encountered in a combined population of 850 from a clinic with a prevalence of 1.2. The median age at diagnosis was 4 years (range, 1 month to 44 years). The cause of the focal weakness was stroke from moyamoya in 2 patients, vascular occlusive disease in 1, venous sinus thrombosis in 1, brain abscess in 1, traumatic subdural hematoma in 1, spinal cord injury from cervical stenosis in 1, atlantoaxial instability in 1, and brachial plexus injury in 1. Thus, the differential diagnosis of a new onset of focal weakness in patients with Down syndrome is broad, and this must be kept in mind because of the multiorgan involvement.

The results of combined neurosurgery and orthopedic investigation were reported at the International Society of Pediatric Neurosurgery in Vancouver in 2005. It was recommended that surgical treatment should be offered to all children with Down’s syndrome and neurological dysfunction.^50,^53,⁵⁴ Craniovertebral junction instability in association with a dystopic os odontoideum, as well as the combination of bifid anterior and posterior atlantal arches, required surgical attention. Reducible basilar invagination mandated primary stabilization. Atlantoaxial instability with sagittal plane excursion of more than 8 mm required surgical attention.⁵⁰^,⁵¹

Proatlas Segmentation Failures or Manifestations of Occipital Vertebrae

Proatlas segmentation failures result in malformation and anomalies of the most caudal occipital sclerotomes. These abnormalities surround the foramen magnum and usually involve the posterior arch of C1.^3,^15,⁵⁵^–⁵⁷ Hindbrain herniation is associated with these abnormalities in 32% to 38% of individuals. At times, the proatlas component of the dens may fail to separate from the portion that forms the basiocciput of the clivus. Consequently, the anterior arch of the atlas comes to rest above the body of the axis. Sometimes the proatlas abnormality is united with the clivus and grossly distorts the cervicomedullary junction ventrally. Variations of this anomaly may be observed in the midline, laterally, and at times dorsally (Fig. 218-4).⁵⁵

FIGURE 218-4 A, Midsagittal T2-weighted magnetic resonance image of the craniocervical junction. This 28-year-old man had spastic quadriparesis with difficulty swallowing, hoarseness of voice, and sleep apnea. He was previously treated with two transoral procedures to resect the odontoid process and had also undergone C1-2 dorsal arthrodesis. A ventral bony mass is extending from the clivus into the ventral cervicomedullary junction. Just below this mass, the uppermost cervical cord is atrophic with an intramedullary high signal change representing myelomalacia. B, Composite of the midline section of a three-dimensional computed tomographic scan of the craniocervical junction (left) and three-dimensional visualization of the craniocervical junction from within foramen magnum. C, The odontoid process apex has never formed because the proatlas component of the odontoid is still unsegmented from the midline clivus.

Thus, one may see paramesial invagination. In our series of 72 individuals, 85% were initially seen between the first and second decades of life. The youngest patient was 3 years of age and the oldest was 23 years. Spastic quadriparesis was seen in 80% and lower cranial nerve palsies in 33% (cranial nerves VIII through XII). Dysfunction of the vertebrobasilar system manifested as vascular syndrome occurred in 40%, and trauma was the cause in 60% of individuals. The latter is important from a legal standpoint.

The best definition of the abnormality was achieved with three-dimensional CT combined with MRI. Hindbrain herniation was present when the volume of the posterior fossa was reduced, especially by distortion of the vertical height of the posterior fossa.⁵⁶ Treatment of this condition consists of precise definition of the abnormality, relief of neurovascular compression, and prevention of recurrence by stabilization. These abnormalities are fairly complex and require careful study of preoperative images.

Assimilation of the Atlas and Klippel-Feil Syndrome

Assimilation of the atlas results from failure of segmentation between the fourth occipital sclerotome and the first spinal sclerotome. It may be unilateral, focal, segmental, or bilateral ³ and occurs in conjunction with abnormalities such as Klippel-Feil syndrome (Fig. 218-5A and B). In this case, basilar invagination is a secondary phenomenon.⁵⁸ Atlas assimilation was encountered in 550 of 6000 patients in the craniovertebral junction database.¹⁵ Hindbrain herniation occurred in 38% and was caused by a reduction in the volume of the posterior fossa. The situation is compounded by failure of segmentation of the second and third cervical vertebrae.⁵⁵^,⁵⁹ When associated with atlas assimilation, atlantoaxial instability occurs as a result of abnormal load placed on the motion segment.^3,^55,⁵⁸ Initially, the atlantoaxial instability will be reducible. The sequence of events is that pannus forms around the odontoid process, and at this time the lesion is reducible. However, as the child grows, grooving occurs behind the occipital condyles secondary to upward migration of the axis vertebra. This was described by Menezes in 1987. It is then followed by reducible basilar invagination up to approximately the age of 14 to 15.⁶⁰ As these children age, the lesion becomes an irreducible basilar invagination. During the phase of reducibility and partial reducibility, a prolific granulation tissue mass sits above the odontoid process surrounding it. It is nature’s attempt to reduce the excursion. This, in turn, compounds the compression on the cervicomedullary junction.^3,^58,⁶¹

FIGURE 218-5 Composite of a midsagittal T2-weighted magnetic resonance image of the head and spine (A) and a two-dimensional sagittal computed tomographic reformatted image of the same area (B). This 2-year-old child had rapidly progressive quadriparesis. The atlas has been assimilated with dorsal compression of the cervicomedullary junction. C, Postoperative lateral craniocervical radiograph taken 4 months after decompression of the foramen magnum along with C1 laminectomy and dorsal occipitoaxial fusion with rib grafts. The child exhibited neurological recovery.

In patients with irreducible basilar invagination under these circumstances, the clivus tends to be horizontally oriented; there is an abnormal groove behind the occipital condyles that pushes up the skull base and results in the horizontal clivus.⁶ This leads to complete irreducibility. Thus, a child who is being evaluated for atlantoaxial dislocation is more likely than a full-grown adult to have a reducible atlantoaxial dislocation or a reducible basilar invagination.⁶⁰ Hence, an operative procedure that relies on posterior decompression for “hindbrain herniation” without addressing the potential for instability can lead to unfortunate results.

Unilateral atlas assimilation may cause torticollis in children. This is critical with the head manipulation that occurs during general anesthesia in patients who require placement of drainage tubes in the tympanic membrane and for adenoidectomy. In this case the trunk seems to stay in one position while the head is rotated 90 degrees to either side. As a result, the child experiences rotary dislocation of the atlas on the axis vertebra. It behooves the treating physician to be aware of these problems.

The Klippel-Feil triad consists of a short neck, webbed neck, and low hairline with limitation of neck motion.⁶¹^–⁶³ There may be associated deafness, a high arched palate, facial palsies, and cardiovascular abnormalities. Abnormal rib fusions and scoliosis are common, and 30% of individuals have genitourinary tract abnormalities.⁶⁴^,⁶⁵

Basilar Invagination

Basilar invagination is a primary developmental defect that implies prolapse of the vertebral column into the base of the skull. It is frequently associated with developmental anomalies of the region, such as occipitalization, assimilation, and block vertebra.^3,^13,^14,³³ The common manifestations of neurodysgenesis, such as a Chiari I malformation and syringohydromyelia, occur in 25% to 30% of patients with basilar invagination (Fig. 218-6).

FIGURE 218-6 Midsagittal T1-weighted magnetic resonance image in 14-year-old boy with headaches, ataxic gait, spastic quadriparesis, and downbeat nystagmus. There is atlas assimilation with basilar invagination, a short clivus, and an abnormal clivus-C1-C2 articulation. The vertical height of the posterior fossa is markedly reduced, and the cerebellar tonsils extend down to the lower border of C2.

The term basilar invagination was used by Chamberlain in 1939 as a synonym for platybasia.² In addition, basilar impression has been used interchangeably with the latter. Basilar impression is an acquired form of basilar invagination caused by softening of the skull, and it occurs in diseases such as osteogenesis imperfecta, rickets, hyperparathyroidism, Paget’s disease, Hurler’s syndrome, and acro-osteolysis (Hajdu-Cheney syndrome).^32,⁶⁶^–⁷² Platybasia refers to an abnormal obtuse basal angle formed by the clivus and the anterior skull base. There are no symptoms or signs attributable to platybasia alone. It is not a measure of basilar invagination, although it may be associated with invagination.¹³

There are two types of basilar invagination. In the ventral variety there is shortening of the basiocciput, so the clivus is short and horizontally oriented and thus displaces the plane of the foramen magnum in an upward direction in relation to the spinal column.⁶ In the paramesial type, condylar hypoplasia may be present, and the clivus becomes dorsally displaced into the posterior fossa and may be of normal length.³ The occipital hypoplasia may be unilateral and leads to torticollis.³⁸ The distinction between these two types is not clinically rigid because a mixture often occurs.

Basilar invagination is commonly associated with an abnormal odontoid process invaginating into the posterior fossa. Of significance is the fact that the body of the axis becomes elongated and the true odontoid process is small.^3,^14,¹⁵ Of greater significance is the abnormal clivus-odontoid articulation. The resultant abnormal clivus-canal angle produces an indentation on the pons, medulla, or cervicomedullary junction in a ventral manner.

The Chiari malformation is associated with basilar invagination in about 25% to 30% of individuals.³^,⁶⁰ In this situation, ventral compression of the cervicomedullary junction by the bony abnormality should be relieved by ventral decompression, which must be performed before any posterior surgical procedure. If a posterior procedure is done before relieving the ventral compression, 30% of individuals have an unfavorable result. The reason for this poor outcome is that the ventral abnormality acts as a peg that indents into the pontomedullary or cervicomedullary junction when the patient is positioned for the posterior procedure. In addition, fusion in a flexed position compromises the surgeon’s ability to perform a satisfactory ventral decompression. The ability to reduce the invagination is related to age, as previously described for atlas assimilation. The presence of syringohydromyelia with hindbrain herniation and basilar invagination should not sway the treating neurosurgeon to perform a posterior operative procedure. In most cases, the syringohydromyelia disappears once the ventral abnormality has been corrected.³⁹

Anomalies of the Odontoid Process

Aplasia-Hypoplasia of the Dens

Dens aplasia-hypoplasia has several degrees of expression, and a rudimentary dens may be present or completely absent.¹³^,¹⁵ Theoretically, the cruciate ligament is incompetent, thereby leading to atlantoaxial instability. This is quite common in patients with atlas assimilation and failure of segmentation of C2 and C3. In these individuals, the body of the axis is abnormal, but the dens itself may be hypoplastic. Significant vascular compromise from stretching and distortion of the vertebral arteries has been seen with such lesions.³^,²⁹ Chronic atlantoaxial dislocation in this situation may result in the formation of granulation tissue at the site of luxation and compression of the cervicomedullary junction.

Os Odontoideum

Os odontoideum refers to an independent bone cranial to the axis in the place of the dens. It is not an isolated dens but exists apart from a small hypoplastic dens. Radiographically, an os odontoideum has rounded, smooth cortical borders separated by a variable gap from a small odontoid process.³^,¹³ It is usually located in the position of the normal odontoid tip or near the basiocciput in the area of the foramen magnum, where it may fuse with the clivus. The gap between the free ossicle and the axis usually extends above the level of the superior facets of the axis. This leads to incompetence of the cruciate ligament and, subsequently, atlantoaxial instability.

Fielding and Griffin described two types of os odontoideum ²²: the orthotopic variety, in which the ossicle lies in the position of the normal dens and moves in unison with the atlas and axis vertebrae, and the dystopic variety, in which the ossicle lies near the inferior end of the clivus, fuses with the occipital bone, and moves in unison with the clivus (Fig. 218-7A and B).

FIGURE 218-7 Composite of craniovertebral junction pluridirectional tomograms in the frontal plane (A) and midline sagittal plane (B). This 13-year-old girl became severely quadriparetic after a minor fall. A dystopic os odontoideum is present. The ossicle has smooth rounded cortical borders and is separated by a gap from a small hypoplastic dens. It is attached to the clivus. C, Lateral radiograph of the same patient at 4 years of age after a rear-end motor vehicle accident. Note the atlantoaxial dislocation. She was treated by immobilization in a Minerva cast.

Evidence favors an unrecognized fracture in the region of the base of the odontoid as the most common cause of an os odontoideum.^4,^15,²² It has been seen in patients who have been shown to have a complete odontoid process in early childhood with the subsequent development of os odontoideum, in one of identical twins who experienced cervical trauma, and in patients with gross ligamentous laxity such as those with Down or Morquio’s syndrome.^14,^35,⁷²

The biomechanics of a particular os odontoideum must be studied carefully because it varies. Movement of an os odontoideum is individual to each patient and cannot be extrapolated to another. In the dystopic variety, dorsal compromise of the spinal cord may occur as a result of the forward position of the posterior arch of the atlas in the flexed position, as well as ventral compromise by the os odontoideum. The axis may be displaced dorsally in extension and increase the ventral compromise. Thus, each patient should be carefully assessed by dynamic studies in the flexed, extended, and neutral positions. Lateral displacement of the atlantoaxial complex is a frequent finding at the time of operative fixation in such individuals.

Direct pathologic examination of the area at the time of ventral decompressive surgery has shown that an irreducible state may be caused by slippage of the transverse portion of the cruciate ligament beneath the ossicle or even in front of it. In addition, intense granulation proliferation may occur ventral to the bony defect because of repeated luxation. In patients with severe, chronic dislocation, it may become fixed over a period of several years, with resultant severe basilar invagination. At its worst, os odontoideum has significant implications regarding compression of the cervicomedullary junction. It is not uncommon for children with asymptomatic os odontoideum to have severe neurological deficits after minor trauma, such as occurs with dental work or sports activities. Thus, all patients with recognizable instability at the craniovertebral junction and an associated os odontoideum should undergo stabilization. If a fixed irreducible abnormality exists, decompression should be performed first.

Basilar Impression and Bone-Softening Syndrome

Basilar impression arises as a consequence of constitutive skeletal disease. These so-called bone-softening disorders may be congenital, as in osteogenesis imperfecta, spondyloepiphyseal dysplasia, acro-osteolysis, Hurler’s syndrome, and achondroplasia, or they may be associated with Paget’s disease, osteomalacia, hyperparathyroidism, and renal rickets.^32,^{66–70,73–75}

In contrast to the acquired forms, in which the underlying metabolic or biochemical abnormalities may be corrected, congenital, developmental basilar impression is characteristically relentless in its progress because of the irreducible nature of the genetically determined errors in bone composition. Thus, any therapeutic intervention must be considered palliative because the fundamental pathology is not addressed.

Anatomically and radiographically, there is infolding of the squama occipitalis; as a result, the posterior fossa floor becomes elevated, and the margin of the foramen magnum curves upward.⁷⁶^,⁷⁷ The basiocciput becomes foreshortened and elevated, and the clivus becomes thinned, truncated, and horizontally oriented, conditions creating an acute craniovertebral angle. Anteroposteriorly, the petrous portion of the temporal bone is also deformed. These changes ultimately permit the clivus-atlas-odontoid complex to assume an abnormally rostral location within the foramen magnum and further restrict the space within the posterior fossa (Fig. 218-8).

FIGURE 218-8 Midsagittal T1-weighted magnetic resonance image of the head and neck in an 11-year-old with osteogenesis imperfecta. There is a severe basilar impression, secondary aqueductal stenosis with hydrocephalus, a horizontal clivus, and hindbrain herniation with marked diminution of the vertical height of the posterior fossa.

The rostral extent of invagination dictates the attendant neurological manifestations.⁷³^–⁷⁵ The brainstem is elevated and splayed over the ventrally situated clivus-atlas-odontoid complex. In addition to causing mechanical compression, it acts as a fulcrum by which traction is applied to the caudal brainstem and the rostral cervical spinal cord and produces bulbar dysfunction and myelopathy.⁶ The lower cranial nerves are stretched and distorted as the brainstem is forced upward, thereby resulting in characteristic lower cranial nerve palsies.⁷¹^,⁷⁴ Cerebellar involvement may be primary and be caused by compression from infolding of the foramen magnum or secondary and be caused by hindbrain herniation (Fig. 218-9). This acquired form of hindbrain herniation may lead to the development of syringohydromyelia. Neurological dysfunction may thus be coupled with hydrocephalus, syringohydromyelia, and hindbrain herniation.

FIGURE 218-9 Composite of a lateral T2-weighted magnetic resonance image (MRI) of the brain and cervical cord (left) and axial T2-weighted MRI through plane of the odontoid/high posterior fossa (right). This 5-year-old with osteogenesis imperfecta has severe basilar impression. The clivus is horizontal, and there is pontine compression with secondary hydrocephalus. The “infolded” skull base has resulted in stenosis of the foramen magnum and syringohydromyelia.

The pathogenesis of secondary invagination and osteochondrodysplasia remains obscure.⁷⁴ Certainly, there is inherent bone fragility, with the load-bearing chondrocranium unable to maintain the weight of the cranium and its contents. Thus, both the skull base and the cranial vault are deformed. It is possible that recurrent microfractures in the region of the foramen magnum underlie the progressive infolding of the posterior skull base.⁷⁵ At surgery, it is not uncommon to find proliferation of bone composing the skull base, a finding reminiscent of exuberant callus.⁷⁶ The high metabolic activity of the reactive bone is confirmed by radioactive bone scanning and is indicative of chronic active bone remodeling.

Osteogenesis imperfecta is noted for its wide clinical and genetic variability; given the low tensile strength and load-bearing properties of bone in these syndromes, it is surprising that basilar impression is not more common. The various types of osteogenesis imperfecta have different clinical manifestations. Infants born with type II osteogenesis imperfecta have the most severe bone fragility and a high propensity for fractures because of a reduction in type I collagen synthesis. Hence, neonates with this variant rarely survive past infancy. In the other subtypes, abnormalities of the atlas and axis region and upper cervical buckling lead to quadriparesis.⁷⁷^–⁷⁹ In young individuals with basilar impression or upper cervical abnormalities, we recommend a custom-fitted modified Minerva brace to “shore up” the skull and prevent progression of the secondary invagination.⁷⁶^,⁸⁰ This must be documented with MRI.⁷⁶

Surgical intervention, regardless of the approach, was well tolerated in our series, with a 100% fusion rate. Neural decompression was accomplished in 32 individuals. All these symptomatic patients improved, with good fusion. However, although the functional improvement persisted, basilar invagination progressed in 80% of patients according to imaging criteria despite successful fusion. In all these patients the entire fusion mass migrated rostrally as a result of further squamous occipital and petrous bone infolding.^3,^6,⁷⁶ Additionally, the posterior fusion mass tends to act as a fulcrum on which the lever arm of the anterior skull base turns, thereby exacerbating the ventral compression. In all patients who were managed with a Minerva orthosis, the brace provided ventral cranial base stability, thus affording relief of symptoms and preventing further skeletal deformity.