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Seminars in Neurosurgery

Seminars in Neurosurgery 2002;  111-118

DOI: 10.1055/s-2002-35808

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Congenital Anomalies of the Craniovertebral Junction

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Ramesh L. Sahjpaul M.D. M.Sc. F.R.C.S.C.

Division of Neurosurgery, University of British Columbia, Vancouver Hospital

Combined Neurosurgical and Orthopedic Spine Program, Vancouver, British

Columbia, Canada

ABSTRACT

Congenital anomalies of the craniovertebral junction (CVJ) include conditions

such as basilar invagination, assimilation of the atlas and other

segmentation defects, atlantoaxial instability, and bony anomalies of the

atlas or axis. They may occur independently or in association with conditions

such as Down syndrome and achondroplasia. Although signs and symptoms can

present at any time in life, these anomalies may also be asymptomatic and

discovered incidentally, which can lead to management dilemmas. This article

discusses the commonly encountered CVJ anomalies with reference to

pathogenesis, clinical presentation, investigation, and management.

Recommended treatment paradigms for common conditions such as atlantoaxial

instability and os odontoideum are presented, recognizing that the natural

history of these conditions is still incompletely understood.

KEYWORDS

Congenital - craniovertebral junction - anomalies - Down syndrome -

achondroplasia

The classic radiographic appearances of craniovertebral junction (CVJ)

anomalies were first described by Chamberlain in 1939[1] and the associated

neurological syndromes were described by List in 1941.[2] These anomalies

result from abnormal fetal development and there is an increased incidence in

conditions such as Down syndrome, achondroplasia, Morquio's disease,

Lesch-Nyhan syndrome, osteogenesis imperfecta, and Klippel-Feil syndrome.[3]

Although CVJ anomalies may appear to be stable if detected in childhood, the

potential exists for progressive deformity and neurologic deficit. Indeed, it

may only be when complications occur, such as with intercurrent trauma, that

the anomaly is detected, and this may not occur until later in life.[4][5]

Successful treatment of congenital CVJ anomalies requires a knowledge of

embryology, neuroanatomy, bony anatomy, and the unique biomechanics of this

region.[6]

DEVELOPMENTAL ANATOMY

A full understanding of the developmental anatomy of CVJ anomalies is beyond

the scope of this chapter, and the interested reader is referred to an

excellent review on this topic.[7] There is a strong relationship between the

development of the bony and ligamentous structures at the CVJ and the

embryology of the central nervous system; abnormalities present in one are

frequently accompanied by abnormalities in the other. For example, between 20

and 30% of patients with Chiari malformations without myelodysplasia have CVJ

anomalies.[6] Because many congenital disorders of the spine involve

malformations of tissues derived from all three embryologic layers, defects

in gastrulation (transformation of the bilaminar embryo into a trilaminar

embryo on day 16 of gestation) may be responsible because all three layers

are available to the same insult at this time.[8] Neurulation begins on day

16 after ovulation with the formation of the notochord, neural plate, and

neural tube. At about 20 days of gestation, the intraembryonic mesoderm on

both sides of the notochord and neural tube thickens to form paraxial

mesoderm, from which paired cuboidal structures called somites develop. These

somites give rise to the sclerotome cell mass and skeletal system, and have

regional specificity early in development, which in animals, is conferred by

expression of Hox and Pax genes.[9] Developmental abnormalities may arise

from errors in expression of these genes or the influence of teratogens on

them. Because of different inductive control mechanisms, developmental

anomalies occurring at this early stage may be limited to or affect

differently the vertebral body or neural arch.[9]

Anomalies of the CVJ are also characterized by errors in chondrification and

ossification, which begin at day 42 and 72, respectively. The ventral arch of

the atlas has a single ossification center, whereas the dorsal arch ossifies

from two centers in the lateral masses. Calcification of the atlas is usually

complete by age 3 years and fusion of the ventral and dorsal arches is

usually complete by 7 years. The axis has five separate ossification centers:

two in the body of the dens that fuse by the 7th embryonic month, two lateral

ossification centers for the neural arch, and one in the body of the axis.

The dens fuses to the body of the axis by the age of 6 years, and by the age

of 12 years the tip of the dens has fused to the body.[9] The relatively

precarious blood supply to the dens (an anastomotic arcade in the region of

the alar ligament) may be responsible for the formation of os odontoideum, in

addition to sequestrum in Type II odontoid fractures.[10]

CLINICAL MANIFESTATIONS OF CONGENITAL CVJ ANOMALIES

Diversity of presentation perhaps best characterizes the presentation of CVJ

anomalies.[6] Although not pathognomonic, an abnormal physical appearance is

commonly seen, including short stature, short neck, webbing of the neck,

asymmetry of the face or skull, torticollis, low hairline and limitation of

neck motion (as part of Klippel-Feil syndrome), Sprengel's shoulder

deformity, and scoliosis. Approximately three fourths of patients with

basilar impression have such physical abnormalities.[5] Presenting symptoms

and signs reflect compression of the brainstem, cervicomedullary junction,

lower cranial nerves, cervical roots, and vertebral arteries, and alteration

of cerebrospinal fluid (CSF) dynamics.[6] Headache, possibly in the

distribution of the greater occipital nerves, and neck pain are common

presenting symptoms in children.[6][11] Myelopathy is a very common feature

in both adults and children, especially in basilar invagination.[11] Sensory

abnormalities, because of posterior column dysfunction from dorsal

compression at the foramen magnum or the C1-2 level, may be exacerbated by

the presence of cerebellar tonsillar herniation from an associated Chiari

malformation.[6] A presentation mimicking central cervical cord syndrome has

also been reported in children with basilar invagination.[6] Other features

of CVJ anomalies include transient brain ischemia with dizziness, confusion,

and syncope secondary to vertebral artery compromise;[12] brainstem and

cranial nerve dysfunction manifesting as sleep apnea, dysphagia, vocal cord

paresis, bouts of aspiration pneumonia, poor feeding, and inability to gain

weight; internuclear ophthalmoplegia, and downbeat nystagmus.[6] Impairment

in CSF flow at the foramen magnum or posterior encroachment on the aqueduct

of Sylvius from basilar invagination may result in hydrocephalus and

increased intracranial pressure.[13] In children the presentation may be

quite subtle and nonspecific, with generalized weakness, lack of physical

endurance, frequent falls, torticollis, or simply a child who prefers to be

carried; signs of pyramidal tract dysfunction may not appear until later in

life.[14] Patients with CVJ anomalies may also be asymptomatic and the

anomaly may be detected during investigations done for other reasons.

IMAGING

Computed tomography (CT) and magnetic resonance imaging (MRI) have replaced

the older techniques (plain radiography and tomography) and are indicated in

patients in whom the routine exam or clinical findings suggest a CVJ anomaly.[

4] Dynamic MRI and CT scan (flexion and extension views) and imaging in

traction have been used to evaluate the biomechanics of the CVJ to help

formulate rational surgical strategies.[15][16] Magnetic resonance

angiography supplemented with selective cerebral angiography, when necessary,

can provide valuable information regarding the vascular supply of this

region. The osseous anatomy of the CVJ is best delineated with thin-section

CT or pleuridirectional tomography.[6]

BASILAR INVAGINATION AND BASILAR IMPRESSION

Basilar invagination and basilar impression have been used interchangeably in

the literature, along with platybasia, but these terms are not synonymous.

Basilar invagination results from a congenital defect in the chondrocranium

and refers to a deformity of the bones at the base of the skull at the margin

of the foramen magnum, which results in prolapse of the vertebral column into

the skull base.[10] The tip of the odontoid lies more cephalad than normal,

sometimes protruding into the opening of the foramen magnum, with possible

brainstem compression. Associated abnormalities include anomalies of the

atlas and odontoid, Klippel-Feil syndrome, Chiari malformation,

syringobulbia, syringomyelia, hydrocephalus, and vertebral artery anomalies.[4

] Anterior (from shortening of the basiocciput and elevation of the plane of

the foramen magnum) or paramedian basilar invagination (from hypoplasia of

the exoccipital bone with medial elevation of this portion of the occipital

bone) may be present simultaneously.[10][17][18]

Basilar impression is an acquired form of basilar invagination occurring

later in life because of softening of the occipital bone in conditions such

as rheumatoid arthritis, Paget's disease, hyperparathyroidism,

achondroplasia, and osteogenesis imperfecta.[19] Platybasia has no clinical

significance and is merely an anthropological term describing the flattening

of the angle formed by the intersection of the plane of the anterior fossa

with the plane of the clivus.[18] It may, however, be found with basilar

invagination.

Radiological Diagnosis

Historically, basilar invagination was diagnosed using various reference

lines from lateral (Fig. [1]) and anteroposterior radiography.[4]

Difficulties with these methods include a wide range of normality,

differences between males and females, and inability to accurately identify

the landmarks necessary to draw these lines on skull radiographs. The use of

McGregor's line, which joins the upper surface of the posterior edge of the

hard palate to the most caudal point of the occipital curve of the skull, is

the best method for routine screening because all of the landmarks can be

clearly identified at all ages on lateral radiography; the tip of odontoid

should not extend more than 4.5 mm above this line.[4][20] McRae's line

defines the opening of the foramen magnum, and if the odontoid lies below

this line, patients will probably be asymptomatic.[20] MRI (Fig. [2]) and

thin-section CT, along with dynamic imaging, are required to adequately

investigate patients with basilar invagination. Recently, a new

classification system for basilar invagination, which divided patients into

two groups, depending on whether they had an associated Chiari malformation,

was found to be useful for surgical planning.[16]

Treatment

Many patients with basilar invagination do not develop symptoms until the

second or third decade in life, presumably because of gradually increasing

instability from ligamentous laxity caused by aging.[4] Controversy exists

regarding the optimal management of basilar invagination. Menezes et al[21]

used a physiologic approach to the treatment of CVJ anomalies that focuses on

the stability of the CVJ, the associated neural abnormalities, and the site

of encroachment in the treatment of almost 2000 patients.[6] Symptomatic

patients are placed in cervical traction with an MRI-compatible halo, and

reducible pathology (as assessed by clinical and radiological improvement) is

treated by posterior stabilization. Surgical decompression is performed in

patients with irreducible pathology. The route of decompression depends on

the direction of compression; transpalatopharyngeal or lateral

extrapharyngeal decompression is used for ventral encroachment, and dorsal

decompression is used for dorsal pathology.[6] Patients with basilar

invagination and an associated Chiari malformation require ventral

decompression prior to a posterior procedure, because performing the latter

may result in a failure to improve or frank neurologic deterioration. Ventral

decompression alone may also adequately treat syringohydromyelia and obviate

the need for a direct posterior procedure. Surgical stabilization may be

required depending on the extent of decompression and the degree of

instability, but the decision to stabilize may be a difficult one.[6] Dickman

et al[22] demonstrated that transoral resection of the odontoid significantly

destabilized the upper cervical spine, even in the absence of preoperative

clinical or radiological evidence of instability. This was especially true in

the presence of a congenital bony CVJ anomaly or if posterior decompression

was required.[21]

In the most recent and largest study of basilar invagination, the presence or

absence of a Chiari malformation was found to be pathologically and

clinically relevant.[16] Patients with basilar invagination but no Chiari

malformation were symptomatic from brainstem compression from an indenting

odontoid process, uniformly and dramatically improved with preoperative

traction (which also suggested relative CVJ instability), and usually

required stabilization. Patients without a Chiari malformation were

symptomatic from crowding of neurologic structures at the foramen magnum and

not from an indenting odontoid process (even though the brainstem was

posteriorly displaced by it), tended not to respond to preoperative traction

(which suggested relative CVJ stability), and responded well to posterior

foramen magnum decompression; stabilization was rarely required. Indeed, this

latter group of patients typically had associated atlanto-occipital or

atlantoaxial assimilation.[16] When performing a dorsal decompression, most

authors recommend opening the dura to divide any possible tight dural bands,[

17] but not routine resection of the cerebellar tonsils.[15]

ASSIMILATION OF THE ATLAS

Assimilation of the atlas results from a failure of segmentation between the

atlas and the base of the skull, and occurs in 0.25% of the population.[23]

Varying degrees of fusion between the occiput and C1 can occur (Fig. [3]).

The odontoid may be posteriorly displaced or of unusual size and cause

brainstem compression. In most instances, this condition is associated with

other CVJ or systemic abnormalities.[9][24] There is an increased incidence

of atlantoaxial instability with assimilation of the atlas, particularly if

there is coincident C2-3 fusion, which occurs in 33 to 72% of patients with

assimilation of the atlas.[10][25] Symptoms typically begin between the ages

of 20 and 40 years and are usually gradual in onset, but may be precipitated

by trauma or inflammation.[4][9] This delayed presentation may be due to

progressive laxity at the atlantodental joint and proliferation of

granulation tissue during childhood, which may eventually lead to an

irreducible state of odontoid invagination.[10] Headache and tenderness in

the greater occipital nerve distribution are common findings. Vertebrobasilar

ischemia and cerebellar stroke secondary to trauma to a relatively fixed

vertebral artery at the C1 level have also been reported.[26][27] Treatment

is directed at symptomatic basilar invagination and/or atlantoaxial

instability, if present.[6]

ANOMALIES OF THE RING OF THE ATLAS AND AXIS

Anomalies of the ring of the atlas and axis are caused by failure of fusion

of the synchondroses of the developing cervical vertebrae. The incidence of

defects of the anterior (Fig. [4]) or posterior arches of the atlas is less

than 1%, and defects of the posterior elements of C2 are rarer still.[24]

They may be incidental findings or may cause symptoms on the basis of

instability.[28] Reconstructed CT scans may be necessary to differentiate a

congenital defect in the ring of C1 or C2 from fractures.[29] If such

anomalies are detected, stability of the C1-2 complex should be assessed with

dynamic radiographs. Angiography may be necessary prior to initiating

treatment because of the increased incidence of vertebral artery anomalies in

patients with C1 ring anomalies.[24] Treatment with preoperative traction to

achieve reduction, if necessary, followed by posterior occiput-C2

stabilization can be successful even in patients in their teen years.[28]

ATLANTOAXIAL INSTABILITY

Atlantoaxial instability may result from odontoid aplasia/ hypoplasia,

assimilation of the atlas, or transverse ligament laxity,[9] and has an

increased incidence in Down syndrome, Klippel-Feil syndrome, skeletal

dysplasias, osteogenesis imperfecta, neurofibromatosis, and congenital

scoliosis.[9] Dynamic stability of C1-2 (Fig. [5]) should be assessed in

these conditions prior to interventions such as spinal traction or

intubation. The distance between the anterior edge of the dens and the

posterior aspect of the anterior ring of the atlas-the atlantodental interval

(ADI)-has been used to diagnose atlantoaxial instability, with the upper

limit of normal being 3 mm in adults and 4 mm in children.[9] However, a

better predictor of instability and potential for neurologic compromise is

the space available for the spinal cord (SAC), because the presence of

associated anomalies, such as os odontoideum, will affect the canal diameter

in this region.[9] The SAC is the distance from the ventral edge of the

posterior arch of C1 to the posterior edge of the dens or anterior arch of

C1, whichever is less. Although lower limits for SAC have been described (14

mm or less in adults), there is considerable variation in sagittal and

transverse diameters of the cervical spine.[9] Haworth and Keillor[30]

studied such normal variations in infants, children, and young adults, and

developed transparencies to provide an efficient screening test for assessing

the transverse diameter of the spinal canal in the growing child. Physicians

must be aware of such normal variations to avoid misinterpreting radiographs

in the growing child.[31] In cases of suspected atlantoaxial instability,

flexion and extension radiographs (performed voluntarily by the patient) are

diagnostic. Alternatively, MRI (including flexion and extension views) should

be obtained to assess ligamentous integrity and in all patients with

neurologic deficit.[9] The treatment of atlantoaxial instability is discussed

in the Craniovertebral Junction Anomalies in Down Syndrome section.

ANOMALIES OF THE ODONTOID (DENS)

Anomalies of the odontoid range from agenesis or aplasia (complete absence)

to hypoplasia (partial absence) to os odontoideum. Distinguishing between

these entities is perhaps of radiological interest only, because they usually

lead to atlantoaxial instability, with an identical clinical presentation and

treatment.[4] Odontoid anomalies are probably more common than is recognized,

but the exact frequency is unknown because they may be asymptomatic. Of the

three conditions, odontoid aplasia is the most rare. Os odontoideum, an

independent ossicle located a variable distance rostral to the C2 vertebral

body in the location of the odontoid process, is the most common odontoid

anomaly.[9] The etiology may be an unrecognized fracture at the base of the

odontoid or an acute ligamentous injury in early childhood, which, because of

distraction forces exerted by the ligaments, pulls the odontoid fragment away

from the centrum of the axis.[6] The precarious blood supply of the dens

further leads to poor healing, callus formation, and failure of closure of

the gap. Recently, a segmentation defect in the midodontoid process has been

theorized to be the cause.[32] Atlantoaxial instability results because the

space between the os odontoideum and the remnant of the odontoid process is

above the level of the superior facet of the axis, which leads to

incompetence of the transverse ligament.[6]

Diagnostic confusion may arise between an os odontoideum and the normal

epiphyseal line in children under 5 years of age. Distinguishing features of

an os include its rounded or oval shape with an intact cortical margin. An

orthotopic ossicle is located near where the odontoid tip normally would be,

whereas a dystopic ossicle is located near the base of the occiput in the

region of the foramen magnum.[24] Os odontoideum has recently been reported

in association with synovial cysts of the C1-2 complex.[33][34] Although

there is little controversy that treatment is indicated in patients with

neurologic signs or symptoms, the decision to intervene surgically in

asymptomatic patients is debated,[6] and there are no prospective studies

comparing conservative to surgical treatment. In a retrospective review of 44

patients with os odontoideum, Dai et al[35] treated 5 asymptomatic patients

successfully with a collar; unfortunately, details regarding duration of

treatment and length of follow-up were not provided. Further consideration

regarding the treatment of os odontoideum appears in the Craniovertebral

Junction Anomalies in Down Syndrome section.

CRANIOVERTEBRAL JUNCTION ANOMALIES IN ACHONDROPLASIA

Achondroplasia occurs in 0.03 to 0.05% of live births and is the most common

skeletal dysplasia leading to rhizomelic short-limbed dwarfism. Inheritance

is autosomal dominant in familial cases; however, approximately 80% of cases

are caused by genetic mutation.[36] Defective enchondral bone formation at

the skull base results in a short basicranium, short clivus, shallow

posterior fossa, and a narrow foramen magnum. A bulbous posteriorly and

superiorly projecting odontoid process may also be present.[36] These

abnormalities result in cervicomedullary and upper cervical cord compression

and hydrocephalus, which can manifest as hypertonia, delayed motor

milestones, or long-tract and bulbar signs and symptoms, including sudden

death.[37][38][39] Prospective studies of patients with achondroplasia have

recently been published.[37][39] In a series of 53 patients studied by i

et al,[39] 10.6% required surgical decompression for CVJ stenosis, and 4 of

11 (36%) children in a study by Keiper et al[37] required decompression.

Recommended investigations for all newborns with achondroplasia include a

thorough neurologic examination, MRI, somatosensory evoked potentials if the

MRI shows cervicomedullary compression, and sleep studies if there is a

history of sleep apnea.[37] Asymptomatic children with abnormal imaging

should be followed by a multidisciplinary team and imaging should be repeated

at least annually until the age of 3 years, and then every 5 years.[37][39]

Posterior fossa decompression (including upper cervical laminectomy, if

necessary) is indicated in symptomatic patients and has yielded satisfying

results, with almost all patients experiencing improvement or resolution of

their symptoms, and no patients showing deterioration. In addition, a

duraplasty does not appear to be necessary.[37][40][41][42] Hydrocephalus may

be present in up to 30% of patients with achondroplasia[41][43] and should be

treated by shunting,[37] although in some patients it resolved following

foramen magnum decompression.[41]

CRANIOVERTEBRAL JUNCTION ANOMALIES IN DOWN SYNDROME

First described in 1961,[44] CVJ anomalies in Down syndrome are characterized

by increased ligamentous laxity and abnormal joint and bony anatomy, which

predispose to instability. The reported incidence of radiologic atlantoaxial

instability ranges from 7 to 40%, although less than 1% of patients are

symptomatic.[45][46][47][48] The incidence of bony anomalies involving the

occipital condyle, C1 ring, and odontoid is also increased in Down syndrome,[6

] as is the incidence of occipitoatlantal instability, which has been

under-recognized until recently.[49] These observations led to the Special

Olympics mandating in 1983 that all Down syndrome patients undergo routine

radiographic cervical spine screening prior to participation in ``high-risk''

sports[50]-a decision supported by the American Academy of Pediatrics in

1984.[51]

This topic has generated considerable controversy since then, partly because

the natural history of CVJ anomalies in Down syndrome is incompletely

understood. A review of the literature suggests that Down syndrome patients

with pure atlantoaxial ligamentous laxity and no bony anomalies undergo

little or no change in the degree of their subluxation (as measured by the

ADI) over time.[52][53][54][55] However, the ADI is an indirect measure of

instability, and natural history studies using ADI alone should be viewed

with caution.[50] Current recommendations for screening Down syndrome

patients include: (1) routine history and physical examinations of all

patients by primary care providers; and (2) screening cervical spine

radiographs, including dynamic views, for individuals who desire to

participate in ``high-risk'' sports according to the Special Olympics

Guidelines[51] and for all patients undergoing otolaryngologic procedures. An

MRI should be obtained if the SAC is found to be less than 14 mm. In

asymptomatic atlantoaxial instability, posterior surgical stabilization

(occipitocervical or C1-2) is recommended if os odontoideum is present, or if

there is significant instability (ADI >4.5 mm or SAC <14 mm) and the MRI

shows evidence of spinal cord injury (increased signal on T2 images). Even if

the MRI is negative, surgery should be considered on the basis of the degree

of atlantoaxial instability. Asymptomatic patients without significant

instability by the above measurements do not need further investigation but

should be examined regularly.

The management of symptomatic atlantoaxial instability is more

straightforward; all patients require CT and MRI, and in the presence of

significant instability, should undergo surgical stabilization. If there is

no significant instability but the MRI is positive as described above,

stabilization is recommended. Observation in this group is acceptable if the

MRI is negative.[50] Occipitoatlantal instability (>7 mm subluxation) may

also warrant stabilization.[50] To dispel previous misconceptions about high

morbidity and mortality in Down syndrome patients undergoing surgical

stabilization, Taggard et al[47] reported good to excellent outcomes on the

basis of improvement or resolution of preoperative symptoms in 24 of 36

patients undergoing transoral resection for irreducible basilar invagination,

occipitocervical stabilization, and/or C1-2 stabilization. Successful fusion

was achieved in 22 patients (96%).

Figure 1 Lateral craniometry depicting the three lines used to determine

basilar invagination. Chamberlain's line connects the dorsal margin of the

hard palate to the posterior edge of the foramen magnum; McGregor's line

joins the dorsal margin of the hard palate to the most caudal point of the

occipital curve of the skull, and is the best method for screening because

the bony landmarks can be clearly seen at all ages on routine lateral

radiographs; McRae's line defines the opening of the foramen magnum.

Chamberlain's line is seldom used because the landmarks are difficult to

identify. (From Hensinger RN. Congenital anomalies of the cervical spine.

Clin Orthop 1991;264: 16-38, with permission.)

Figure 2 Basilar impression, an acquired form of basilar invagination, is

demonstrated in this midsagittal T1-weighted MRI of the brain in a

16-year-old patient with Paget's disease. (From Menezes AH, Ryken TC.

Craniovertebral junction abnormalities. In: Weinstein SL, ed. The Pediatric

Spine: Principles and Practice. New York: Raven Press; 1994:307-321, with

permission.)

Figure 3 Coronal CT demonstrating bilateral assimilation at the

atlanto-occipital level.

Figure 4 Axial CT showing a congenital defect in the anterior arch of C1 due

to failure of fusion of the synchondrosis of the developing vertebra.

Figure 5 Lateral dynamic radiographs demonstrating atlantoaxial instability

with an associated os odontoideum.

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