1. Craniovertebral Junction
Ayberk Özkavaklı

2. What is craniovertebreal junction?
The craniocervical (craniovertebral) junction represents the complex transitional zone between the cranium and the spine and comprises a complex balance of different elements: it should be considered anatomically and radiologically a distinct entity from both the cranium and, in particular, the cervical spine. It is composed of osseous structures articulated with synovial joints, intrinsic ligaments and membranes and muscles. As well as housing the spinal cord and multiple cranial nerves, it is also approximated by critical vasculature supplying both the brain and the cervical spinal cord parenchyma.
 The requirements placed on the craniocervical junction are onerous—not only must it house, protect and support structures critical for function, it must also simultaneously provide significant mobility.

3. Atlas
The atlas is the first cervical vertebra, commonly called C1. It is an atypical cervical vertebra with unique features. It articulates with the dens of the axis and the occiput, respectively allowing rotation of the head, and flexion, extension and lateral flexion of the head.  Unlike the rest of the cervical vertebrae, with exception to the similarly structured axis (C2), the anterior rami are sent posteriorly instead of anteriorly.

4. Gross anatomy of Atlas
The atlas is composed of an anterior arch and a posterior arch, paired lateral masses, and paired transverse processes. It has the dens of the axis sit where a centrum (body) of a typical vertebra would be. The transverse ligament holds the dens of the axis against the anterior arch of the atlas and divides its vertebral canal into two parts. The anterior 1/3 is occupied by the dens. The posterior 2/3 contains the spinal cord, which occupies 1/3 of the total vertebral canal space.

5. anterior arch posterior arch
anterior tubercle: sits on the anterior aspect of the anterior arch and is the site of attachment of the anterior longitudinal ligament
posterior facet for the dens: sits on the posterior aspect of the anterior arch
upper border: attachment of the anterior atlanto- occipital membrane and lateral parts of the anterior longitudinal ligament
lower border: attachment of the anterior atlanto- occipital membrane and lateral parts of the anterior longitudinal ligament
posterior arch
3/5th of circumference of the ring
posterior tubercle: sits posteriorly to the posterior arch, is a rudimentary spinous process and attachment site for the ligamentum nuchae
superior surface: contains paired grooves for the C1 nerve  and vertebral artery, sits just posterior to the lateral mass
superior border: attachment for the posterior atlanto-occipital membrane
inferior border: attachment for the ligamentum flava

6. lateral masses transverse processes paired, ovoid
superior articular facet: kidney-shaped, concave and articulates with the occipital bone
inferior articular facet: circular, with a flat or slightly concave surface articulating with the lateral atlanto-axial joint
medial surface: marked by vascular foramina and a tubercle for the attachment of the transverse ligament
transverse processes
longer than all of the transverse processes of the cervical vertebrae except C7
typically covered by costal lamella 
transverse foramina: contains the vertebral arteries
anterior tubercle: sometimes present on the anterior aspect of the transverse process

7. Articulations
atlanto-occipital joint:  hyaline-covered synovial joint between the occipital condyle and concave facet of the lateral mass of the atlas. Covered by a capsule and innervated by C1, this joint allows for flexion, extension and lateral flexion.
median atlanto-axial joint: hyaline-covered synovial joint between the dens of axis back of the anterior arch of the atlas which allows for the rotation of the head. The dens is held in place by the transverse ligament, with a bursa between the two. 
lateral atlanto-axial joint: hyaline-covered synovial joint between the inferior articular facet of the atlas and the superior articular facet of the axis which allows for the rotation of the head. A capsule innervated by the C2 nerve surrounds the joint.

8. Ligaments
transverse ligament: strong band that runs posterior to the dens of the axis, holding it in place. Each end is attached to tubercles on the anterior arch of the atlas. The former is attached to the basilar part of the occipital bone, in close relation with the membrana tectoria; the latter is fixed to the posterior surface of the body of the axis; hence, the whole ligament is named the cruciate ligament of the atlas.
atlanto-axial ligaments: attach from the lower border of the anterior arch of the atlas to front of the body of the axis. Provides tertiary support against ventral translation of the dens

9. Musculotendinous
anterior atlanto-occipital membrane: attached to upper border of the anterior arch to the outer margins of foramen magnum
posterior atlanto-occipital membrane: attached to upper border of the posterior arch to the outer margins of foramen magnum. At each lateral margin, there is a gap for the passage of the C1 nerve and vertebral artery, which sometimes ossifies and become foramen. Innervated by C1.
longus colli: deep grooves of the anterior surface of the body
levator scapulae: tip of the transverse processes
splenius cervicis: transverse processes
obliquus capitis superior: transverse processes
obliquus capitis inferior: transverse processes 
rectus capitis posterior minor: tubercle on the posterior arch of the atlas

10. Axis
The axis is the second cervical vertebra, commonly called C2. It is an atypical cervical vertebra with unique features and important relations that make it easily recognisable. Its most prominent feature is the odontoid process, which is embryologically the body of the atlas (C1)1,2. It plays an important role in rotation of the head with the majority of movement occurring around the dens and at the atlanto-axial joint.

11. Gross anatomy of axis
The axis is formed by a body with the attached dens, two lateral masses, a posterior neural arch (formed by the pedicle and a thick lamina), and a large spinous process, which is commonly bifid.
Anterior components of the axis are composed of: 
dens: conical in shape
body
lateral mass
transverse process with foramina transversarium
superior articular facets
inferior articular facets
Posterior elements of the axis are composed of: 
pedicle
lamina
spinous process
has several muscle attachments
semispinalis cervicis
rectus capitis posterior major
inferior oblique

12. Articulations
superior articular facet with inferior articular facet of C1
dens articulates with the posterior aspect of the anterior arch of C1
inferior articular facet with superior articular facet of C3
uncovertebral articulation with C3

13. Foramen Magnum
The foramen magnum is the largest foramen of the skull and is part of the occipital bone. It is oval in shape with a large antero- posterior diameter 2.
Gross Anatomy
The foramen magnum is found in the most inferior part of the posterior cranial fossa 3. It is traversed by vital structures including the medulla oblongata. In the midline, the anterior margin is the basion and the posterior margin is the opisthion.
Relations
anteriorly: basilar part of occipital bone
anterolaterally: occipital condyles, hypoglossal canal, jugular foramen
posteriorly: squamous part of occipital bone with the internal occipital crest
Contents
medulla oblongata
meninges
spinal root of the accessory nerve
vertebral arteries
anterior and posterior spinal arteries
tectorial membrane and alar ligaments

15. Tectorial membrane and Alar ligament
The tectorial membrane of atlanto-axial joint (occipitoaxial ligaments) is situated within the vertebral canal.
It is a broad, strong band which covers the dens and its ligaments, and appears to be a prolongation upward of the posterior longitudinal ligament of the vertebral column.
It is fixed, below, to the posterior surface of the body of the axis, and, expanding as it ascends, is attached to the basilar groove of the occipital bone, in front of the foramen magnum, where it blends with the cranial dura mater.
Its anterior surface is in relation with the transverse ligament of the atlas, and its posterior surface with the dura mater.
The alar ligaments join the lateral margins of the sloping upper margin of the dens of C2 to the lateral margins of the foramen magnum (adjacent to the occipital condyles) and lie on either side of the apical ligament. They are paired ligaments that are very strong and limit rotation of the head.

16. Vertebral Artery
origin: branch of the 1st part of the subclavian artery
course: ascends posterior to the internal carotid artery in the transverse foramina of the cervical vertebrae
branches
numerous small branches
radicular/spinal branches
posterior inferior cerebellar artery (PICA)
termination: combines with the contralateral vertebral artery to form the basilar artery
key relationships: posterior to the internal carotid artery; ascends anterior to the roots of the hypoglossal nerve (CN XII)

17. Segments
The vertebral artery is typically divided into 4 segments:
V1 (preforaminal): origin to transverse foramen of C6
V2 (foraminal): from the transverse foramen of C6 to the transverse foramen of C2
V3 (atlantic or extradural): from C2 to the dura
V4 (intradural): from the dura to their confluence to form the basilar artery

18. Chiari Malformations Background
Chiari malformations, types I-IV, refer to a spectrum of congenital hindbrain abnormalities affecting the structural relationships between the cerebellum, brainstem, the upper cervical cord, and the bony cranial base.
Epidemiology
Although Chiari malformation is still listed as a rare disease by the Office of Rare Diseases of the National Institutes of Health, this categorization is based on outdated data from before the MRI era. With routine use of MR imaging, Chiari malformation is discovered with increasing frequency. For Chiari I, prevalence rates of % with a slight female predominance are suggested by recent studies. [5]Chiari II is found in all children with myelomeningocele, although less than one-third develop symptoms referable to this malformation.
Etiology
Based on analysis of familial aggregation, a genetic basis for Chiari I has been suggested.  Recent studies suggest linkage to chromosomes 9 and 15. It is hypothesized that Chiari type I originates as a disorder of para-axial mesoderm, which subsequently results in formation of a small posterior fossa. The development of the cerebellum within this small compartment results in overcrowding of the posterior fossa, herniation of the cerebellar tonsils, and impaction of the foramen magnum. This theory is consistent with the observed association of Chiari I and other hereditary mesodermal connective tissue disorders, such as Ehlers-Danlos syndrome.

19. Theories regarding embryogenesis of Chiari II malformation must take into account its invariable association with myelomeningocele. An attractive theory is the "CSF loss" theory. It is hypothesized that escape of fluid through the open placode in myelomeningocele results in an inadequate stimulus for mesenchymal condensation at the skull base. The disordered and inadequate growth of the posterior fossa results in upward herniation of vermis, downward herniation of brainstem, and distortion of tectum (tectal beaking). Furthermore, collapse of the developing ventricular system because of fluid loss results in associated abnormalities such as agenesis of corpus callosum and enlargement of massa intermedia
Pathophysiology
Symptoms of Chiari I develop as a result of 3 pathophysiological consequences of the disordered anatomy: (1) compression of medulla and upper spinal cord, (2) compression of cerebellum, and (3) disruption of CSF flow through foramen magnum. Compression of cord and medulla may result in myelopathy and lower cranial nerve and nuclear dysfunction. Compression of cerebellum may result in ataxia, dysmetria, nystagmus, and dysequilibrium. Disruption of CSF flow through foramen magnum probably accounts for the most common symptom, pain.
Accordingly, headache and neck pain in Chiari I are often exacerbated by cough and Valsalva maneuver. Hydrocephalus occurs less frequently. Furthermore, the disordered flow of CSF through foramen magnum may result in formation of syringomyelia and central cord symptoms such as hand weakness and dissociated sensory loss. These symptoms are usually asymmetrical, as a syrinx has a tendency to develop in the side of the spinal cord that is more significantly affected by tonsillar ectopia

20. Medical Therapy
Patients with Chiari I malformations who have minimal or equivocal symptoms without syringomyelia can be treated conservatively. Mild neck pain and headaches can be treated with analgesics, muscle relaxants, and occasional use of a soft collar. Frankly symptomatic patients should be offered surgical treatment.
Surgical Therapy
The goals of surgical treatment are decompression of cervicomedullary junction and restoration of normal CSF flow in the region of foramen magnum. Considerable controversy has existed throughout the years about the surgical steps that are required to achieve these goals.

21. Characteristic
Chiari I
Chiari II
Usual age of diagnosis
Adults and older children
Infants and young children
Clinical findings
Headache and neck pain (worsened by cough or Valsalva maneuver)
Myelopathy
Cerebellar symptoms
Lower brainstem symptoms (eg, dysarthria, dysphagia, downbeat nystagmus)
Central cord symptoms (eg, hand weakness, dissociated sensory loss, cape anesthesia)
In infants, signs of brainstem dysfunction predominate: swallowing/feeding difficulties, stridor, apnea, weak cry, nystagmus
Weakness of extremities
Primary anatomical abnormalities
Herniation of cerebellar tonsils through foramen magnum, producing compression of cervicomedullary junction
Herniation of lower brainstem through foramen magnum
Cephalad course of cranial nerves
Kinking of cervicomedullary junction
"Beaking" of tectum
Upward herniation of vermis through incisura
Nearly vertical tentorium
Myelomeningocele No
Always
Hydrocephalus
Less than 10% of cases
Very common
Syringomyelia
30-70%
Common
Associated abnormalities
Craniocervical hypermobility syndromes
Klippel-Feil anomaly
Hereditary connective tissue disorders and neurofibromatosis type II
Callosum corpus pellucidum septum of agenesis
Hypoplasia or
Enlargement of massa intermedia
Heterotopias and gyral abnormalities
Shared associated abnormalities
Basilar invagination
Occipitalization of atlas
Bifida of C1 posterior arch
Foramen magnum variant anatomy

22. Sagittal and coronal MRI images of Chiari type I malformation
Sagittal and coronal MRI images of Chiari type I malformation. Note descent of cerebellar tonsils (T) below the level of foramen magnum (white line) down to the level of C1 posterior arch (asterisk).

23. Axial MRI image at the level of foramen magnum in Chiari type I malformation. Note crowding of foramen magnum by the ectopic cerebellar tonsils (T) and the medulla (M). Also note the absence of cerebrospinal fluid.

24. T2 hyperintense region on MRI (arrow) depicting edema in central cord region of a patient with Chiari I malformation. Left untreated, this patient is likely to develop cavitation of the edematous central cord, resulting in syringomyelia.

25. Intraoperative photograph of duraplasty with pericranial graft
Intraoperative photograph of duraplasty with pericranial graft. The duraplasty provides additional room for cerebellar tonsils at the craniocervical junction, while achieving closure of dura and prevention of cerebrospinal fluid leak.
Intraoperative photograph of Chiari type 1 malformation showing descent of cerebellar tonsils well below the level of foramen magnum

26. Syrinx
Terminology
The use of the term has grown out of the difficultly in distinguishing between the two conditions using current imaging modalities. It may also be known as hydrosyringomyelia or syringohydromyelia. Diagnosis is made by MRI and electromyography.
In hydromyelia, there is dilatation of the central canal of the spinal cord. In syringomyelia and syringobulbia, there is dissection through the ependymal lining of the central canal and a CSF collection within the cord itself. 
Clinically, there is no difference between the two and severity of symptoms is related to the location and size of the syrinx.
Clinical presentation
Symptoms, which may not occur immediately, include flaccid weakness of the hands and arms as well as "cape-like" distribution of pain and temperature sensory loss with preservation of position, vibration and touch (dissociated sensory loss).

27. Pathology
They may be congenital (90%) or acquired.
Congenital causes include:
myelomeningocoele
Chiari I malformation
Chiari II malformation
Dandy-Walker malformation
Klippel-Feil syndrome
Acquired (secondary) causes include:
post-traumatic: occurs in ~5% of patients with spinal cord injury usually from a whiplash type injury; symptoms may start many months or years after injury 
cervical canal stenosis
post-inflammatory
secondary to a spinal cord tumour
secondary to a haemorrhage
due to vascular insufficiency

28. Chiari I malformation with syrinx
This is a sagittal T2-weighted MR image of the cervical spine and shows abnormal position of the cerebellar tonsils (yellow arrow) below the foramen magnum (dashed line), indicating a Chiari I malformation. In this case, the malformation is associated with a huge syrinx (red arrows) that extends throughout the spinal cord. In some cases of Chiari I malformation, the patient will be asymptomatic however many cases are associated with a syrinx and/or hydrocephalus and may therefore present with symptoms such as upper or lower limb weakness or headaches.
Syrinx. This fluid-filled cavity (arrows) in the cervical spinal cord is a syrinx, which can be caused by dilatation of the central canal (hydromyelia) or a build-up of fluid within the substance of the cord (syringomyelia). Distinguishing between the two on MRI can be very difficult, and doesn’t really matter as their clinical behaviour is identical. These may be congenital or acquired; acquired causes include trauma and spinal cord tumours.

29. References
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Maingard, J. (n.d.). Axis (C2) | Radiology Reference Article. Retrieved March 30, 2017, from
Maingard, J. (n.d.). Foramen magnum | Radiology Reference Article. Retrieved March 30, 2017, from
Wong, A. (n.d.). Alar ligament | Radiology Reference Article. Retrieved March 30, 2017, from
Tectorial membrane of atlanto-axial joint. (2017, March 28). Retrieved March 30, 2017, from
Gaillard, F. (n.d.). Vertebral artery | Radiology Reference Article. Retrieved March 30, 2017, from
Chiari Malformation. (2017, January 06). Retrieved March 30, 2017, from
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Chiari I malformation with syrinx. (n.d.). Retrieved March 30, 2017, from

30. TEŞEKKÜRLER
AYBERK ÖZKAVAKLI