**1. Anatomy of the craniocervical junction**

The cervical spine is made up of seven vertebrae divided into upper and lower sections. The upper cervical spine includes the first two vertebrae, classically named atlas (C1) and axis (C2). The CCJ links the skull to the upper cervical spine and therefore the foramen magnum

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to the spinal canal. The atlas is essentially a ring of bone formed by two arches that are flanked and joined by the lateral masses, which contain the superior and inferior facets [1].

The transverse processes of the atlas are attached to the lateral masses and contain the intertransverse foramina. The intertransverse foramina of the cervical spine form a flexible protective tunnel for the passage of the vertebral arteries. The alignment of occiput (C0) with the atlas and axis is crucial to the integrity and functional architecture of the spinal cord and mid brain structures [2]. Like the entire spine, this connection is primarily ligamentous and membranous in nature. The atlantoaxial joint (C1–2) is arguably the most unique and complex of all spinal intersegmental relationships. The relative horizontal to biconvex orientation of the opposing weight-bearing facets allows excellent rotation at the expense of osseous stability [3]. The transverse band of the cruciate ligament arises from tubercles on the atlas lateral masses and stretches across and behind the dens of C2 holding the odontoid process (dens) against the anterior arch preventing migration of the dens into the spinal canal [4–6].

The alar ligaments are much larger and stronger than the apical or accessory ligaments. Damage to the alar ligaments can cause joint instability and excess motion [7]. Excess motion can lead to kinking or compression of the vertebral arteries and irritation of nociceptor and mechanoreceptors, which may play a role in symptoms such as headache, neck pain and dizziness associated with head/neck trauma and whiplash-type injuries (**Figure 1**).

> The tectorial membrane is a continuation of the posterior longitudinal ligament and ultimately coalesces with the periosteum lining along the anterior margin of the foramen magnum at the basion [6, 12, 13]. The Tectorial Membrane (TM) plays a substantial role in stabilizing the cranio-cervical junction, especially by limiting flexion. During head/neck trauma, hyperextension/hyperflexion and translation take place at the cranio-cervical junction. Hyperflexion alone or combined with anterior translation is the presumed mechanism for injury/damage to

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**1.** Partial thinning involving less than 1/3rd the width of the TM (grade I lesion) can represent

**2.** Lesions involving up to 2/3rd's of the width (grade II) may be seen as a consequence of

**3.** Complete absence of or disruption of greater than 2/3rds of the membrane (grade III lesion), accompanied by a normal or partially ruptured dura mater, has not been described

Weakening and disruption of the key stabilizers of the CCJ can lead to a head forward posture resulting in loss or reversal of the cervical lordosis. This straightening effectively lengthens the spinal canal. The dentate ligaments stabilize the position of the spinal cord in the center of the spinal canal. The spinal cord subsequently can become tethered to each spinal segment by way of the dentate ligaments, and such loss of the cervical lordosis may create traction on the spinal cord resulting in a caudal downward pulling of the brain and cranial elements (brainstem/cerebellar tonsils) downward into the foramen magnum [16, 17]. This can result in

the TM (**Figures 3** and **4**) [14].

**Figure 2.** Disruption of the alar ligaments.

a normal variant,

Grading of ligament disruption is as follows:

in the normal patient population [15].

head/neck trauma and or repetitive micro-stress.

**Figure 1.** Coronal illustration of the ligamentous stabilizers of the Cranio-cervical junction.

The anterior and posterior spinal longitudinal ligaments (ALL and PLL) are major stabilizers of the anterior and middle columns of the entire spinal axis [8]. The posterior longitudinal ligament transcends into what becomes the anterior dura-mater/tectorial "membrane" complex cephalad to the mid C2 vertebral body (the longitudinal collagenous architecture of the tectorial "membrane" is indistinguishable from the posterior longitudinal "ligament"). The ALL and PLL are two "paired" ligaments known as the suboccipital stabilizers to flexion and extension stress [9, 10]. The capsular ligaments stabilize the facet joints by limiting flexion and rotation (**Figure 2**) [11].

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**Figure 2.** Disruption of the alar ligaments.

to the spinal canal. The atlas is essentially a ring of bone formed by two arches that are flanked

The transverse processes of the atlas are attached to the lateral masses and contain the intertransverse foramina. The intertransverse foramina of the cervical spine form a flexible protective tunnel for the passage of the vertebral arteries. The alignment of occiput (C0) with the atlas and axis is crucial to the integrity and functional architecture of the spinal cord and mid brain structures [2]. Like the entire spine, this connection is primarily ligamentous and membranous in nature. The atlantoaxial joint (C1–2) is arguably the most unique and complex of all spinal intersegmental relationships. The relative horizontal to biconvex orientation of the opposing weight-bearing facets allows excellent rotation at the expense of osseous stability [3]. The transverse band of the cruciate ligament arises from tubercles on the atlas lateral masses and stretches across and behind the dens of C2 holding the odontoid process (dens) against the

The alar ligaments are much larger and stronger than the apical or accessory ligaments. Damage to the alar ligaments can cause joint instability and excess motion [7]. Excess motion can lead to kinking or compression of the vertebral arteries and irritation of nociceptor and mechanoreceptors, which may play a role in symptoms such as headache, neck pain and diz-

The anterior and posterior spinal longitudinal ligaments (ALL and PLL) are major stabilizers of the anterior and middle columns of the entire spinal axis [8]. The posterior longitudinal ligament transcends into what becomes the anterior dura-mater/tectorial "membrane" complex cephalad to the mid C2 vertebral body (the longitudinal collagenous architecture of the tectorial "membrane" is indistinguishable from the posterior longitudinal "ligament"). The ALL and PLL are two "paired" ligaments known as the suboccipital stabilizers to flexion and extension stress [9, 10]. The

capsular ligaments stabilize the facet joints by limiting flexion and rotation (**Figure 2**) [11].

**Figure 1.** Coronal illustration of the ligamentous stabilizers of the Cranio-cervical junction.

and joined by the lateral masses, which contain the superior and inferior facets [1].

28 Hydrocephalus: Water on the Brain

anterior arch preventing migration of the dens into the spinal canal [4–6].

ziness associated with head/neck trauma and whiplash-type injuries (**Figure 1**).

The tectorial membrane is a continuation of the posterior longitudinal ligament and ultimately coalesces with the periosteum lining along the anterior margin of the foramen magnum at the basion [6, 12, 13]. The Tectorial Membrane (TM) plays a substantial role in stabilizing the cranio-cervical junction, especially by limiting flexion. During head/neck trauma, hyperextension/hyperflexion and translation take place at the cranio-cervical junction. Hyperflexion alone or combined with anterior translation is the presumed mechanism for injury/damage to the TM (**Figures 3** and **4**) [14].

Grading of ligament disruption is as follows:


Weakening and disruption of the key stabilizers of the CCJ can lead to a head forward posture resulting in loss or reversal of the cervical lordosis. This straightening effectively lengthens the spinal canal. The dentate ligaments stabilize the position of the spinal cord in the center of the spinal canal. The spinal cord subsequently can become tethered to each spinal segment by way of the dentate ligaments, and such loss of the cervical lordosis may create traction on the spinal cord resulting in a caudal downward pulling of the brain and cranial elements (brainstem/cerebellar tonsils) downward into the foramen magnum [16, 17]. This can result in

an acquired cerebellar tonsillar ectopia, which can interfere with the cerebral spinal fluid flow of CSF, resulting in a disequilibration of arterial and venous flow while degrading the nutritive, restorative and support function of the CSF for the central nervous system (**Figure 5**) [18].

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Rotary misalignments of C1–2 can impair the normal CSF flow as well as contributing to an

A tortuous vertebral artery may be visible on MRI imaging. Pulsatile compression of the brain stem by the vertebral artery is associated with cerebellar dysfunction, hydrocephalus, ischemic

insufficiency of blood flow of the vertebro-basillar system (**Figure 6**).

**Figure 6.** Rotary misalignment of atlas (C1) and axis (C2).

**Figure 5.** Brain stem compressed by the right vertebral artery. Low cerebellar tonsils.

**Figure 3.** Sagittal illustration of the ligamentous stabilizers of the Cranio-cervical junction.

**Figure 4.** Disruption of the tectorial membrane.

an acquired cerebellar tonsillar ectopia, which can interfere with the cerebral spinal fluid flow of CSF, resulting in a disequilibration of arterial and venous flow while degrading the nutritive, restorative and support function of the CSF for the central nervous system (**Figure 5**) [18].

**Figure 5.** Brain stem compressed by the right vertebral artery. Low cerebellar tonsils.

Rotary misalignments of C1–2 can impair the normal CSF flow as well as contributing to an insufficiency of blood flow of the vertebro-basillar system (**Figure 6**).

**Figure 6.** Rotary misalignment of atlas (C1) and axis (C2).

**Figure 4.** Disruption of the tectorial membrane.

30 Hydrocephalus: Water on the Brain

**Figure 3.** Sagittal illustration of the ligamentous stabilizers of the Cranio-cervical junction.

A tortuous vertebral artery may be visible on MRI imaging. Pulsatile compression of the brain stem by the vertebral artery is associated with cerebellar dysfunction, hydrocephalus, ischemic stroke, transient or permanent motor deficits, central sleep apnea, trigeminal neuralgia, as well as brain stem compression syndrome [19–21].

FONAR upright weight bearing MRI has been shown to be most sensitive in detecting cerebellar tonsillar ectopia since weight- bearing posture presents the cerebellar tonsils further distended into the foramen magnum [18]. Visualization of misalignment of the craniocervical junction and its effects on the nervous system is also demonstrated when images are acquired under the effects of gravity. Imaging of the sagittal, coronal and axial planes ensure a fulsome evaluation of the adequacy of the foramen magnum and provides good sensitivity in the evaluation of the cerebellar tonsils (**Figures 7**–**10**).

**Figure 7.** Normal position of cerebellar tonsils.

**2. CSF flow**

In 1891, Chiari discovered anomalies involving the cerebellar tonsils while performing postmortem examinations on children and adolescents with cerebral hydrocephalus. He recognized that the size of these structural defects in the brain was not related to the severity of the hydrocephalus [18]. The classic definition of Chiari malformation is herniation of the cerebellar tonsils 3 to 5 mm below the foramen magnum. This excess tissue in the upper cervical spinal canal creates pressure and disrupts the flow of cerebrospinal fluid (CSF). Blocked spinal fluid can cause hydrocephalus

**Figure 10.** Bilateral cerebellar tonsillar ectopia demonstrated on the coronal view (left) and on the axial view (right).

**Figure 9.** Coronal view demonstrating misalignment of C0-C1 with left cerebellar tonsillar ectopia.

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or, as is more common in Chiari malformation, a fluid-filled cyst known as a syrinx [22].

**Figure 8.** Cerebellar tonsillar ectopia.

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**Figure 9.** Coronal view demonstrating misalignment of C0-C1 with left cerebellar tonsillar ectopia.

**Figure 10.** Bilateral cerebellar tonsillar ectopia demonstrated on the coronal view (left) and on the axial view (right).
