The C-V junction is a transition site between mobile cranium and relatively rigid spinal column. It is also the site of the medullo spinal junction. CV anomalies are defects of development, not necessarily congenital and may not manifest at birth.

Development:

It is a complex process.

Mesodermal somites numbering 42 appear at the 4th week

Ventromedial part of the somatomes migrate and cluster around notochord-Sclerotomes

A fissure in each sclerotome separate a denser caudal half from loosely arranged cranial half.

Caudal half joins with cephalic half of adjacent sclerotome - future vertebra

Mesenchymal cells of the fissure condense to form I.V.D.

Notochord disappears at the vertebral bodies, but persist as disc (nucleus pulposus)

This membranous stage is followed by chondrification and ossification.

Out of 4 occipital sclerotomes the first 2 form basiocciput, the III Jugular tubercles and the IV (Proatlas) form parts of foramen magnum, atlas and axis.

Dysplasia of the occiptal segments may flatten the clivus - platybasia.

When the basiocciput and rim of foramen magnum are underdeveloped, the odontoid and arch of atlas may grow normally to over hang along the sides. Odontoid and arch of atlas invaginate-Basilar invagination.

The proatlas may develop into separate vertebrae - Occipital vertebra, hypochondral bow of proatlas may persist to gain attachment to the atlas, clivus or even to the apical segment of the dens - responsible for anti-cervico medially compression.

The axis has a fully developed center from the second sclerotome which form the caudal part of the body and articular facets.

At birth odontoid base is separate from the body of axis by a segment of cartilage which persists until the age of eight and the center gets ossified., may remain separate as Os- odontoidium.

The apical segment is not ossified until 3 years of age. At 12 years if fuses with odontoid to form normal odontoid., failure leads to Os terminale.

 Classification CV anomalies:

Often the anomalies are in different combinations and hence the difficulty in an appropriate terminology in every case. Certain terms are conventionally used to express the anomalies, as follows

1. Platy basia:

Flatness of the base of skull. Angle formed by the clival line and a line drawn along the floor of ant. cranial fosse exceeds 140 degrees.

Platy basia alone not associated with other conditions does not produce any symptoms.

2. Basilar Invagination:

Vertebral column invaginates into the posterior fossa which is of 2 types:

The anterior type has a short clivus horizontally placed, with the anterior lip of the foramen magnum invaginated in relation to the spinal column.

The other is paramedian invagination associated with hypoplasia of the occipital condyles. Thus the atlas may get invaginated. The hypochondral bow of the proatlas may persist to gain articulation or fusion with lower end of clivus, ant. arch. The mass of bone may cause ant. compression. There is associated soft tissue anomalies of hind brain in 25-30% of cases. In certain diseases of bone like hyperparathyroidism, pagets or osteomalacia, there is softening of the base of skull which gets invaginated. This is called basilar impression or secondary basilar invagination.

3. Assimilation of atlas:

Assimilation of atlas with the occiput is an expression of nonsegmentation of certain parts of the proatlas and fusion of the first spinal sclerotome with the proatlas. It occurs in 0.25% or less. However its occurrence along with other CV anomalies is frequent. It could be partial or complex and may restrict occiptial movement. It is frequent in Klippel-feil syndrome, involving the second and the third vertebral bodies and may affect the atlanto axial joint. This combination of assimilation of atlas and segmental failure of the II and III vertebral bodies exert an abnormal strain on the atlanto axial joints from childhood. In course of time the ligaments become lax and mobility increases predisposing to atlanto axial dislocation.

4. Os Terminale:

Refers to the nonfused terminal part of the odontoid derived from the centrum of the IV occipital sclerotome. This apical segment is usually about 12mm in length, but can be very small. In case of disruption at the interface and if atlanto axial dislocation occurs, the remaining part of the odontoid may compress the cervico meduallary region.

5. Os odontoideum:

This term has been used to denote a separate piece of bone present posterior to the anterior arch of atlas. The odontoid base fails to fuse with the axis. These are only few odontoid base falls to the above specification. But the diagnosis of Os odontoideum is much more frequent in clinical practice. Careful exam reveals a small hypoplastic odontoid at the upper border of the body of the axis. It is generally believed that traumatic fracture leaves an irregular margin, though the margin may be rounded enough to be indistinguishable from the developmental anomalies. Many of the patients have history of fall. Dens lacks a good nutrient artery. Blood supply thro' the body of axis is limited due to interposition of cartilage between body and the odontoid process. An injury in early childhood probably leads to Os-odontoideum in later life due to avascular necrosis. In the absence of strong reasons to consider embryological basis, traumatic theory is more rationale.

Pathogenesis of Neural involvement:

Neural involvement is basically due to 3 mechanisms.

Mal aligned bony components of the spinal canal compress underlying cord due to dislocation of the joints, the commonest is Atlanto axial dislocation.

Encroachment into the spinal canal may also occur due to formation of the abnormal bone masses around the CV junction. Occasionally the foramen magnum may be narrowed or the rest of arch of the atlas may be deformed to cause compression of spino medullary junction.

Lastly, associated Chiari malformation and syrinx may cause further neural compression.

A-A dislocation is the commonest abnormality, be it congenital or acquired. A dysplastic odontoid provides a vulnerable situation. The transverse ligament may be basically in- competent or become so after a minor trauma. If the dislocation is sudden and severe, an acute quadriparesis may occur. Abnormal mobility in flexion may cause transient neural compression resulting in sudden transient deficit and occasionally Lhtermitte's sign. In course of time the dislocation may become fixed leading to progressive deficit. If it goes on for years vascular damage may happen with no recovery even after adequate decompression. Deformities involving the facet joints between atlas and axis may give rise to rotatory dislocation . Such deformities cause only a neck tilt and pain without neuro deficit.

Occipito-atlantal dislocation is rare.

In basilar invagination, there is crowding of structures in the small post. fossa resulting in compression of medulla, long tract involvement and lower cranial nerve deficits. Rarely there is vertebral art. compression leading onto VBI.

Clinical features:

Prevalence appears to be high as observed by neurosurgeons in India although it is yet to be corroborated by epidemiological studies. The abnormalities may have a familial occurrence. In one series they affect children and young adults primarily. Majority of patients present in their second or third decade. There is male preponderance (1:5:1). H/o. trauma is often available. Many children have URI preceding the onset of symptoms.

Symptoms:

Progressive weakness of limbs, stiffness, difficulty in walking and neck pain are the modes of presentation in 75-85% of cases. Smaller number present with neck tilt, neck pain, cough headaches, occipital cephalalgia without any deficit. Symptoms of lower brain stem dysfunction, such as dysphagia, dysphonia, nasal regurgitation, sleep apnoea are due to basilar invagination.

Signs:

Physical appearance is often striking. Short stature, short neck, low hairline, head tilt, facial asymmetry, web neck, or scoliosis occur in different combinations.

High arched palate, hemiatrophy of tongue, and syndactyly may be associated. Evidence of high cervical cord compression with or without sensory involvement is common. Involvement of one limb or one sided limbs may be misleading . Crossed hemiparesis may suggest cervico medullary junction involvement. Small muscle atrophy due to 'central cord syndrome' like effect of the lower cord level due to upper cord compression is seen in 22%. Spino thalamic involvement is uncommon. Posterior column involvement is seen in 60%. Sphincter disturbance is rare. Associated syrinx may produce dissociated or suspended sensory loss. Lower cranial nerves are involved in about 18%. Cerebellar involvement is seen in about 16%. Mirror movements of the hands are often seen in klippel Feil syndrome due to inadequate decussation of pyramidal tract at medulla.

Imaging:

MRI has totally changed the prospects of investigation of spinal lesions and more so at CV junction. The soft tissue details can be imaged with a high degree. CT scan with reconstruction is still preferred by some to study bone details. X ray with chamberlin line, mcgregor line etc have become history.

Atlanto-Axial subluxation	platibasia	basilar invagination(odontoid at the level of IAM)
3D CT transoral view of odontoid(basilar invagination)	AP view	lat. view
	posterior fixation with wiring and plating after odontoidectomy

Management:

Though a number of bony and ligamentation anomalies have been described, consequences are mainly due to (A) A -A dislocation (B) malformed components of bone producing compression.

In early stages of A-A dislocations, most of them are reducible and require only stabilization.

Irreducible types require open reduction (operative reduction). Operative treatment has involved thro' several modifications. The first effort was in 1910 by Osgood, who tried to reduce the dislocation by pushing backwards the atlas via the pharynx while the posterior arch of atlas was pulled back with a thick silk thread, which was then tied to spinous process of axis. This was the beginning.

Gallow popularized the technique of midline wiring which kept the atlas and axis is opposition. The wire retains an interposed only bone draft. Several modification were suggested. Screw fixation of facet junctions, Halifax clamps, contoured rods are the latest. Hartshell frame is still being used by many. Some have recommended methylmethacrylate use. All these methods are effective when reduction of dislocation is adequate.

Instances of redislocation by snapping of wire, loosening of screws are not rare.

In the fixed or irreducible variety foremen magnum decompression along with C1 & C2 laminectomy is recommended by few and claim to have satisfactory results, if done after a period of skull traction. Oppel was probably the first to operate by ant. route. Removing the compressing element form it front is more rationale.

Recently this has become the preferred procedure. The arch of the atlas, the odontoid and part of the axis can be excised. In addition, the thick ligament and chronic granulation tissue which contribute to compression can be excised.

The ant aspect of CV junction can be approached by Trans pharyngeal, Transpalato pharyngeal, Trans maxillary (Le fort -  with maxillary down fracture) routes. It is generally accepted that a stabilization procedure is necessary following ant. decompression either in the same sitting or as a II stage procedure. Of late ant. stabilization following decompression has been tried in some centers. Some feel a stabilization procedure is not required in selected cases. 

Basilar invagination -   Achondroplasia	AP view	LAT view
	posterior wiring after odontoidectomy

Irrespective of methods used it is essential to immobilize its CV junction with collar till bony fusion occurs, which may take 3 months. Ideal will be the Halo frame.

Outcome:

Significant relief in 70% of cases following ant. decompression can be expected. Reduction in spasticity is appreciated in the immediate post operative period.

Fatal meningitis, post operative dislocation are possible complications.