The craniocervical junction is the region where the base of the skull connects to the upper cervical spine. It includes the occiput, the first cervical vertebra called the atlas, and the second cervical vertebra called the axis.
This area protects the lower brainstem, upper spinal cord, important nerves, and vertebral arteries while allowing the head to bend, rotate, and remain balanced over the spine.
Craniocervical junction disorders may result from congenital abnormalities, degeneration, trauma, inflammatory disease, tumors, infection, poor bone quality, or previous surgery. Some conditions remain stable and cause few symptoms. Others may lead to pain, instability, deformity, spinal cord compression, brainstem compression, or progressive neurological dysfunction.
Major Diagnosis Types
Below are the major types we see in our clinic. Click on available links to learn about our approach to the problems.
Chiari Malformation
Chiari malformation occurs when part of the cerebellum extends downward through the opening at the base of the skull.
This may interfere with the normal flow of cerebrospinal fluid and place pressure on structures near the craniocervical junction.
Possible symptoms include:
- Headaches at the back of the head
- Headaches worsened by coughing, sneezing, or straining
- Neck pain
- Dizziness
- Balance problems
- Numbness or weakness
- Difficulty swallowing
- Ringing in the ears
- Sleep-related breathing problems
Chiari malformation may be associated with syringomyelia, a fluid-filled cavity within the spinal cord.
Not every patient with low-lying cerebellar tonsils has a symptomatic Chiari malformation.
Minor Diagnosis Types
These terms are often encountered, but are either less often treated or described within one of the above diagnoses.
Atlantoaxial Instability
Atlantoaxial instability affects the relationship between the first and second cervical vertebrae.
This region normally allows a large amount of head rotation. Stability depends on the shape of the joints and several strong ligaments, especially the transverse ligament.
Possible causes include:
- Trauma
- Rheumatoid arthritis
- Congenital abnormalities
- Down syndrome
- Os odontoideum
- Infection
- Previous surgery
- Ligament injury
Symptoms may include:
- Upper neck pain
- Pain with head rotation
- Headaches
- A sensation of movement or instability
- Arm or leg weakness
- Balance difficulty
- Loss of coordination
- Signs of spinal cord compression
Significant atlantoaxial instability may place the upper spinal cord at risk.
Atlantoaxial Subluxation
Atlantoaxial subluxation occurs when the atlas and axis become partially misaligned.
The abnormal alignment may be fixed or may worsen with neck movement.
Potential causes include:
- Rheumatoid arthritis
- Trauma
- Ligament injury
- Congenital abnormalities
- Infection
- Inflammatory conditions
Some patients have pain alone, while others develop neurological symptoms if the spinal canal becomes narrowed.
Atlantoaxial subluxation may occur together with cranial settling or basilar invagination.
Basilar impression may cause symptoms similar to basilar invagination, particularly when the brainstem or upper spinal cord is compressed.
Basilar Invagination
Basilar invagination occurs when the upper cervical spine moves upward toward the base of the skull.
The odontoid process of the second cervical vertebra may project abnormally high and reduce the space available for the brainstem or upper spinal cord.
Possible causes include:
- Congenital bone abnormalities
- Rheumatoid arthritis
- Atlantoaxial instability
- Bone-softening disorders
- Paget disease
- Trauma
- Cranial settling
Symptoms may include:
- Upper neck pain
- Headaches
- Weakness
- Numbness
- Difficulty walking
- Loss of coordination
- Difficulty swallowing
- Abnormal reflexes
- Symptoms related to brainstem compression
The clinical importance depends on the degree of compression, instability, and neurological dysfunction.
Basilar Impression
Basilar impression also describes upward displacement of the upper cervical spine toward the skull base.
The term is often used when the abnormality results from softening, remodeling, or acquired deformation of the skull base rather than a congenital bone abnormality.
Possible causes include:
- Rheumatoid arthritis
- Paget disease
- Osteomalacia
- Severe osteoporosis
- Bone tumors
- Other metabolic bone disorders
Occipitocervical Instability
Occipitocervical instability is abnormal movement between the skull and the upper cervical spine.
This condition is uncommon but potentially serious because the brainstem, upper spinal cord, lower cranial nerves, and vertebral arteries are located nearby.
Possible causes include:
- High-energy trauma
- Congenital abnormalities
- Inflammatory arthritis
- Connective tissue disorders
- Tumors
- Infection
- Previous surgery
- Destruction of bone or ligaments
Symptoms may include:
- Severe upper neck pain
- Headaches
- A sensation that the head is unstable
- Neurological symptoms
- Difficulty swallowing
- Difficulty maintaining head position
- Balance or walking problems
Traumatic occipitocervical instability may be life-threatening and requires urgent evaluation.
Atlas Assimilation
Atlas assimilation is a congenital condition in which part or all of the first cervical vertebra is fused to the base of the skull.
Many patients have no symptoms and discover the condition incidentally.
In some cases, atlas assimilation may alter the mechanics of the upper cervical spine and contribute to:
- Reduced neck motion
- Atlantoaxial instability
- Basilar invagination
- Spinal cord compression
- Early degeneration at nearby levels
- Vertebral artery abnormalities
The significance depends on the anatomy and whether neurological or mechanical symptoms are present.
Os Odontoideum
Os odontoideum is a condition in which the upper portion of the odontoid process is separated from the main body of the second cervical vertebra.
It may be congenital or may result from an unrecognized childhood injury.
The separated bone fragment may move abnormally with the atlas, leading to atlantoaxial instability.
Possible symptoms include:
- Upper neck pain
- Headaches
- A sensation of instability
- Weakness or numbness
- Loss of hand coordination
- Difficulty walking
- Symptoms after relatively minor trauma
Some patients remain asymptomatic, while others develop significant spinal cord compression.
Odontoid Abnormalities
The odontoid process, also called the dens, is a bony projection from the second cervical vertebra that helps stabilize rotation between C1 and C2.
Odontoid abnormalities may include:
- Odontoid fractures
- Odontoid hypoplasia
- Os odontoideum
- Congenital absence or malformation
- Degenerative erosion
- Tumor involvement
- Infection
- Inflammatory pannus
These disorders may produce instability, pain, deformity, or compression of the spinal cord.
Odontoid abnormalities are especially important because even small changes in alignment can affect the limited space available at the craniocervical junction.
Platybasia
Platybasia is an abnormal flattening of the skull base.
It may occur by itself or together with basilar invagination and other congenital abnormalities.
Platybasia may alter the relationship between the skull, brainstem, and upper cervical spine. Many patients have no symptoms.
When associated with compression or abnormal alignment, symptoms may include:
- Headaches
- Neck pain
- Balance difficulty
- Weakness
- Numbness
- Difficulty swallowing
- Signs of brainstem or spinal cord dysfunction
Cranial Settling
Cranial settling occurs when the skull shifts downward relative to the upper cervical spine or when the upper cervical spine migrates upward toward the skull base.
It is most commonly associated with advanced rheumatoid arthritis but may also develop from severe bone or ligament destruction.
Cranial settling may occur together with:
- Atlantoaxial instability
- Basilar invagination
- Odontoid erosion
- Occipitocervical instability
Possible symptoms include:
- Severe upper neck pain
- Headaches
- Neurological weakness
- Difficulty walking
- Trouble swallowing
- Changes in breathing
- Signs of brainstem compression
This condition can be progressive and may place critical neurological structures at risk.
Retro-Odontoid Pseudotumor
A retro-odontoid pseudotumor is a mass of thickened soft tissue located behind the odontoid process.
Despite the name, it is usually not a true tumor.
It may develop from:
- Atlantoaxial instability
- Rheumatoid arthritis
- Calcium crystal deposition
- Long-standing mechanical stress
- Ankylosing spinal conditions
- Reduced motion in the lower cervical spine
The mass may narrow the spinal canal and compress the upper cervical spinal cord.
Possible symptoms include:
- Neck pain
- Hand clumsiness
- Arm or leg weakness
- Balance problems
- Difficulty walking
- Abnormal reflexes
The underlying instability or mechanical condition is often as important as the soft tissue mass itself.
Craniocervical Junction Tumors
Tumors may arise from the bones, nerves, meninges, brainstem, spinal cord, or other tissues near the craniocervical junction.
Examples include:
- Meningioma
- Schwannoma
- Chordoma
- Metastatic cancer
- Bone tumors
- Spinal cord tumors
- Tumors involving the skull base
Symptoms depend on the location and growth pattern of the tumor.
Possible symptoms include:
- Persistent upper neck pain
- Headaches
- Weakness
- Numbness
- Loss of coordination
- Difficulty swallowing
- Changes in speech
- Balance problems
- Progressive neurological dysfunction
Because many important structures are concentrated in this region, even a benign tumor may become clinically significant as it grows.
Craniocervical Junction Infection
Infection at the craniocervical junction may involve the bones, joints, discs, ligaments, or epidural space.
Possible causes include:
- Bloodstream infection
- Spread from nearby tissues
- Previous surgery
- Immune suppression
- Intravenous drug use
- Infection elsewhere in the body
Symptoms may include:
- Severe upper neck pain
- Fever
- Chills
- Reduced neck motion
- Headache
- Neurological weakness
- Difficulty walking
- Difficulty swallowing
Infection may damage the supporting structures and lead to instability, epidural abscess, or spinal cord compression.
Congenital and Acquired Disorders
Craniocervical junction disorders may be congenital or acquired.
Congenital disorders are present from birth and may include:
- Chiari malformation
- Atlas assimilation
- Os odontoideum
- Odontoid hypoplasia
- Platybasia
- Congenital basilar invagination
Acquired disorders develop later and may result from:
- Trauma
- Degeneration
- Rheumatoid arthritis
- Infection
- Tumor
- Metabolic bone disease
- Previous surgery
Some congenital conditions do not become symptomatic until adulthood, when degeneration or trauma increases stress on the region.
Stable and Unstable Disorders
Not every craniocervical junction abnormality causes instability.
A stable disorder maintains normal alignment during movement and does not place neurological structures at increasing risk.
An unstable disorder allows abnormal motion between the skull, C1, and C2. This motion may intermittently or continuously compress the brainstem or spinal cord.
Instability may develop from:
- Ligament disruption
- Fracture
- Bone erosion
- Congenital malformation
- Inflammatory arthritis
- Infection
- Tumor
The distinction between stable and unstable conditions is important because it affects the risk of neurological injury.