Basilar Invagination / Impression (BI)

FACTS

  • most common congenital anomaly of CCJ
  • Craniovertebral junction (CVJ) abnormality where the odontoid projects abnormally upward into the foramen magnum.
  • Can be congenital (e.g., Chiari, Klippel-Feil, Down syndrome) or acquired (RA, bone softening disorders, trauma, degenerative settling).
  • Concern is for ventral brainstem compression, craniocervical instability, neuro deficits, and compromised CSF flow
  • Surgical need is based on: myelopathy, instability, ventral compression magnitude, and failure of traction realignment.

HPI

Focused, high-yield questions:
  • Symptoms of myelopathy: gait imbalance, hand clumsiness, falls, bowel/bladder changes.
  • Cranial nerve symptoms: dysphagia, dysarthria, facial numbness, vertigo.
  • Neck pain, occipital headaches; worse with neck motion?
  • History of RA, congenital syndromes, trauma.
  • Progression speed (weeks? months? acute?).
  • Signs of cord compression: Lhermitte’s, hyperreflexia, sensory levels.
  • Prior cervical spine surgeries.
  • Any respiratory difficulty or central sleep apnea (brainstem compression).
 
🚩Emergent Red Flags: Acute respiratory compromise, Rapid neurological decline, New severe occipital headache with brainstem signs, Loss of protective airway reflexes.

Associated congenital syndrome and acquired pathologies:
  • Congenital: Down syndrome, Klippel-Feil syndrome, Chiari, syringomyelia, RA
  • Acquired: Paget’s, osteogensis imperfecta, osteomalacia, rickets, hyperparathyroidism

PHYSICAL EXAM

  • universal spine neuro exam
  • HA

IMAGING

CT C-spine:
  • Look for odontoid migration above key lines:
    • Chamberlain line (normal ≤ 3mm of odontoid above this line, 6mm+ definitely pathologic): hard palate to opisthinon
    • McGregor line (normal ≤ 4.5mm dens above this line): hard palate to most caudal point of occiput
Chamberlain / McRae / McGregor lines: source https://radiopaedia.org/articles/chamberlain-line
Chamberlain / McRae / McGregor lines: source https://radiopaedia.org/articles/chamberlain-line
  • Basilar impression vs invagination differentiation.
  • Congenital anomalies, occipital assimilation, facet alignment.
MRI Cervical spine / brain:
  • Evaluate:
    • Ventral brainstem compression, cord flattening.
    • Myelomalacia.
    • Syrinx / CSF flow compromise.
    • Ligamentous laxity or pannus (e.g., RA).
    • for platybasia (may or may not be associated with BI)
BASAL ANGLE (θ) = posterior clivus vs. floor of anterior fossa to dorsum sella. Normal mean: 130˚, platybasia > 145˚
BASAL ANGLE (θ) = posterior clivus vs. floor of anterior fossa to dorsum sella. Normal mean: 130˚, platybasia > 145˚

A/P

Initial Management
  • Immobilize the neck (C-collar) if symptomatic or unstable.
  • Avoid excessive manipulation until full imaging reviewed.
  • Operative indications: Myelopathy or progressive neuro deficit, Significant ventral cord/brainstem compression, demonstrated atlantoaxial instability, failure of traction to reduce deformity.
  • Decide what surgery is needed based on presence of Chiari
 
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Type 1 Basilar Invagination
Type 2 Basilar Invagination
Definition
no Chiari malformation
with Chiari malformation
Notes
Tip of odontoid tends to be above Chamberlain’s and McRaes
Tip of odontoid tends to be above Chamberlain’s but not McR
Treatment
transoral surgery + PSF
foramen magnum decompression
If not surgical:
  • Rigid cervical immobilization.
  • RA-associated pannus → aggressive rheumatologic management.
  • Close follow-up with serial neuro exams and imaging.