Definition
Cranial prominence of odontoid into region of foramen magnum
- upper cervical spine encroaches on brain stem
Aetiology
Congenital syndromes
- Klippel Feil Syndrome
- Achondroplasia
- OI
- Downs
- Morquio
- SED
- NF
Congenital abnormalities
- atlas hypoplasia
- bifid posterior arch atlas
- occipitocervical synostosis
Acquired
- JRA
- Rickets
- RA
Clinical Features
Become symptomatic in third decade
- neurological compression 15%
Xray
McRae's line
- line along foramen magnum
- Basion to Opisthion
- dens above line = basilar invagination
MacGregor' s Line
- hard palate to inferior aspect occiput
- < 4.5mm above line
Ranawat Criterion
- distance between centre of C2 pedicle & transverse axis of C1
- men < 15mm
- women < 13mm
MRI
Delineate pathology
Management
Consider surgery if symptomatic
Indications
1. Severe pain
2. Neurology
3. Progression
- basilar invagination > 5mm
Technique
Surgery complex & fraught with complications
1. Fusion
- Gallie C1/2 fusion if reducible
- occipitocervical fusion if not with C1 laminectomy
2. Excision of odontoid