Definition
- A radiological finding that has many aetiologies
- a developmental anomaly of the craniovertebral junction in which the odontoid abnormally prolapses into the foramen magnum
- Atul Goel 1998 definition
- tip of the odontoid process was at least 2.5 mm above Chamberlain’s line
- Normal bone quality skull base
Due to
- basiocciput (clivus) hypoplasia,
- occipital condyle hypoplasia,
- atlas hypoplasia,
- incomplete ring of C1 with spreading of the lateral masses,
- achondroplasia,
- atlanto-occipital assimilation
Definition
- Basilar Impression
- A type of Basilar invagination
- the acquired form of basilar invagination, which results from softening of the bone at the base of the skull.
- Aetiologies
- Common
- Paget disease
- Osteomalacia
- other
- tumors,
- infection,
- hyperparathyroidism,
- osteogenesis imperfecta,
- Hurler syndrome,
- rickets,
- skull base infection
- Cranial settling:
- A type of basilar invagination
- Used instead of basilar impression in patients with rheumatoid arthritis;
- affects 5-10% RA patients
- Ryken 1993: Erosive changes of the atlantal lateral masses result in downward telescoping of the atlas onto the axis body, anterior displacement of the atlantal posterior arch, and subsequent ventral and dorsal cervicomedullary compression.
Associated condition
- 25% and 35% of patients with basilar invagination also have associated neural axis abnormalities, including Chiari malformation, syringomyelia, syringobulbia, and hydrocephalus
Aetiology
- RA
- Klippel- Feil
- osteogenesis imperfecta
- Achondroplasia
- Chiari malformations
- cleidocranial dysostosis
- Schwartz- Jampel syndrome 2
- Morquio’s syndrome
- Hyperparathyroidism
- Osteomalacia
- Rickets
- Paget’s disease.
- Congenital conditions (BI is the most common congenital anomaly of the craniocervical junction; it is often accompanied by other anomalies
- Down syndrome
- Klippel-Feil syndrome
- Chiari malformation: 92%
- Syringomyelia
- Acquired conditions
- Rheumatoid arthritis (in part due to incompetence of transverse ligament, see Basilar impression in rheumatoid arthritis
- posttraumatic
- Conditions with BI associated with softening of bone include30:
- Paget’s disease
- Osteogenesis imperfecta:
- patients have blue discolored sclera and early hearing loss due to a genetic defect that causes defective Type 1 collagen.
- Bones are weak (“brittle-bone disease”). Autosomal dominant inheritance.
- There are 4 common types of OI and some uncommon ones
- Osteomalacia
- Rickets
- Hyperparathyroidism
Pathophysiology
- Atul goel classification: Two types
- Type I: BI without Chiari malformation.
- Odontoid tip tends to be above CL, McR, and WCCL
- Brainstem compression is due to odontoid process invagination.
- patients were younger (adolescents)
- F:M= 1:3
- 85% can be reduced with traction.
- More acute presentation
- Treatment:
- transoral surgery is recommended, usually accompanied by posterior fusion
- Type II: BI + Chiari malformation.
- Odontoid tip tends to be above CL, but not McR or WCCL.
- Brainstem compression is due to reduced p-fossa volume.
- Patients older
- F:M= 1:1
- Less acute presentation
- Only 15% can be reduced with traction.
- Foramen magnum decompression is appropriate
- Pang et al classification:
- Anterior basilar impression
- caused by changes in the basioccipital complex (Platybasia), which derives from the axial (median) occipital sclerotomes. → A short + flattened clivus → The basion, which normally lies below the nasion–opisthion line (of Boogaard), is raised way above it and moves cranially →causing the plane of the Foramen magnum to tilt upwards in a lordotic angle (same time this causes the planes of the occipital condyles and C0/C1 articulation to be tilted upwards → (The fusion and chondrification of the basioccipital and the exoccipital sclerotomes occur slightly earlier than the resegmentation of the C1–C2 sclerotomes) forces the emerging upper cervical sclerotomal column to bend backwards “in sympathy”, especially its centra complex that will ultimately form the dens–axis → forming a retroflex and lordotic dens → in severe cases can cause compression of the brainstem
- Platybasia
- Most dramatic basioccipital complex change found
- When the nasion–tuberculum–basion angle (NTB angle of Welcker) is increased and the sphenoclival block appears severely flattened.
- Often accompanied by a shortened basioccipital or clivus, which normally measures more than 3.2 cm from the sphenooccipital synchondrosis to the basion.
- Posterior basilar invagination
- 2 mech
- The dens is secondarily elevated towards the cranial cavity without being lordotic or retroflexed because
- Exoccipitals, derived from the lateral sclerotomes of the proatlas, rise up towards the foramen magnum, bringing with them the occipital condyles and opisthion.
- Exoccipital bones may be thin and the condyles flat and hypoplastic.
- The opisthion often invaginates into the cranial aperture (basilar invagination).
- CT of CVJ. Left: coronal CT shows severely upslanting and elevated left occipital condyle–C1 joint (arrows). C4 is a hemi-vertebra. Right: sagittal CT shows invagination of the opisthion (Op) (basilar invagination). The posterior arch of C1 is assimilated into the posterior rim of the foramen magnum. Note slightly high-riding odontoid but absence of platybasia, short clivus or retroflexed dens.
- Sagittal and axial MRI shows compression of the medulla by the invaginating opisthion and syringomyelia
- Combined anterior–posterior form of basilar impression
- Severe basilar impression by a retroflexed dens + posterior invagination of the opisthion → neural compression from both front and back → reduces posterior fossa volume (Drop in index of Klaus from normal of >30mm to below 25mm) →Crowding of cerebellum and brainstem → herniation of tonsils (ectopic cerebellar tonsils)
- Combined anterior–posterior form of basilar impression.
Sagittal CT shows extreme platybasia (NTB angle=180°), short clivus (<1.5 cm) and forward folding of the clivus–axis angle of Wackenheim (80°), causing lordotic tilt of the foramen magnum plane and plane of the occipital condyles, resulting in a retroflexed dens and severe basilar impression. - Note violation of McGregor’s, Chamberlain’s and McRae’s lines by the dens. Also, extreme invagination of the opisthion (O) and high posterior C1 arch (C1).
- Sagittal MR shows distortion and compression of brainstem by both the dens and the opisthion
Clinical presentation
- Associated Without chiari malformation
- Age of onset
- 58% presented in the 20s
- 86% presented within the first 30 yrs of life
- Trauma was the principal precipitating factor in initiating symptoms in 48% of patients with basilar invagination without associated Chiari malformation
- Common signs and symptoms
- weakness (100%)
- neck pain (59%)
- only posterior column dysfunction (39%)
- bowel and bladder disturbance (28%)
- paresthesia (25%)
- Common localized findings
- torticollis (69%)
- restricted neck movements (59%)
- low hairline (48%)
- webbed neck (47%)
- short neck (41%)
- Associated with chiari malformation
- Age of onset
- 44% presented in the third decade of life
- 88% presented between second and fourth decades of life.
- duration of symptoms was relatively long-standing and slowly progressive,
- Common signs and symptoms
- weakness (94%)
- paresthesia (79%)
- posterior column and spinothalamic tract disturbance (56%),
- ataxia (47%)
- Common localized findings
- short neck (50%)
- webbed neck (38%)
- low hairline (37%).
- Hydrocephalus
- Syringomyelia
- Progressive myelopathy
Radiology
- Radiographic measurements have been described for identifying basilar invagination
- If Dens tip cannot be identified
- Due to erosion and pannus.
- Use the position of the anterior C1 arch in relation to the C2 body.
- A severe abnormality is identified when the anterior arch of C1 is adjacent to the lower third of the C2 body.
- Radiographic measurements (See Skull base measurements)
Management
- There is little consensus on the best way to treat basilar invagination
- The location of the vertebral arteries must be considered.
- Combination including
- Traction
- Anterior decompression
- The presence of ventral tissue contraction may make an anterior approach essential.
- Where cranial nerve palsies exist or where traction is considered unsafe then an anterior transnasal or transoral resection of the odontoid peg should be considered.
- Craniocervical fusion with or without foramen magnum decompression.
- When there are no cranial nerve palsies a posterior fusion is usually adequate, preoperative traction may help to predict whether adequate reduction can be achieved intraoperatively.
- In some cases C1 and C2 will already have fused but in others surgical C1/ 2 fusion will be required either anteriorly or with a combined posterior approach.
- Most authors recommend performing the anterior decompression first followed by the posterior fusion, but a few authors do recommend the reciprocal.
- Posterior decompression for Chiari I malformation with coexisting basilar impression carries a much higher late complication rate than for pure cerebellar ectopia.
- Even if the anterior vector is not initially symptomatic, the slightest amount of post-operative cranial settling after nullification of the dorsal tension band will deliver the brainstem straight on to the pointing dens to cause rapid onset of new brainstem signs
- If symptomatic post-decompression cranial settling does occur despite precautions, calipers skull traction with the neck in slight extension should be rendered under muscle relaxation and sedation for a few days, aiming at reversing the kyphotic deformity and telescoping effect at the foramen magnum. Occiput–C1–C2 fusion should be done after reduction. If brainstem indentation persists, transoral resection of the dens may be necessary.
Differentials
Atlantoaxial instability / subluxation radiological features | Basilar invagination radiological features |
ADI difference of >3.5 mm between flexion and extension suggests instability (not an indication for surgery). | Ranawat C1–C2 index <13 mm (most reproducible; normally 17 mm in men, 15 mm in women). |
ADI >5 mm is diagnostic of subluxation. | Tip of dens >4.5 mm above McGregor’s line. |
ADI difference of >7 mm of motion may indicate disruption of alar ligament. | Dens is above McRae’s line. |
ADI difference of >10 mm motion associated with increased risk of neurologic injury and an indication for surgery. | Dens >3 mm above Chamberlain’s line. |
PADI (SAC) <14 mm associated with increased risk of neurologic injury and is an indication for surgery. | Cervicomedullary angle <135°. |
PADI (SAC) >13 mm may predict complete neural recovery after decompressive surgery. | ㅤ |