Basilar invagination is a condition where the base of the skull is indented by the upper cervical spine. This can cause the tip of the odontoid process to protrude into the foramen magnum, putting pressure on the brainstem and spinal cord. There are two types - primary, which is a congenital structural abnormality of the craniocervical junction, and secondary, which is acquired due to conditions like Paget's disease or rickets that weaken the skull bones. Symptoms range from being asymptomatic to severe and include headaches, weakness, and difficulties with cranial nerves. Diagnosis involves X-rays, CT scans, and MRI to measure the degree of basilar invagination.

1. Basilar invagination
(BASILAR IMPRESSION)
DR.SAJIL KRISHNA K

2. • Indentation of the skull floor by the upper cervical spine
• Tip of the odontoid is more cephalid than normal, may protrude into
foramen magnum
• Neurological damage caused by direct pressure/ circulatory
compromise of vertebral artery/ impairment o CSF
• Misdiagnosed as posterior fossa tumor, bulbar polio palsy,
amyotrophic lateral sclerosis, spinal cord tumor, or multiple sclerosis

3. • Two types Primary and secondary
• Primary ( incidence 1%) is the congenital structural abnormality of
craniocervical junction
• primary may associated with other vertebral defects like,
atlantooccipital fusion ,
klippel – feil syndrome,
arnorld chiary malformation,
syringomyelia,
odontoid anomalies,
hypoplasia of atlas,
bifid posterior arch of atlas

4. • Secondary is the acquired type resulting from systemic disease that
cause softening of osseous structures at the base of skull such as
Pagets disease
osteomalacia
rickets
Osteogenic imperfect ( mainly type 3 and type 4)
Rheumatoid arthritis
Neurofibramatosis
Ankylosing spondylitis

5. Clinical presentation
• Varies from totally asymptomatic to severe
• Symptoms starts usually during second and third decades
( increased ligamentous laxity and instability with age and decreased
tolerance to compression spinal cord and vertebral arteries)
• Most have short neck, asymmetry of face or skull, torticollis (non
specific)
• Headache in distribution of greater occipital nerve

6. • DeBarros et al. divided the sign and symptoms into two categories
1) caused by pure basilar impression
2) caused by Arnold chiari malformation

7. • By pure basilar impression :- primary motor and sensory disturbances
such as weakness and paresthesia in the limbs, involvement of lower
cranial nerves such as trigeminal, vagus, glossopharyngeal and
hypoglossal nerves
• Due to Arnold chiari malformation had symptoms of cerebellar and
vestibular disturbance such as ataxia, dizziness, and nystagmus
• Sexual disturbances such as impotence and reduced libido also seen

8. • Higher incidence of vertebral artery anomalies seen
• Symptoms of vertebral artery insufficiency such as dizziness, seizures,
mental deterioration and syncope

9. Radiographic findings
• Xrays
• CT
• MRI
(functional MRI obtained with spine in flexion and then extension
shows dynamics of spinal cord compression caused by vertebral
instability or anomaly)

10. Measurement for diagnosing basilar impression
On lateral radiograph

16. On AP radiograph / CT

17. • Clark station, redlund – johnell criterion, ranawat criteria are useful
to measure basilar impression in adult with rheumatoid arthritis

18. • By dividing odontoid process into three equal parts in sagittal plane
• Positive if anterior rim of atlas is at the level of middle third( station 2)
or caudal third ( station 3)
Clarks stations

20. • Distance between McGregor line and midpoint of caudal margin of
second cervical vertebral body .
• Basilar invagination present if measurement less than 34 ( in men)
and less than 29 ( in women).
• Not applicable in children
Redlund- johnell criterion

21. • Distance between center of second cervical pedicle and transverse
axis of atlas.
• Basilar invagination present if distal is less than 15 mm ( in men)
and less than 13 ( in women)
• Not applicable in children
Ranawat criterion

23. Treatment
• Conservative in asymptomatic patients with periodic examination
• Surgery if clinical symptoms worsen and not based on degree of invasion.
• If symptoms caused by anterior impingement from odontoid , stabilization
in extension by an occipital C1-2 fusion.
• If symptoms persists anterior excision of odontoid can be done after
posterior stabilization.
• Posterior impingement requires suboccipital craniectomy and laminectomy
C1 and possibly C2 to decompress brain stem and spinal cord
• Posterior fusion is recommended in addition to decompression if stability is
doubtful

24. • Thank you