Basilar invagination occurs when the superior part of the cervical spine migrates upward.  More specifically, it is the odontoid, part of the second cervical vertebra, that migrates above the junction between the skull and cervical spine (craniocervical junction).  Basilar invagination and basilar impression are often used interchangeably since they refer to same upward movement of the the odontoid with either softened skull base bone (B. invagination) or normal bone ( B. impression). 

Basilar invagination may be related to congenital conditions such as Down syndrome, may result of an acquired condition such as rheumatoid arthritis or may occur following a trauma.

Symptoms
This upward movement of the dens decreases the space available for structures that normally course down the foramen magnum (opening at the skull base enabling brainstem, vertebral arteries and lower cranial nerves to transit towards the cervical spine).  Headaches are frequently reported. Dizziness, swallowing difficulty, change in voice numbness/tingling or weakness noted in the extremities may also be noted and reflect compression of neurological structures within the foramen magnum..

Symptoms may also be dynamic in nature and worsen with head flexion which further compresses the brainstem and/or spinal cord on the odontoid.

The fourth ventricle, one of the four cavities filled with cerebrospinal fluid (CSF), is located just between the inferior brainstem (medulla) and the cerebellum.  The upward movement of the odontoid may also compress the inferior portion of the fourth ventricle, resulting in accumulation of CSF and possibly symptomatic hydrocephalus. 

Diagnosis
Imaging is required to diagnose basilar invagination.

Plain lateral cervical spine x-ray with odontoid view is the first test performed and enables to locate the odontoid in regard to various anatomical landmarks.  CT scanner with sagital reconstructions is best to assess bony structures.  MRI scanner is best to evaluate neural tissue such as the brainstem.  Dynamic exams with flexion-extension may give more information regarding the dynamic aspect of the invagination and have a higher yield of positive findings. 

Surgery is recommended when neurological symptoms and signs are present and compression of the lower brainstem and upper spinal cord is confirmed by MRI.

If the brainstem is compressed anteriorly, like is the case in basilar invagination, removal of the compressing structure (the odontoid) has been proposed.  Until recently, odontoid resection has been achieve by a transoral approaches.  This approach is however associated which is with a high complication rate.  More recently, minimally invasive techniques have been used to treat basilar invagination.  The endoscopic endonasal route uses the nose as a natural corridor to approach this pathology.

Posterior cranio-cervical fusion is required in most patients to stabilize the articulation between the skull and upper cervical spine.