Basilar invagination (as associated bony anomaly)

What it is

Basilar invagination is a bony abnormality in which the top of the spine — specifically the odontoid process of the second vertebra — migrates upward into the foramen magnum and skull base. It can accompany Chiari malformation and contribute to crowding of the brainstem.

Understanding Basilar invagination (as associated bony anomaly)

In basilar invagination, the odontoid pushes upward into the foramen magnum (the opposite direction of the tonsils pushing out of the skull), narrowing the opening and crowding or compressing the lower brainstem. It is one of the bony abnormalities seen in "complex Chiari" and can occur alongside tonsillar descent. Its presence is important because it can contribute to symptoms (including brainstem-related ones) and may require treatment beyond standard decompression — sometimes including craniocervical fusion for stability and procedures to address the odontoid (ventral decompression). Basilar invagination is measured on imaging using craniometric lines. Recognizing it ensures the full craniocervical picture is addressed. Understanding basilar invagination and Chiari clarifies an important associated bony anomaly that can change the treatment approach.

Why it matters

Understanding basilar invagination and Chiari helps families see how upward migration of the odontoid can crowd the brainstem alongside Chiari, and why its presence can require more extensive surgery, such as fusion.