Basilar invagination is a relatively rare condition in which the upper portion of the second cervical vertebra migrates upward and posteriorly into the intracranial space. It can be associated with a number of other conditions such as rheumatoid arthritis, Chiari malformation, syringomyelia, C1-2 instability, or congenital abnormalities.
The terms basilar impression and basilar invagination are often used interchangeably in the literature: historically, basilar invagination (AKA cranial settling) denoted upward indentation of skull base usually due to acquired softening of bone, often associated with atlanto-occipital fusion, while basilar impression implied normal bone. Making a distinction seems pointless (the abbreviation (BI) will be used for either). Common feature: upward displacement of the upper cervical spine (including odontoid process, AKA cranial migration of the odontoid) through the foramen magnum into the p-fossa.
Platybasia: flattening of the skull base. Originally assessed on plain x-rays (which are subject to error due to skull rotation or difficulty identifying landmarks), now more commonly evaluated on CT or MRI. May or may not be associated with BI, and may occur in association with craniofacial abnormalities, Chiari malformation, Paget’s disease…
Quantitated by measuring the basal angle, which on plain x-rays, measured the angle between lines drawn from the nasion to center of sella and then to the anterior foramen magnum, 1) but on MRI was felt to be better represented by the angle between a line drawn along the floor of the anterior fossa to the dorsum sellae and a second line drawn along the posterior clivus. 2) Normal mean basal angle: 130°. Platybasia: >145° (abnormally obtuse basal angle).
Patients generally become symptomatic when the displaced vertebral segment causes sufficient pressure on the upper spinal cord or lower portion of the brainstem. The most common symptoms include headache, dizziness, swallowing problems, numbness/tingling in the extremities, and paralysis. Symptoms can become worse with flexion of the head, which even further drapes the spinal cord over the upper portion of C2.
1. congenital conditions (BI is the most common congenital anomaly of the craniocervical junction, it is often accompanied by other anomalies 3).
2. Acquired conditions
3. conditions with BI associated with softening of bone include 5)
b) Osteogenesis imperfecta: patients have blue discolored sclera and early hearing loss and due to a genetic defect that causes defective Type 1 collagen. Bones are weak (”brittle-bone dis- ease”). Autosomal dominant inheritance. There are 4 common types of OI and some uncommon ones
Basilar impression occurs when parts of the cervical (neck) vertebrae enter the hole in the skull base (foramen magnum).
Basilar invagination is the same problem, except that it’s caused by softening in the bones at the skull base.
Cranial settling occurs when the bone at the top of the neck on which the skull rotates (dens) extends into the foramen magnum.