Platybasia with Basilar invagination

MRI Mid sagittal section of CV junction shows:

• Platybasia (abnormal flattening of the skull base) evident by wide NTB (Nasion – Tuberculum – Basion) angle,
• Forward folding of the Clivus – Axis Angle (Angle of Wackenheim) causing lordotic tilt at CV junction, Elevated plane of the foramen magnum with retroflexed dens. 
• Basilar invagination evident by violation of Chamberlain’s (Hard palate - Ophisthion) line by the Dens. 
• Among two types of basilar invaginnation (Ventral and Paramesial), this is a Ventral variety of Basilar Invagination characterized by shortening of basiocciput due to short and horizontally placed clivus leading to forward facing plane of foramen magnum. 
• Abnormal sharp Clivus – Odontoid Angle causing exteme indentation over cervico medullary junction. However signal abnormality not very obvious in compressed medulla or cervico medullary junction.
• Normal Atlanto- Odontoid Distance. No obvious Atlanto Dental subluxation or dislocation. 

Imaging diagnosis : Platybasia with Basilar invagination.

Basal angle or NTB (Nasion – Tuberculum – Basion) angle. 

Chamberlain's line (Hard plate to Ophisthion)

Clivus – Axis Angle or ‘ Angle of Wackenheim’

Similar Post : Basilar-impression

Basilar Impression (BI)

MRI Cervical Spine sagittal T2 image shows:
Basilar Impression (BI) with Atlantoaxial Subluxation (AAS).
An associated congenital C5-6 fusion, exaggerated cervical lordosis, upwards and posterioly pointed Odontoid tip contributing to marked foramen magnum stenosis.
Significant compression over Medulla and CV junction cord.

Terms Basilar invagination and Basilar impression are often used interchangingly for BI, because in both cases there is abnormal upward migration of the upper cervical spine, the tip of the odontoid process projects above the foramen magnum.  But precisely, basilar impression is defined as upward displacement of C2 into the normal foramen magnum with normal bone, while basilar invagination is a similar displacement due to softening of bones at the base of the skull. Thus different terms are used according to whether bone is normal or not.

Ultimately there is stenosis of the foramen magnum and compression of the medulla / cord resulting in neurological symptoms, obstructive hydrocephalus, syringomyelia or even death.
Neurological symptoms vary depending upon level of compression, degree of compression and structures compressed whether the spinal cord, brain stem, nerves or blood vessels.

Symptoms become more apparent when the neck is bent. They include Posterior head pain, Neck weakness,  Episodes of confusion, Difficulty swallowing or talking due to loss of muscle control, Dizziness, Cranial nerve disturbance, Loss of sensation of joint positioning, Lhermitte's sign (tingling when the neck bends backward), Weakness of the arms and legs, Orthostatic hypotension.

BI may be present at birth. If develops after birth, is usually the result of injury or bone diseases like osteomalacia, rheumatoid arthritis, Paget's disease, Ehlers-Danlos syndrome, Marfan syndrome, and Osteogenesis imperfecta.

Platybasia, is a related term and may be an associated finding. Basically is a flattening of the base of the skull due to increase in the basal angle of the skull. Depicted on a lateral skull film or saggital reformatted CT sections.

Atlantoaxial Subluxation (AAS) may be an associated finding. Abnormal increased distance between C1 anterior arch and anterior surface of Odontoid. Distance greater than 3mm in adult and 5mm in children is abnormal. Results from an associated transverse ligament laxity or destruction. Depicted on lateral x-rays taken in extension and flexion.

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