Basilar Invagination

What happens in basilar invagination

The skull rests on the first neck bone (the atlas, C1), which sits on the second (the axis, C2). The dens of C2 projects upward through the ring of C1, just in front of the brainstem. In basilar invagination, the dens and upper spine rise above their normal position into the foramen magnum, the large opening where the brainstem passes into the spinal canal. This narrows the space and can press on:

• the brainstem, which controls breathing, swallowing, heart rate and many cranial nerves
• the upper spinal cord
• the cerebellum
• the vertebral arteries that supply the back of the brain

It can also block the flow of cerebrospinal fluid, contributing to syringomyelia (a fluid cavity in the spinal cord) and hydrocephalus.

Causes

Congenital (primary) basilar invagination

Present from birth because of abnormal development of the bones at the skull base and upper spine. It often occurs with:

• Chiari malformation, where the lower cerebellum extends into the spinal canal
• Klippel-Feil syndrome (fused neck bones)
• atlas assimilation (C1 fused to the skull)
• achondroplasia and other skeletal dysplasias
• Down syndrome

Acquired (secondary) basilar invagination, or cranial settling

Develops later when bones soften or joints are destroyed:

• Rheumatoid arthritis: the most common acquired cause. Erosion of the joints between C1, C2 and the skull lets the skull settle downward onto the spine
• Osteogenesis imperfecta: brittle bones
• Paget's disease of bone
• osteomalacia, rickets and hyperparathyroidism
• tumors, infections or trauma at the skull base

Symptoms

Many people with mild basilar invagination have no symptoms, and it is found on imaging done for another reason. When symptoms occur, they often develop gradually in adulthood, sometimes after minor neck injury:

• neck pain and stiffness, with reduced neck movement
• headaches at the back of the head, often triggered by coughing, sneezing or straining
• numbness, tingling or weakness in the arms and legs
• clumsy hands and difficulty with fine tasks
• unsteady walking and falls
• difficulty swallowing, choking, hoarse voice or slurred speech
• dizziness, vertigo and nystagmus (jerky eye movements)
• sleep apnea or breathing problems from brainstem compression
• in congenital forms: a short neck, low hairline and tilted head

Diagnosis

• Examination: reflexes, strength, sensation, coordination, cranial nerve function and gait.
• X-rays: lateral views of the skull and neck with measurements against reference lines (such as Chamberlain's and McGregor's lines). Flexion and extension views assess instability.
• CT scan with 3D reconstruction: shows the bony anatomy in detail and is essential for surgical planning.
• MRI: shows compression of the brainstem and spinal cord, Chiari malformation, syrinx and hydrocephalus.
• Swallowing and sleep studies when relevant.

Treatment

Observation

People without symptoms and with mild findings may be monitored with periodic examination and imaging, and advised to avoid activities with a high risk of neck injury.

Surgery

Surgery is recommended when there is compression of the brainstem or spinal cord, progressive symptoms, or instability. Options depend on the type of deformity:

• Reduction and fusion: realigning the joints from behind using screws and rods to pull the spine down and stabilize the skull and upper neck (occipitocervical fusion). Many cases can now be reduced this way without a front approach.
• Anterior decompression: removing the dens through the mouth or nose (transoral or endoscopic endonasal odontoidectomy) when compression from the front persists, followed by fusion.
• Posterior fossa decompression: when Chiari malformation coexists.

Surgery carries risks, including injury to nerves or arteries, swallowing problems and infection, and fusion reduces neck rotation. It is performed at specialist neurosurgical centers.

Rheumatoid arthritis

Good control of rheumatoid arthritis with disease-modifying medicines has reduced cranial settling. People with long-standing rheumatoid arthritis should tell anesthetists before surgery, as neck manipulation during intubation needs care.

Outlook

Asymptomatic, stable basilar invagination may never cause problems. Once neurological symptoms develop, they tend to progress without treatment. Surgery usually halts progression and often improves symptoms, especially when done before severe damage occurs.

Frequently asked questions

What is basilar invagination?

Basilar invagination is a condition where the top of the spine, especially the dens of the second neck bone, pushes up into the opening at the base of the skull. It can press on the brainstem and upper spinal cord, causing neck pain, headaches, numbness, weakness and swallowing problems.

What causes basilar invagination?

It can be congenital, from abnormal development of the skull base and upper spine, often with Chiari malformation or Klippel-Feil syndrome. It can also be acquired from conditions that soften bone or destroy joints, especially rheumatoid arthritis, osteogenesis imperfecta and Paget's disease.

What are the symptoms of basilar invagination?

Symptoms include neck pain and stiffness, headaches at the back of the head, numbness or weakness in the arms and legs, unsteady walking, dizziness, difficulty swallowing and speech changes. Some people have no symptoms and it is found incidentally on a scan.

Is basilar invagination the same as Chiari malformation?

No, but they often occur together. Basilar invagination is a bony abnormality of the skull base and upper spine. Chiari malformation is a downward displacement of the cerebellum. Both crowd the foramen magnum, and treatment may need to address both.

Does basilar invagination always need surgery?

No. Mild cases without symptoms are often monitored. Surgery is recommended when the brainstem or spinal cord is compressed, symptoms are worsening, or the joints are unstable. It decompresses the nerves and stabilizes the skull and upper spine.