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Basilar Invagination (BI)

Basilar (meaning "situated at the base, often of the skull") invagination (meaning "the action or process of being turned inside out or folded back on itself to form a cavity or pouch") is a rare condition that affects the vertebrae--the 33 bones that make up the spine. These bones and the discs that provide cushioning between them make up a passage for the spinal cord and nerves.

Image from Boston Children’s Hospital (2)


What is Basilar Invagination?


In basilar invagination, the second cervical vertebra (also called C2 or the atlas) moves upward and back. The C2 vertebra has a nob on it called the odontoid process. It’s about the width of a pencil and helps hold the first cervical vertebra (also called C1 or the axis) in place. The odontoid process of the C2 vertebra can then press against the lower brain stem, which is the connection between the brain and the spinal cord. This pressure reduces the amount of space available for the brainstem and top of the spinal cord. If the brain stem doesn’t have enough room and is compressed, the messages it transmits to and from the brain can't make their way to where they need to be. They may be disrupted or distorted.


Causes of Basilar Invagination

Basilar invagination presents frequently with other conditions, including Chiari malformation. Cases that appear at birth can be due a heritable condition, such as Ehlers-Danlos syndrome, osteogenesis or Marfan's syndrome. It can occur in patients with bone diseases such as rheumatoid arthritis, tumors, or Paget's disease, or with irregularity of the cervical spine.(2) According to an article in the National Library of Medicine about basilar invagination, other conditions associated with basilar invagination are atlantoaxial dislocation atlas occipitalization, Klippel-Feil syndrome, atlanto-occipital hypoplasia, Klippel-Feil syndrome, clival hypoplasia, os odontoideum, and platybasia.(3) 


Basilar invagination can occur later in life due to injury or disease, most commonly a vehicle or bicycle accident, a fall, or an accident while doing some kind of recreational activity, like diving.1 Sometimes, there is no other cause for basilar invagination, which is called idiopathic basilar invagination.


Symptoms of Basilar Invagination (1,2,3,4)

Symptoms of basilar invagination range from no or minor symptoms to paralysis and death. They can depend on what part of the body is affected by the pressure from the C2 vertebra--spinal cord, brain stem, nerves, or vasculature. Symptoms sometimes only appear when the neck is bent or otherwise moved to cause the compression.


Symptoms can include:

  • Headaches

  • Dizziness/light headed

  • Neck weakness and/or stiffness

  • Pain in the back of the head

  • Patient may have a short neck or an asymmetrical face or skull

  • Difficulty swallowing or saying words due to loss of muscle control

  • Confusion or conscious awareness that doesn't seem normal

  • Back pain or shock when bending the neck forward

  • Loss of body sense--the ability to know where the body is (proprioception)

  • Numbness, tingling or loss of sensation in the fingers, arms, toes, or legs

  • Paralysis in the arms or legs

  • Weakness or stiff, awkward movements of the arms and legs

  • Muscle spasms

  • Pain, numbness, or tingling when coughing, laughing, lifting weights, or changing the position of the head

  • Gait instability

  • In a body of water, like a pool, the patient will notice that the water below the belly button is not as cold as the water above it


Complications of basilar invagination can include hydrocephalus or syringomyelia due to the spinal fluid being blocked from flowing freely around the brain and spinal cord.


Diagnosis and Treatment of Basilar Invagination (1)

Basilar invagination is usually diagnosed by a specialist in spine care, neurology, or neurosurgery. They will take a medical history of the patient and ask about what symptoms they are experiencing. Then, the medical provider will perform a patient exam to check the neck, reactions to the movement of the neck as well as the reaction of muscles and nerves.


Imaging is most likely the next step for treatment. A medical provider may have one or more of the following tests done to see what’s going on in the brain, neck, and spinal column: an X-ray, a magnetic resonance imaging (MRI) and/or computed tomography (CT) scan of the head and brain, neck and/or spine.


Basilar invagination can get worse as a patient get older. They will need to check in with a spine specialist over time to make sure their condition remains stable and discuss any new symptoms or treatment options.


Treatments for basilar invagination involve conservative, non-surgical approaches as well as invasive surgical approaches which may be considered if symptoms are unbearable or especially detrimental to the patient's quality of life.


Conservative treatments--usually for those with no evidence of pressure on the spinal cord--include:


  • Medications, such as aspirin and steroids to relieve inflammation

  • Neck traction--the neck is pulled along its length, relieving pressure on the spinal cord

  • Physical therapy

    using a neck collar


Surgical treatment is usually reserved for when there is pressure on the spinal cord. It is recommended to relieve the pressure on the patient's brain stem, to stabilize the cervical spine, and restore the flow of the spinal fluid around the brain and spinal cord. Both an orthopedic surgeon and a neurosurgeon are often present to perform this surgery.(2)


The surgery for basilar invagination involves realigning the spine and decompressing any pressure on the brain stem. The cervical spine is then stabilized with a spinal fusion. Vertebrae will be joined together so they are more stable. This can be done using hardware, such a screws and rods as well as bone grafts to fuse with the native bone to create stability when healing.(4) According to Columbia University's Neurosurgery Department, an odontiodectiomey may also be performed to remove the small bit of the C2 vertebra that is sticking out and causing problems. It’s important for patients and medical providers to work together to find a treatment for basilar invagination that takes into consideration the patient’s symptoms, severity, goals, and lifestyle.







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