Last Updated on October 24, 2023
Basilar invagination is a craniocervical junction abnormality where the tip of the odontoid process projects above the foramen magnum. The tip of the dens is more cephalad [towards the head] and sometimes protrudes into the opening of the foramen magnum. This may cause brainstem encroachment and then risk neurologic damage from direct injury, vascular compromise, or cerebrospinal fluid flow alteration.
Basilar invagination can be congenital or due to secondary causes. The basilar invagination due to secondary causes is also termed as basilar impression. Similarly, the entity caused due to rheumatoid arthritis is called cranial settling.
Basilar invagination is the most common form of craniovertebral junction malformation.
Relevant Anatomy
The head and neck are an area that harbors a junction between the nervous system tissue of the brain and spinal cord between bones of the skull and upper cervical spine.
The brain stem is a part of the brain that emerges from its base through the foramen magnum and is a connection between the brain and the spinal cord. Below the brainstem spinal cord starts to continue to the lumbar region.
The first and second cervical vertebrae, C1 and C2 form the upper cervical spine and are important contributors to the anatomy of the craniovertebral junction. C1 and C2 articulation and relation are unique to spine anatomy.
A unique feature is dens which is a projection from C2 verbra and fits into a groove in C1. It’s called the dens, or odontoid process. This arrangement allows greater rotational motion than otherwise would have been possible.
In basilar invagination, C2 and the dens move back and up, toward the foramen magnum. This may lead to compression of the braineste and top of the spinal cord.
Types of Basilar Invagination
- Primary
- A congenital abnormality
- often associated with other vertebral defects like
- Klippel-Feil syndrome
- Odontoid abnormalities
- Atlanto-occipital fusion
- Atlas hypoplasia.
- The primary basilar impression is found in 1% of the general population.
- Secondary or acquired [Also called basilar impression]
- less common than primary
- Causes
- Paget’s disease
- Renal osteodystrophy
- Rickets
- osteomalacia
- Bone Dysplasias
- Osteogenesis imperfecta
- Achondroplasia & hypochondroplasia
- Neurofibromatosis
- Rheumatologic disorders
- Ankylosing spondylitis
Associations
- Chiari malformation I& II malformation
- Syringomyelia
- Hydrocephalus
Classification
The classification is based on whether the Chiari malformation is present or not
- Type I or A – Chiari formation is absent
- Type II or B – Chiari formation is present
Pathophysiology
The most recent theory is that type I basilar invagination is due to atlantoaxial facet joint instability that predisposes odontoid peg migration through the foramen magnum. This results in brainstem compression.
A developmental anomaly is responsible for type 2. This anomaly leads to the crowding of the posterior fossa and compression at the foramen magnum
Presentation of Basilar Invagination
The presentation in this condition varies and patients may be totally asymptomatic in spite of severe basilar invagination. Symptoms usually appear during the second and third decades of life.
- Neck pain with or without torticollis (most common symptom)
- Spastic muscle weakness
- Numbness
- Tingling sensations (Pins and needles)
- Gait instability
- Difficulty in speaking and/or swallowing
Type 1 basilar invagination has a comparatively acute presentation when compared to type 2.
Type 1 has neurological symptoms due to direct compression of the brainstem by the odontoid process whereas type 2 is due to crowding of posterior fossa.
The neurologic examination would reveal the deficit.
- In cases of isolated basilar impression, the neurologic involvement is basically a pyramidal syndrome associated with proprioceptive sensory disturbances [motor weakness, limb paresthesias.
- In cases of basilar invagination associated with Arnold-Chiari malformations, there is usually cerebellar (motor in coordination with ataxia, dizziness, and nystagmus).
Ataxia is a very common finding in children.
Minor trauma may precipitate the symptoms in previously asymptomatic patients.
Imaging in Basilar Invagination
The basilar impression is difficult to assess radiographically. CT scan with sagittal plane reconstructions can show the osseous relationships at the occipitocervical junction more clearly. Magnetic resonance imaging shows the neural anatomy. Occasionally, vertebral angiography is needed to evaluate the blood flow.
Five important cervical lines help to evaluate basilar invagination. These can be assessed by X-rays, CT, and MRI as well.
Digastric and bimastoid lines are seen in the anteroposterior image whereas Mcrae, Chamberlain, and Mcgregor lines are seen on lateral views.
- Digastric line
- The line is drawn between the right and left digastric grooves, located medial to the mastoid process tip
- Tip of the odontoid process projects normally not more than 10 mm above this line
- Bimastoid line
- A line is drawn between tips of mastoid process processes
- The tip of the odontoid process projects up to 10 mm above this line
- McRae line
- Line connecting anterior and posterior margin of foramen magnum
- The tip of the odontoid process normally projects below this line
- Chamberlain line
- Line connecting the posterior border of the hard palate and posterior margin of the foramen magnum
- The tip of the odontoid process should not be more than 3 mm above this line
- McGregor line
- Line connecting the posterior edge of the hard palate to the most caudal point of the occipital curve
- The tip of the odontoid process does not project > 5 mm above this line
Treatment of Basilar Invagination
Surgical treatment is usually necessary though some asymptomatic cases can be managed by nonoperative treatment. The noon operative treatment includes
physical therapy, nonsteroidal anti-inflammatory medications (NSAIDs), and/or a neck brace called a cervical collar.
Conservative treatment of symptomatic patients is not effective. Symptomatic basilar invagination is treated by surgery.
The goals of surgery are
- Decompression of foramen magnum decompression,
- Stabilization of craniovertebral junction after alignment is restored.
- Restoring normal cerebrospinal fluid flow
The various procedures are
- Cervical traction and posterior stabilization
- Anterior decompression via a transoral or endonasal fixation
- Transoral odontoid removal
- atlanto-axial facet distraction with fusion [Type 1]
- Suboccipital and foramen magnum decompression[type 2]
Recently endoscopic has been used and looks promising.
Surgery can lead to improvement of symptoms but complete recovery may not be possible.
References
- Botelho RV, Botelho PB, Hernandez B, Sales MB, Rotta JM. Association between Brachycephaly, Chiari Malformation, and Basilar Invagination. J Neurol Surg A Cent Eur Neurosurg. 2023 Jul;84(4):329-333. [Link]
- Jian Q, Zhang B, Jian F, Bo X, Chen Z. Basilar Invagination: A Tilt of the Foramen Magnum. World Neurosurg. 2022 Aug;164:e629-e635. [Link]
- Lin JY, Bao MG, Lin SY, Liu JH, Liu Q, Li RY, Huang ZC, Zhu QA, Zhang ZM, Ji W. Cervical Alignment of Patients with Basilar Invagination: A Radiological Study. Orthop Surg. 2022 Mar;14(3):566-576. [Link]
- Joaquim AF, Ghizoni E, Giacomini LA, Tedeschi H, Patel AA. Basilar invagination: Surgical results. J Craniovertebr Junction Spine. 2014 Apr;5(2):78-84. [Link]
