What is Basilar Invagination

Basilar invagination is an abnormality where the skull floor is indented by the upper cervical spine. 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 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.

There are two types of basilar invagination

• Primary
• Secondary or acquired [also called as basilar impression]

Primary basilar invagination is a congenital abnormality often associated with other vertebral defects like Klippel-Feil syndrome, odontoid abnormalities, atlanto-occipital fusion, and atlas hypoplasia. The primary basilar impression is found in 1% of the general population.

Secondary basilar invagination is less common than primary. The common causes are metabolic bone diseases like

• Paget’s disease
• Renal osteodystrophy
• Rickets & osteomalacia
• Bone Dysplasias
• Mesenchymal syndromes
  • Osteogenesis imperfecta
  • Achondroplasia & hypochondroplasia
• Neurofibromatosis
• Rheumatologic disorders
• Ankylosing spondylitis

Presentation of Basilar Invagination

The presentation in this condition varies and patients may be totally asymptomatic patients with severe basilar invagination may be totally asymptomatic. Symptoms usually appear during the second and third decades of life.

Neurologic signs and symptoms are often present.

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, the neurologic involvement is usually cerebellar (motor in coordination with ataxia, dizziness, and nystagmus).

In both types of basilar invaginations, the patients complain of neck pain and headache in the distribution of the greater occipital nerve and cranial nerve involvement.  Ataxia is a very common finding in children. Hydrocephalus may develop due to obstruction of the cerebrospinal fluid flow by obstruction of the foramen magnum from the odontoid.

A number of children present following acute onset of symptoms precipitated by minor trauma.

Imaging in Basilar Invagination

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.

Treatment of Basilar Invagination

Conservative treatment of symptomatic patients is not effective

The indications for surgery are based on the clinical symptoms and not on the degree of  basilar impression

– If symptoms are caused by anterior impingement from the odontoid, stabilization in extension by an occipital C1-2 fusion is done.

– If symptoms and impingement persist, anterior excision of the odontoid can be done after posterior stabilization.