Basilar Invagination

Basilar invagination is one presentation of a disordered craniocervical junction, and the neighbouring entities are managed differently: craniocervical junction instability for the broader chronic picture, atlantoaxial instability for the C1-C2 component that Goel showed is often the real lesion, atlanto-occipital dislocation for the acute traumatic disruption, and occipitocervical fusion for the operation most of these converge on.

Definition. Basilar invagination is the cephalad migration of the odontoid process through the foramen magnum, compressing the cervicomedullary junction. Strictly, the term belongs to the primary, developmental anomaly; basilar impression is the secondary, acquired form, in which softened bone at the skull base (rheumatoid arthritis, Paget disease, osteogenesis imperfecta) allows the skull base to settle onto the cervical spine. Both compress the brainstem and upper cervical cord, and a missed diagnosis can end in catastrophic brainstem compression.

What sits alongside it. Atlantoaxial instability may coexist, particularly in RA and in the congenital forms. Platybasia, a flattening of the skull base, often coexists as well but is a distinct measurement, described with the angular lines under Investigations.

Why it is examined. The condition tests three things at once: craniovertebral junction anatomy, the radiographic measurement lines, and the surgical principle that stabilisation, not decompression alone, is the treatment.

Who. The age at presentation is bimodal. The congenital form, often associated with the Klippel-Feil triad, Chiari I malformation and os odontoideum, presents in the second to third decade with no strong sex predilection. Rheumatoid arthritis is the most common acquired cause, affecting up to 10 percent of patients with severe RA; it presents in the fifth to seventh decade and follows RA epidemiology, so it is female predominant.

The junction. The craniovertebral junction (CVJ) comprises the occiput, the atlas (C1) and the axis (C2). The odontoid process projects upward from the body of C2 into the ring of C1 and articulates with the anterior arch of C1. Stability depends on the transverse ligament, the primary restraint against anterior translation of C1; the alar ligaments, which limit rotation and lateral flexion; and the tectorial membrane, the continuation of the posterior longitudinal ligament.

What the migrated odontoid does. The foramen magnum is the narrowest point in the craniospinal axis. Once the odontoid tip protrudes above it, the cervicomedullary junction, the lower cranial nerves (IX-XII) and the vertebral arteries are compressed, and even a few millimetres of migration can produce devastating neurological compromise.

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Congenital mechanisms. Failure of segmentation between C1 and the occiput (occipitalisation of C1) forces the odontoid cephalad. In os odontoideum the dens is a separated fragment and the body of C2 migrates upward. Klippel-Feil fusion of cervical vertebrae concentrates stress at the CVJ. A Chiari I malformation, herniation of the cerebellar tonsils below the foramen magnum, often coexists and worsens the brainstem compression.

Rheumatoid mechanism. Synovitis attacks the C1-2 articulations, the atlantodental and facet joints. Destruction of the transverse ligament allows anterior atlantoaxial subluxation, pannus erodes the dens and weakens the fulcrum of CVJ stability, and progressive erosion allows C2 to migrate cephalad through the foramen magnum. The pannus is also a mass in its own right, adding ventral cord compression beyond the bony impingement alone.

History. Neck pain is suboccipital and radiates to the vertex, and neck pain with restricted motion is a common early complaint. Myelopathy is the most common presentation, and neurological progression shows as numbness, weakness, a change in gait and clumsiness. Ask specifically for the lower cranial nerve symptoms (dysphagia, dysarthria, a hoarse voice, tongue weakness) and the vertebrobasilar ones (vertigo, visual disturbance, syncope on rotation).

Examination. On inspection look for a short neck, a low hairline and restricted cervical rotation, the Klippel-Feil triad. The motor picture is a spastic quadriparesis with upper motor neuron signs in all four limbs. Examine the cranial nerves for palatal weakness, tongue wasting and an impaired gag reflex, and look for the cerebellar signs of brainstem and cerebellar compression: ataxia, dysmetria and nystagmus.

Differential diagnosis. MRI is the key discriminator between basilar invagination and its mimics:

• Atlantoaxial instability without invagination - the odontoid is in a normal position but C1 translates anteriorly
• Chiari I malformation - cerebellar tonsillar herniation without odontoid migration (the two may coexist)
• Cervical spondylotic myelopathy - cord compression lower in the cervical spine, with no cranial nerve involvement
• Foramen magnum tumours (meningioma, chordoma) - similar compression but a different imaging appearance

Radiographs. The first-line study is the lateral cervical radiograph in neutral, flexion and extension. Draw the Chamberlain, McGregor and McRae lines, and in RA add the Ranawat index. The dynamic views answer the question that shapes the operation: does the odontoid reduce on extension?

Why there are three lines. Chamberlain, McGregor and McRae were all defined on the lateral radiograph, and that is why there are three. Chamberlain's is the most specific, but the opisthion is frequently not visible on a plain film, so McGregor substituted the lowest point of the occipital curve, which always is; the price is a less specific line with a lower threshold. On a midsagittal CT or MRI reformat the opisthion is unambiguous, so Chamberlain and McRae become the usable lines and McGregor loses the problem it was invented to solve. McRae's line is the most clinically relevant because it is the actual boundary of the foramen magnum: an odontoid tip below it is normal, and one above it is invagination.

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MRI. Mandatory for every suspected basilar invagination, with flexion-extension if tolerated. It shows the cervicomedullary junction compression, cord signal change (T2 hyperintensity is myelomalacia), the pannus mass in RA and any associated Chiari malformation, and a CSF flow study assesses obstruction at the foramen magnum. Measure the space available for the cord at the CVJ: under 13 mm is critical.

CT. Thin-slice CT of the craniovertebral junction with sagittal and coronal reconstruction is the pre-operative planning study for screw trajectory: the bony anatomy, the size of the C1 lateral masses, the thickness of the occipital bone and the course of the vertebral arteries. A 3D reconstruction is essential for understanding complex congenital anatomy, and CT assesses reducibility, whether the odontoid can be reduced on traction.

CT angiography. Obtain it when a vertebral artery anomaly is suspected, to map the artery before surgery. An anomalous vertebral artery at C1-2 occurs in up to 20 percent and affects screw placement; the patterns are a persistent first intersegmental artery, an arcuate foramen (posterior ponticulate) and an aberrant course. The purpose is to avoid vertebral artery injury during C1 lateral mass or C2 pedicle screw placement.

What predicts recovery. The duration and severity of myelopathy before surgery is the strongest predictor of neurological recovery, and early intervention, before cord signal change appears on MRI, gives the best outcomes. The best prognosis belongs to the young patient diagnosed early before a fixed neurological deficit, with a reducible invagination and good bone quality for fusion.

Poor prognosis. Severe pre-operative myelopathy (especially Nurick grade 4-5), a fixed irreducible deformity, RA with poor disease control, and the elderly patient with osteoporotic bone.