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Craniocervical junction abnormalities: Congenital or acquired bony abnormalities of the occipital bone, foramen magnum, or first two cervical vertebrae that decrease the potential space for the lower brain stem and cervical cord and can result in cerebellar, lower cranial nerve, and spinal cord symptoms.
Because the spinal cord is flexible and therefore susceptible to intermittent compression, several types of lesions at this level can cause symptoms that vary from patient to patient and that can be intermittent. Fusion of the atlas and occipital bone produces symptoms of cervical myelopathy when the anterior-posterior diameter of the foramen magnum behind the odontoid process decreases to < 19 mm. Platybasia is an asymptomatic flattening of the skull base; ie, the angle formed by the intersection of the plane of the clivus and the plane of the anterior fossa is > 135° on lateral skull x-ray. Basilar invagination (protrusion of the odontoid process into the foramen magnum) leads to a short neck and combinations of cerebellar, brain stem, lower cranial nerve, and spinal cord signs. The Klippel-Feil malformation (fusion of cervical vertebrae) is usually asymptomatic except for a neck deformity with limited range of motion. Atlantoaxial subluxation or dislocation (displacement of the atlas anteriorly in relation to the axis) causes acute or chronic spinal cord compression.
A craniocervical abnormality should be considered when fixed or progressive neurologic deficits refer to the lower brain stem, high cervical spinal cord, or cerebellum.
Plain x-rays (lateral view of the skull showing the cervical spine, anteroposterior view, and oblique views of the cervical spine) are used to identify factors that influence treatment. These factors include reducibility of the abnormality (ability to achieve a normal osseous alignment, thus relieving compression on neural structures), bony erosion, the mechanics of compression, and the presence of abnormal ossification centers and epiphysial growth plates with anomalous development. CT after intrathecal administration of contrast dye provides anatomic detail of the neural structure abnormality and associated bony distortion. Sagittal MRI best identifies associated neural lesions (hindbrain herniation, syringomyelia, and vascular abnormalities). MRI can correlate bone and soft tissue pathology and define the level and extent of a malformation and associated neural defect (eg, Chiari malformation, syringomyelia). Vertebral angiography or MRI is used selectively to identify fixed or dynamic vascular compromise.
Congenital abnormalities include os odontoideum, atlas assimilation or hypoplasia, and the Chiari malformations (cerebellar tonsils or vermis descend into the cervical spinal canal). Achondroplasia can cause narrowing of the foramen magnum and neural compression. Down syndrome, Morquio's syndrome (mucopolysaccharidosis IV), and osteogenesis imperfecta can cause atlantoaxial instability and spinal cord compression.
Acquired abnormalities may be due to trauma or disease. When the occipitoatlantoaxial complex is injured, mortality at the accident scene is high. Such injuries may be osseous (fractures), ligamentous (luxations), or complex (subluxation of C-2, transaxial cervicomedullary junction injury, and osteoligamentous disruptions). Half are caused by vehicle or bicycle accidents, 25% by falls, and 10% by recreational activities, particularly diving accidents. Minor neck injury can precipitate variably progressive symptoms and signs in patients with an underlying craniocervical junction abnormality. RA and metastatic disease of the cervical spine can cause atlantoaxial dislocation. A slowly growing tumor (eg, meningioma, chordoma) at the craniocervical junction produces symptoms by impinging on the brain stem and the spinal cord. RA and Paget's disease can result in basilar invagination with spinal cord or brain stem compression. RA is the most common cause of craniocervical instability, which can also result from trauma or erosion by tumor or Paget's disease.
Symptoms and Signs
Presentation varies because bony and soft tissue abnormalities in various combinations can compress the cervical spinal cord, brain stem, cranial nerves, cervical nerve roots, or their vascular supply. An abnormal head posture is common, and in some patients, the neck is short or webbed. The most common manifestations are neck pain and cord impingement (myelopathy). Compression of motor tracts causes weakness, spasticity, and hyperreflexia in the arms and/or legs. Lower motor neuron involvement causes muscular atrophy and weakness in the arms and hands. Sensory abnormalities (including changes in joint position sense and vibration sense) often reflect posterior column dysfunction. Patients may describe tingling down the back, and often into the legs, on neck flexion (Lhermitte's sign). Spinothalamic tract involvement (eg, with loss of pain and temperature sense) is unusual but can be reflected in stocking-and-glove paresthesias or numbness. Brain stem and cranial nerve deficits include sleep apnea, internuclear ophthalmoplegia (ipsilateral eye adduction weakness and horizontal nystagmus in the contralateral abducting eye on lateral gaze), downbeat nystagmus (fast component downward), hoarseness, and dysarthria and dysphagia (due to weakness and incoordination of the tongue, soft palate, pharynx, and larynx). Neck pain spreading to the arms and suboccipital headache radiating to the skull vertex are common. Symptoms worsen with head movement and can be precipitated by coughing or bending forward. Pain is attributed to compression of the C-2 root and greater occipital nerve and to local musculoskeletal dysfunction.
Vascular symptoms include syncope, drop attacks, vertigo, intermittent periods of confusion and altered consciousness, episodic weakness, and transient visual disturbance. Vertebrobasilar ischemia may be provoked by moving or by changing head position.
Certain craniocervical junction abnormalities (eg, acute traumatic atlantoaxial luxations and acute ligamentous injuries) can be realigned and reduced, relieving compression of neural structures, with head positioning alone. Most patients require skeletal traction with a crown halo ring in gradual increments up to 8 to 9 lb to achieve reduction. Traction usually achieves results in 5 to 6 days. If reduction is achieved, immobilization in a halo vest is maintained for 8 to 12 wk; then x-rays must be taken to confirm stability. If reduction does not relieve neural compression, surgical decompression, using a ventral or a dorsal approach, is necessary. If instability is present after decompression, posterior fixation is required. With other abnormalities (such as RA), external immobilization alone is unlikely to achieve permanent reduction, and posterior fixation (stabilization) or anterior decompression and stabilization is required.
There are various techniques for fusion in the craniocervical region. In general, all unstable levels must be fused. Instrumentation provides immediate stability until bony fusion develops and provides long-term stability.
Radiation therapy and a hard cervical collar often help in metastatic disease. Calcitonin, mithramycin, and bisphosphonates may help patients with Paget's disease.