Atlantooccipital anomaly
Atlanto-axial subluxation is present in up to 50% of congenital atlantooccipital anomalies. Flexion-extension MRI is often necessary to fully evaluate the pathology.
Anterior compression of the brainstem or cervical cord occurs from posterior displacement of the odontoid process. This results in a wide range of manifestations, depending on the site and severity of compression. Pyramidal tract signs and symptoms are most common, although signs of lower cranial nerve involvement can be seen. Compression from the posterior lip of the foramen magnum or dural constricting band can disturb the posterior columns, with a loss of proprioception, vibration and tactile senses. Nystagmus may occur as a result of posterior cerebellar compression. Vascular disturbances from vertebral artery involvement can result in brainstem ischemia, manifested by syncope, seizures, vertigo and unsteady gait. Cerebellar tonsil herniation can occur. The altered mechanics of the cervical spine may result in a dull, aching pain in the posterior neck, with intermittent stiffness and torticollis. Irritation of the greater occipital nerve may cause tenderness in the posterior scalp.