In mucopolysaccharidosis IV, or the Morquio syndrome (Chap. 36), a typical feature is the absence or severe hypoplasia of the odontoid process. This abnormality, combined with laxity or redundancy of the surrounding ligaments, results in atlantoaxial subluxation and compression of the spinal cord. Affected children refuse to walk or develop spastic weakness of the limbs. Early in life they excrete an excess of keratan sulfate, but this may no longer be detectable in adult life. In certain of the mucopolysaccharidoses, we have also seen a true pachymeningopathy with great thickening of the dura in the basal cisterns and high cervical region with spinal cord compression. Surgical decompression and spinal immobilization have been curative.
Achondroplasia This dominantly inherited form of dwarfism is caused by a mutation in one of the fibroblastic growth factors, which causes a failure of conversion of fetal cartilage to bone at the growth plate. It occasionally results in great thickening of the vertebral bodies, neural arches, laminae, and pedicles because of increased periosteal bone formation. The spinal canal is narrowed in the thoracolumbar region, often with kyphosis, leading sometimes to a progressive spinal cord or cauda equina syndrome. Another complication, which results from a small foramen magnum, is hydrocephalus (or markedly widened subarachnoid spaces). In young children, a syndrome of central apnea and spasticity of the legs is characteristic. These complications may require ventricular shunting. Narrowing of the lumbar canal tends to present later in life.
Platybasia and basilar invagination Platybasia refers to a flattening of the base of the skull (the angle formed by intersection of the plane of the clivus and the plane of the anterior fossa is greater than 135 degrees). Basilar impression or invagination has a somewhat different meaning, namely, an upward bulging of the occipital condyles; if the condyles, which bear the thrust of the spine, are displaced above the plane of the foramen magnum, basilar invagination is present. Each of these abnormalities may be congenital or acquired (as in Paget disease); frequently they are combined. They give rise to a characteristic shortness of the neck and a combination of cerebellar and spinal signs. A normal-pressure hydrocephalus may also develop.
In the Klippel-Feil syndrome there is fusion of the upper cervical vertebrae or of the atlas to the occiput. The anomaly is easily identified by substantial foreshortening of the neck. Affected individuals are susceptible to compression of the cervical cord after minor trauma. Many such patients demonstrate mirror movements of their hands, comparable to those described earlier in cervical spondylosis.
Tethered cord This developmental anomaly is discussed more fully and illustrated in Chap. 37. A progressive cauda equina syndrome with prominent urinary difficulties and varying degrees of spasticity are the usual presentations.