Diastematomyelia is another unusual abnormality of the spinal cord often associated with spina bifida. Here a bony spicule or fibrous band protrudes into the spinal canal from the body of one of the thoracic or upper lumbar vertebrae and divides the spinal cord into halves for a variable vertical extent. In extreme examples, the division of the cord may be complete, each half with its own dural sac and complete set of nerve roots. This longitudinal fissuring and doubling of the cord are spoken of as diplomyelia. With body growth, the restriction created by the bone spicule leads to a traction myelopathy, presenting with pain and progressive sensory, motor, and bladder symptoms, sometimes as late as adult life. Removal of the fibrous-bony spicule and untethering of the spinal cord have been beneficial in some cases.
Syringomyelia (see also Chap. 42) is a developmental cavity within the cervical cord, extending a variable distance caudally or rostrally, and usually associated with an Arnold-Chiari malformation that is described below. There are a variety of other neurodevelopmental spinal abnormalities in the high cervical region, such as fusion of atlas and occiput or of cervical vertebrae (Klippel-Feil syndrome), congenital dislocation of the odontoid process and atlas, platybasia, and basilar impression. These abnormalities are discussed in Chap. 42, with other diseases of the spinal cord.
Encompassed by this term are a constellation of related congenital anomalies at the base of the brain, the most consistent of which are (1) extension of a tongue of cerebellar tissue posterior to the medulla and cord that extends into the cervical spinal canal; (2) caudal displacement of the medulla and the inferior part of the fourth ventricle into the cervical canal; and (3) a frequent but not invariable association with syringomyelia or a spinal developmental abnormality. These and associated anomalies were first clearly described by Chiari (1891). Several translations of his original material have been made, but they have been criticized as inaccurate. Arnold’s name is attached to the syndrome, but his contribution to our understanding of these malformations was relatively insignificant. Use of the double eponym Arnold-Chiari malformation is so entrenched that a dispute over its propriety serves little purpose. Chiari recognized four types of abnormalities. In recent years, the term has come to be restricted to
Chiari’s types I and II—that is, to cerebellomedullary descent without and with a meningomyelocele, respectively. Type III Chiari malformation is no more than a high cervical or occipitocervical meningomyelocele with cerebellar herniation, and type IV consists only of cerebellar hypoplasia and is probably not related to the other types.
The incidence among adults, acquired from autopsy series and more recently, from incidentally discovered descent of the cerebellar tonsils on imaging studies, is about 0.6 percent of the population. It should be emphasized that a proportion of normal individuals have a small tongue of the posterior cerebellum protruding by a few millimeters below the lower lip of the foramen magnum; this is usually of no significance and does not justify inclusion as a Chiari malformation. A historical account of the clinical, pathologic, and imaging aspects of this malformation and the evolution of ideas concerning it has been given by Bejjani.