Syringomyelia (Syrinx) (See Also Chap. 37)
Syringomyelia (from the Greek syrinx, “pipe” or “tube”) is defined as a chronic progressive degenerative or developmental disorder of the spinal cord, characterized clinically by painless weakness and wasting of the hands and arms (brachial amyotrophy) and segmental sensory loss of dissociated type (loss of thermal and painful sensation with sparing of tactile, joint position, and vibratory sense, as described later). The cause is a cavitation of the central parts of the spinal cord, usually in the cervical region, but extending upward in some cases into the medulla and pons (syringobulbia) or downward into the thoracic and even into the lumbar segments. Frequently, there are associated developmental abnormalities of the vertebral column (thoracic scoliosis, fusion of vertebrae, or Klippel-Feil anomaly), of the base of the skull (platybasia and basilar invagination), and there is a special relationship to developmental deformations of the cerebellum and brainstem (particularly type I Chiari malformation). A large proportion of cases of developmental syringomyelia have type I Chiari malformation, consisting of a descent of cerebellar tonsils below the foramen magnum as discussed in
Chap. 37. There is also a group of less frequent but well-described syringomyelias that derives from the acquired processes mentioned earlier such as intramedullary tumor (astrocytoma, hemangioblastoma, ependymoma) and from preceding traumatic or hemorrhagic necrosis of the spinal cord.
Wider experience with the pathology of developmental syringomyelia has led to the following classification, modified from Barnett and colleagues that unfortunately creates some confusion because it simulates the Roman numeral classification of the Chiari malformations, with which it is sometimes allied:
Type I. Syringomyelia with obstruction of the foramen magnum and dilatation of the central canal (developmental type)
A. With type I Chiari malformation
B. With other obstructive lesions of the foramen magnum, usually bony anomalies
Type II. Syringomyelia without obstruction of the foramen magnum (idiopathic developmental type)
Type III. Syringomyelia with other diseases of the spinal cord (acquired types)
A. Spinal cord tumors (usually intramedullary, especially hemangioblastoma)
B. Traumatic myelopathy
C. Spinal arachnoiditis and pachymeningitis
D. Secondary myelomalacia from cord compression (tumor, spondylosis), infarction, hematomyelia
Type IV. Pure hydromyelia (developmental dilatation of the central canal), with or without hydrocephalus
Historical note Although pathologic cavitation of the spinal cord was recognized as early as the sixteenth century, the term syringomyelia was first used to describe this process in 1827 by Ollivier d’Angers (cited by Ballantine et al). Later, following recognition of the central canal as a normal structure, it was assumed by Virchow (1863) and by Leyden (1876) that cavitation of the spinal cord had its origin in an abnormal expansion of the central canal, and they renamed the process hydromyelia. Cavities in the central portions of the spinal cord, unconnected with the central canal, were recognized by Hallopeau (1870); Simon suggested in 1875 that the term syringomyelia be reserved for such cavities and that the term hydromyelia be restricted to simple dilatation of the central canal. Thus, a century ago, the stage was set for an argument about pathogenesis that has not been settled to the present day.