Gardner’s hydrodynamic theory could not explain syringomyelia. In some cases, for example, the foramina of Luschka and Magendie are found to be patent, and other abnormalities of the posterior fossa or foramen magnum that block CSF flow are not in evidence. Furthermore, in many cases, including several we have inspected, serial histologic sections have failed to demonstrate a connection between the fourth ventricle and the syrinx in the spinal cord or of a widening of the central canal above the syrinx (see also Hughes). Gardner’s theory has been questioned on other grounds. Ball and Dayan calculated the pulse-pressure wave transmitted into the cord to be of such low amplitude as to be unlikely to produce a syrinx. In their view, the CSF around the cervical cord, under increased pressure during strain or physical effort because of subarachnoid obstruction at the craniocervical junction, tracks into the spinal cord along the Virchow-Robin spaces or other subpial channels. Over a prolonged period, abetted perhaps by traumatic lesions, small pools of fluid coalesce to form a syrinx. In their view, originally the syrinx forms independently of the central canal, but eventually the two may become connected, allowing secondary enlargement of the canal (hydromyelia ex vacuo). The findings of Heiss and colleagues lend support to this theory. They found that progression of syringomyelia is produced by the compressive effect of the cerebellar tonsils, which partially occlude the subarachnoid space at the foramen magnum and create pressure waves that compress the spinal cord from without and not from within; the pressure waves propagate syrinx fluid caudally with each heartbeat. This hardly exhausts the list of hypotheses that have been offered over the years but none of them has been confirmed.
The authors favor the type of hydrodynamic mechanism as postulated originally by Gordon Holmes and elaborated by Ball and Dayan. In this view, a relationship exists between basal cranial, cervical spine, the cerebellospinal Chiari malformation, syringomyelia, and disturbed hydrodynamics of perispinal CSF. Logue and Edwards documented several cases of syringomyelia in which the foramen magnum was obstructed by a lesion other than a Chiari formation, for example, by dural cyst, localized arachnoiditis, atlantoaxial fusion, simple cerebellar cyst, and basilar invagination (see Williams for a review of the numerous hypotheses of causation).
Irrespective of its mode of origin, the syrinx first occupies the central gray matter of the cervical portion of the spinal cord, usually independent of the central canal but sometimes extending into it. It interrupts the crossing pain and temperature fibers in the anterior commissure at several successive cord segments. As the cavity enlarges, it extends symmetrically or asymmetrically into the posterior and anterior horns and eventually into the lateral and posterior funiculi of the cord. It may enlarge the spinal cord. The cavity is lined with astrocytic glia and a few thick-walled blood vessels, and the fluid in the cavity is clear and in our patients, has had relatively low protein content, like intracranial CSF.
The cavitation nearly always arises in the cervical portion of the cord and can only reach the thoracic and lumbar portions by its extension from the cervical region, sometimes by a small, flat and thin, and eccentrically placed. Either a cavity or a glial septum may extend asymmetrically into the medulla, usually in the vicinity of the descending tract of the fifth cranial nerve to create a syringobulbia.
The clinical picture of syringomyelia is so characteristic that diagnosis is seldom in doubt. Now one can obtain spectacular demonstrations of the syrinx, either traumatic or developmental (Fig. 42-14), Chiari malformations, and other foramen magnum lesions by MRI of the sagittal planes of the brain and spinal cord (see Fig. 37-4). Also, hours after a CT myelogram, contrast material fills the syrinx and the central canal directly, possibly by diffusion from the surface of the cord.