Several theories, nothing proven: The exact pathophysiological mechanisms behind syrinx formation remain poorly understood, but several theories have been proposed, particularly for Chiari-associated syringomyelia. It is likely that, given the different etiologies, there exist different mechanisms responsible for syringomyelia.
Hydrodynamic theory: In the late 1950s, Gardner proposed his hydrodynamic theory, which was expanded in the 1970s. This theory attributes Chiari I-related syrinx formation to a constantly pulsating CSF flow through the obex opening and into the spinal cord in the presence of compression of posterior fossa structures and closure of the lateral fourth ventricular foramina (62, 72, 73).
Williams’s modification of Gardner’s theory: On the basis of a series of invasive measurements of cranial and spinal pressures, Williams proposed a similar theory in which hindbrain adhesions could result in epidural venous congestion, leading to transient pressure differentials between the cranial and spinal compartments, particularly during Valsalva maneuvers. This, in turn, would cause a delay of caudad CSF flow, while normal cranial flow is maintained. As a result, fluid is “sucked” from the ventricle into the central canal (63).
Piston theory of Oldfield: Oldfield, using dynamic MRI observations and intraoperative ultrasound information, further expanded the perivascular CSF dissection theory as he postulated that the downward displacement of CSF into the spinal cord and subarachnoid space happens routinely and not only during Valsalva maneuvers. According to his group, normal CSF pulsations provide a more or less continuous reason for fluid to enter the spinal cord, and the displaced cerebellar tonsils act like a piston as they are propelled caudally with systole, thus creating a pressure wave within the entrapped subarachnoid space and syrinx (63).
Venous compression theory: A more recent theory combines some of the above-mentioned concepts and explains syrinx formation as a result of increase in hydrostatic venous pressure that results from epidural vein compression at a particular location in the spinal cord depending on the exact etiology and that in turn results in transudation with accumulation of extracellular fluid and inability to drain. The fluid buildup causes progressive cyst enlargement. This cyst may or may not communicate with the central canal. One of the most important features of the venous congestion theory is that it might explain syrinx formation due to most causes, unlike the previous theories that mainly referred to posterior fossa/foramen magnum pathology (17, 47).
Enlarged spinal cord: Macroscopically, the spinal cord often appears enlarged, tense, and with no leptomeningeal thickening.
Etiology influences location: Location of the syrinx depends on the etiology, as syringomyelia related to a cranial abnormality such as Chiari I malformation tends to occur in the cervical region, while syringomyelia associated with tethered cord or a more distal abnormality tends to occur in the lower third of the spinal cord (terminal syringomyelia).
Indirect evidence for genetic factors: Existence of familial cases indicates possible genetic factors, but no clear associations have been described. This is particularly true in Chiari-related syringomyelia (48, 49).
Effacement and thinning of long tracts: In transverse section, a syrinx typically crosses the midline. As it enlarges, it causes effacement of the grey matter located anteriorly with the anterior motor neurons remaining visible ventrally and thinning of the posterior and lateral columns (50). Pressure from within the fluid-filled cavity can damage traversing nerve fibers and cause retrograde degeneration and damage to motor and sensory tracts.
Degeneration and gliosis: Walls of the cavity may contain degenerated neuroglia and neural elements as well as sheaths of Schwann cells. Astrocytic hyperplasia is present in more chronic cavities, and these cavities may have collagen-lined septations with hyalinized blood vessels traversing them. These can give a septated appearance on imaging (50).