[History, controversy and pathogenesis]

In this paper, we discuss the historical and pathophysiological aspects of syringomyelia. Defined as fluid cavities extending beyond several segments within the spinal cord this pathological entity is a condition with many possible causes. Hindbrain herniation is the commonest foramen magnum abnormality associated with the so called "hindbrain related syringomyelia". A history of birth injury, a small posterior fossa, an arachnoid scarring of the basal cisterna, a segmentation abnormality of the superior cervical vertebrae, a hydrocephalus or arachnoid cyst of the posterior fossa are often present in this context. Previous theories of the origin and the mechanism of syringomyelia progression have been controversly proposed. Gardner and colleagues postulated that the primary event is the incomplete embryonic opening of the outlets of the fourth ventricle. The fluid arrived in the syrinx along the embryologically natural route down the central canal. Their hydrodynamic theory states that with each arterial pulse, the outflow of CSF is transmitted from the fourth ventricle down to the syrinx via the central canal. Most patients have patent fourth ventricle foramina and evidence of communication between the ventricle and the syrinx is rare. Williams then proposed his "cranio-spinal pressure dissociation theory". Significant pressure differential occur daily during activities that increase intrathoracic pressure such as sneezing, coughing and could be transmitted to the spinal fluid from the epidural spinal veins. The progression of the cavity is better understood and analyzed with dynamic MR imaging and quantitative analysis. The CSF flow from the cranial to the spinal subarachnoid space results from the expansile motion of the brain during the cardiac cycle. The progression of the cavity is based on pressure acting on the surface of the cord and does not require any communication of the fourth ventricle with the central canal and the syrinx. The origin of fluid cavity remain questionable. Aboulker but also Ball and Dayan pointed out the role of the perivascular spaces and the DREZ which are involved in the communication between the perimedullar CSF, the spinal cord extracellular spaces and the central canal. Other causes of syringomylia include acquired conditions which could be grouped under the heading of "non-hindbrain related syringomyelia". Arachnoid scarring is related in many case to spinal trauma or occurs after spinal meningitis, spinal intradural surgery, peridural anesthesia, subarachnoid hemorrhage. Rarely an extra medullary compression is discussed. The mechanism involved is here again an alteration of the CSF flow at the spinal level.