Chapter 134 Posttraumatic and Idiopathic Syringomyelia
Syringomyelia denotes a collection of heterogeneous pathologic conditions characterized by abnormal, longitudinally oriented, fluid-filled cavities within the spinal cord (Fig. 134-1). Syringomyelia may be the result of congenital, traumatic, or neoplastic processes and therapy is directed at correcting the underlying pathology. However, in many cases the etiology of the condition remains unknown, and treatment results are usually unsatisfactory.1 This chapter offers guidance for diagnosis and management of patients with posttraumatic and idiopathic syringomyelia and provides an update on current research.
Although Charles Estienne first described the condition in 1546, the term syringomyelia was suggested by Olivier d’Angers in 1827 from the Greek syrinx, meaning “pipe,” “tube,” or “channel,” and myelos, meaning “marrow.”2,3 The term is restricted to this condition and should not be used to describe similar entities such as proteinaceous cysts or a terminal ventricle. The term hydromyelia refers to cystic dilation of the ependyma-lined central canal by cerebrospinal fluid (CSF; Fig. 134-2). However, the syrinx may dissect into the parenchyma of the spinal cord and its original connection with the central canal may disappear. Furthermore, the type of cellular lining is not a reliable criterion to distinguish between syringomyelia and hydromyelia. Because of these difficulties, the term syringohydromyelia has been used in the literature to refer to both entities.4–7 In this chapter, syringomyelia is used to denote all abnormal fluid-filled cavities in the spinal cord.8
Epidemiology
Syringomyelia affects primarily children and young adults, presenting on average before the third decade of life. Before the widespread availability of MRI, a prevalence of 9 per 100,000 and an incidence of approximately 0.44 cases per year were cited in the literature.9–13 Approximately 50% of patients presenting with syringomyelia have a Chiari malformation, whereas 25% present with a history of spinal cord trauma or arachnoiditis.9 The incidence of syringomyelia appears to be similar after quadriplegia or paraplegia. There is a statistically significant male preponderance in posttraumatic syringomyelia (80%), which follows the gender distribution in spinal cord injury.6,10,12 Of patients with a spinal cord injury investigated between 1 and 30 years after the initial insult, 21% to 28% have a syrinx and 30% to 50% have some degree of spinal cystic change. However, symptomatic syringomyelia is reported in only 1% to 9% of the spinal cord injury population.8,14–18
Classification
A distinction based on pathologic and MRI findings is made between communicating and noncommunicating syringomyelia.19–21 Communicating syrinx cavities are central canal dilations in continuity with the fourth ventricle and are often associated with hydrocephalus. They occur in children and young adults with conditions that obstruct CSF outflow from the fourth ventricle, such as Chiari II malformations or Dandy-Walker cysts. Only 10% of the lesions are of the communicating type.19
Noncommunicating syringomyelia can be further classified into central canal and extracanalicular syringomyelia.19,20 Noncommunicating central canal syrinx cavities are associated with Chiari I malformations, cervical spinal stenosis, spinal arachnoiditis, and basilar impression. The cavities are dilations of the central canal and are partially or completely lined by ependymal cells. Noncommunicating extracanalicular syrinx cavities are associated with spinal trauma, infarction, hemorrhage, or transverse myelitis. Most extracanalicular syrinx cavities are found in the vascular watershed zones of the central and dorsolateral gray matter. Extracanalicular syrinx cavities are irregular in shape and commonly have microglia, hemosiderin-containing macrophages, and gliosis of the cyst wall.20 In addition, the perivascular spaces may be enlarged and direct communication with the subarachnoid space at the dorsal nerve root entry zones or the ventromedian fissure can occur.20 Posttraumatic syrinx cavities are usually near the injury site and extend rostrally in 81%, caudally in 4%, and in both directions in 15% of cases.8,20,21
Etiology
The exact pathophysiologic process underlying posttraumatic syrinx formation is not clear, and various theories have been advanced. Two popular explanations are the hydrodynamic theory and a theory that assumes a differential between intracranial and spinal pressure caused by a valvelike effect at the foramen magnum.8 The second of these theories does not adequately account for the occurrence of noncommunicating syrinx cavities. The hydrodynamic theory proposes that fluid flows into the central canal from the fourth ventricle because of a “water-hammer–like” transmission of arterial or respiratory pressure. These theories have concentrated on canalicular syringomyelia associated with Chiari malformations, frequently at the expense of ignoring posttraumatic cases.22–24
Historically, the explanation for syrinx cavities isolated from the fourth ventricle was a secondary obstruction of the rostral central canal. However, this conclusion is difficult to reconcile with the observation that the central canal in humans is obstructed by segmental occlusions by the end of the third decade of life, and that there is a clear anatomic distinction between the central canal and syrinx cavity in most cases of posttraumatic syringomyelia.19,20,25 The initial formation of a spinal cord cavity may involve different mechanisms than the subsequent enlargement of the cavity to form a syrinx. Inflammatory responses to traumatic injury in the central nervous system result in localized edema and may lead to cyst formation.26–28
It has been reported that an intramedullary hematoma after spinal cord trauma increases the likelihood of developing a syrinx.6 Natural dissolution and absorption of the hematoma can leave a cystic cavity that can predispose the patient to syrinx formation. Furthermore, posttraumatic syrinx cavities are usually found in vascular watershed regions in the spinal cord.6,19 This finding implicates ischemia during the primary injury or subsequent inflammatory response as a contributor to syrinx formation.6,8,19
