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Syringomyelia is best defined as a confluent collection of fluid within the spinal cord. The fluid closely resembles or is identical to cerebrospinal fluid (CSF). As such, the clinician must distinguish syringomyelia from spinal cord edema, a condition in which the increased tissue fluid is not identified as confluent but is interstitial, and from tumor-associated cysts. The fluid in tumor cysts generally has higher protein content than CSF, and it may also have other tumor-related constituents. Most importantly, the treatment of tumor cysts is quite different from that of syringomyelia.
Classification
Both from the diagnostic point of view and with respect to treatment planning, it is useful to classify syringomyelia as follows:
- 1.
Syringomyelia related to abnormalities at the foramen magnum
Tonsillar descent (Chiari malformation); arachnoid veil with fourth ventricle outlet obstruction
- 2.
Primary spinal syringomyelia
- a.
Posttraumatic, including postsurgical
- b.
Postinflammatory: infection, neoplastic meningitis
- c.
Related to abnormalities of the arachnoid: arachnoid cysts, presumably developmental in origin
- d.
Related to focal structural lesions narrowing the subarachnoid space
- (1)
Tumor
- (2)
Disk
- (1)
- e.
Idiopathic
- a.
Two other conditions must be noted when considering a classification of syringomyelia: presyrinx and hydromyelia. Presyrinx is defined on the basis of imaging technology as a focal area of spinal cord edema often adjacent to a confluent syrinx cavity. A mechanism of fluid accumulation similar or identical to that postulated for syringomyelia is considered the basis of the presyrinx state. The potential for progression of such tissue fluid accumulation over time to form a confluent cavity is the reason for the designation of the presyrinx state.
Hydromyelia, which is also defined as a confluent CSF cavity within the spinal cord, is considered a remnant of the central canal of the spinal cord, which is a normal structure in embryogenesis. It has a characteristic imaging appearance, fusiform in the longitudinal axis and round and central within the spinal cord on axial images ( Figs. 29-1 and 29-2 ). The spinal cord is generally not expanded by these small, slitlike cavities, which are not associated with symptoms and are not considered pathologic entities. When these findings are present in adults, they generally do not change over time. Hydromyelia is not uncommonly encountered in children, but involution of the central canal occurs most rapidly during the first 10 years of life.
The rostro-caudal extent of the syrinx cavity must be considered. Syrinx cavities may be confined to one region of the spinal cord, such as cervical or thoracic, or they may involve both these areas. Cavities may also extend through the entire length of the spinal cord, a condition often referred to as holocord syringomyelia.
These various entities are discussed in the following sections.
Pathophysiology
A general understanding of the formation of syringomyelic cavities is very important to a consideration of treatment principles and therapeutic options.
Formation
The mechanism of formation of syrinx cavities associated with Chiari malformations has been studied more extensively than has that of other types of syringomyelia. The theory that syrinx cavities fill from the fourth ventricle is of historical interest and has mostly been abandoned, largely because such a communication cannot be demonstrated by modern imaging studies in most patients with Chiari malformation–related syringomyelia. Progressive enlargement of a syringomyelic cavity frequently occurs even in the absence of such a communication.
The concept proposed by Oldfield and colleagues is that the pulsatile action of the cerebellar tonsils acts like a piston on an essentially enclosed CSF compartment, the spinal subarachnoid space below the tonsils. Severe constriction of the subarachnoid space by the cerebellar tonsils within the dura and bony confines at the level of the foramen magnum prevents wide dispersion of the fluid pressure wave. This piston-like action is postulated to force fluid into the spinal cord parenchyma along the Virchow-Robin (V-R) spaces, and the fluid ultimately coalesces to form a confluent cavity. Investigators have suggested that the presence of a segment of the residual central canal within the spinal cord may favor the coalescence of fluid migrating along the V-R spaces. Arteriolar pulsations along the V-R spaces appear to aid in propelling the fluid centrally, but the work by Bilston, Brodbelt, Stoodley, and Fletcher also makes it clear that the mechanism for fluid accumulation within the spinal cord is likely to be far more complex.
Current treatment of Chiari malformation–related syringomyelia is based on the premise that reducing the piston-like action of the cerebellar tonsils on the spinal subarachnoid space will inactivate the filling mechanism of the syringomyelic cavity. This treatment is accomplished by (1) enlarging the subarachnoid space at the level of the foramen magnum so that the subarachnoid space of the posterior fossa is in unobstructed continuity with the spinal subarachnoid space and (2) by reducing the size of the cerebellar tonsils to diminish their effectiveness as pistons acting on the spinal subarachnoid fluid. Although the technical aspects of treatment vary widely among surgeons, depending on the patient’s age group (pediatric versus adult) and other considerations, the syrinx cavities generally respond well to treatment based on these principles. Long-standing cavities and cavities in older patients are less likely to undergo complete collapse.
The formation of primary spinal syringomyelia can be considered analogous to that described for Chiari malformation–related syringomyelia, with an arachnoid barrier fulfilling the same role as the cerebellar tonsils in producing an incomplete but significant obstruction of the spinal subarachnoid space. This can be most clearly visualized when one considers an arachnoid band or web, presumably but not necessarily developmental in origin, stretching across the subarachnoid space to form an arachnoid cyst. Such a web would then propagate the pulsatile pressure wave of the CSF to the subarachnoid fluid compartment just caudal to the web, which behaves as an enclosed fluid compartment. The reason that such pulse waves still exert significant pressure on the CSF is that compartmentalization of the spinal subarachnoid space by the web reduces the size of the compliance reservoir as compared with the intact subarachnoid space. A different mechanism may apply in some patients with trauma to the spinal cord. Trauma can result in focal tissue disruption within the spinal cord, thus permitting more direct entry of fluid into the cord tissue. A cavity, once established, may extend, as discussed later.
An arachnoid cyst and web comprise the simplest and most straightforward example of a focal obstruction of the spinal subarachnoid space. Traumatic scars following spinal injury may also be focal, but because of the crushing nature of many such injuries, as well as associated subarachnoid bleeding that promotes scar formation, the rostrocaudal extent of subarachnoid scarring may be much greater and may extend over several vertebral levels. Scarring can occur ventral or dorsal to the spinal cord, it may be circumferential, or it may develop as a combination of these distributions. Scar tissue tends to thicken over time, perhaps because it is exposed to the continuous pulsations of CSF, and this may explain the time interval between spinal injury and the development of syringomyelia. It is not uncommon for years to elapse between injury and symptoms of syringomyelia. A strict correlation may not necessarily exist between the severity of spinal injury and the development of a syrinx cavity.
Postinflammatory syringomyelia may have an even more complex distribution of scar formation. When scarring follows meningitis, it obviously can take place throughout the spinal subarachnoid space. The same can be said of scarring that may follow spontaneous subarachnoid hemorrhage or neoplastic meningitis, even when this disorder has been treated successfully. Of infectious organisms, some, such as the tubercle bacillus, seem to evoke a much stronger scar tissue response than do other acute bacterial or viral infections.
Tumors, whether or not they are accompanied by a true tumor cyst, may compress the subarachnoid space and thereby set the stage for similar development of syringomyelia. Not uncommonly, a spinal cord tumor may have both a true cyst containing somewhat proteinaceous fluid and a syrinx cavity. Syringomyelia has been reported to form in relation to disk protrusion, with the disk acting similarly to narrow the subarachnoid space.
Progression
A large, fluid-filled cavity within the spinal cord is exposed to complex dynamic forces that may propel the fluid rostrally, caudally, or in both directions, thereby contributing to the rostrocaudal enlargement of the syrinx cavity over time. Williams particularly studied the role of distention of spinal epidural veins (Batson plexus) in propelling the fluid cavity within the spinal cord, dissecting through the spinal cord, and enlarging the cavity. Alterations in CSF pressure may contribute by externally compressing the spinal cord containing a cyst and thus extending the syrinx caudally. Dural compliance may also play a role in this process. The presence of a potential space between rests of ependymal cells, or even a distinct residual central canal, may facilitate rostrocaudal enlargement of a syrinx cavity.
The treatment of primary spinal syringomyelia consequently is also predicated on removing the partial obstruction of the subarachnoid space and thereby allowing the CSF pressure wave to be propagated along the length of the spinal canal. This acts to inactivate the force driving fluid into the spinal cord. Only when such an approach is technically not feasible must other fluid diversion strategies be considered.
Preoperative Considerations
Clinical Presentation
The clinical manifestations of syringomyelia are varied and relate, in part, to the underlying pathogenesis. Thus, patients with syringomyelia related to Chiari malformation and similar abnormalities may have symptoms of partial CSF obstruction at the foramen magnum, symptoms related to compression of the brainstem by the descended and impacted cerebellar tonsils, and symptoms resulting from the associated syringomyelia. The last type of symptoms also may vary, depending on the anatomic level of the syrinx cavity.
Only the most commonly encountered symptoms are listed here. They may be categorized as follows:
- a.
Symptoms resulting from partial obstruction of CSF flow at the foramen magnum
Tussive headaches and other strain-related activities
- b.
Symptoms resulting from direct brainstem compression
Swallowing difficulty
Voice changes
Nystagmus
Balance problems
Sleep apnea
- c.
Symptoms related to syringomyelia
Sensory loss, which classically involves the upper limbs, but may extend further down
Upper extremity weakness
Hand and upper extremity atrophy
Gait impairment
Spasticity of lower extremities
Bowel and bladder control problems
Dysesthetic pain
Symptoms in patients with primary spinal syringomyelia most commonly fall into category c, but they may vary to some degree, depending on the underlying origin. In types of syringomyelia related to scarring of the arachnoid, a significant time interval may occur between the insult (i.e., trauma, infection, subarachnoid hemorrhage) and the development of symptoms related to syringomyelia. In patients with posttraumatic syringomyelia, the clinical presentation often is a mixture of symptoms and signs attributable to the spine and spinal cord injury and symptoms related to the development of the syrinx cavity. The time interval between injury and recognition of symptoms may be measured in years and is sometimes masked by neurologic deficit resulting directly from the injury, such as paraplegia. In such patients, the first manifestation of the presence of syringomyelia may be a subtle ascent of an existing sensory level.
Findings on examination related to syringomyelia are essentially findings of spinal cord dysfunction. They include motor findings of weakness and atrophy, long tract signs such as spasticity, and findings of sensory deficit, which may or may not be asymmetric. Asymmetry of neurologic deficit is sometimes seen, and the Brown-Séquard syndrome is a classic example.
Postinflammatory syringomyelia tends to be quite extensive in rostrocaudal extent, and dysesthetic pain often is an early and dominant symptom. Symptoms and signs of cauda equina arachnoid scarring would not be unexpected in some of these patients.
Diagnostic Evaluation
Syringomyelia Related to Chiari Malformation
Magnetic resonance imaging (MRI) is the most widely used imaging modality to detect syringomyelia associated with Chiari malformations. The obvious advantage of MRI is that it is noninvasive and causes no disturbance of CSF dynamics. Depending on the particular case series, tonsillar descent is accompanied by true syrinx formation in approximately half of adult patients with Chiari malformation, but the relationship between severity of tonsillar descent and development of a syrinx cavity is not linear. Syringomyelia may also develop as a result of posterior fossa abnormalities other than tonsillar descent, such as outlet obstruction of the fourth ventricle or an arachnoid membrane at the level of the foramen magnum. T2-weighted MRI images, which highlight the fluid spaces including the cisterns surrounding the base of the cerebellum as well as the fluid in the syrinx cavity, tend to exaggerate the size of the fluid compartments, whereas T1-weighted images are anatomically more precise. It is uncommon to see cystic spinal cord tumors in combination with tonsillar descent, but such coincidental findings do occur and justify the use of a gadolinium contrast–enhanced study to rule out tumor in selected cases in which the presence of a tumor may be suspected.
Imaging of the brain is important in patients with Chiari malformation to determine whether they have coexisting hydrocephalus or a mass lesion and to assess the particular architecture of the posterior fossa that may be critical in determining the optimal surgical procedure for a particular patient. Although the midsagittal image presents a classic view of the descended tonsils, often with an associated “medullary beak” resulting from long-term compression, the axial image at the level of the foramen magnum is also very important. It often shows distortion of the lower brainstem at the cervicomedullary junction by the tonsils, with obliteration of the subarachnoid space between the brainstem and the tonsils. Asymmetric descent of the tonsils is frequently identified. Cardiac gated CSF flow studies are particularly helpful in sorting out borderline cases in which the tonsils may be somewhat low in position but not clearly pointed or peglike as a result of chronic pressure. When cardiac gated flow studies demonstrate the presence of a normal CSF flow pattern dorsal to the tonsils and the lower cerebellum, tonsillar descent is not likely to be the underlying factor responsible for development of the syringomyelic cavity. Constructive interference with steady-state MRI sequences may help to define obstructive disease that is not otherwise recognized.
Primary Spinal Syringomyelia
MRI is currently also the most widely used imaging modality for the diagnosis of primary spinal syringomyelia. T1-weighted images demonstrate the intramedullary fluid-filled cavity. A study with intravenously administered contrast medium (gadolinium) is frequently necessary to rule out the presence of an associated spinal cord tumor. This is particularly true when the patient has no evidence of tonsillar descent. T2-weighted images may show the presence of an arachnoid web near the lower end of the syrinx cavity, a capability improved by use of the high-resolution T2 sequence scan, which is very useful in demonstrating fine anatomic details, such as septa, in the subarachnoid space. Cardiac gated flow studies, such as those used to study CSF flow at the level of the foramen magnum, have not been widely available for exclusively spinal studies because overlying vertebral bone interferes with imaging the flow patterns of CSF around the spinal cord.
The configuration of the syrinx cavity, particularly when the caudal end looks blunt, may suggest obstructive subarachnoid pathology, such as a web ( Fig. 29-3 ). In such cases, consideration should be given to performing a myelogram, followed by a thin-section computed tomography scan of the region of interest. Such a study may give a very clear delineation of obstructive arachnoid disease and may indicate the precise level for a surgical approach ( Fig. 29-4 ). Performing myelography through a C1-C2 puncture, rather than by the lumbar route, has the advantage of allowing pooling of contrast material at the level of the web. This pooling may not occur when contrast material is introduced by the lumbar route in situations in which the obstructive subarachnoid membrane may act as a one-way valve.
