Syringomyelia

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Chapter 28 Syringomyelia

Spyros Sgouros

Clinical Pearls

• The presence of syringomyelia requires lifelong neurological follow-up with repeat magnetic resonance imaging scans.

• When a patient with Chiari I or Chiari II malformation, with hydrocephalus and syringomyelia, has deteriorated radiologically, the shunt should be investigated first before any treatment for syringomyelia is considered.

• Chiari I patients with syringomyelia require decompression of the cerebrospinal fluid (CSF) obstruction at the craniovertebral junction.

• Treatment of syringomyelia caused by trauma requires decompression of the CSF at the level of the deformity or block followed by spinal stabilization, when clinically indicated. Treatment with a syringosubarachnoid or syringopleural shunt may stop the progression of the syrinx and neurological deterioration but the long-term prognosis remains guarded.

• Chiari II malformation is very different from that of Chiari I with respect to syringomyelia management.

Syringomyelia is the cystic cavitation of the spinal cord extending over a distance of more than two spinal segments. The syringomyelia cavity contains fluid with a consistency similar or equivalent to that of cerebrospinal fluid (CSF). The term is derived from the word syrinx, an ancient Greek word meaning wooden reed or tube (also describing a wooden ancient Greek musical organ, pan pipes, similar to a flute), and the word myelos, meaning the spinal cord. The word syrinx has its origins in Greek mythology in the tale of a beautiful and chaste nymph, Syrinx, being pursued to the river-bank by Pan, an amorous creature, half man and half goat. Syrinx, the nymph, in order to escape the approaching Pan, turned to the river nymphs for assistance. They turned her into a bundle of hollow reeds, which Pan cut down to make pan pipes. This new musical instrument of hollow tubes played plaintive music as the frustrated god’s breath blew over them. These pan pipes are known as syrinx. The word syringe is derived from this term, denoting a hollow tube.

The condition, syringomyelia, was described in 1876 by Leyden ¹ and in 1891 by Abbe and Coley.² Although several theories had been proposed in the 1960s and 1970s on the pathophysiology surrounding the formation and propagation of syringomyelia, the real progress came after the advent of magnetic resonance imaging (MRI) in the past decade, which facilitated noninvasive high-definition imaging of the condition, and later with the development of dynamic in vivo imaging of CSF movement.

Syringomyelia is most frequently associated with hindbrain herniation. Hindbrain herniation, or Chiari I malformation, is the herniation of the cerebellar tonsils for 5 mm or more beyond the foramen magnum below a line connecting the basion and opisthion (Figs. 28.1 to 28.3). In large clinical series 50% to 80% of patients with symptomatic Chiari I malformation (CMI) have syringomyelia. However, because these series contained only symptomatic patients it is difficult to know the precise incidence of syringomyelia among all patients with Chiari I malformation. It is unclear also if syringomyelia represents an evolutionary stage in the natural history of the Chiari I malformation or it starts ab initio, from the beginning of the creation of the malformation. In most patients, Chiari I malformation is regarded as congenital. It is unclear if the malformation changes with time, if the cerebellar herniation increases. Certainly, from studies in asymptomatic patients and from the author’s unpublished experience of following over 20 asymptomatic children for many years with yearly MRI scans, in most patients the cerebellar herniation does not increase with time.^3,⁴ Very few occasions of spontaneous resolution of Chiari I malformation have been reported.⁵ On the other hand, syringomyelia frequently changes with time. Acquired Chiari I malformation can develop following lumbar shunt placement or failure of a ventricular shunt, if the hindbrain is pulled down through the foramen magnum.^6,⁷ Syringomyelia may be associated also with various other conditions such as spinal arachnoid cysts; traumatic paraplegia (Fig. 28.4); postinfectious, postinflammatory, or tuberculous meningitis; chemical insults following use of intrathecal agents, antibiotics, or myelographic contrast agent Myodil-Pantopaque; epidural abscess; Pott’s disease; and idiopathic meningeal fibrosis. In addition, various forms of spinal dysraphism such as spinal lipoma (Fig. 28.5), diastematomyelia (Fig. 28.6), recurrent tethering after myelomeningocele repair (Fig. 28.7), thickened filum terminale, rare malformations affecting the foramen magnum region such as achondroplasia, posterior fossa arachnoid cysts, Chiari II malformation, syndromic craniosynostosis, osteogenesis imperfecta, mucopolysaccharidosis, and spinal cord tumors can also be associated with syringomyelia (Fig. 28.8).

FIGURE 28.1 Chiari I malformation, syringomyelia, and scoliosis. Magnetic resonance images of a 12-year-old boy who presented with progressive scoliosis and a limp. A, Head T1-weighted sagittal sequence. There is cerebellar tonsillar herniation below the foramen magnum down to the lower border of the arch of C1, and an extensive syringomyelia with septations in its cavity, that commences at the level of C4. B, Spine T2-weighted sagittal sequence. The syringomyelia cavity extends down to the lower thoracic spine. It is difficult to calculate the lower level, as the spine is not seen in one image due to the scoliosis. C, Spine T1-weighted coronal sequence. There is significant scoliosis and the syringomyelia cavity is seen in the midthoracic region. D, Postoperative spine T2-weighted sequence, 1 year after craniovertebral decompression. The syringomyelia cavity has decreased in size in comparison to the image in B.

FIGURE 28.2 Chiari I malformation without syringomyelia. Magnetic resonance images of a 12-year-old boy who presented with occipital headaches, severe enough to miss significant time from school. A, Head T1-weighted sagittal sequence. There is significant cerebellar tonsillar herniation below the foramen magnum down to the upper border of the lamina of C2. B, Head cardiac gated phase contrast cine sequence. There is reduced cerebrospinal fluid flow in the posterior aspect of the foramen magnum. C, Operative photograph taken through the microscope during craniovertebral decompression. The patient is in prone position, so the top of the head is toward the bottom of the photograph. The view is that of the region of the foramen magnum after opening the dura and the arachnoid. Toward the top of the image the two cerebellar tonsils are seen, protruding well below the foramen magnum. For orientation, the “burned” edges of the dura either side toward the top of the photograph are the site of the dural venous sinus, commonly seen in children and young adults, which have been coagulated for hemostasis, marking the level of the foramen magnum. The tips of the tonsils are well into the upper cervical subarachnoid space.

FIGURE 28.3 Syringomyelia that progressed to syringobulbia. Magnetic resonance images of an 11-year-old boy who presented with weakness of the right upper limb. A, Head T1-weighted sagittal sequence. There is significant cerebellar tonsillar herniation below the foramen magnum down to the lamina of C2. B, Spine T1-weighted sagittal sequence. There is extensive syringomyelia cavity down to T10 at least. The patient had craniovertebral decompression. He developed postoperative cerebrospinal fluid (CSF) leak and meningitis that was treated with antibiotics and lumbar punctures to relieve the CSF pseudomeningocele. C, Head T1-weighted sagittal sequence 6 months after operation. There are adhesions—arachnoid fibrosis at the craniovertebral junction—which cause tethering of the dorsal aspect of the lower medulla and upper cervical cord to the operation site. The syringomyelia cavity has extended cranially (compare with image in A) and is invading the lower medulla.

FIGURE 28.4 Post-traumatic syringomyelia. Magnetic resonance images of a 36-year-old man who suffered a motor vehicle accident that led to spinal injury and paraplegia. A, Spine T1-weighted sagittal sequence. There is wedge fracture of the body of T7 and corresponding fracture of the upper-posterior aspect of the body of T8. There is corresponding narrowing of the spinal canal and compression of the spinal subarachnoid space. B, Cervical spine T1-weighted sagittal sequence. There is a significant syringomyelia cavity extending cranially up the level of C2.

FIGURE 28.5 Syringomyelia with spinal lipoma. Magnetic resonance images of a 9-year-old boy who had surgical untethering for spinal lipoma at the age of 1 year, and now his walking has been deteriorating for several months. A, Lumbar spine T2-weighted sagittal sequence. There is tethering of the lower spinal cord at the region of the lower lumbar-sacral spine, at the site of the previously operated spinal lipoma. At the level of T10 there is the lower end of a syringomyelia cavity. B, Cervical spine T2-weighted sagittal sequence. The syringomyelia cavity extends cranially up to the level of C2; essentially, it is a holocord syringomyelia.

FIGURE 28.6 Syringomyelia with diastematomyelia. Spine T1-weighted coronal magnetic resonance image of a 6-year-old girl who had a hairy patch in the midline of the back at the level of the upper lumbar spine and developed a limp in her walking. There is a bony spur in the region of L1, which splits the spinal cord in two hemicords. Just cranially to the cord split there is a syringomyelia cavity, extending for two spinal levels.

FIGURE 28.7 Syringomyelia with Chiari II malformation. Magnetic resonance imaging (MRI) studies of a 7-year-old boy who was born with myelomeningocele and Chiari II malformation. He had closure of the myelomeningocele at birth and placement of a ventriculoperitoneal shunt for hydrocephalus. Owing to the low level (lumbosacral) of the myelomeningocele, he developed to become fully ambulant, walking with no support. For the last 4 months prior to this MRI examination, he has been complaining of pain in his legs and deteriorating walking. A, Head T1-weighted sagittal sequence. There is a Chiari II malformation, with cerebellar herniation down to the level of C3. Other features of the Chiari II malformation are seen such as hypoplastic corpus callosum, small posterior fossa, caudal migration of the medulla, and hypertrophic interthalamic adhesion. B, Spine T1-weighted sequence. There is a syringomyelia cavity extending from T2 to T7.

FIGURE 28.8 Syringomyelia with spinal cord astrocytoma. Spine T1-weighted sagittal sequence with intravenous contrast agent in a 5-year-old girl who presented with back pain and a limp. There is a contrast-enhancing intramedullary spinal cord tumor at the levels T8-L1. Cranially to the tumor there is a large syringomyelia cavity extending up to C2. The tumor was excised and proved to be grade II astrocytoma.

Syringomyelia affects commonly the cervical and upper thoracic spinal cord, although it can extend throughout the entire spinal cord, creating a holocord syringomyelia (see Figs. 28.3B and 28.5). If untreated, it follows a progressive and often relentless course over a number of years. Usually the neurological deterioration is gradual, but sudden deterioration has been described. The neurological function of arms and legs is at risk and patients in whom, despite treatment, the syringomyelia cavity continues to extend end up quadriplegic. The cavity may extend both proximally and distally and the tension of the fluid inside it continues to increase. Longstanding syringomyelia provokes gliosis formation in the cavity walls and the surrounding cord. Gliosis formation has been seen in postmortem examinations, in which the cavity walls are seen to be clearly lined by gliotic tissue.⁸ A drain inserted in the syringomyelia cavity may be encased by gliotic tissue.⁸ Even though surgical treatment may lead to radiological collapse of the syringomyelia cavity, sometimes this is not associated with clinical improvement. Gliosis inside the cord may lead to progressive neurological loss even after successful surgery and collapse of the cavity.⁹ The syringomyelia cavity can reach the upper cervical cord and medulla, becoming syringobulbia (Fig. 28.3C); it may cause deterioration of neurological function in the arms and eventually affect the function of the lower brainstem, producing bulbar palsy, respiratory failure, and death.

Epidemiology

The incidence of Chiari I malformation in the general population is estimated at 7%, but not all subjects are symptomatic. The increasing use of MRI scanning in the investigation of headaches has led to an increase in the incidence of this diagnosis. It is unclear, though, what percentage of patients with radiological evidence of Chiari I malformation have symptoms attributable to it. Syringomyelia has a prevalence of 8.4 cases per 100,000 population. It is estimated that in the United States, 21,000 Americans suffer from syringomyelia. More accurately, the figure is likely to be considerably higher, as even today, despite the wide availability of MRI scanning, the condition is underdiagnosed. The incidence of post-traumatic syringomyelia is around 1% to 3% of cases of paraplegia with a male predominance and relatively young age, and parallel to the incidence or trauma for age and gender.¹⁰^–¹² Why so few patients develop syringomyelia following spinal cord injury is not known. The average time between spinal injury and symptomatic presentation is 7 years (range: 3 months to 30 years). Post-traumatic syringomyelia is rare in children.

Pathophysiology of Syringomyelia

In the majority of patients syringomyelia is associated with an obstruction of CSF flow in the subarachnoid space. Surgical treatments are designed to remove such a subarachnoid obstruction. In most patients the block in CSF flow is in the region of the foramen magnum and is due to hindbrain hernia (Chiari I and Chiari II) or in other sites in any of the other conditions mentioned before.

The mechanism of formation of syringomyelia has not been adequately explained yet. Most of the studies that attempt to explain the development of syringomyelia focus on the model of hindbrain hernia in Chiari I malformation. In experimental studies, dye injected in the spinal subarachnoid space has been found in the central canal, believed to have traveled there along the Virchow-Robin spaces beside the vessels of the cord. It is unclear when and how the creation of the cavity commences.¹³ There are reports of a presyringomyelia state seen in MRI scans of patients with Chiari I malformation, with upper cervical spinal cord swelling, which subsequently developed into syringomyelia.¹⁴^–¹⁶ These reports are very few, in contrast to the very high numbers of patients with established syringomyelia that have been reported and are seen in clinical practice. This by implication may signify that the presence of a presyringomyelia state may be the exception rather than the rule. Several theories have been proposed on the formation and propagation of syringomyelia. The most notable ones, in chronological order, are Gardner’s hydrodynamic theory, Williams’ theory of “pressure dissociation,” and Oldfield’s theory of increased subarachnoid CSF pressure wave:

In the 1960s Gardner observed that in patients with hindbrain hernia the foramen of Magendie was partially obstructed by a membrane. He believed that fluid entered the central canal in the obex at the floor of the fourth ventricle due to the “water hammer” effect of the arterial pulsations on CSF flow. Fluid accumulated inside the central canal creating the syringomyelia cavity—communicating syringomyelia or hydromyelia.^17,¹⁸ This led to the surgical concept of obex plugging, where a piece of muscle or cotton wool was used to plug the obex. Syringomyelia not communicating with the central canal was called noncommunicating. MRI studies have shown that in the majority of the patients there is no direct communication between the obex and the syringomyelia cavity. Cardiac gated cine phase contrast MRI (cine PC-MRI) studies have shown that during systole the syrinx cavity contracts, whereas if Gardner’s theory was correct it should expand under the force of inflowing CSF. Gardner’s theory is regarded as imperfect and the term communicating syringomyelia has been used with less frequency. The technique of obex plugging has fallen out of favor, if for no other reason than the material used to plug the obex caused significant arachnoid fibrosis and adhesions, with subsequent recurrence of the syringomyelia.

In the early 1980s Williams described the concept of “pressure dissociation” between the subarachnoid CSF spaces of the head and the spine secondary to the block of hindbrain hernia, based on in vivo CSF pressure measurements and in vitro biomechanical studies, which he performed.¹⁹ Every time a Valsalva maneuver takes place (e.g., coughing) the pressure from the chest and abdomen is transmitted to the spinal canal through the valveless venous plexus around the vertebral bodies. Normally, pressures are equalized within the spine rapidly. In the presence of a partial subarachnoid block, fluid is forced upward past the block more efficiently than it can run down again. This leads to a collapsed theca below the block, which exerts a suction effect on the spinal cord (the “suck” mechanism). Fluid that has entered the cord cavity can extend the cavity (the “slosh” mechanism).^9,^20,²¹ Williams’ theories were partly verified by MRI scan observations, and Williams’ demonstrations of his “suck and slosh” theory with models were impressive. Using cine PC-MRI it has been shown that in the presence of hindbrain hernia CSF movement in the region of the foramen magnum differs between systole and diastole. However, Williams’ observations of pressure differential were not confirmed by other studies of intraoperative CSF pressure measurements.²² It is possible that the sitting position he used to obtain his readings contributed to exaggerated cranial-spinal pressure difference readings, owing to CSF leakage from the site of the spinal measuring needle.

Using preoperative PC-MRI scanning and intraoperative CSF pressure measurements in patients with syringomyelia and Chiari I malformation, Oldfield’s group observed that CSF velocity was increased at the foramen magnum but CSF flow was decreased. Cervical subarachnoid CSF pressure was increased, spinal CSF compliance was decreased, and syrinx CSF flowed caudally during systole and cranially during diastole.^22,²³ They surmised that the prolapsed cerebellar tonsils act like a piston, partially occluding the subarachnoid space, and create a state of increased subarachnoid pressure waves, which compress the spinal cord from without and force CSF into the syrinx cavity and propagate it with each heartbeat.

All these theories do not explain precisely how and where CSF enters the cord and the mechanism of formation of septations inside the syringomyelia cavity. In the past decade, research with in vitro computer simulations using parameters gained from MRI has focused on the hydrodynamic properties of CSF, how they are altered in the presence of a subarachnoid block, and how the spinal cord viscoelastic properties relate to these changes.²⁴^–³¹ It is hoped that as our noninvasive imaging of CSF flow evolves, more insights will be gained on the formation and propagation of syringomyelia.

In patients who suffer spinal cord trauma, cord contusion leads to edema, blood effusion, and subsequent cord liquefaction during the first few months after injury in up to 50% of patients. Subsequently, cavity formation results and may be referred to as the primary cyst.³²^–³⁴ Commonly, the cyst at the site of the injury is separated from the syringomyelia cavity by an intact septum. The syringomyelia cavity may decompress above and below the injury site following treatment, but the primary cysts may remain unchanged.^35,³⁶ As the acute injury settles, inflammation is followed by adhesions in the subarachnoid space and gliosis in the cord. Obstruction of the CSF pathways may be compounded by narrowing of the bony spinal canal from the spinal fracture. Fluid may enter the cord at the fracture site, which is often thin-walled.

Syringomyelia is associated with other pathological conditions such as dysraphic states, spinal cord tumors, and spinal arachnoid cysts. There is no unifying, satisfactory explanation of how CSF enters the spinal cord, and it may be analogous to the situation in hindbrain-related syringomyelia, but that remains unproved. The presence of arachnoid adhesions appears to be an important event in the formation of syringomyelia. The presence of syringomyelia associated with tethered cord or low-lying cord from a thickened filum terminale is difficult to explain applying the concept of subarachnoid CSF block. Thus, the exact pathophysiology of a terminal syrinx or dilatation of the central canal associated with a low-lying spinal cord remains unknown.